organic chemistry structures 1. what do i need to know? 1.translate between molecular, structural...

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Organic Chemistry

Structures

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What do I need to know?

1. Translate between molecular, structural and ball and stick representations of simple organic molecules

2. Describe how the functional group affects the property of an organic compound and understand that alkanes are unreactive towards aqueous reagents because C—C and C—H bonds are unreactive;

3. Write balanced chemical reactions including for burning hydrocarbons including state symbols

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Representations of organic molecules

• There are a number of different ways to represent organic molecules.

• Ball and stick – this is just like molymods

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Representations of organic molecules

• Structural formula – this is where we show the covalent bonds between atoms as a line

• Semi-structural (molecular) – this is where we write out the formula but do not include bonds; these are implied eg CH3CH2OH

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• Molecular formula – this simply counts the numbers of each sort of atom present in the molecule, but tells you nothing about the way they are joined together.

• Eg C2H6O• This is the least helpful type of formula as it could be one

of two (or more) different chemicals

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Example question

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Mark scheme

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Rules of organic molecules

Generally speaking

Carbon must make four bondsNitrogen must make three bondsOxygen must make two bondsHydrogen must make one bond

A double bond counts as two bonds eg C=C or C=O. A triple bond counts as three bonds.

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AfL - Quiz

1. Draw the structural formula for butanol2. Write the molecular formula for butanol3. Draw the structural formula for hexane4. Write the molecular formula for hexane5. Write the molecular formula for an alkane with 25

carbon atoms.6. How many bonds does oxygen make in methanol?7. Give an example of a use for ethanol8. Give an example of a use for methanol

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1. Butanol2. C4H10O

3. Hexane4. C6H14

5. C25H52

6. 27. Fuel/feedstock for synthesis/solvent/used in

perfume8. Solvent, antifreeze, feedstock for adhesives and

plastics

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Understanding reactivity

• Alkanes are unreactive towards aqueous reagents because C-C and C-H bonds are unreactive.

• What about organic molecules that have different bonds?

• We call families of different types of bonded atoms FUNCTIONAL GROUPS

• An example is the –OH group or alcohol group.

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Different functional groupsName Functional group Properties

Alkane C-H Relatively unreactive, burns in air due to hydrocarbon chain

Alkene C=C Used as a feedstock to make polymers

Alcohol -OH Good solvent, volatile, burns in air due to hydrocarbon chain

Carboxylic acid -COOH Weak acid such as vinegar

Ester RCOOR’ Have distinctive smells such as fruits

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Alkanes and combustion

• Because of the hydrocarbon chain alkanes burn readily releasing large amounts of energy.

• Alkanes are therefore used as fuels.• When they burn completely they make carbon

dioxide and water.eg octane (found in petrol) C8H18 +12 ½ O2 8CO2 + 9H2O

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Example question

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Mark scheme

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Example question

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Balanced chemical equations

Write the balanced chemical equation for burning ethanol in air as a fuel and burning pentane as a fuel (include state symbols).

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Answers

Ethanol

2C2H5OH(l) + 6O2(g) 4CO2(g) + 6H2O(l)

Pentane

C5H12(l)+ 8O2(g) 5CO2(g) + 6H2O (l)

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Example questions

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Alcohols and the Manufacture of Ethanol

C7.1 and C7.5

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What do I need to know?

1. The characteristic properties of alcohols are due to the presence of an –OH functional group2. Know a range of methods for synthesising ethanol and limitations of fermentation reactions3. Be able to explain why bioethanol is important for sustainability

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Functional groups - reminder

• Look back at your table of functional groups.

• Write a short paragraph to explain why different organic chemicals have different properties in terms of functional groups.

• Use examples such as “carboxylic acids are acidic because they have a –COOH group”.

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Can you recognise the functional group?

• Circle which of these are alcohols?

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Answer

• Alcohols have an –OH group

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Properties and uses of alcohols

Properties: • volatile liquid (evaporates quickly at room

temperature – more than water) • colourless• burns readily in air because of the hydrocarbon

chain • good solvent

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Example question

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Mark scheme

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Uses of ethanol and methanol

Ethanol: biofuels, solvents, feedstock for synthesis

Methanol: cleaner, feedstock for synthesis

Feedstock is the name we give to an “ingredient” on a chemical plant

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Reactions of different functional groups

• This is illustrated very well by comparing the reaction of sodium with ethanol, hexane and water.

• You have seen this reaction. Fill in the following table and compare with the mark scheme:

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Observations with sodium

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Mark scheme

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Comparing functional groups

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How do we make ethanol?

• Fermentation is a key process for obtaining ethanol. It is relatively cheap and requires wheat or beet sugar.

