Redox Equilibria - 14 Flashcards
what does an electrochemical cell consist of
*A cell has two half–cells.
*The two half cells have to be connected with a salt bridge.
*Simple half cells will consist of a metal (acts an electrode) and a solution of a compound containing that metal (eg Cu and CuSO4).
*These two half cells will produce a small voltage if connected into a circuit. (i.e. become a Battery or cell).
Why use a high resistance voltmeter?
- The voltmeter needs to be of very high resistance to stop the current from flowing in the circuit.
- In this state it is possible to measure the maximum possible potential difference (E).
- The reactions will not be occurring because the very high resistance voltmeter stops the current from flowing.
Why does a voltage form when a zinc half-cell is connected to a copper half-cell?
- When connected together the zinc half-cell has more of a tendency to oxidise to the Zn2+ ion and release electrons than the copper half-cell. (Zn —–> Zn2+ + 2e-)
- More electrons will therefore build up on the zinc electrode than the copper electrode.
- A potential difference is created between the two electrodes.
- The zinc strip is the negative terminal and the copper strip is the positive terminal.
- This potential difference is measured with a high resistance voltmeter, and is given the symbol E.
- this same concept applies to any other cell
what is a salt bridge
A salt bridge is a piece of filter paper soaked in potassium nitrate
what is the function of a salt bridge
- Completes the circuit by allowing the flow of ions between the half-cells.
- Maintains electrical neutrality by preventing charge buildup in each half-cell.
- Allows movement of ions, but does not react with the solutions or interfere with the electrode reactions.
Without a salt bridge, the cell would stop working as charge would build up and prevent electron flow.
why is a wire not used instead of a salt bridge
A wire is not used because the metal wire would set up its own electrode system with the solutions and wires do not allow the flow of ions
if a system does not contain an electrode that acts as a metal what can be used instead and why
- Platinum
- As: It provides a conducting surface for electron transfer
- it is unreactive and can conduct electricity
what is the standard hydrogen electrode (SHE)
is an electrode that assigned the potential of 0 volts which used to measure the potential of all electrodes by comparing their potential to that of the standard hydrogen electrode
what is the hydrogen electrode equilibrium
H2 (g) <——> 2H+ (aq) + 2e-
what is the hydrogen electrode represented by in a standard electrode
Pt |H2 (g) | H+ (aq)
conditions of standard hydrogen electrode
- Hydrogen gas at pressure of 100 kPa
- Solution containing the hydrogen ion at 1.00 mol dm-3 (solution is usually 1 mol dm-3 HCl)
- Temperature at 298K
what does a SHE half cell consist of
a platinum wire is used which is coated in finely divided platinum (platinum black) which acts as a catalyst, because it is porous and can absorb the hydrogen gas
Why are secondary standard electrodes used, and what are some common examples?
- The Standard Hydrogen Electrode (SHE) is difficult to use, so easier secondary standards are used instead.
- These are calibrated (measured in comparison) against the SHE to ensure accuracy.
- Common secondary standards:
- Silver/Silver Chloride Electrode (Ag/AgCl)
- Calomel Electrode (Hg₂Cl₂)
what is the standard electrode potential
the potential difference measured between an electrode system and hydrogen electrode system under standard conditions
what are standard conditions
- All ion solutions at 1.00 mol dm-3
- temperature 298K
- gases at 100 kPa pressure
- No current flowing
how to calculate the e cell
Ecell = Ered – Eox
how to identify which half cell is being oxidised and reduced
- The more negative half cell will always oxidise
- The more positive half cell will always reduce
Which half cell always oxidises in a redox reaction?
the more negative half cell (so if putting in an equation write it backwards)
Which half cell always reduces in a redox reaction?
The more positive half cell (so if putting in an equation write it forwards)
Where are the strongest reducing agents found in the electrochemical series?
At the most negative end (right side)
Where are the strongest oxidising agents found in the electrochemical series?
At the most positive end (left side)
on the electrochemical series which substance is more likely to reduce and which is more likely to oxidise
the more positive electrochemical will reduce (write forwards) and more negative electrochemical will oxidise (write backwards)
What does E°cell measure in a redox cell?
How far the reaction is from equilibrium; the more positive the E°cell, the more likely the reaction is to occur.
What happens to E°cell as current flows through a cell?
The reaction proceeds, reactant concentrations drop, and E°cell falls to zero as the reaction occurs
What effect does increasing temperature have on E°cell for most cells?
It decreases E°cell because most redox reactions are exothermic, so equilibrium shifts backward.
Does a positive E°cell always mean a reaction will occur?
No; a reaction might not occur if the activation energy is too high or the reaction is too slow
Why might the actual E°cell differ from the calculated standard E°cell?
Because the reaction is carried out under non-standard condition
What do electrochemical cells do?
Provide a commercial source of electrical energy by converting chemical energy into electrical energy.
What are the three types of electrochemical cells?
Non-rechargeable (irreversible), rechargeable (secondary), and fuel cells.
When is a cell considered non-rechargeable?
