1 Phenylamine is an important intermediate compound for the production of dyes.

(a) Phenylamine can be synthesised from benzene in two steps. Draw the structural formula of the intermediate Y in the scheme below, and suggest reagents and conditions for steps I and II.

reagent + conditions for step I conc. HNO3 + H2SO4 [(aq) negates]

reagent + conditions for step II Tin/Sn or iron/Fe [NOT Zn] + (conc.)HCl or LiAlH4, [NOT NaBH4] or H2 + Ni [NOT Pt]

(b) Phenylamine is a weak base.

(i) Write an equation showing phenylamine acting as a base.

C6H5NH2 + H+/HCl/H2O C6H5NH3+ [+ Cl-/OH-]

(ii) How would you expect its basicity to compare with that of ammonia? Explain your answer.

less basic than NH3.................................................................................................................................................

.................................................................................................................................................

(c) Dyes can be made from phenylamine by first converting it into benzenediazonium chloride, and then coupling this with a phenol.

Class Reg Number

Candidate Name .......................................................................

Chemistry H2 9746Tutor TuteeRevision Exercise 15: Miscellaneous

1

(i) State the reagents and condition needed for step III.

HNO2 or nitrous (nitric(lII)) acid or NaNO2 + HCI0·C < T < 10·C

(ii) What reagent is the phenol dissolved in for step IV to be effective?

NaOH (aq) or dilute or in solution (or in words} [NOT NH3(aq)]

(iii) Suggest the structural formula of the dye formed when benzenediazonium chloride is coupled with 2-methylphenol.

S02/qp4/Q72(a) What do you understand by the term standard electrode potential?

The EMF of a cell made up of the test electrode and a standard hydrogen electrode. EMF measured under standard conditions of T, P and concentration

(b) The following cell was set up between a copper electrode and an unknown metal electrode M2+(aq) /M(s). The standard cell potential was found to be 0.76 V, and the copper foil was the positive electrode.

(i) Use the Data Booklet to calculate the standard electrode potential of theM2+(aq) /M(s) system.

2

Eleft = Eright – Ecell = 0.34 - 0.76 = -0.42 (V)

(ii) Draw an arrow over the voltmeter symbol in the above diagram to show the direction of electron flow through the voltmeter.

(iii) Predict the outcomes of the following situations. Describe what you might see and write ionic equations for any reactions that occur.

I A rod of metal M is dipped into a solution of 1moldm–3 CuSO4.

pink/red solid/ppt or copper will be formed or blue solution fades or M dissolves/corrodes

Cu2+ + M → Cu + M2+

II Dilute sulphuric acid is added to a beaker containing a powdered sample of metal M.

hydrogen/gas evolved or M dissolves

(do not allow "M dissolves" for [2] marks in both I and II)

M + 2H+ → M2+ + H2

(c) Because of its increased scarcity, cheaper copper ornaments are no longer made from the solid metal, but from iron that has been copper plated.

(i) Complete the following diagram showing the set-up for a copper electroplating process. Show clearly the polarity (+/–) of the power source, and suggest a suitable electrolyte.

polarity of d. c. source: - is on the left, + is on the right

electrolyte is Cu2+(aq)/CuSO4/CuCl2/Cu(NO3)2 etc. or name

(ii) A current of 0.500 A is passed through the electroplating cell. Calculate the time required to deposit a mass of 0.500 g of copper on to the ornament.

moles of Cu = 0.5/63.5 = 7.87 x 10-3

moles of e- = 2 x 7.87 x 10-3 = 1.57 x 10-2

no. of coulombs = 96500 x 1.57 x 10-2 = 1517 (C)time = 1520/0.5 = 5034 seconds = 50.7 min

S03/qp4/Q1

3

3(a) Give an expression for Ka as applied to the weak acid RCO2H.

.................................................................................................................................................

(b) The Ka values for three carboxylic acids are listed in the table below.

(i) Describe and explain the trend in acid strength illustrated by these values.

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

(ii) Calculate the pH of a 0.100moldm–3 solution of ClCH2CO2H.

(iii) Calculate the pKa value for Cl2CHCO2H.