• The process involves the anaerobic respiration of yeast at temperatures between 20 and 40°C and at pH 7.

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Conditions for fermentation

Why is temperature important?• Outside an optimum temperature the yeast does not work (high

temperatures kill the yeast).Why do you think pH is important?• Outside an optimum pH the yeast does not work (extremes of pH kill

the yeast).Why do you think it is important to shut out oxygen?• To make ethanol the yeast must respire anaerobically (without oxygen).What effect will increasing ethanol concentration have on the yeast?• Eventually the ethanol concentration will be too high for the

fermentation to continue. This means only a dilute solution can be made.

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Example question

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Example question

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How do we obtain a concentrated solution?

• Ethanol has a different boiling point to water. We can therefore separate water and ethanol using distillation.

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Example question

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Making ethanol using ethane from crude oil

Ethane to ethene by CRACKING

C2H6 CH2=CH2

• zeolite catalyst OR

• heatEthene to ethanol by reaction with STEAM

CH2=CH2 + H2O CH3CH2OH

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Example question

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Mark scheme

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Working out masses

• We can use the useful relationship

• Where Mr is the molecular mass• eg Mr of ethane C2H6 is (2 X 12) + (6 x 1) = 30

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Example question

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Explanation • In this question every ethene molecule that reacts makes one

molecule of ethanol.• We need to relate the number of molecules to mass using our

equation.

• Mass 1 is mass of ethene = 1 tonne• Mr 1 is Mr of ethene = 28• Mass 2 is mass of ethanol = ?• Mr 2 is Mr of ethanol = 46

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Example question

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Other alternatives

• Ethanol has also been synthesised using genetically modified e-coli bacteria and sugars from seaweed.

• This process is sustainable as the seaweed and bacteria are renewable sources

• Like yeast, bacteria can be killed by high concentrations of alcohol and high temperatures

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Example question

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Ethanol – Key facts• Ethanol is made on an industrial scale as a fuel, a solvent and as a

feedstock for other processes; • There is a limit to the concentration of ethanol solution that can be

made by fermentation and there are optimum conditions of pH and temperature.

• Ethanol solution can be concentrated by distillation to make products such as whisky and brandy;

• Genetically modified E. coli bacteria can be used to convert waste biomass from a range of sources into ethanol and recall the optimum conditions for the process;

• Ethane from crude oil can be converted into ethanol • Evaluating the sustainability of each process is important.

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Plants photosynthesise •Remove CO2 from atmosphere

Fermentation•produces ethanol fuel

Burning•Releases CO2 into atmosphere

Replanting•Photosynthesis removes CO2

Bioethanol cycle

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Balancing carbon cycle equations

• Glucose (a simple sugar) is created in the plant by photosynthesis.

• Can you balance the following equation for photosynthesis?

__CO2 + ___ H2O → C6H12O6 + __ O2

6 CO2 + 6 H2O → C6H12O6 + 6 O2

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Balancing carbon cycle equations

During ethanol fermentation, glucose is decomposed into ethanol and carbon dioxide.

Can you balance this equation?C6H12O6 → __ CH3CH2OH+ __CO2

C6H12O6 → 2 CH3CH2OH+ 2 CO2

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Balancing carbon cycle equations

During combustion ethanol reacts with oxygen to produce carbon dioxide, water, and heat:

Can you balance this equation?CH3CH2OH + __ O2 → __ CO2 + __ H2O

CH3CH2OH + 3 O2 → 2 CO2 + 3 H2O

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Carboxylic acids

C7.1

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What do I need to know?

1. understand that the properties of carboxylic acids are due to the presence of the –COOH functional group;

2. recall the names and formulae of methanoic and ethanoic acids;

3. recall that many carboxylic acids have unpleasant smells and tastes and are responsible for the smell of sweaty socks and the taste of rancid butter;

4. understand that carboxylic acids show the characteristic reactions of acids with metals, alkalis and carbonates;

5. recall that vinegar is a dilute solution of ethanoic acid.

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Can you recognise the functional group?

• Circle which of these is a carboxylic acid?

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Answer

• This is a carboxylic acid

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Methanoic and Ethanoic

Methanoic acid Ethanoic acid (VINEGAR)

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Organic or CARBOXYLIC acids are part of life itself and can be found in many

animals and plants.

Many acids are part of life itself, they are known as CARBOXYLIC acids

Acids in nature

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Reactions of carboxylic acids

Reaction of carboxylic acids 1) Acid + metal salt + hydrogen Ethanoic acid + magnesium magnesium ethanoate + hydrogen

2) Acid + metal oxide salt + waterEthanoic acid + copper oxide copper ethanoate + water

3) Acid + metal carbonate salt + water + carbon dioxideEthanoic acid + sodium carbonate sodium ethanoate + water + carbon dioxide

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Example Question

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Example question

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Esters, Fats and Oils

C7.1

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What do I need to know?

