Energy Changes

Question 1

what happens to temperature in an endothermic reaction?

Answer 1

the temperature of the surroundings decreases

Question 2

what happens to temperature in an exothermic reaction?

Answer 2

the temperature of the surroundings increases

Question 3

what is activation energy?

Answer 3

The minimum amount of energy that colliding particles must have for them to react

Question 4

what is collision theory

Answer 4

For a chemical reaction to occur, the reactant molecules must collide with enough energy

Question 5

how would you draw an exothermic reaction profile?

Answer 5

• y axis- heat energy
• x axis- progress of reaction or time
• reactants higher than products
• activation energy- reactant line to highest point
• overall energy change- arrow going down between reactants and products line

Question 6

how do you draw an endothermic reaction profile?

Answer 6

• y axis- heat energy
• x axis- progress of reaction or time
• reactants lower than products
• activation energy- from reactant line to highest point
• overall energy change- arrow going up between reactants and products

Question 7

explain the energy changes in breaking and making bonds

Answer 7

• breaking bonds is endothermic so heat energy is taken in
• making bonds is exothermic so heat energy is given out

Question 8

how do youcalculate the overall energy change using bond energies?

Answer 8

• add together the bond energies for all the bonds in the
  reactants
• this is the ‘energy in’
• add together the bond energies for all the bonds in the
  products
• this is the ‘energy out’
• energy change = energy in - energy out

Question 9

what is a simple chemical cell/ battery and how do they produce electricity?

Answer 9

• A store of internal energy that can be transferred as an electric current in a circuit
• use chemical reactions to transfer energy by
  electricity

Question 10

why are alkaline batteries non-rechargeable?

Answer 10

because the chemical reactions stop when one of the reactants has been used up

Question 11

why are some cells rechargeable?

Answer 11

• because the chemical reactions are reversed when an external electrical current is supplied
• The reactants in rechargeable batteries can undergo a reversible reaction (⇌)
• This allows the battery to be recharged, as the products can reform the original reactants

Question 12

what are three types of cell/ battery

Answer 12

• alkaline
• rechargeable
• hydrogen fuel cell

Question 13

evaluate the use of an alkaline cell

Answer 13

• Cheaper to manufacture
• May end up in landfill sites once fully discharged; recyclable though it is expensive

Question 14

evaluate the use of a rechargeable cell

Answer 14

• Can be recharged many times before being recycled, reducing the use of resources
• Costs more to manufacture

Question 15

evaluate the use of a hydrogen fuel cell

Answer 15

• Easy to maintain as there are no moving parts; small size; water is the only chemical product
• Very expensive to manufacture; need a constant supply of hydrogen fuel, which is a flammable gas

Question 16

what is a fuel cell?

Answer 16

Device that produces a voltage continuously when supplied with a fuel and oxygen

Question 17

describe the overrall reaction in a hydrogen fuel cell

Answer 17

• Hydrogen and oxygen are supplied to the fuel cell.
• The fuel is oxidised to produce a potential difference (voltage).
• Overall, the hydrogen is oxidised to form water through conversion of light to electricity (the hydrogen and oxygen can be replaced by the
  electrolysis of water)
• 2H2 + O2 → 2H2O + electrical energy

Question 18

what are the half equations and overall equation for a hydrogen fuel cell?

Answer 18

• negative electrode: 2H2 + 4OH- → 4H2O + 4e-
• positive electrode: O2 + 2H2O + 4e- → 4OH-
• 2H2(g) + O2(g) → 2H2O(l)