(c) The acid ClCH2CO2H features in the industrial synthesis of the important weedkiller 2,4-D.

(i) Suggest a possible reagent for reaction I.

.................................................................................................................................................

(ii) What type of reaction is

reaction I, .................................................................................................................

4

reaction II .................................................................................................................

(iii) Describe a test (reagents and observations) that would distinguish phenol from compound A.

reagents ...................................................................................................................

observation with phenol ...........................................................................................

observation with compound A ..................................................................................

S05/qp4/q5

4 The antipyretic (fever-reducing) drug antifebrin can be made from benzene and ethanoic acid by the following route.

(a)(i) What type of reaction is reaction I?

Electrophilic substitution or nitration

(ii) Suggest the reagents and conditions for reaction I.

HNO3 + H2SO4(both) conc., and at 50°C ≤ T ≤ 60°C

(iii) Complete the following scheme showing the mechanism of reaction I, by drawing appropriate formulae in the three boxes.

5

(b)(i) What type of reaction is reaction II?

Reduction

(ii) Suggest the reagents and conditions for reaction II.

Sn/Fe/Zn/SnCl2 + HCl/H+/H2SO4 (but not conc. H2SO4) or H2 + Ni/Pt (not LiAlH4)

(c) Suggest the reagents and conditions for reaction III.

PCl5/PCll3/SOCll2/POCl3 (+ heat) aq negates

(d)(i) Apart from the benzene ring, name the functional group in antifebrin.

An amide, not peptide

(ii) What reagents and conditions are needed to hydrolyse antifebrin?

Heat with H3O+ or heat with OH-(aq)Or warm (not heat/reflux) with aqueous amidase/peptidase/protease notenzyme/trypsin/chymotrysin/pepsin/papain etc.

S05/qp4/q6

5 This question is about the elements of Group VII, the halogens.

(a) Complete the following table.

Halogen Colour Physical state at room temperature

Chlorine green/yellow Gas

Bromine orange/red Liquid

iodine black Solid

(b) Concentrated sulphuric acid is added to separate solid samples of magnesium chloride, magnesium bromide, and magnesium iodide.

(i) Describe, in each case, one observation you would be able to make.

6

MgCl2 white fumes/steamy fumes (of HCl)

MgBr2 red colour (of Br2) or steamy fumes (of HBr)

MgI2 purple colour (of I2) or black solid (I2) or yellow solid (S) or stinking gas (H2S)

(ii) Give an equation for the reaction of concentrated sulphuric acid with magnesium chloride.

MgCl2 + H2SO4 → MgSO4 + 2HCl

Allow

MgCl2 + 2H2SO4 → Mg(HSO4)2 + 2HCl(c) When dilute nitric acid and aqueous silver nitrate are added to a solution of a magnesium

halide, MgX2, a pale cream precipitate is formed. This precipitate is soluble in concentrated aqueous ammonia but not soluble in dilute aqueous ammonia.

(i) What is the identity of the precipitate?

AgBr/silver bromide

(ii) Give an equation, with state symbols, for the reaction of the precipitate with concentrated aqueous ammonia.

AgBr(s) + 2NH3(aq) → Ag(NH3)2Br(aq)

(d) A hot glass rod is plunged into separate gas jars, one containing hydrogen chloride and one containing hydrogen iodide.

(i) For each gas, state what you would observe, if anything, and write an equation for any reaction that takes place.

HCl no reactionHI purple vapour/black solid2HI → H2 + I2

(ii) Explain your answer to (i) in terms of enthalpy changes.

bond energy in HCl is highbond energy in HI is lower/more easily broken

(iii) What is the role of the hot glass rod in any reaction that occurs?

hot glass rod provides activation energy

S06/qp2/q36 Compounds containing the allyl group, CH2=CHCH2

−, have pungent smells and are found in onions and garlic. Allyl alcohol, CH2=CHCH2OH, is a colourless liquid which is soluble in water. Allyl alcohol behaves as an alkene and as a primary alcohol.

(a) Give the structural formula of the organic compound formed when allyl alcohol is

(i) reacted with Br2,

BrCH2CHBrCH2OH

7

(ii) heated under reflux with an acidified solution of Cr2O72– ions.