1. Recall the method for producing an ester using reflux

2. Describe how fats and oils are all types of ester and explain how margarine is made

3. Explain how bromine water can be used to test whether a fat is saturated or unsaturated.

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Making esters

What type of organic chemicals do you need to mix together?

Can you name the ester made from ethanoic acid and methanol?

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Making esters

What type of organic chemicals do you need to mix together?• A carboxylic acid and an alcohol with an acid

catalystCan you name the ester made from ethanoic acid and methanol?• Methyl ethanoate

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Esters

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Example question

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Mark scheme

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Making esters

Reflux Distillation Purification Drying

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Reflux apparatus

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How do I describe reflux for an exam?

1. Mixture heated in flask (1) …2. with condenser above (1) …3. so no liquid is lost by evaporation and allows

longer time for the reaction (1)

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Distillation

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Describing distillation

1. The mixture is heated2. At the boiling point of the ester is becomes a

vapour 3. The vapour is condensed in the condenser 4. The liquid is collected

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Purification

1. Collected ester is shaken in a separating funnel with distilled water.

2. Impurities dissolve in the water3. Impurities are tapped off

Ester

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Drying

1. Solid drying agent is added to the product2. This could be calcium chloride or sodium

sulphate3. This removes water from the product

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Example question

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Fats and oils

• These are a special type of ester made from glycerol and fatty acids.

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Fats and oils

• Removal of water in the condensation reaction makes a fat or oil

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Saturated or unsaturated?

• Have you heard these terms on the television?

• Vegetable oil is mostly unsaturated• Animal fat is mostly saturated

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Double bonds or not

• A saturated fat has no C=C double bonds (alkene functional groups) and is usually a solid fat like margarine or animal fat.

• An unsaturated fat has C=C double bonds and is usually an oil like vegetable oil.

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Example question

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Making margarine

• To make margarine we have to saturate vegetable oil by bubbling hydrogen gas through the oil.

• This process is called hydrogenation

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Is a fat or oil saturated or not?

• We can test for this by adding bromine water.

• If there are double bonds present the bromine water changes from orange/brown to colourless.

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Example question

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Mark scheme

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Hydrolysis

• When an ester is hydrolysed it goes back to an acid and alcohol

• We can hydrolyse by adding acid or alkali (NaOH).

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Example question

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Mark scheme

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Energy changes in chemistry

C7.2

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Quiz

• When a chemical reaction takes place heat may be given out or taken in.

1. Can you remember the word we use when heat is given out?

2. Can you remember the word we use when heat is taken in?

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What do I need to know?

1. Recall and use the terms ENDOTHERMIC and EXOTHERMIC

2. Describe examples of ENDOTHERMIC and EXOTHERMIC reactions.

3. Use simple energy level diagrams to represent ENDOTHERMIC and EXOTHERMIC reactions.

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Change in energy

• Chemical reactants have a certain amount of chemical energy stored within them.

• When the reaction has taken place they have either more or less energy stored within them than before.

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Definitions

When heat is given out (exothermic) then the products have less energy than they did before. They have lost it to the surroundings.

When heat is taken in (endothermic) then the products have more energy than they had before. They have taken it from the surroundings.

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Energy level diagrams

Which diagram do you think is endothermic and which is exothermic?

Heat taken inHeat given out

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Energy level diagrams

Endothermic Exothermic

Heat taken in Heat given out

Energy level of products is higher than reactants so heat taken in.

Energy level of products is lower than reactants so heat given out.

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Example question

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Mark scheme

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Bond enthalpies

C7.2

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Quick quiz

1. Reactions where the products are at a lower energy than the reactants are endothermic (TRUE/FALSE)

2. Activation energy is the amount of energy given out when a reaction takes place (TRUE/FALSE)

3. A reaction which is exothermic transfers heat energy to the surroundings (TRUE/FALSE)

4. How can we tell if a reaction is exothermic or endothermic?5. Sketch the energy profile for an endothermic reaction.6. When methane (CH4) burns in oxygen (O2) bonds between

which atoms need to be broken?

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Answers1. Reactions where the products are at a

lower energy than the reactants are endothermic (TRUE/FALSE)

2. Activation energy is the amount of energy given out when a reaction takes place (TRUE/FALSE)

3. A reaction which is exothermic transfers heat energy to the surroundings (TRUE/FALSE)

4. How can we tell if a reaction is exothermic or endothermic?

5. Sketch the energy profile for an endothermic reaction.

6. When methane (CH4) burns in oxygen (O2) bonds between which atoms need to be broken?

FALSE

FALSE

TRUE

Measure the temperature change

C—H bonds and O=O bonds

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What do I need to know?

