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CHEMISTRY U3/4 > REDOX > Flashcards

REDOX Flashcards

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1
Q

electrochemical cell

A

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

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2
Q

galvanic cell

A

type of electrochemical cell that converts chemical energy into electrical energy

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3
Q

battery

A

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

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4
Q

salt bridge

A
  • 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

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5
Q

galvanic vs direct reaction

A

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

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6
Q

half cells

A
  • 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
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7
Q

primary cells - non rechargeable

A
  • 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
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8
Q

electrochemical series

A
  • 25 degrees celsius
  • only applies under standard conditions
    • 1M solutions
    • 100kPa
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9
Q

relative oxidising and reducing strengths

A
  • 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
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10
Q

standard electrode potential

A
  • 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
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11
Q

standard hydrogen electrode

A
  • 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)
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12
Q

calculating voltage of a cell

A

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

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13
Q

limitations of prediction

prediction of voltage using electrochemical series

A
  • 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
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14
Q

what is a fuel cell

A
  • 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
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15
Q

applications of fuel cells

A
  • 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
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16
Q

features of fuel cells

A
  • 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-
17
Q

difference between fuel cell and galvanic cell

A
  • reactants aren’t stored in fuel cells
  • must be continuously supplied
18
Q

challenges when using hydrogen as a fuel

A
  • production
  • distribution
  • storage
  • safety
19
Q

advantages and disadvantages of fuel cells compared to engines

A
  • 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

20
Q

limitations of non-renewable fuel sources

A
  • 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
21
Q

limitations of renewable fuel sources

A
  • 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
22
Q

hydrogen production

A
  • 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
23
Q

hydrogen storage

A
  • 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)
24
Q

materials based storage

A
  • alternative hydrogen storage methods
  • metal organic frameworks can capture and store large quantities of gases
  • gases can then be released as needed
25
Q

designing better fuel cells

A
  • 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
26
Q

external circuit vs internal circuit

A
  • external circuit: where electrons flow (through wire)
  • internal circuit: part of cell where current is due to movement of ions (salt bridge)
27
Q

why an expected redox reaction doesn’t occur

A
  • reaction rate is slow
  • it is an equilibrium reaction
  • electrodes may be greasy - reduce electrolytic conduction
  • not under standard conditions
28
Q

secondary cell polarities

A
  • 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
29
Q

function of electrolyte

A
  • complete circuit by allowing flow of charged particle through cell
  • carry current between electrodes through the movement of charged ions
30
Q

why are electrodes conducting and porous in fuel cells

A
  • 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.
31
Q

design features of fuel cells

A
  • 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.
32
Q

how to make fuel cells more green

A

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

CHEMISTRY U3/4 (11 decks)

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