energy changes

exothermic

reaction which releases energy to the surroundings making temperature increase

eg. combustion of methane: CH₄ + O₂ → H₂O + CO

endothermic

reaction which absorbs energy from the surroundings making the temperature decrease

eg. thermal decomposition of zinc carbonate: ZnCO₃ + heat → ZnO + CO₂

exothermic reaction notation

change in heat = +285 KJ/mol

endothermic reaction notation

change in heat = -285 KJ/mol

exothermic energy profile diagram

products have less energy than reactants because energy has been transferred to surrounding

endothermic energy profile diagram

products have more energy than reactants because energy is taken in from the surroundings

activation energy

the energy required for a chemical reaction to happen; it is the minimum amount of energy colliding particles must have in order to react

cells

• two different metals in an electrolyte can produce electricity

→ eventually the chemicals run out and the reaction (electricity) stops

→ cells only produce electricity if the metals used have different reactivities

The greater the different between the reactivity of the metals,

the greater the potential difference produced by the cell.

battery

two or more cells connected in series; produces a greater voltage

alkaline batteries

non rechargeable: at some the point reactants run out

rechargeable batteries

we can reverse the chemical reaction when we apply an electrical current

advantages of rechargeable batteries

• no dangerous fuels are required

• can produce a higher voltage than a hydrogen fuel cell

disadvantages of rechargeable batteries

• run out and need to be recharged

• can store less electricity the more charging cycles they go through and eventually must be replaced

fuel cell

react a fuel such as hydrogen with oxygen/air to form a chemical reaction inside the cell and create an electric current

2H₂ + O₂ → 2H₂O : hydrogen is oxidised

negative electrode half equation

2H₂ → 4H⁺ + 4e^-

positive electrode half equation

O₂ + 4H⁺ + 4e^- → 2H₂O

advantages of fuel cells

• produce electricity for as long as you provide hydrogen

• do not get less efficient the longer they run

• can be a source of drinking water

disadvantages of fuel cells

• hydrogen is an explosive gas which is difficult to store safely

• produce a relatively low voltage so several are needed