Energy Changes

Question 1

what is an exothermic reaction?

Answer 1

a reaction that release energy into its surroundings and increases in temp eg. combustion, neutralisation and most oxidation reactions

Question 2

what are the everyday uses of exothermic reactions?

Answer 2

self-heating cans
hand warmers

Question 3

what is an endothermic reaction reaction?

Answer 3

a reaction that takes in energy from its surroundings and decrease in temp eg. thermal decomposition, and citric acid and sodium hydrogencarbonate reacting

Question 4

what are the everyday uses of endothermic reactions?

Answer 4

sport injury packs
chilling drinks

Question 5

required practical: energy (temp) change

Answer 5

1. put 25 cm3 of 0.25 mol/dm3 of HCl and sodium hydroxide (NaOH) in separate beakers
2. place beakers in a water bath set to 25 C until they are both at the same temp
3. add HCl and NaOH to a polystyrene cup with a lid - reduces energy loss
4. measure temp of solution every 30 secs and record highest temp
5. repeat and calculate a mean
6. repeat with 0.5 mol/dm3 and 1 mol/dm3 of HCl

Question 6

reaction profile of an exothermic reaction

Answer 6

reactants are at higher energy levels than products

Question 7

reaction profile of an endothermic reaction

Answer 7

reactants are at lower energy level than products

Question 8

what is activation energy?

Answer 8

minimum amount of energy required for particles to collide and cause a reaction

Question 9

what is the difference between bond energies in an exothermic and an endothermic reaction?

Answer 9

exothermic
- energy released from forming new bonds is greater than energy needed to break existing bonds

endothermic
- energy needed to break existing bonds is greater than energy released from forming new bonds

Question 10

what is triangleH?

Answer 10

enthalpy change

Question 11

how do you calculate triangleH?

Answer 11

energy taken in - energy released

Question 12

how can cells be made?

Answer 12

• by connecting two different metals in contact with an electrolyte
  Batteries consist of two or more cells connected together in series to provide a greater voltage

Question 13

differences between non-rechargeable and rechargeable cells?

Answer 13

non-rechargeable cells and batteries the chemical reactions stop when one of the reactants has been used up eg. alkaline batteries

rechargeable cells and batteries can be recharged because the
chemical reactions are reversed when an external electrical current is supplied

Question 14

what factors affect the voltage on a cell?

Answer 14

bigger difference in reactivity=bigger voltage
type of electrodes

Question 15

how do fuel cells work?

Answer 15

• supplied by an external source of fuel (eg hydrogen)
  and oxygen or air
• fuel is oxidised electrochemically within the
  fuel cell to produce a potential difference
• overall reaction in a hydrogen fuel cell involves the oxidation of
  hydrogen to produce water

Question 16

what happens inside a fuel cell?

Answer 16

• electrolyte=acid eg. phosphoric acid
• electrodes= porous carbon+catalyst
• hydrogen into anode compartment=loses electrons
• oxygen in cathode compartment=gains electrons+reacts with H+ ions in acid=water
• electrons flow through external current from anode to cathode=current

Question 17

pros and cons of fuel cells

Answer 17

pros
- x produce as many pollutants eg. greenhouse gases, SO2, nitrogen oxides
- only release water+heat
- electric cars x polluting but batteries are polluting to dispose=release toxic chemicals
- cheaper than batteries
- batteries have a limit to how many times they can be recharged before replacement

cons
- hydrogen=gas=takes more space than rechargeable battery
- hydrogen=explosive when reacted with air=hard to store safe
- hydrogen fuel made from hydrocarbons from fossil fuels or electrolysis of water=electricity