Briefly covers C3 4.1-4

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Energy produced by fuels

Heat energy is produced when fuel burns or foods are eaten, each fuel can produce different amounts of energy.

Energy is measured in Joules which can be converted to calories (1 Cal = 4.2 J)

We can use a calorimeter to measure the amount of heat produced when a substance is burned and we can compare the energy produced to other substances to see which has the most energy.

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Energy Changes in Reactions 1

In chemical reactions, new substances are formed, however bonds have to be broken first for this to happen. This requires energy.

When new products are formed from chemical reactions, energy is released or takes from the surroundings:

  • If more energy is needed to break bonds than is released when new bonds form, the chemical reaction is endothermic (A chemical reaction that absorbs energy, therefore making the product cold.)
  • If more energy is released when the bonds are made in the products than is used in breaking the bonds in the reactants, the chemical reaction is exothermic (A chemical reaction which gives off heat)
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Energy Changes in Reactions 2

These changes can be represented on a energy level diagram.

This is a exothermic reaction: http://gcserevision101.files.wordpress.com/2009/02/energy-level-diagram-exothermic.jpg

This is a endothermic reaction: http://gcserevision101.files.wordpress.com/2009/02/energy-level-diagram-endothermic1.jpg

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Calculating Energy Changes 1

Looking back at the calorimeters above, when chemicals react and give off/take in energy, we can use calculations to work out exactly how much energy has changed. There is one vital piece of information we need to know to do this:

4.2 joules of energy raises 1g of water by 1°C

Hence the units involved in this energy change will be kJ/g/°C (kilojoules per gram per degree). A simple calorimeter is used to measure energy change in a reaction A + B → C. So let’s calculate an example of such a reaction:

Question : 60cm³ of a solution containing 0.1 moles of A is mixed with 40cm³ of a solution containing 0.1 moles of B. Prior to mixing, their temperature was 19,6°C. After mixing, the maximum temperature reached was 26.1°C.

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Calculating Energy Changes 2

First, calculate the temperature change:
26.1°C – 19.6°C = 6.5°C
Since 60cm³ of A added to 40cm³ of B makes 100cm³ overall, we are looking at 100g (assuming the density of the solution is the same as water density). And we know that 4.2J raises 1g by 1°C
So energy change = 100g x 6.5°C x 4.2J/g/°C = 2,730J = 2.73kJ
BUT – don’t forget the solutions are only 0.1 molar – so we have to multiply our value by 10 to find out a 1.0M solution
2.73kJ x 10 = 27.3kJ
So the final energy change was -27.3kJ (this was a exothermic reaction for the temperature increased)

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