Chemistry Topic 5- Energy Changes

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What is an exothermic reaction?
A reaction which transfers energy to the surroundings, usually by heating. This is shown by a rise in temperature.
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Give examples of exothermic reactions.
Combustion, neutralisation reactions, oxidation reactions, hand warmers, self heating cans.
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What is an endothermic reaction?
A reaction which takes in energy from the surroundings. This is shown by a fall in temperature.
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Give examples of endothermic reactions.
The reaction between citric acid and sodium hydrogen carbonate, thermal decomposition, sports injury packs.
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What is activation energy?
The minimum amount of energy the reactants need to collide with each other and react.
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What does the difference in height represent on a reaction profile?
The overall energy change in the reaction.
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Bond breaking and bond making
Bond breaking is an endothermic process because energy must be supplied to break them. Bond making is exothermic because energy is released when they are formed.
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How do you calculate overall energy change?
The sum of the energies needed to break bonds minus the energy released when the new bonds are formed.
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What is an electrochemical cell?
A basic system made up of two different electrodes in contact with an electrolyte.
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What are the electrodes made out of?
They are usually metals because they need to be able to conduct electricity.
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What is an electrolyte?
A liquid that contains ions which react with the electrodes.
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How is electricity produced in the cell?
The chemical reactions between the electrodes and the electrolyte set up a charge difference between the electrodes. If they are connected by a wire, the charge is able to flow and electricity is produced.
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What affects the voltage of a cell?
Type of electrodes, difference in reactivity of the electrodes, electrolyte used, number of cells in the battery
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What happens in a non-rechargeable battery?
The chemical reactions are irreversible so over time the reactants get used up and full turn into the products. Therefore, the reaction stops and no more electricity is produced.
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Why are rechargeable cells able to be recharged?
The reaction can be reversed by connecting it to an external electric current.
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What is a fuel cell?
An electrical cell that's supplied with a fuel and oxygen and uses energy from the reaction between them to produce electrical energy.
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How is energy released from a fuel cell?
The fuel enters the cell and becomes oxidised, setting up a potential difference within the cell.
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Which electrode is negative and which is positive?
The anode is positive and the cathode is negative.
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What happens at the negative electrode (cathode)?
Hydrogen loses electrons to produce H+ ions. This is oxidation. The H+ ions then move to the cathode (positive electrode).
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What is the half equation for what happens at the cathode?
H2 --> 2H+ + 2e-
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What happens at the positive electrode (anode)?
Oxygen gains electrons from the cathode and reacts with H+ ions to make water. This is reduction.
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What is the half equation for what happens at the anode?
O2 + 4H+ + 4e- --> 2H2O
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How is electricity produced from this?
The electrons flow through an external circuit from the anode to the cathode- this is the electric current.
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What is the overall reaction in a fuel cell?
Hydrogen + oxygen -> water 2H2 + O2 --> 2H2O
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What are the advantages of using hydrogen-oxygen fuel cells?
-they don't produce as many pollutants -they don't have limited rechargeability -they are less expensive than electrical vehicle batteries -they don't have to be recharged as often
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What are the disadvantages of using hydrogen fuel cells?
-gas takes up lots more space to store -hydrogen is explosive when mixed with air -hydrogen fuel is often made from fossil fuels or electrolysis of water
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Card 2

Front

Give examples of exothermic reactions.

Back

Combustion, neutralisation reactions, oxidation reactions, hand warmers, self heating cans.

Card 3

Front

What is an endothermic reaction?

Back

Preview of the front of card 3

Card 4

Front

Give examples of endothermic reactions.

Back

Preview of the front of card 4

Card 5

Front

What is activation energy?

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Preview of the front of card 5
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