These are reactions where the thermal energy from the reactants is transferred to the surroundings and so we observe an increase of temperature.
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Uses of exothermic reactions
Hand warmers, self heating cans
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Endothermic reactions
These are reactions where thermal energy is transferred from the surroundings to the reactants and so we observe a decrease in temperature.
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Uses of endothermic reactions
Sports injury packs, sherbet sweets
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Producing and breaking chemical bonds
Breaking chemical bonds requires energy and producing chemical bonds releases energy.
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Activation energy
Chemical reactions can only occur when particles collide with each other with enough energy to react. Activation energy is the minimum energy required to kick-start a chemical reaction
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Exothermic reactions in terms of chemical bonds
A reaction is exothermic when the amount of energy released during bond-producing is greater than the energy required to break the bonds(i.e the activation energy)
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Endothermic reactions in terms of chemical bonds
A reaction is endothermic when the energy required to break bonds (i.e the activation energy) is greater than the energy released during bond-producing.
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Bond energies
the amount of energy needed to break or create a chemical bond is known as the bond energy. Different chemical bonds have different bond energies.
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Overall energy change of a reaction
This is the difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed.
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Interpreting overall energy changes in reactions
If the overall energy change for a reaction is a negative number it means it is an exothermic reaction and if the number is positive it is an endothermic reaction.
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Chemical cells and batteries
A chemical cell is made by placing two electrodes in an electrolyte, producing electricity. A battery is two or more cells connected together
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Factors affecting voltage produced by cells and batteries
The bigger the difference in the reactivity in the metals (electrodes) , the bigger the voltage. Changing the electrolyte or the concentration of the electrolyte can affect the voltage.
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Rechargeable cells and batteries
In rechargeable cells and batteries, the chemical reactions can be reversed by supplying an external electrical current to reform these chemicals, meaning that they can be used over and over again.
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Fuel cells
A chemical cell with a continuous supply of chemicals,so that those being used up are replaced allowing the cell to work forever.
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Hydrogen fuel cell reactions
hydrogen+oxygen=water 2(h2)+02=2(h20)
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What happens in a hydrogen fuel cell
At one electrode, hydrogen loses electrons and forms ions. The hydrogen ions move to the other electrode through the electrolyte. The electrons move to the other electrode through a wire. The hydrogen ions and electrons react with oxygen making water
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Half equations for hydrogen fuel cell
At the anode:2(h2)=4h(+)+4e- At the cathode: 02+4h++4e-=2(h2o)
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Advantages of hydrogen fuel cells
The only waste product is water. Electricity is produced continuously as long as fuel is supplied.
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Disadvantages of hydrogen fuel cells
They are very expensive. They require hydrogen gas which is flammable and so difficult to transport and store. Hydrogen is produced from methane, a fossil fuel.
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Other cards in this set
Card 2
Front
Hand warmers, self heating cans
Back
Uses of exothermic reactions
Card 3
Front
These are reactions where thermal energy is transferred from the surroundings to the reactants and so we observe a decrease in temperature.
Back
Card 4
Front
Sports injury packs, sherbet sweets
Back
Card 5
Front
Breaking chemical bonds requires energy and producing chemical bonds releases energy.
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