CH2=CHCO2H

(b) When allyl alcohol is reacted with MnO2 at room temperature, propenal, CH2=CHCHO is formed.

What type of reaction is this?

Oxidation

(c) Allyl alcohol may be converted into propanal, CH3CH2CHO, by using a ruthenium(IV) catalyst in water.

The reactant and the product are isomers.What form of isomerism do they display?

structural or functional group isomerism

(d) Allyl alcohol can be converted into propanal in two steps without the use of a ruthenium(IV) catalyst.

What reagents and conditions would be used for each step?

step I

reagent(s) H2

condition(s) Ni catalyst and heat or Pt at room temperature

step II

reagent(s) acidified dichromate(VI)

condition(s) heat not under reflux or distil off aldehyde as it is formed

(e) By considering your answers to (b) and (d), suggest what is unusual about the single-step reaction in (c).

both oxidation and reduction have occurred

(f) Suggest the structural formula of the organic compound formed when allyl alcohol is

(i) reacted with cold, dilute MnO4– ions,

HOCH2CHOHCH2OH formedCH2=CH- forms HOCH2CHOH-

8

-CH2OH is unchanged

(ii) heated under reflux with acidified MnO4– ions.

HO2CCO2H

S06/qp2/q47 The amino acids tyrosine, lysine and glycine are constituents of many proteins.

(a) State the reagents and conditions you could use to break proteins down into amino acids.

HCl or H2SO4 or H+ or acidconc(if HCl only)/dilute/aqueous + heat

(b) Draw an asterisk (*) around each chiral centre in the above molecules.

two rings only (1 ring around the α-C of tyrosine & 1 around the α-C of lysine)

(c) In aqueous solution amino acids exist as zwitterions. Draw the zwitterionic structure of glycine.

+NH3CH2CO2- (or displayed formula)

(d) For each of the following reactions, draw the structure of the organic compound formed.

(i) glycine + excess NaOH(aq)

NH2CH2CO2- (Na+) (either -CO2

- Na+ or -CO2Na but NOT –CO-O-Na)

(ii) tyrosine + excess NaOH(aq)

(Na+) –O-C6H4-CH2CH(NH2)CO2- (Na+)

(iii) lysine + excess HCl (aq)

(Cl-)+NH3(CH2)4CH(NH3+)CO2H (Cl-)

9

(iv) tyrosine + excess Br2(aq)

HO-C6H2Br2-CH2CH(NH2)CO2H (if shown, Br at 2,6 to OH group)

(e) Draw the structural formula of a tripeptide formed from all three of these amino acids, showing clearly the peptide bonds.

(f) The formula of part of the chain of a synthetic polyamide is shown below.

(i) Identify the repeat unit of the polymer by drawing square brackets around it on the above formula.

(ii) Draw the structures of the two monomers from which the polymer could be made.

S06/qp4/q48 This question concerns the following reaction scheme, which shows the synthesis of

compound B and some subsequent reactions.

10

(a) A structural isomer of B is also obtained in Step 1. Suggest a possible structure for this isomer.

(b)(i) C contains an alcohol group.

Give the reagent(s) required to carry out Step 2, classify the type of reaction occurring and state the type of alcohol produced.

LiAlH4 / NaBH4 Reduction / nucleophilic additionsecondary alcohol

(ii) C exhibits stereoisomerism. Draw the 2 isomers formed by C.

?

(d)(i) Give the reagent(s) required for Step 3. Name the type of reaction occurring.

11

concentrated H2SO4

OR conc. H3PO4

OR Al2O3

OR names

dehydration OR elimination

(ii) Explain why D does not exhibit geometrical isomerism.

has two H atoms/two atoms the same at one end of the double bond

(e) The reagent for Step 4, to produce E, is hydrogen bromide, HBr.

Draw the mechanism for this reaction.

(f) Explain, in terms of all the intermolecular forces present, why C, CH3CH(OH)C6H4OCH3, has a higher boiling temperature than A, C6H5OCH3.

