Chemistry : topic 5 = energy changes

What is an exothermic reaction?

• they give off energy to surroundings, this makes the surroundings hotter

• They give off more energy after making new bonds than they took in too make the bonds

What is an endothermic reaction?

• take in energy from sorroundings, makes temp of sorroundings cooler

• They need more energy to break bonds than they give off after making bonds

What are fuel cells?

• supplied by external source of fuel e.g. hydrogen or oxygen

• Fuel is oxidised electrochrmically to produce electricity

• Overall reaction involves oxidation of hydrogen into water

• Offer alternative to oxidation of rechargeable electric batteries

What is the half equation for fuel cells at the anode?

• hydrogen loses electrons

• H2 —> 2H+ + 2e-

• Oxidised

What is half equation for fuel cells at cathode?

• oxygen gains electrons

• O2 + 4H+ + 4e- —> 2H2O

• Reduction

What is the overall equation for fuel cells?

• 2H2 + O2 —> 2H2O

What are the advantages of fuel cells?

• less pollution

• Very effective

What are the disadvantages of fuel cells?

• storing + transportation of hydrogen has safety issues

• Storing pressurised liquid is hard

• Limited lifecycle

• High production costs

• Use of toxic chemicals in production

Exothermic reaction example?

• combustion

• Neutralisation

• Oxidation

Endothermic reaction example?

• reaction between citric acid and sodium hydrogen carbonate

• Thermal decompasition

• often used in instant ice packs

What is needed in a cell?

• two different electrode (usually metal as conductive electricty)

• An electrolyte ( liquid contains ions that react with the electrodes)

What increases Voltage of a cell?

• the bigger the difference in reactivity of the electrodes

What is a battery?

• two or more cells joined up together in series

Non - rechargeable batteries?

• use irreversible reactions, once reactant is used up they stop producing charge and need replacing

Rechargeable cell?

Reaction can be reversed by connecting to an external electrical current