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C6 Chemistryout there

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1.3 Electrolysis, current and time

Electrolysis uses dc current (if ac was used, the ions would be changing direction fifty timesa second!)

� Electrolysis involves moving electrons to discharge ions. The more electrons that move, themore ions that get discharged

� Current (in amps) is a measure of how many electrons are moving each second

� To increase the amount of chemical discharged, you can:

a) increase the current

b) increase the time

1 Electrolysis

1.2 Electrolysis words

Electrolysis - lysis is splitting, electro iselectricity, so electrolysis means ‘splittingwith electricity’

Positive electrode is the anode

Negative electrode is the cathode

Electrolyte - the liquid

As opposite charges attract, the positive ions(cations) go to the negative electrode(cathode)

Negative ions (anions) go to the positiveelectrode (anode)

In this case, zinc is discharged at the cathode and chlorine is discharged at the anode

1.1 Electrolysis and ions

Ions are formed when atoms gain or loseelectrons

� Positive ions (cations) are formed when atomslose electrons e.g.

Mg - 2e- →  Mg2+

Which is the same as:-

Mg →   Mg2+ + 2e-

� Negative ions (anions) are formed when atomsgain electrons e.g.

Cl2 + 2e- → 2Cl-

In electrolysis we use the energy of thebattery or power pack to change the ionsback into atoms e.g.

Mg2+ + 2e- → Mg

Positive ions have electrons added tochange them back into atoms

� Negative ions have electrons taken awaywhen they are turned back into atoms

Discharged - when ions are turned backinto atoms they lose their charge, so

we say the ions are discharged

electrolyte

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1.4 Electrolysis of water

� A very few water molecules separate into H+ and OH- ions

H2O → H+ + OH-

� As there are so few ions in pure water, dilute sulfuric acid (H2SO4) or dilute sodium hydroxide(NaOH) solution is used as the electrolyte to increase the conductivity

The H+ ions are discharged at the cathode to make H2 gas

2H+ + 2e- → H2

The OH- ions are discharged at the anode to make O2 gas (and water)

4OH- - 4e- → 4OH

4OH → 2H2O + O2

1.5 Half equations (HT)

The equation for each electrode only gives halfthe story, so is called a half equation

Cathode = reduction (as electrons added)

Anode = oxidation (as electrons removed)

OILRIG - oxidation is loss, reduction is gain

1.6 Electrolysis of lead bromide

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1.7 Electrolysis of copper sulfate solution

Oxygen gas is produced at the anode. The cathode gains mass ascopper is added to the surface from the solution. The electrolyte(copper sulfate solution) gradually changes to a dilute solution ofsulfuric acid.

Electrolysis of Copper Sulfate - Carbon ElectrodesAnode

Copper formed

Cu2+ + 2e- → Cu

Oxygen formed

4OH- - 4e-→ O2 + 2H2O

Cathode

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2.2 Fuel cells in spacecraft

� Fuel cells provide electrical energy inspacecraft as they are:-

lightweight

compact

have no moving parts

produce water for the astronauts

2 Fuel cells

2.1 Fuel cells - basic ideas

� Reaction between hydrogen and oxygen isexothermic - gives out energy

Hydrogen + oxygen → water + energy

� If hydrogen and oxygen are mixed togetherand reacted, heat energy is given out

In a fuel cell the hydrogen and oxygen arekept apart (one at each electrode) - the energy isreleased as electrical energy

Advantages of the fuel cell compared to a petrol engine:-

1) no pollutants such as carbon dioxide produced (only water)

2) more efficient

3) fossil fuels are non-renewable

4) lots of hydrogen from decomposingwater

HT only:-

Disadvantages of fuel cells

1) making hydrogen and oxygen for the fuel cells uses energy which may have come from fossil fuels

2 fuel cells often contain poisonous catalysts - pollute when fuel cell finished with

2.3 Reactions at the electrodes in a fuel cell(HT)

Anode (+ve electrode)

H2 - 2e- → 2H+

The H+ ions move through the electrolyte to the cathode.

Cathode (-ve electrode)

2O2 + 4e- → 2O2-

The O2- ions then react with the H+ ions that have moved through the electrolyte to make water

O2- + 2H+ → H2O

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3 Redox reactions

3.1 Oxidation and reduction

� Foundation definition (FT)

oxidation - adding oxygen

reduction - removing oxygen

� Higher definition (HT)

oxidation - loss of electrons

reduction - gain of electrons

OILRIG - oxidation is loss, reduction isgain

Redox - oxidation and reduction alwaysoccur together

3.2 Redox equations (HT)

Reactions at electrodes are redox reactions

H2 - 2e- → 2H+ oxidation

2O2 + 4e- → 2O2- reduction

� Other redox reactions:-

Fe - 2e- → Fe2+ oxidation

Fe2+ - e- → Fe3+ oxidation

Cl2 + 2e- → 2Cl- reduction

3.3 Rusting - a redox reaction

Iron and steel need oxygen and water to rust

Iron + oxygen + water → hydrated iron(III) oxide (rust)

Iron loses electrons (oxidised)

Oxygen gains electrons (reduced)

Preventing rusting:-

1) oil and grease (barrier to oxygen and water)

2) paint (barrier to oxygen and water)

3) tin plate (barrier to oxygen and water)

4) galvanising (zinc layer provides barrier to oxygen and water, zinc reacts before iron)

5) sacrificial protection (more reactive metal reacts before iron)

6) alloying (alloying chemicals prevent rusting e.g. stainless steel)

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3.4 Rusting (HT)

Tin plating - tin only acts as a barrier to waterand oxygen.

