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

Question 30

fuel cell considerations

Answer 30

SESESGPT Source Energy released Safety Energy needed Storage Greenhouse gas/waste Price Transport