Energy from Reactions (C3)

This set is for GCSE, Chemistry, AQA Energy for Reactions and uses the Nigel English book as reference. The sub-topics of this topic are:

Calorimetry          Energy Level Diagrams          Bond Energy Calculations          Hydrogen Power

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Calorimetry is a method used to measure energy released by a chemical reaction, like burning feuls. This can be done by having a known mass of water heated by the burning fuel.

The equation is: energy (J) = mass of water (g) x heat capcity of water x temperature change

The experiment of energy released by different fuels is set up by attatching a glass or metal container of a known amount of water to a clamp stand above a burning fuel underneath, which you would weigh before and after to know the mass of feul used.

Different fuels can then be compared if the energy is worked out per mole for each fuel.

Calorimetry can also be measure energy produced or absorbed in a reaction by reactions in solutions.

This is done by the reagents being mixed in an insulated container and recording the temperature change. This method is suitable for neutralisation reactions and reactions with a solid and water.

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Energy Level Diagrams

In a reaction, reactant bonds are broken and product bonds are formed. For this energy must be supplied to break the bonds and then released to form them. These changes are shown on an energy level diagram.

In exothermic reactions, more energy is in the reactants than products. Therefore, on the diagram the reactants would be higher and as the reaction takes place energy is transferred to the surroundings. This is shown on the diagram by a downwards pointing arrow. Examples of an exothermic reaction is combustion and neutralisation.

In an endothermic reaction, energy is absorbed from the surroundings as there is less chemical energy in the reactants than products. This is shown on a diagram by an upwards pointing arrow.

Activation energy is the energy required for the reaction to start and is often made by a flame. If a reaction is reversible the activation energy is the same for both ways.

Catylists work by providing a different route for the reaction with a lower activation energy.

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Bond Energy Calculations

The bond energy is the energy needed to break a bond. The amount of energy needed to break the bond is the same amount as the energy given off when the bond forms.

Bond energy is measured in kJ/mole.

There are three steps to measure the energy change:

1. Calculate the total amount of energy needed to break all the bonds in the reactants.

2. Calculate the total amount of energy needed to make all the bonds in the products.

3. Overall Energy Change = Energy In (1.) - Energy Out (2.)

If the overall value is positive, the reaction is endothermic, if the overall value is negative, the reaction is exothermic.

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Hydrogen Power

Burning hydrogen seems a useful idea to create energy as it does not produce greenhouse gases so does not accelerate climate change.

Hydrogen, however, must be luiqidised or pressurised so it takes up a smaller volume.

Fuels can be compared by energy densities in which case hydrogen releases far more energy than other fuels but because it is a gas its energy density is lower.

This would mean hydrogen-fuelled vehicles would need huge tanks to keep hydrogen as a gas and to keep it cold, so vehicles may use pressurised hydrogen instead. But because the energy density is much less when liquified, hydrogen still is a highly beneficial fuel.

Fuel cells produce electricity by reacting hydrogen with oxygen where water is the only waste product. This reaction has a partially permeable membrane that only lets hydrogen ions through. Fuel cells are very efficient and are used to power vehicles.

Because hydrogen is not truly clean from the way it is made (directly or indirectly using electricity) it still uses a non-renewable source and produces carbon-dioxide emissions.

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