C3.3 Calculating and Explaining Energy Change

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  • Created by: Fiona S
  • Created on: 19-04-15 22:16

Energy from Reactions

The relative amounts of energy released when substances burn can be measured by simple calorimetry, e.g. by heating water in a glass or metal container.

You should be able to calculate and compare the amount of energy released by different fuels given the equation:

Q = mcΔT

This method can be used to compare the amount of energy released by fuels and foods.

The amount of energy released or absorbed by a chemical reaction in solution can be calculated from the measured temperature change of the solution when the reagents are mixed in an insulated container.

If the change in temperature is negative it is an exothermic reaction. If the change in temperature is positive it is an endothermic reaction.

This method can be used for reactions of solids with water or for neutralisation reactions.

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Exothermic Reactions

An exothermic reaction is one where energy is transferred from the reacting chemicals to the surroundings. Reactants --> Products

In an exothermic reaction the reactants have more energy than the products. As the reactants react to become products they transfer energy to the surroundings.


Energy change is represented by ΔH and units are KJmol^-1 (KJ per mole).

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Endothermic Reactions

An endothermic reaction is one where energy is transferred from the surroundings to the reacting chemicals.


Using a catalyst reduces the activation energy but not overall the energy change.

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Breaking Bonds and Making Bonds

H2 + Br2 --> 2HBr

For a reaction to take place we need to break the bonds in the reaction. The energy to do this is known as the activation energy.

New bonds are then formed in the products. This releases energy. Whether a reaction is endo or exothermic depends if more energy is needed to break bonds than is released by making bonds.

break>make = endothermic

break<make = exothermic

The energy change for an exothermic reaction will always be a negative value. This is because energy has been lost to the surroundings.

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Hydrogen Fuel Cell

  • Uses electrochemical reaction
  • Reactants are H2 and O2... H2 + O2 --> H2O
  • Electrons are transferred and this can create an electrical current
  • It can be used in Engines.


  • No CO2 produced when fuel is burned
  • Water is the only waste products
  • Renewable


  • Hydrogen is produced by electrolysis of water --> needs energy --> releases CO2
  • Not truly carbon neutral
  • Explosive gas
  • Gas stored under pressure, takes up room
  • Not as energy efficient
  • Engines need to be modified
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