A has van der Waals’/dispersion/London forces and dipoledipole forcesC has H bonding and van der Waals/dispersion/London and dipole-dipole forcesH bonding stronger than van der Waals/dispersion/ London forces and dipole-dipole forces

Edexcel 6246(02)9(a) The following data were obtained by studying the reaction between compounds A, B and C

at a constant temperature.

12

(i) Deduce the order of reaction with respect to A.

2

(ii) Deduce the order of reaction with respect to B.

1

(iii) Deduce the order of reaction with respect to C.

0

(b) The rate equation for the reaction between compounds D and E at a given temperature is rate = k[D]2[E]

The initial rate of reaction is 8.36 × 10–4 mol dm–3 s–1 when the initial concentration of D is 0.84 mol dm–3 and the initial concentration of E is 1.16 mol dm–3.

Calculate a value for the rate constant, k, at this temperature and deduce its units.

AQA CHM4W QPJan 07 10 The amino acid alanine is shown below.

(a) Give the systematic name for alanine.

2-aminopropanoic acid or 2-aminopropionic acid

(b)(i) Draw the structure of the dipeptide formed from two alanine molecules, showing clearly the full structure of the peptide linkage.

13

(ii) Draw the structure of the organic compound formed by the reaction of alanine with propan-2-ol in the presence of a small amount of the catalyst concentrated sulphuric acid.

(iii) Draw the structure of the N–substituted amide formed by the reaction of alanine with ethanoyl chloride. Name the type of mechanism involved.

(nucleophilic) addition-elimination

(c) A solution was prepared by reacting alanine with an equal number of moles of hydrochloric acid. This solution was titrated with aqueous sodium hydroxide. The titration curve obtained is shown below.

14

(i) Draw the structure of the alanine species present at point X on the curve.

(ii) Draw the structure of the alanine species present at point Y on the curve.

AQACHM4WQPJan0711 An acylium ion has the structure R–C+=O where R is any alkyl group.

In the conversion of benzene into phenylethanone, C6H5COCH3, an acylium ion CH3CO reacts with a benzene molecule.

(a)(i) Write an equation to show the formation of this acylium ion from ethanoyl chloride and one other substance.

(ii) Name and outline the mechanism of the reaction of this acylium ion with benzene.

15

(b) Phenylethanone, C6H5COCH3, reacts with HCN according to the equation below.

(i) Name and outline the mechanism of this reaction.

Nucleophilic addition

(ii) The product formed exists as a racemic mixture. State the meaning of the term racemic mixture and explain why such a mixture is formed in this reaction.

AQACHM4WQPJan0712 Two Australian chemists have developed a method for measuring the ethanol content of

alcoholic drinks which is based on the oxidation of ethanol by acidified dichromate(VI) ions.

In this method of analysis, the intensity of colour of a solution of dichromate(VI) ions, to which alcoholic drink has been added, is compared with that of a solution of dichromate(VI) ions alone.

This comparison is made using a colorimeter. Because solutions of dichromate(VI) ions have an intense orange colour, light with a wavelength of 440 nm is chosen for use in the colorimeter. The difference in absorbance at this wavelength between the two solutions is measured. Under the conditions used in the analysis, this change in absorbance is directly proportional to the amount (in moles) of dichromate(VI) ions which has reacted. It is also, therefore, directly proportional to the amount (in moles) of ethanol which was added in the form of alcoholic drink.

Fig. 12.1 summarises the wavelength ranges of the colour of visible light.

16

Fig. 12.1: Wavelengths and colours of visible light

(a) Using the information in Fig. 12.1, explain why light with a wavelength of 440 nm is used for measuring the concentration of the orange solution of dichromate(VI) ions.

.................................................................................................................................................

.................................................................................................................................................

The method used to analyse the contents of a bottle of white wine is given below.

25.0 cm3 of a solution containing dichromate(VI) ions were pipetted into a 50.0 cm3 volumetric flask. 50.0 l of white wine were added to the volumetric flask from a micropipette. (1 l = 1 x 10–6 dm3 = 1 x 10–3 cm3.) Dilute sulphuric acid was added to the volumetric flask to bring the volume of

the contents up to 50.0 cm3. The absorbance of a sample of this mixture was recorded. The absorbance of a

'blank' solution made up in the same way but without the addition of any ethanol was also recorded.

