C3 Chemistry - Thermodynamics

Chemistry C3 (Unit 3) : Thermodynamics Unit (Energy and Fuel)

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Endo- and Exo- thermic reactions

Reactions are exothermic or endothermic.

An exothermic reaction is one which gives out energy to the surroundings usually in the form of heat and is usually shown by a rise in temperature.

An endothermic reaction is one which takes in energy from the surroundings usually in the form of heat and is usually shown by a fall in temperature.

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Calculating fuel energy: Calorimetry

You can find out how much energy a fuel's got by using it to heat up water.You can measure the temperature change (increase) of the water.

Then you can use the temperature change to calculate the energy change using:

E = M x S x T

E: energy change (in joules)
M: mass of water
S: specific heat capacity of water. How many joules it takes to heat 1g of water 1 degree celcius. S= 4.2 j/g/degree celcius
T: temperature change

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Consequences of energy from fuels

Fuels release energy which we use in loads of ways - e.g. to generate electricity, to heat our homes and to power cars, lorries, trains, planes, etc.

Burning fuels has various effects on the environment. Burning fossil fuels releases carbon dioxide, a greenhouse fas. This causes global warming and climate change. It'll be expensive to slow down these effects, and to put things right. Developing alternative energy sources (e.g. tidal power) costs money.

The price of crude oil has a ig economic effect. We use lots of fuels made from crude oil (e.g. petrol and diesel). When the price of oil goes up they get more expensive as does everything that needs to be transported, Price is linked to the limited supply.

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Energy level diagrams

Energy level diagrams show it its endo- or exo- thermic.

In exothermic reactions the energy change is negative

1) On an exothermic energy level diagram the products are at a lower energy that the reactants. Also, the difference in height represents the energy given out and this will be negative as it goes down because the reactants are higher up.

2) The initial rise in the line on the diagram represents the energy needed to break old bonds.

In endothermic reactions the energy change is positive

1) On this diagram the products are at a higher energy level than the reactants so the energy change is positive.

2) The difference in the height represents the energy taken in during the reaction

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Food energy

Food energy is usually measured in calories or kilocalories.

The calorimetry calculation for the amount of energy in a fuel gives the answer in joules. The joule is the standard unit of energy used by all scientists but it replaced the calorie.

1 calorie - amount of energy needed to raise the temperature of 1g of water by 1 degree celcius.

1 calorie = 4.2 joules

The dietry information on food labels is in kilocalories. But they don't usually write it as kilocalries, or kcal, they usually write it as Calorie.

1 Calorie = amount of energy needed to raise the tempertature of 1kg of water by 1 degree celcius

1 Calorie = 4200 Joules

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Bond energy and calculating energy change (delta H

 The energy needed to break the bond between 2 atoms is known as the bond energy for that bond.

Bond energies are measured in kj/mol.

They are used to calculate the energy change (delta H) in chemical reactions.

Calculating Energy Change (delta H)

To calculate the energy change for a chemical reaction you need to work out:

1. How much energy is needed to break the chemical bonds in the reactants.
2. How much energy is released when the new bonds are formed in the products.

Overall energy transfer = 

energy supplied to break bonds - energy released in making bonds 

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Bond energies

Bond: Bond energy (kJ/mol)

C-C: 347                                       Cl-Cl: 243

C-O: 358                                       H-Cl: 432

C-H: 413                                       H-O:464

C-N: 286                                       H-N: 391

C-Cl: 346                                      O=O: 498

Cl-Cl: 243                                     N(triple line)N: 945

H-H: 436                                       C=O: 805

The data above is the energy required to BREAK bonds.

When bonds are formed the number is the same but the sign is negative

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Activation energy

The activation energy is the minimum energy required to start a chemical reaction.

It is lowered by a catalyst although the overall energy change remains the same.

Catalysts make the reaction happen easier (and therefore quicker) by reducing the initial energy needed.
This is represented by a lower curve on the energy level diagram - showing a lower activation energy.

(See your Chemistry C2 (additional) notes for more information  on catalysts)

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Bond making and bond breaking

Energy must always be supplied to break bonds...

...and energy is always released where bonds form.

During a chemical reaction, old bonds are broken and new bonds are formed.

Energy must be supplied to break existing bonds - so bond breaking is an endothermic process.

Energy is released when new bonds form - so bond formation is an exothermic process.

In an endothermic reaction the energy required to break bonds is greater than the energy released by bond formation.

In and exothermic reaction the energy released in making bonds is greater than the energy required to break bonds.

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