� If tin is scratched, iron loses electrons (oxidises)instead of tin as iron is the more reactive metal

Sacrificial protection - more reactive metal thaniron (e.g. magnesium or zinc) loses electrons(oxidises) instead of iron

3.5 Displacement reactions

� A more reactive metal will displace aless reactive metal from a compound e.g.

zinc + iron chloride → zinc chloride + iron

Order of reactivity:-

magnesium

zinc

iron

tin

morereactive

3.6 Displacement reactions (HT)

� Symbol equations needed for displacementreactions e.g.:-

Mg + ZnCl2 → MgCl2 + Zn

The metal (Mg above) is oxidised (loses electrons)

The metal ion (Zn above) is reduced (gainselectrons)

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4 Alcohols

4.1 Alcohols

Alcohols have an -OH group

� Molecular formula of ethanol C2H5OH

Alcohols are not hydrocarbons (ashydrocarbons have carbon and hydrogen atoms

only)

4.2 Not just ethanol (HT)

� There is a whole series of alcohols, not justethanol

General formula for an alcohol is CnH(2n+1)OH

Displayedformulaof ethanol

4.3 Uses of alcohols

Alcoholic beverages (drinks)

Solvent to dissolve chemicals (industrialmethylated spirits)

Fuel for cars

4.4 Making alcohol - fermentation

To make alcohol by fermentation need:-

glucose

yeast

water

no oxygen - otherwise alcohol turned to ethanoic acid (vinegar)

25 - 50 ºC as reaction catalysed by enzymes in yeast

� Equation for fermentation:-

glucose → carbon dioxide + ethanol

C6H12O6 → 2CO2 + 2C2H5OH

Ethanol produced is separated by fractional distillation

Ethanol produced byfermentation is a renewable fuelas the glucose is extracted from

plants

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4.5 Making ethanol fromethene

Ethanol for industrial use can also be madefrom ethene by hydration

Ethene and steam are passed over aphosphoric acid catalyst at high temperatureand pressure

ethene + water → ethanol

C2H4 + H2O → C2H5OH

4.6 Comparing the two methods of makingethanol (HT)

Makingethanol from ethene is non-

renewable as ethene comes fromcrude oil

Fermentation

Fromethene

Process Batch Continuous

ConditionsLowtemperatureand pressure

Hightemperatureand pressure

Speed Slow FastSustainability Renewable Non-

renewablePurification Distillation

neededLittlepurificationneeded

% yield andatomeconomy

Low High

4.7 Conditions used in fermentation (HT)

If temperature is too low, yeast is inactive

If temperature is too high, enzymes in yeast are denatured

� If oxygen gets in, ethanoic acid (vinegar) is formed

� Apparatus for fractional distillation of ethanol:-

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5.1 CFCschlorofluorocarbons

� CFCs (chlorofluorocarbons) are moleculeswith carbon, fluorine and chlorine atoms

� When discovered, CFCs were used a lot inaerosols and refrigerators as:-

a) were chemically inert (did not react)

b) had a low boiling point (so were gases)

c) were insoluble in water

However, was later discovered that CFCs weredestroying the ozone layer

Scientists and the rest of the world agreed CFCs should be banned

CFCs now replaced by alkanes or HFCs

5 CFCs and the depletion of the ozone layer

5.2 Why ozone is important

� Ozone (O3) is a form of oxygen with 3atoms in each molecule

Ozone forms a layer high up in the stratospherethat blocks harmful ultraviolet (UV)radiation

UV radiation can cause:-

a) increased risk of sunburn

b) accelerated ageing of skin

c) skin cancer

d) increased risk of cataracts

5.3 CFCs and ozone

� When UV light hits a CFC molecule in the stratosphere, a very reactive chlorine atom

(called a chlorine radical) is formed

CFCl3 → CFCl2 + Cl

(CFC) (chlorine radical)

Chlorine radicals then react with the ozone molecules, breaking them down

The chlorine radicals are regenerated(reformed) so can react with more ozone

molecules (each chlorine radical can destroy 10,000 - 100,000 ozone molecules!)