The change in absorbance between the blank solution and the sample was compared with the calibration graph shown in Fig. 12.2.

The amount of ethanol (in mol), which was added to the solution of dichromate(VI) ions in the 50.0 l of the white wine, was read off from the calibration graph. The ethanol concentration in the white wine was calculated from this.

Fig. 12.2: Calibration graph for the absorbance, at 440 nm, of solutions of dichromate(VI) ions to which ethanol has been added.

(b) Describe what measurements you would make in order to plot a calibration graph such as the one in Fig. 12.2.

.................................................................................................................................................

17

.................................................................................................................................................

.................................................................................................................................................

(c) The bottle of white wine which was analysed was stated on the label to contain 12% by volume of ethanol. The change in absorbance recorded in the analysis of this wine was 0.45. Calculate, showing the steps clearly, the concentration of ethanol in the wine (in cm3

per dm3), and confirm that the value quoted on the label is correct.

(Density of ethanol = 0.789 g cm–3; Mr(ethanol) = 46)

(d) This method of analysis for ethanol concentration uses the same chemistry as that of a 'breathalyser' which measures the ethanol content of breath. The suspect blows through a tube containing potassium dichromate(VI) crystals. If alcohol is present in the breath, the crystals turn green as the orange Cr2O7

2– ions are oxidised to Cr3+ ions.

The half equation for the oxidation of ethanol is shown below:

Oxidation of ethanol:

CH3CH2OH + H2O CH3COOH + 4H+ + 4e–

Complete the equation below for the reduction of dichromate(VI) ions and write a balanced overall equation for the process that takes place in a breathalyser tube.

Reduction of dichromate(VI) ions:

Cr2O72– + .......... + .......... .......... + ..........

Overall equation:

.................................................................................................................................................

OCR Salters 2000 Chemistry By Design SAM (?)13

(a)

The decomposition of dinitrogen pentoxide, N2O5, at 45°C was investigated. The reaction that takes place is shown below.

2N2O5 4NO2 + O2

In an experiment, N2O5 with a concentration of 0.60moldm–3 was decomposed at 45°C.At this temperature, the reaction has a constant half-life of 1200 s.

(i) How can you tell that this reaction is first order with respect to N2O5?

constant half-life

(ii) Write down an expression for the rate equation of this decomposition.

rate = k [N2O5]

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(iii) Complete the graph opposite to show how the [N2O5] changes over the first 3600 s of the reaction.

curve downwards getting less steepcurve goes through 1200,0.30; 2400,0.15; 3600,0.075

(iv) The rate of this reaction can be determined from this graph.Show on the graph how the rate can be measured after 1200 s.

tangent shown on graph at t = 1200 s

(v) The rate can also be calculated from the rate equation. The rate constant for this reaction is 6.2 × 10–4 s–1.

Calculate the initial rate of this reaction. State the units.

3.7(2) x 10−4 mol dm–3 s–1

ecf possible from (ii) using [N2O5]x

(2nd order answer: 2.2(3) x 10−4)

19

20

(b) A student investigated the hydration of 2-methylpropene, (CH3)2C=CH2, with dilute aqueous acid to form 2-methylpropan-2-ol, (CH3)3COH.

The following mechanism has been proposed for this hydration.

step 1 (CH3)2C=CH2 + H+(aq) (CH3)3C+ rate determining stepstep 2 (CH3)3C+ + H2O (CH3)3COH + H+(aq)

(i) Step 1 is the rate-determining step for this hydration.

What is meant by the term rate-determining step?

slow step

(ii) Write a balanced equation for the overall hydration reaction.

(CH3)2C=CH2 + H2O → (CH⎯ 3)3COH

(iii) Suggest the role of H+(aq) in this mechanism. Explain your reason.

H+ is a catalystH+ used in first step and formed in second step/regenerated/ not used up

(iv) Use the mechanism above to suggest the rate equation for this hydration.

rate = k [(CH3)2C=CH2] [H+]

OCR 2816/ Jun 200514 Groundwater usually contains iron compounds and therefore water from wells will contain

significant amounts of iron compounds. The main problem with household water containing iron compounds is the staining it causes to laundry, porcelain and plumbing fittings.

