Energy Changes (Year 11) - C7
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What is an exothermic reaction
It is a reaction that transfers energy to the surroundings This causes the temperature of the surroundings to go up
What is an endothermic reaction
It is a reaction that transfers energy from the surroundings This causes the temperature of the surroundings to go down
Examples of Endothermic reactions
Melting and Boiling
Examples of Exothermic reactions
Combustion, Rusting and Neutralisation
What type of reaction is breaking bonds
Endothermic
What type of reaction is making bonds
Exothermic
What is the equation for energy change
Energy change = Energy of bonds broken - Energy of bonds made If the answer is negative then the reaction is exothermic
Why do reactions need activation energy
To break the bonds in the reactants
What do fuel cells use
They use fuel and oxygen to produce electrical energy
What happens in hydrogen cells
Hydrogen is oxidised to produce water and electricity
What is the overall equation for hydrogen cells
2H 2 + O 2 = 2H20
What are the half equations for hydrogen cells
2H 2 + 4OH (N) = 4H20 + 4 electrons O2 + 2H20 + 4 electrons = 4OH (N) (N) = negative
What are the advantages of hydrogen fuel cells
No pollution Can be used for a range of things Don’t need to be electrically recharged
What are the disadvantages of hydrogen fuel cells
Hydrogen is highly flammable Hydrogen is difficult to store The hydrogen used in the cells can come from non-renewable sources
What is a battery
It is 2 or more cells connected together in a series
What does the voltage of cells depend on
Reactivity of metals Electrolyte Surface area of electrons
What is an electrolyte
A solution that conducts electricity e.g. NaCl
What are electrodes
They are 2 different metals which react together in an electrolyte which build up a charge difference The positive metal is called the anode The negative one is called the cathode
What happens in non-rechargeable batteries
They use irreversible reactions Once one reactant is used up, they stop producing a charge