Energy changes - Chemical and fuel cells

Question 1

what is an electrochemical cell

Answer 1

a basic system made up of two different electrodes in contact with an electrolyte

Question 2

what must the two electrodes in a cell be able to do

Answer 2

conduct electricity

Question 3

Factors that affect voltage or charge difference of a cell

Answer 3

• difference in reactivity of the two electrodes, the higher the difference between them, the bigger the voltage of the cell
• the electrolyte as different ions in the solution will react differently with the metal electrodes used

Question 4

how is a battery formed

Answer 4

by connecting two or more cells together in series

Question 5

what are non - rechargeable batteries

Answer 5

batteries where the reactants get used up- e.g an alkaline battery

Question 6

properties of non - rechargeable batteries

Answer 6

• irreversible
• over time, the reacting particles get used up and turn into the product of the reaction
• once any one of the reactants get used up, the reaction can’t happen, no electricity produced
• products cant be turned back to reactants, so the cell cannot be recharged

Question 7

what is a rechargeable cell

Answer 7

a cell where the chemical reaction can be reversed by connecting it to an external current - e.g a phone or laptop battery

Question 8

what is a fuel cell

Answer 8

an electrical cell thats supplied with fuel and oxygen and uses energy from the reaction between them to produce electrical energy efficiently

Question 9

what is a hydrogen oxygen fuel cell

Answer 9

a fuel cell which combines hydrogen and oxygen to produce clean water and releases energy

Question 10

properties of a hydrogen oxygen fuel cell

Answer 10

• involves a redox reaction
• hydrogen goes to the anode (oxidation as it loses electrons ), oxygen to the cathode ( reduction
• at the cathode, oxygen gains electrons from the cathode and reacts with the H+ ions to make water ( reduction)

Question 11

advantages of using hydrogen oxygen fuel cells in vehicles

Answer 11

• don’t produce as many pollutants as other fuels, only by- products are water and heat
• better than electric vehicles as their batteries are polluting to dispose of
• less charging time than electric cars as they store more energy
• electric car batteries are more expensive to make then fuel cells

Question 12

disadvantages of hydrogen oxygen fuel cells in vehicles

Answer 12

• hydrogen is a gas so takes more space to store than a rechargeable battery
• hydrogen is explosive when mixed with air, harder to store
• the hydrogen fuel is made from hydrocarbons which use electricity which come from fossil fuels

Question 13

what is the half equation for oxygen in a hydrogen- oxygen

Answer 13

O2 + 4H+ + 4e&raquo_space;»2H2O

reduction

Question 14

what is the equation for hydrogen + oxygen

Answer 14

2H2 + o2 —-2H2O

Question 15

how can electricity be made

Answer 15

by taking 2 metals of different reactivity and placing them in an electrolyte

Question 16

what is an electrolyte

Answer 16

a solution that can conduct electricity - eg, the solution of an ionic compound

Question 17

what are characteristics of a cell

Answer 17

• a cell can only produce electricity for a certain period of time, eventually, the chemicals in the cell run out and the reaction stops
• cells only produce electricity when metals of different reactivity are used

Question 18

what do cells contain

Answer 18

chemicals which react to produce electricity

Question 19

what are batteries

Answer 19

f two or more cells connected together in series which produces greater voltage

Question 20

what is an electrolyte

Answer 20

a solution that can conduct electricity

Question 21

how does a fuel cell work

Answer 21

• react a fuel such as hydrogen with pure oxygen or air
• inside the fuel cell, a chemical reaction takes place, producing an electric current
• only waste product is water

Question 22

what happens at the negative electrode

Answer 22

• hydrogen molecules split into hydrogen ions and electrons
• 2H2 —> 4H+ +4e

Question 23

what happens at the positive electrode

Answer 23

• oxygen molecules combine
  o2 + 4H+ 4e - —> 2H2o

Question 24

the hydrogen is being oxidised

Answer 24

2H2 + O2 —-> 2H2O

Question 25

advantages of fuel cells compared to rechargeable batteries

Answer 25

• HFC will produce electricity for as long as you provide hydrogen but rechargeable batteries run out and need to be recharged
• HFC do not get less efficient the longer they run whilst rechargeable batteries can store less energy the more charging cycles they go through
• HFC can also he a source of drinkable water like for astronauts on a rocket

Question 26

problems with hydrogen fuel cells

Answer 26

• they run on hydrogen which is an explosive gas and is difficult to store safely, whilst no dangerous fuels are required with rechargeable batteries
• hfc produce a relatively low pd or voltage so several are needed together, whilst rechargeable batteries can produce a greater voltage than a hfc