(a) Water containing iron in an oxidation state of +2 is known as ‘clear water’ since it appears colourless. However, on leaving the tap it may become coloured and it is then referred to as ‘red water’. ‘Red water’ contains iron(III) compounds.

(i) Give the formula of the complex ion which causes the ‘red’ colour.

[Fe(H2O)6]3+

(ii) What causes the colour change when clear water leaves the tap?

Oxidation of Fe(II) ions/Fe(II) ion loses electron/ Fe(II) converted to Fe(III) by oxygen/air

(iii) If the ‘red water’ is made slightly alkaline, for example by adding sodium hydroxide solution, a red–brown precipitate will form.

Write an ionic equation, including state symbols, for the formation of the red–brown precipitate.

21

Fe3+(aq) + 3OH¯(aq) → Fe(OH)3(s)

(b) ‘Iron stains’ contain iron(III) compounds and can be removed using a variety of products available in the supermarket. One commonly used chemical is ethanedioic acid. It is used in stain removers as the disodium salt.

(i) Draw the full structural formula of the ethanedioate ion, C2O42–.

(ii) Name the type of reaction that occurs when ethanedioate ions in aqueous solution react with ‘red water’ to form green [Fe(C2O4)3]3– ions.

Ligand exchange/complex formation/ligand substitution/Ligand displacement

(iii) Explain why a green substance looks green.

particular frequencies/wavelengths of light/radiation in visible region absorbed; hence colour transmitted is light NOT absorbed,(in this case green/ complementary colour is seen

(c) Iron stains can be removed by adding a solution of a suitable reducing agent. Use the data in the table below to identify a reducing agent which can reduce iron(III) ions under standard conditions. Explain your answer and write an equation for the reaction that occurs.

SO32- ; Eө for half-reactions are more negative than Eө for Fe2+/ Fe3+ half-reaction / electrons will flow to Fe3+ . 2Fe3+ + SO3

2- + H2O → SO42- + 2H+ + 2Fe2+

AS Level Chemistry B SAM Unit F33415 Hydrogen peroxide is a mild oxidising agent which is used in the restoration of oil paintings.

Paintings darken over time as some of the metal ions in the paints react with atmospheric pollutants. Hydrogen peroxide can be used to convert these unwanted dark coloured compounds to white products.

22

(a) Write down the half-equation (ion-electron equation) for hydrogen peroxide, H2O2, acting as an oxidising agent. This reaction takes place in acid solution and water is the only product.

H2O2 + 2H+ + 2e–→ 2H2O

(b) The concentration of an H2O2 solution can be found by titration of samples of this with aqueous potassium manganate(VII) of known concentration using acidic conditions.

(i) 25.0 cm3 of a concentrated solution of hydrogen peroxide is diluted to 250 cm3. 10.0 cm3 of this diluted H2O2 reacted with exactly 17.2 cm3 of 0.0200 mol dm–3 MnO4

− solution.

The equation for the reaction taking place is given below.

2MnO4¯(aq) + 6H+(aq) + 5H2O2(aq) 2Mn2+(aq) + 8H2O(l) + 5O2(g)

Calculate the concentration of the concentrated H2O2 solution.

Give your answer to an appropriate number of significant figures.

Moles of MnO4− = (17.2/1000) x 0.0200 ;

moles of H2O2 = 2.5 x (17.2/1000) x 0.0200 ecf; mark is for the 2.5 ratio concentration of undiluted = 2.5 x (17.2/1000) x 0.0200 x (1000/10.0) x 10 ; concentration 0.860 mol dm-3

(ii) The concentration of the hydrogen peroxide solution used for treating paintings must not be greater than 3.0%. Assume this means 3.0 g of H2O2 in 100 cm3 of solution.

Is the undiluted solution of H2O2 suitable to be used for treating paintings? Show your working.

Mr of H2O2 = 34 ; mass of H2O2 in 100 cm3 of water = 34 x 0.86 x 100/1000 = 2.9 g or max moles of H2O2 allowed in 100 cm3 of water = 3.0/34 = 0.088 mol therefore YES ecf from (iii) and Mr of H2O2.

