REDOX

Question 1

electrochemical cell

Answer 1

device that converts chemical energy into electrical energy - or vice versa

Question 2

galvanic cell

Answer 2

type of electrochemical cell that converts chemical energy into electrical energy

Question 3

battery

Answer 3

combination of several cells in series to obtain a higher potential difference or voltage

Question 4

salt bridge

Answer 4

• purpose: to balance the charge in the half cells by preventing charge build up in the half cell
  
  • without a salt bridge, the reaction wouldn’t proceed due to build up of charge
• anode: electrons are lost → negative ions of the electrolytes move into half cell to replenish it
• cathode: electrons are coming into the half cell → positive ions of the electrolytes move in to neutralise it
• negative ions move in the opposite direction as the electrons

Question 5

galvanic vs direct reaction

Answer 5

if the reactants in a galvanic cell are allowed to come into direct contact, chemical energy is converted into heat energy rather than electrical energy

Question 6

half cells

Answer 6

• each half cell consists of an electrode in contact with a solution
• species present in each half cell forms a conjugate redox pair
• if one of the pairs is a metal - usually used as the electrode - the solid block (active electrode)
• if no solid is involved, an inert electrode (unreactive electrode) can be used → platinum/graphite
• when one of the conjugate pairs is a gas or has no solid metal → inert electrode used
  
  • most half cells contain additional species that don’t take part in the reaction → spectator ions and solvent

Question 7

primary cells - non rechargeable

Answer 7

• disposable → goes flat when the cell reactions reach equilibrium (quantities of reactants and products don’t change anymore)
• cells are prevented from being recharged as products migrate away from electrode or are consumed by side reactions occurring in the cell

Question 8

electrochemical series

Answer 8

• 25 degrees celsius
• only applies under standard conditions
  
  • 1M solutions
  • 100kPa

Question 9

relative oxidising and reducing strengths

Answer 9

• species that more readily give up electrons are more likely to oxidise → stronger reductants
• species that are more able to attract electrons are more likely to reduce → stronger oxidants
• chemical species being reduced always sits on top of the one being oxidised on the electrochemical series

Question 10

standard electrode potential

Answer 10

• the potential difference of a cell measured under the standard conditions → known as electromotive force (emf) or voltage (v)
• potential difference measures the tendency to push electrons into the external circuit than the other cell
• it is the electromotive force between two points in a circuit

Question 11

standard hydrogen electrode

Answer 11

• standard hydrogen HALF-CELL
• used to measure standard electrode potential of half cells
• all Eo values are relative to this arbitrary standard
• above H+(aq)/H2(g) → chemical species gets reduced (electrons flow into this half cell)
• below H+(aq)/H2(g) → chemical species gets oxidised (electrons flow out of half cell)

Question 12

calculating voltage of a cell

Answer 12

cell potential difference = higher half cell Eo - lower half cell Eo

Question 13

limitations of prediction

prediction of voltage using electrochemical series

Answer 13

• standard electrode potentials given in the electrochemical series are only under standard conditions
• the values and order on the series will change under other conditions
  
  • doesn’t tell us anything about the reaction rate - how fast/slow

Question 14

what is a fuel cell

Answer 14

• a type of galvanic cell which generates electricity from redox reactions
• fuel cells use the chemical energy of hydrogen or other fuels to cleanly and efficiently generate electricity
• continue to produce electricity as long as fuel is supplied to them
• two types : acidic and alkaline
• considered zero emission device as electricity, heat and water are the only by products

Question 15

applications of fuel cells

Answer 15

• transport using fuel cells as an alternative to the internal combustion engine
  
  • better fuel efficiency, lower emission of greenhouse gases + other pollutants
  • not fossil oil reliant

Question 16

features of fuel cells

Answer 16

• two separate compartments
  
  • one for inflow of combustible gaseous fuel (negatively charged anode)
  • the other for oxygen/ air (positively charged cathode)
• electrodes are conducting and porous to allow contact between reactant gases and the ions in the electrolyte
  
  • electrode size determines the size of current drawn from a fuel cell
• catalysts are used to increase cell efficiency
  
  • increase rate of reaction + the current produced by the cell
• an electrolyte carries charge between the electrodes

Question 17

difference between fuel cell and galvanic cell

Answer 17

• reactants aren’t stored in fuel cells
• must be continuously supplied

Question 18

challenges when using hydrogen as a fuel

Answer 18

• production
• distribution
• storage
• safety

Question 19

efficiency of a fuel cell

Answer 19

• reduced green house gas emission
• 40-60% efficiency
• energy losses like those in coal fired power stations are avoided

Question 20

limitations of non-renewable fuel sources

Answer 20

• coal:
  
  • reserves are extensive
  • low efficiency
  • concerns about emissions
• crude oil
  
  • more limited reserved - likely to decline in coming decades
• natural gas
  
  • deposits likely to be exhausted over coming decades
  • reserves of coal seam gas + shale gas could potentially provide natural gas into the next century
  • environmental concerns about fracking

Question 21

limitations of renewable fuel sources

Answer 21

• solid biofuels produces low quantities of energy
• biogas has poor energy output
• growing large amounts of crops for biofuel → land degradation , forest clearing and potential food insecurity

Question 22

hydrogen production

Answer 22

• production uses a lot of energy
• hydrogen is produced from fossil fuels by steam reforming
• steam reacts with fossil fuel at high temperatures in the presence of a catalyst
• steam reacts w fossil fuels at high temperatures in the presence of a catalyst
  
  • CH4(g) + H2O(g) → (Ni) CO(g) + 3H2 (g)
  • CO (g) + H2O(g) →(Cu/Fe) CO2(g) + H2(g)
• ISSUES: hydrogen produced has a lower energy content than the original fuel (chemical energy lost as heat)
• alternative methods to produce hydrogen:
  
  • use electrical energy to convert water to hydrogen
  • use biogas from landfills as fuel for steam reforming

Question 23

hydrogen storage

Answer 23

• gas at room temp - storage is challenging
• compressed hydrogen:
  
  • stored in high pressure tanks
  • tanks must be very large
• liquid hydrogen
  
  • requires large amount of energy to liquefy
  • tanks must be well insulated
• safety consideration
  
  • highly flammable and potentially explosive
  • require strict code and standards for use (leak detection)

Question 24

materials based storage

Answer 24

• alternative hydrogen storage methods
• metal organic frameworks can capture and store large quantities of gases
• gases can then be released as needed

Question 25

designing better fuel cells

Answer 25

• use of materials that pose environmental and humanitarian risk
  
  • avoid heavy metals and use less risk metals
• use of renewable energy to supply the energy
• combining emerging tech w established electrochemical devices
  
  • reduce environmental impact and increases energy use

Question 26

external circuit vs internal circuit

Answer 26

• external circuit: where electrons flow (through wire)
• internal circuit: part of cell where current is due to movement of ions (salt bridge)