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

It is worth remembering that a piece of filter paper used for a salt bridge is a cheap, but imperfect substitute for a gel-filled salt bridge

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
• max voltage produced - voltage is usually less due to loss of energy

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
• potential decrease of voltage as reaction proceeds due to decrease in reactants and build up of products
• 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
• 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 - for hydrogen fuel cell
• Fuel cells always involve combustion reactions
• the oxidising agent is always oxygen, so oxygen will be reduced (at cathode)
• The fuel will always be oxidised (at anode)
• The products of the fuel cell will be the same as for the combustion reaction

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 - increase cell efficiency (often catalysts as well)
  
  • 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
• acidic or alkaline electrolyte determines if half equations have H+ or OH-

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

advantages and disadvantages of fuel cells compared to engines

Answer 19

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

disadvantage
- hydrogen is difficult to store and distribute
- cost

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)

Question 27

why an expected redox reaction doesn't occur

Answer 27

* reaction rate is slow * it is an equilibrium reaction * electrodes may be greasy - reduce electrolytic conduction - not under standard conditions

Question 28

secondary cell polarities

Answer 28

* polarity of an electrode remains the same * the name of electrode changes (anode to cathode) * acts as anode during discharge - reaction is reversed and acts as cathode and vice versa

Question 29

function of electrolyte

Answer 29

* complete circuit by allowing flow of charged particle through cell * **carry current** between electrodes through the movement of charged ions

Question 30

why are electrodes conducting and porous in fuel cells

Answer 30

* Electrodes are porous to increase available surface area for reactions * improving the efficiency of the cell * they are porous to allow for catalyst particles to be embedded in them, increasing the efficiency of reaction * they allow gaseous reactants to enter the cell to make contact with the electrolyte. * Electrodes are conductive to allow for the flow of charge through the cell / to allow for the movement of electrons from anode to cathode.

Question 31

design features of fuel cells

Answer 31

* **Role of electrode** (must be conducting and porous. e.g. porous electrodes increase surface area and provide greater efficiency in transforming chemical energy to electrical energy; embedding catalyst to speed up reaction rates and current produced by the cell) * **Operating temperature** (e.g. higher operating temperature results in greater efficiency) * **Electrolytes** (allows ions from anode and cathode to come into contact. Does not allow electron transfer) carry current - complete internal circuit.

Question 32

how to make fuel cells more green

Answer 32

**design for energy efficiency** * stacking of fuel cells for energy efficiency **renewable feedstocks** * Using renewable feedstocks (Green hydrogen) - Currently, the hydrogen feedstock is mostly produced from methane (fossil fuels) by steam reforming. - To get renewable feedstock, green hydrogen gas can be obtained from: 1) electrolysis using sustainable energy (solar, wind, biomass) 2) biogas from landfill sites instead of fossil fuels 3) alcohol made from renewable feedstock such as sugar cane, cereal grains, potatoes 4) biomass using renewable feedstock such as crop residues and algae