(c) Restorers of paintings are instructed to make up the solutions of hydrogen peroxide in a polythene bottle with pure water rather than tap water. Traces of transition metal ions, such as Fe2+, present in tap water, can catalyse the decomposition of hydrogen peroxide.

(i) Write the equation for the decomposition of hydrogen peroxide into water and oxygen. Give

23

the state symbols.

2H2O2(aq) → 2H2O(l) + O2(g)

(ii) The decomposition of hydrogen peroxide is a redox reaction. Explain how Fe2+(aq) ions can catalyse a redox reaction in aqueous solution.

Fe2+ can reduce/lose electrons to one reactant to form a product and Fe3+ ; Fe3+ can then oxidise/lgain electrons from a reactant to reform Fe2+ ; both reactions are faster than the uncatalysed reaction/ Ea for both is lower AW

(iii) A solution of hydrogen peroxide stored in a glass bottle at room temperature was found to be completely decomposed after two weeks.

Describe an experimental procedure you could use to measure the oxygen produced when hydrogen peroxide decomposes.

Show how you would use your results to find the initial rate of the reaction.

Measure volume of oxygen by syringe/over water (1); plot graph of volume of O2 v time (1); find gradient at time = 0 (1).

(iv) The reaction is found to be first-order with respect to hydrogen peroxide with a rate

constant of 2.0 × 10–6 s–1 at 298 K.

Calculate the rate of decomposition of a 2.0 mol dm–3 hydrogen peroxide solution at 298 K.

Rate of decomposition = k x [H2O2] (1); = 2.0 x 10-6 (x 5.0) mol dm-3 s-1 = 4.0 x 10-6 mol dm-3 s-1

AS Level Chemistry B SAM Unit F33416 Sea squirts (Ascidia) have an extensive biochemistry based on vanadium, and some of

them have blood containing vanadium compounds. Vanadium can be present in three oxidation states in sea squirts. The effectiveness of a biochemistry based on vanadium compounds is thought to arise, in part, from the ease with which these three oxidation states can be interconverted.

Electrode potential values are given below for the reduction of oxygen to water and for half-reactions which involve V3+, VO2

+ and VO2+.

These three ions are commonly found in aqueous solutions of vanadium compounds. These electrode potential values were obtained under standard conditions, except that pH = 7.

24

(a) In the space below, draw a labelled diagram of the apparatus which could be used to measure the standard electrode potential of a half-reaction involving ions in solution.

High resistance voltmeter (or potentiometer) Salt bridge Hydrogen half-cell (or other reference cell; allow copper ions/copper) Vanadium ions half-cell (or general ion/ion half-cell) Standard conditions

(b)(i) Predict, using the electrode potentials given above, the oxidation state of vanadium that would be stable in aqueous solution at pH = 7 in the presence of air.

Stable oxidation state would be expected to be +5

(ii) Explain how the electrode potential data allow this prediction to be made.

Electrons flow from more negative to more positive helf-cell Both vanadium half-cells are more negative than oxygen half-cell

or V(+4) to V(+5) is still sufficiently negative to supply electronsto oxygen half-cell)

(c) In reactions in living cells, transition metal ions are often present in the form of biochemical complexes. State why the electrode potential data given above do not allow a reliable prediction to be made of the oxidation state of vanadium that would be stable in the presence of oxygen in such reactions.

Electrode potential values depend on the ligands present in a complex

(d)(i) Vanadium has an atomic (proton) number of 23. Complete the space below to show the electron configuration of a vanadium atom.

1s2 2s2 2p6 3s2 3p6 4s23d 3 (or 3d 34s2)

(e) Ionisation enthalpy changes for vanadium and calcium are given in the table below.

(i) Write the chemical equation for the third ionisation of vanadium.

V2+(g) V3+(g) + e–

25

(ii) Explain, using the ionisation enthalpy data in the table, why vanadium can form high oxidation states in its compounds whereas calcium does not form oxidation states greater than +2 in its compounds.

