Energy ChangesEnergy is measured in Joules (J). For comparing the energy content of substances, we may use kiloJoules per mole or kiloJoules per gram.

Calories are the ‘old’ units for energy. 1 calorie = 4.2 Joules

Calorimetry is the technique of measuring how much energy a food or fuel releases when it burns.

Foods also release energy when they are used for respiration - this is also an exothermic reaction, exactly the same as burning, but taking place inside the body.

We calculate the energy released during a reaction, Q, by measuring the temperature change when a known mass of water is heated using this energy.

Q = m c T Q is in Joulesm is the mass of water (in g)T is the change in temp. (in ˚C)c is a constant = 4.2 (J/g/˚C)

A simple calorimeter not very accurate, but can be used to compare the amount of energy released by different fuels or foods.

What are the sources of error that make it inaccurate ?

c is called the specific heat capacity

Working out energy released:e.g. In a calorimetry experiment, a fuel sample was burned and produced a temperature change of 25°C in the calorimeter, which contained 100g of water. c = 4.2 J/g/˚C

Q = m x c x T = 100 x 4.2 x 25 = 10,500 J (or 10.5 kJ)

Comparing energy contents of fuels or foodsTo make a fair comparison, we will need to calculate the energy released per gram or per mole of fuel burnt. To do this we work out the energy released as before, then divide by the mass or moles of fuel burnt.

Example:A 0.5g sample of ethanol is burnt, raising the temperature of 250g of water from 10˚C to 28˚C but when 0.8g of butane is burnt, the temperature of 250g of water increases from 10˚C to 40˚C. Which fuel produces more energy per gram ? c = 4.2 J/g/˚C

For ethanol: energy per g = (4.2 x 250 x 18) ÷ 0.5 = 37,800 J per g

For butane: energy per g = (4.2 x 250 x 30) ÷ 0.8 = 39,375 J per g

Practice:Which produces the greater amount of energy per mole of fuel burnt ?

The amount of energy produced by a chemical reaction in solution can be found by mixing the reagents in an insulated container and measuring the temperature change caused by the reaction.

This method can be used for - reactions of solids with solutions (or water)- neutralisation reactions etc.

Practice:A solution of sodium hydroxide was mixed with a solution of hydrochloric acid. 100g of each solution was mixed in the beaker. The temperature rose from 15˚C to 18.5˚C. Calculate the energy change for this reaction, in Joules. Hint: what mass of solution was being heated ?

water

insulation

lid

Temperature changes in other reactions

Hydrogen – fuel of the future

Fossil fuels provide the majority of our energy needs currently, but there are sustainability issues: Two main problems with using fossil fuels ?

- non-renewable, using up a finite resource- produce carbon dioxide, leading to climate change

The most promising alternative is hydrogen. When hydrogen is burnt the only product is water, which has no environmental impact.

2 H2(g) + O2(g) 2 H2O(l)

Hydrogen therefore solves the second problem listed above, but doesn’t solve the first one. There is no natural source of hydrogen on this planet. We’d have to make it – usually by electrolysis of water (which of course requires the use of electrical energy).

Scientists are working on the problems with hydrogen:• safe use – hydrogen is extremely flammable• storage – hydrogen is a gas, so takes up a large volume unless compressed and cooled into a liquid• there is no natural supply of hydrogen – we need a sustainable way of making it (probably from water)• if we make hydrogen by electrolysis, generating the electricity at the power station may still use fossil fuels

Hydrogen fuel cellsThe most efficient way to get energy out of hydrogen is not to burn it directly, but to react it with oxygen in a fuel cell. The reaction produces electrical energy rather than heat energy which can drive an electrical motor in a car.

The only product is water, as before.

Recommended viewing:www.youtube.com/watch?v=ODi-P5QNDx8www.youtube.com/watch?v=8rofx6Gaz40

How reactions work – bond breaking and makingWe know that when a reaction takes place, bonds in the reactants have to be broken, and new bonds in the products can then formed. Consider the reaction between hydrogen and chlorine molecules to make hydrogen chloride:

H2 + Cl2 → 2 HCl

Energy is needed to break bonds - that’s why molecules have to collidewith sufficient energy for a successful reaction to take place. Activation energy is the energy required to break all the necessary bonds in the reactants. The stronger the bonds are, the more energy is needed to break them. We call this the bond energy – values are published for eachdifferent bond.

Bond breaking requires heat energy to be taken from the surroundings andused to break the bonds. The surroundings get cooler. Bond breaking isENDOTHERMIC

When new bonds form, energy is given out. This causes the surroundings to heat up. Bond forming is EXOTHERMIC. (The amount of energy given out is equal to the bond energy for that bond)

Look at the list of bonds broken and made in the reaction H2 + Cl2 2 HClHow much energy overall is needed to do the breaking ?How much energy overall is released when the new bonds are made ?

One H-H bond is broken: 436One Cl-Cl bond is broken: 242TOTAL ENERGY TAKEN IN = 678 kJ/mol

Two H-Cl bonds are made 2 x 431TOTAL ENERGY GIVEN OUT = 862 kJ/mol

Overall energy changes for reactions can be calculated: Energy change = energy taken in – energy given out

The overall energy change tells us whether the reaction will be exothermic or endothermic.

The energy change is -184 kJ/mol. Overall, energy is being given out so this is an exothermic reaction.

ΔH The symbol ΔH is used for the energy change;

In an exothermic reaction, ΔH is always negative (the energy of the products is lower than that of the reactants). Explanation: Less energy is used to break the bonds in the reactants than is released when the bonds in the products are formed.

Similarly in an endothermic reaction, ΔH is always positive (the energy of the products is higher than that of the reactants)Explanation: More energy is used to break the bonds in the reactants than is released when the bonds in the products are formed.

Energy level diagramsThese show the energy changes during reactions:

Effect of a catalystA catalyst increases the rate of a reaction, without being used up itself.

It does this by lowering the activation energy for the reaction, meaning that particles don’t need to collide with so much energy in order to react – so successful collisions are more frequent.