C3: Calculating and Explaining Energy Change

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Measuring Energy by Calorimetry

The unit of measurement for energy is the joule (J). It takes 4.2 joules of energy to heat up 1g of water by 1 degrees celsius. This amount of energy is called a calorie (cal), i.e. 1 calorie = 4.2 joules.

Information about the energy provided by food products is given in kilocalories (kcal). When any chemical change takes place it is accompanied by an energy change, i.e. energy can be taken in or given out. The relative amounts of energy produced by food or fuels can be measured using calorimetry.

To measure the temperature change that takes place when a fuel burn0,s follow this method:

1. Place 100g of water in a calorimeter (a container made of glass or metal) and measure the temperature change of the water.

2. Find the mass (in grams) of the fuel to be burned.

3. Burn the fuel under the in the calorimeter for a few minutes.

4. Record the new temperature of the water and calculate the temperature change of the water.

5. Weigh the fuel and calculate how much fuel has been used.

6. Calculate the energy released (Q) with the following formula: Q = m x c x temperature change

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Making and Breaking Bonds

In a chemical reaction, new substances are produced. In order to do this, the bonds in the reactants must be broken and new bonds are made to form products.

Breaking a chemical bond requires a lot of energy - this is an endothermic process.

When a new chemical bond is formed, energy is given out - this is an exothermic process.

If more energy is required to break old bonds than is released when the new bonds are formed, the reaction must be endothermic.

If more energy is released when the new bonds are formed than is needed to break the old bonds, the reaction must be endothermic.

Endothermic - a reaction takes in heat from surroundings ('En' sounds like 'in' --> heat is taken in)

Exothermic - a reaction that gives out heat to its surroundings ('Ex' = 'exit --> heat leaves and is given out)

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Measuring Energy of Chemical Reactions

The amount of energy produced in a chemical reaction in solution can be measured by mixing the reactants in an insulated container. This enables the temperature change to be meausured before heat is lost to the surroundings. This method would be suitable for neutralisation reactions and reactions involving solids, e.g. zinc and acid.

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Energy Level Diagrams

The energy changes in a chemical reaction can be illustrated using an energy level diagram:

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/add_21c_chem_exothermic.jpg)                       Energy diagram for an endothermic reaction (http://www.bbc.co.uk/schools/gcsebitesize/science/images/add_21c_chem_endothermic.jpg)                           

In an exothermic reaction, energy is given                  In an endothermic reaction, energy is

out. This means energy is being lost, so the               taken in. This means energy  is being

products have less energy than the reactants.           gained, so the products have more energy.

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Energy Level Diagrams (cont...)

(http://4.bp.blogspot.com/_uzrWbt2ZlyE/TInsf-GBK8I/AAAAAAAAAGQ/nuRopXPltaI/s320/energy-level-diagram-activation-energy.jpg)

The activation energy is the energy needed to start a reaction, i.e. break old bonds.

Catalysts reduce the activation energy needed for a reaction - this makes the reaction go faster.

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Hydrogen as a Fuel

Hydrogen can be used...  as a fuel in combustion engines or in fuel cells, which produce electricity that can power vehicles.

A hydrogen fuel cell is made from an anode and a cathode with en electrolyte contained between them. The fuel cell generates electricity.

hydrogen  +  oxygen  ---->  water  +  heat

The fuel cell needs to be continuously supplied with a fuel and oxygen which will react together to produce electricity.

Pros: - It can be a renewable source of energy

         -  High level of efficiency and produces less pollution than fossil fuels.

Cons: - It's exspensive due to needing a large number of fuel cells

          - Hydrogen is explosive and difficult to store

          - It can come from methane which would make it non-renewable.

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