5.4 UV radiation and bond breaking (HT)

� When UV radiation hits an ozone molecule, it breaks an oxygen-oxygen covalent bond, formingordinary oxygen (O2) and an oxygen atom

Other reactions then cause the ozone molecule to be regenerated

When two atoms form a covalent bond, they share two electrons. When UV radiation breaks the C-Clbond in a CFC molecule, the C gets one electron and the Cl also gets one electron, forming a radical

� The equations for the Cl radical reacting with ozone are:-

(notice that Cl is regenerated, it is a catalyst)

Cl + O3 → ClO + O2

ClO + O → O2 + Cl

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6 Hardness of water

6.1 Hard and soft water

Soft water lathers with soap, hardwater doesn’t lather

Testing hardness of water - add liquid soap adrop at a time, shake, record how many dropsit takes for a lather to form. More soap =harder water

Hard water contains dissolved calcium ions(Ca2+) and/or magnesium ions (Mg2+). Theseions react with soap and stop it lathering

� Soapless detergents are not affected byhard water

Temporary hard water - hardnessremoved by boiling

Permanent hard water - hardness notremoved by boiling

6.2 Causes of hard water

Temporary hardness - caused by calcium hydrogen carbonate Ca(HCO3)2 which is soluble

Calcium hydrogen carbonate is formed when calcium carbonate in rocks reacts with dissolved carbondioxide in water

calcium carbonate + water + carbon dioxide → calcium hydrogen carbonate

(HT) CaCO3 + H2O + CO2 →               Ca(HCO3)2

Permanent hardness - caused by dissolved calcium sulfate, not affected by boiling

6.2 Removing hard water

Temporary hardness - boiling causes the soluble calcium hydrogen carbonate to break down intoinsoluble calcium carbonate (limescale), water and carbon dioxide

calcium hydrogen carbonate → calcium carbonate + water + carbon dioxide

(HT) Ca(HCO3)2 → CaCO3 + H2O + CO2

Permanent hardness - caused by dissolved calcium sulfate

Ion exchange resin - resin packed full of sodium ions, when hard water runs through resin, calciumand magnesium ions stick to the resin, sodium ions take their place in the water

Washing soda (sodium carbonate) softens water (HT only - because the calcium andmagnesium ions react to form insoluble calcium and magnesium carbonate)

limescale

heat

heat

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7 Natural fats and oils

7.1 Natural oils and fats

Natural fats and oils are important raw materials in the chemical industry and vegetable oils canbe used to make biodiesel

Oils are liquid, fats are solid. Both are esters

Saturated oils and fats - have single C-C bonds only. Bromine water stays orange when added tosaturated fats or oils

� Unsaturated oils and fats - have at least one double C=C bonds only. Bromine water goes colourlesswhen added to unsaturated fats or oils (HT only - the two bromine atoms add either side of the doublebond to give a dibromo compound. This is an addition reaction)

Saturated fats and oils are more unhealthy as they are converted into cholesterol which can blockarteries, leading to heart disease

7.2 Making soap

Soap is made by reacting vegetable oil withhot sodium hydroxide

Reaction is called saponification:-

fat/oil + sodium hydroxide → soap + glycerol

This is a hydrolysis reaction (as a watermolecule is added to split the fat/oil into soap

+ glycerol)

7.3 Making margarine

� Margarine - unsaturated oils (liquid) arereacted with hydrogen using a nickel catalystto make saturated fats (solid) which are used inmargarine

As the reaction adds hydrogen, the reaction ishydrogenation

7.4 Emulsions

Oil and water are immiscible - they do not mix

Emulsion - where small droplets of a liquid are dispersed (spread out) in another liquid

Milk - oil droplets dispersed in water (oil-in-water enulsion)

Butter - water droplets dispersed in oil (water-in-oil emulsion)

Water in oil Oil in water

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8 Detergents

8.1 Washing powder

Washing powder contains:-

1) active detergent - to clean

2) bleach - to remove coloured stains

3) water softener - softens water

4) optical brightener - makes whites brighter

5) enzymes - break up food and protein stains in low temperature washes

� Low temperature washes save energy and are better for delicate fabrics

8.2 Solutions

� Solvent - the liquid chemicals dissolve in

� Solute - the chemical that dissolves

� Solution - the solvent and the solute together

� Soluble - the chemical will dissolve in the solvent

� Insoluble - the chemical won’t dissolve inthe solvent

Different solvents will dissolve differentstains

8.3 Washing-up liquid

Washing-up liquid contains:-

1) active detergent - to clean

2) water - to thin out detergent

3) rinse agent - helps water drain off crockery

4) colour and fragrance - to make product more attractive

8.4 Detergents

Detergents have a hydrophilic (waterloving) head and a hydrophobic (water

hating) tail

HT only:-

Hydrophobic tail formsstrong intermolecularforces with moleculesof oil and fat

Hydrophilic head formsstrong intermolecularbonds with water

molecules

8.5 Dry cleaning

� Dry cleaning - cleaning clothes without water - different solvent (not water) used, works on stainsthat will not dissolve in water

HT only:-

Weak intermolecular forces between molecules of grease

Weak intermolecular forces between solvent molecules

� Solvent molecules form intermolecular forces with grease, solvent molecules surroundmolecules of grease and remove them from the fabric

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Topic

1 Electrolysis L K J2 Fuel cells L K J3 Redox reactions L K J4 Alcohol L K J5 CFCs and thedepletion of theozone layer

L K J

6 Hardness of water L K J7 Natural fats and oils

L K J8 Detergents

L K J