1. Recall that energy is needed to break chemical bonds and energy is given out when chemical bonds form2. Identify which bonds are broken and which are made when a chemical reaction takes place.3. Use data on the energy needed to break covalent bonds to estimate the overall energy change for a reaction.

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Activation energy revisited

• What is the activation energy of a reaction?• The energy needed to start a reaction.• BUT what is that energy used for and why does the

reaction need it if energy is given out overall?• The activation energy is used to break bonds so

that the reaction can take place.

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Burning methane

Consider the example of burning methane gas.

CH4 + 2O2 CO2 + 2H2O

This reaction is highly exothermic, it is the reaction that gives us the Bunsen flame. However mixing air (oxygen) with methane is not enough. I need to add energy (a flame).

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What happens when the reaction gets the activation energy?

Bond Forming

BondBreaking

Progress of reaction

En

erg

y in

ch

emic

als

OO

OO

H

CH

HH

O OOO

C H H H H

O C OO

O

H H

H H

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Using bond enthalpies

By using the energy that it takes to break/make a particular bond we can work out the overall enthalpy/energy change for the reaction.

Sum (bonds broken) – Sum (bonds made) = Energy change

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BIN MIX

Breaking bonds is ENDOTHERMIC energy is TAKEN IN when bonds are broken

Making bonds is EXOTHERMIC energy is GIVEN OUT when bonds are made.

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Bond enthalpiesBond Bond enthalpy (kJ) Bond Bond enthalpy (kJ)

C—H 435 Cl—Cl 243

C—C 348 C—Cl 346

H—H 436 H—Cl 452

H—O 463 O=O 498

C=O 804

C=C 614

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Can you work out the energy change for this reaction?

CH4 + Cl2 CH3Cl + HCl

Tip: Draw the reactants and products and work out the bonds you are breaking and the ones you are making.

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The answer is -120 kJ

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Example question part 1

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Question part 2

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Question part 3

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Mark scheme

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Challenge question

• The true value for the energy change is often slightly different from the value calculated using bond enthalpies.

• Why do you think this is?

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Example question

The calculated value is 120 kJ

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Mark scheme

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Definitions

Write each of these phrases in your book with a definition in your own words:

• Exothermic reaction• Endothermic reaction• Activation energy• Catalyst• Bond energy/enthalpy

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How did you do?Exothermic reactionA reaction which gives energy out to the surroundings.Endothermic reactionA reaction which takes in energy from the surroundings.Activation energyThe energy required to start a reaction by breaking bonds in the reactantsCatalystA substance that increases the rate of a reaction by providing an alternative pathway with lower activation energy. It is not used up in the process of the reactionBond energy/enthalpyThe energy required to break a certain type of bond. The negative value is the energy given out when that bond is made.

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Popular exam question

1. Explain why a reaction is either exothermic or endothermic?

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Popular exam question

1. Explain why a reaction is either exothermic or endothermic?

① In a chemical reactions some bonds are broken and some bonds are made.

② Breaking bonds takes in energy.③ Making bonds gives out energy.④ If the energy given out making bonds is higher than the

energy needed to break them the reaction is exothermic.⑤ If the energy needed to break bonds is higher than the

energy given out making them the reaction is endothermic.

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Chemical Equilibria

C7.3 Reversible Reactions & Dynamic Equilibria

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What do I need to know?

1. State that some chemical reactions are reversible2. Describe how reversible reactions reach a state of

equilibrium3. Explain this using dynamic equilibrium model.

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Reversible or not reversible

Until now, we were careful to say that most chemical reactions were not reversible –

They could not go back to the reactants once the products are formed.

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Example

In the case of the vast majority of chemical reactions this is true, the reaction of methane and oxygen for example:

CH4(g) + O2(g)   CO2(g) + 2H2O(l)

It is almost impossible to return the carbon dioxide and water to the original methane and oxygen.

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Reversible

• Some chemical reactions, however, will go backwards and forwards depending on the conditions.

• CoCl2·6H2O(s) CoCl2(s) + 6H2O(l) pink blue

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How do we write them down?

• This is the symbol for used for reversible

reactions.

CoCl2·6H2O(s) CoCl2(s) + 6H2O(l)

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What is equilibrium?

• Reversible reactions reach a balance point, where the amount of reactants and the amount of products formed remains constant.

• This is called a position of equilibrium.

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Dynamic Equilibrium.

• In dynamic equilibrium the forward and backwards reactions continue at equal rates so the concentrations of reactants and products do not change.

• On a molecular scale there is continuous change. • On the macroscopic scale nothing appears to be

happening. The system needs to be closed – isolated from the outside world.

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Example question