3.1.3 Primary galvanic cells and fuel cells as sources of energy

Question 1

oxidation number rules

Answer 1

• free element 0
• hydrogen +1
• oxygen -2 (except peroxide -1)
• most electronegative element is negative (e.g. OF2)

Question 2

redox

Answer 2

oxidation: loss of electrons, rise in ON
reduction: gain electrons, decrease in ON

Question 3

conjugate redox pair

Answer 3

reactant and product of half equation

Question 4

balancing acidic

Answer 4

1. Key elements
2. Oxygen with H2O
3. Hydrogen with H+
4. Electrons
5. States

Question 5

balancing basic with OH

Answer 5

1. Key elements
2. OH- by adding OH-
3. Electrons
4. States

Question 6

balancing basic without OH

Answer 6

1. Key elements
2. Oxygen with H2O
3. Hydrogen with H+
4. Electrons
5. OH to neutralise H+
6. Cancel H2O
7. States

Question 7

electochemical cell

Answer 7

device that converts chemical energy into electrical energy or vice versa
(uses spontaneous reaction where no external energy source required to start it)

Question 8

battery

Answer 8

combination of cells connected in a series

Question 9

galvanic cell

Answer 9

type of electrochemical cell converting chemical into electrical energy

Question 10

galvanic cell components

Answer 10

• half cell: electrode in contact with electrolyte solution (conjugate redox pair)
• external circuit wire
• salt bridge inner circuit

Question 11

salt bridge

Answer 11

• material soaked in electrolyte solution containing soluble ions that will not react with chemicals in either half cell
• completes circuit and balances charges so that charges don’t accumulate and prevent reaction from occurring

Question 12

primary vs secondary cell

Answer 12

• primary: galvanic cell that can only be used once and is non rechargeable
• secondary: rechargeable and reusable

Question 13

half cells if one reactant is gas or both are solutions

Answer 13

• platinum or graphite (inert) electrode used
• tube for gas
• solutions mixed if both solutions

Question 14

potential difference

Answer 14

tendency to push electrons into external circuit (electromotive force)

Question 15

standard electrode potential

Answer 15

• voltage of half cell in standard conditions (100pka, 25 degrees, 1.0 M) when connected to standard hydrogen half cell
• tendency to react as reduction reaction
  higher-lower

Question 16

limitations

Answer 16

• if conditions are not standard, cell potentials will be different and order of reactions will differ
• rate of reactions not considered

Question 17

galvanic vs fuel cell

Answer 17

• ## reactants are stored in galvanic cell whereas reactants are constantly supplied to fuel cell

Question 18

fuel cell components

Answer 18

• porous/catalytic electrodes
• alkaline or acidic electrolyte
• fuel (oxidised at anode)(reducing agent)
• oxygen gas (reduced at cathode)(oxidising agent)

Question 19

electrode

Answer 19

• porous: allowing ions in electrolyte and reactant gases to come into contact with each other and react
• also increases surface area for reaction to occur increasing rate of reaction
• catalysts: increase rate of reaction and thus current (but not voltage) produced
• inert so it doesn’t react with reactants

Question 20

electrolyte

Answer 20

• carries ions from one electrode to the other
• acidic: anode to cathode
• alkaline: cathode to anode

Question 21

advantages of fuel cells

Answer 21

• only one energy transformation from chemical to electrical so less waste heat lost
• waste heat can be used to produce steam which heats the car or operates a turbine
• only water and heat emitted so no greenhouse gases
• don’t need to be recharged or replaced
• use variety of fuels including biofuels
• quiet operation

Question 22

disadvantages of fuel cells

Answer 22

• require constant fuel supply
• not portable
• expensive electrolytes or electrodes
• electrodes not mined sustainably or ethically
• require new infrastructure in vehicles and filling stations
• hydrogen is sourced from fossil fuels
• storage and safety of hydrogen
• toxic electrodes or electrolytes

Question 23

steam reforming

Answer 23

• methane + water -> (nickel catalyst) carbon monoxide + water
• carbon monoxide + water -> (copper or iron catalyst) carbon dioxide + water

Question 24

steam reforming cons

Answer 24

• lower energy content due to energy transformations losing heat
• CO2 emissions in production

Question 25

sustainable ways of forming hydrogen

Answer 25

- electrolysis using electrical energy (from solar or wind) to convert water to hydrogen - methane from biogas from landfill sites

Question 26

hydrogen storage and safety

Answer 26

- higher energy content by mass than other fuels so highly flammable and potentially explosive needing leak detectors - gas at room temperature so must be liquid or compressed which has lower energy per litre so larger tanks needed

Question 27

alternative to storing hydrogen

Answer 27

- materials based storage where hydrogen is absorbed by materials and released when needed - store at smaller volumes and lower pressure at room temp

Question 28

humanitarian issues

Answer 28

- cobalt mining in Central Africa where practices are unsafe - exploitative - expose local communities to health problems like birth defects

Question 29

voltage

Answer 29

energy carried by fixed amount of charge between two cells