Comment to the effect that: after the loss of 2 electrons from calcium, furtherionisation requires a large energy input Comment to the effect that: vanadium can lose a greater number of electronsbefore a large energy input is required

L A Level Chemistry Salters SAM17 The South American bombadier beetle has an unusual way of dealing with predators.

When threatened, it releases an explosive spray of boiling hot corrosive liquid, aimed with precision from a gland on the tip of its abdomen.

The gland has two separate chambers; one containing a solution of hydroquinones and excess hydrogen peroxide, the other containing a mixture of two enzymes, catalase and peroxidase. When the beetle is attacked, the mixture of hydroquinones and hydrogen peroxide is released into the reaction chamber containing the enzymes. As the chemicals mix, a hot spray is released explosively from the gland.

The enzyme peroxidase catalyses the reaction of hydroquinones with hydrogen peroxide. The reaction is exothermic:

Equation 17.1At the same time the decomposition of hydrogen peroxide is catalysed by the enzyme catalase in a second exothermic reaction:

Equation 17.2(a) Explain (in terms of the reactions shown in Equations 17.1 and 17.2) why the spray

released from the gland of the bombadier beetle is:(i) Hot

Reactions are exothermic or heat/energy given out by reactions

(ii) Released explosively

Oxygen (not just gas) (or steam) is produced (as well) so pressure builds up in gland (or forced out)

(b) Name the functional group which is present in:

hydroquinone Phenol (or hydroxyl)benzoquinone Ketone (or carbonyl)

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(c) The effect of the concentration of hydrogen peroxide on the rate of the decomposition reaction in Equation 17.2 was investigated in the laboratory. This was done by adding solid catalase to hydrogen peroxide solution and measuring the rate at which oxygen was given off.

The table below shows the results of a series of experiments in which the initial rate of the reaction was found for different starting concentrations of hydrogen peroxide. The concentration of catalase and the temperature were the same in each experiment.

(i) Plot this data on the grid below.

(ii) What is the order of the reaction with respect to hydrogen peroxide? Explain your reasoning.

Initial rate of reaction is proportional to starting concentration of H2O2 so reaction is first order with respect to hydrogen peroxide

(iii) Using the decomposition of hydrogen peroxide as an example, explain the meaning of the term rate constant.

It is the constant in the rate equation that relates rate to concentrations or by writing the rate equation:rate of reaction = k [H2O2] [catalase]n and identifying k

L A Level Chemistry Salters SAM18 The lead-acid battery was invented by French physicist Gaston Planté in 1859. It consisted

of nine cells connected in parallel, each of which was constructed by loosely rolling a sandwich made of two long electrodes of lead foil, separated by strips of flannel. The coiled assembly was then inserted into a cylindrical jar, which was filled with dilute sulfuric acid. The battery was then charged by connecting the cells to a supply of electricity. The same basic design is still used in car batteries today over one hundred years later.

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(a) Use the standard electrode potential below to calculate the Ecell of a single cell of a typical

lead/acid car battery.

PbO2 (s) + SO42– (aq) + 4H+ (aq) + 2e– PbSO4 (s) + 2H2O (l) E = +1.685V

PbSO4 (s) + 2e– Pb (s) + SO42– (aq) E = –0.356V

(b)(i) The solubility product of lead (II) sulfate at 25oC is 1.3 x 10–8. Write an expression for the solubility product, including its units and calculate its solubility in distilled water.

(ii) How will the solubility of lead (II) sulfate be altered in the presence of sulfuric acid electrolyte present in lead/acid batteries? Explain your answer.

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(iii) If the sulfuric acid of a lead/acid battery were replaced by hydrochloric acid of the same concentration, the battery would not function as a rechargeable cell but as an electrolytic cell.

What essential property of lead (II) sulfate allows sulfuric acid to be used as an electrolyte

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in a lead/acid battery, but not lead (II) chloride and hydrochloric acid, even though both are ionic salts and strong acids, respectively?

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(c) The electrolyte in a car battery is analysed and is found to have a concentration of 3.75 mol dm–3 sulfuric acid and a density of 1.23 g cm–3.

(i) Calculate the percentage by mass of sulphuric acid in the battery electrolyte.

A Level Chemistry Notes and Questions Topic 13End of Paper

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