C5 - Energy Changes

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What happens if the products of a reaction store more energy than the original reactants?
Then they must have taken in the difference in energy between the products and reactants from the surroundings during the reaction
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Why does the overall amount of energy not change in a reaction?
Because energy is conserved in reactions - it can't be created or destroyed, only moved around (transferred - the law of conservation of energy). This means that the amount of energy in the universe always stays the same.
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What is an exothermic reaction?
A reaction that transfers energy to the surroundings usually by heating. It has a rise in temperature (increase).
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An example of an exothermic reaction - combustion
Burning fuels - aka Combustion. This gives out a lot of energy - very exothermic
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Other examples of exothermic reactions
Neutralisation reactions (acid + alkali) are also exothermic. Most oxidation reactions are exothermic. E.G. adding sodium to water releases energy and the sodium moves about on the surface of the water as it is oxidised.
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Everyday uses of exothermic reactions
Hand warmers - they use the exothermic oxidation of iron in air (with a salt solution catalyst) to release energy. Self heating cans - rely on exothermic reactions between chemicals in their bases.
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What is an endothermic reaction?
A reaction that takes inn energy from the surroundings. It has a fall temperature (decrease).
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An example of an endothermic reaction
The reaction between citric acid and sodium hydrogencarbonate. Thermal decompostition - E.G. heating calcium carbonate causes it to decompose into calcium oxide and carbon dioxide.
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Everyday uses of endothermic reactions
Sports injury packs - the chemical reaction allows the pack to become instantly cooler without having to put it in the freezer.
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How can you measure the amount of energy released by a chemical reaction?
By taking the temperature of the reagents, mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction.
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What is the biggest problem with energy measurements?
The amount of energy is lost to the surroundings
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How can you reduce the amount of energy lost to the surroundings?
By putting the polystyrene cup into a beaker of cotton wool to give more insulation, and putting a lid on the cup to reduce energy lost by evaporation.
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How can you test the effect of acid concentration on the energy released in a neutralisation reaction between HCl and NaOH?
1)Put 25cm^3 of 0.25 mol/dm^3 of HCl and NaOH in separate beakers. 2)Place the beakers in a water bath set to 25 degrees C until they are both at the same temperature. 3)Add the HCl followed by the NaOH to a polystyrene cup with a lid.
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How can you test the effect of acid concentration on the energy released in a neutralisation reaction between HCl and NaOH? (cont)
4)Take the temperature of the mixture every 30 sec and record the highest temperature. 5)Repeat steps 1-4 using 0.5 mol/dm^3 an then 1 mol/dm^3 of HCl.
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Other cards in this set

Card 2

Front

Why does the overall amount of energy not change in a reaction?

Back

Because energy is conserved in reactions - it can't be created or destroyed, only moved around (transferred - the law of conservation of energy). This means that the amount of energy in the universe always stays the same.

Card 3

Front

What is an exothermic reaction?

Back

Preview of the front of card 3

Card 4

Front

An example of an exothermic reaction - combustion

Back

Preview of the front of card 4

Card 5

Front

Other examples of exothermic reactions

Back

Preview of the front of card 5
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