When the redox reactions inside are non-reversible.
examples of primary non rechargeable cells
zinc dry cell
Lithium –manganese dioxide cel
example of secondary rechargeable cells
Lead acid Cell
nickel–cadmium cells
Lithium ion cell
what is the overall reaction for a zinc dry cell and what is being reduced and oxidised
overall equation: 2MnO₂ + 2NH₄⁺ + Zn → Mn₂O₃ + 2NH₃ + H₂O + Zn²⁺
Zn is oxidised (E° = -0.76 V)
MnO₂ is reduced (E° = +0.75 V)
What is the overall reaction for a lithium-manganese dioxide primary cell?
Li + MnO₂ → LiMnO₂
Write the conventional cell diagram for a lithium-manganese dioxide cell
Li+(aq) + e- —> Li(s) E=-3.04V
Li+aq + MnO2 (s) +e- —> LiMnO2(s) E = - 0.13 V
Li(s) | Li⁺(aq) || Li⁺(aq), MnO₂(s), LiMnO₂(s) | Pt(s)
What happens during discharge and charging in a rechargeable cell?
Discharge: forward reaction (releases energy)
Charge: reverse reaction (requires energy)
What is the overall reaction for a lead–acid cell (discharging)?
PbO₂ + Pb + 2SO₄²⁻ + 4H⁺ → 2PbSO₄ + 2H₂O
what makes cells reversible
due to the products staying attached onto the electrode and do not disperse
What is the overall reaction in a nickel–cadmium cell?
2NiO(OH) + Cd + 2H₂O → 2Ni(OH)₂ + Cd(OH)₂
What is the discharge reaction in a lithium-ion cell?
Li + CoO₂ → LiCoO₂
how does lithium ion cells work to power cameras and mobile phones
The reagents in the cell are absorbed onto powdered graphite that acts as a support medium. The support medium allows the ions to react in the absence of a solvent such as water.
Why can’t water be used as a solvent in lithium-ion cells?
Water would react with the lithium metal.
What is the overall reaction in a hydrogen fuel cell using potassium hydroxide and using acidic electrolyte?
2H₂ + O₂ → 2H₂O
Why is a higher temperature used in hydrogen fuel cells
as using standard conditions: The rate is too slow to produce an current so a higher temperature is used to increase the reaction rate
Why can higher temperature decrease the EMF (overall voltage) in hydrogen fuel cells and how can it be counteracted
The reaction is exothermic; higher temperature shifts equilibrium, reducing voltage (Le Chatelier’s Principle).
- increasing the pressure can help counteract this
Why does voltage remain constant in a hydrogen fuel cell compared to ordinary cells?
- as they are continuously fed with fresh O2 and H2 so maintaining constant concentration of reactants.
- This differs from ordinary cells where the voltage drops over time as the reactant concentrations drop
limitations of hydrogen fuel cell
- hydrogen is very flammable so not safe to store and transport
- To store hydrogen as a liquid, it needs to be: Cooled and pressurised in strong, heavy containers which is expensive
- materials store hydrogen through adsorption and absorption, which can wear out the material and may be a slow release of hydrogen from material when needed
- hydrogen fuel cells have a limited lifetime so require regular replacements and are expensive to make so have a Hugh production cost
- use of toxic chemicals in their production
advantages of hydrogen fuel cells over petrol/diseal
- less pollution and less CO2 as Pure hydrogen emits only water whilst hydrogen-rich fuels produce only small amounts of air pollutants and CO2
- greater efficiency
Name 3 ways hydrogen can be stored
- As a pressurised liquid
- Adsorbed onto a solid
- Absorbed within a solid
What are the benefits of ethanol fuel cells over hydrogen?
- Made from renewable sources
- ethanol is less explosive so easier and safer to store than hydrogen
- Raw materials to produce ethanol by fermentation are abundant
What is the anode half-equation in an ethanol fuel cell?
C₂H₅OH + 3H₂O → 2CO₂ + 12H⁺ + 12e⁻
What is the cathode half-equation in ethanol fuel cells?
O₂ + 4H⁺ + 4e⁻ → 2H₂O
What is the colour change during the MnO₄⁻/Fe²⁺ titration?
Purple (MnO₄⁻) → Colourless (Mn²⁺)
Why is the MnO₄⁻/Fe²⁺ titration self-indicating?
Because MnO₄⁻ is purple and Mn²⁺ is colourless, giving a clear visible colour change.
What practical difficulty occurs when reading MnO₄⁻ in a burette?
The purple colour makes it hard to see the bottom of the meniscus.
Which acid should be used in manganate titrations and why?
Dilute sulfuric acid – it provides enough H⁺ ions (8 per MnO₄⁻) and doesn’t interfere with redox reactions.
Why can’t ethanoic acid be used in manganate titrations?
It’s too weak to provide enough H⁺ ions — leads to brown MnO₂ forming instead of Mn²⁺ leading to a greater (inaccurate) volume of manganate being used in the titration
Why can’t conc. HCl be used in manganate titrations?
Cl⁻ gets oxidised to toxic Cl₂ gas by MnO₄⁻, giving an inaccurate greater volume of manganate being used and poisonous Cl2 being produced
Why is nitric acid (HNO₃) unsuitable for manganate titrations?
It’s an oxidising agent and can oxidise Fe²⁺ to Fe³⁺, reducing the volume of MnO₄⁻ used
What is the overall equation for the reaction between MnO₄⁻ and H₂O₂?
2MnO₄⁻ + 6H⁺ + 5H₂O₂ → 5O₂ + 2Mn²⁺ + 8H₂O
What is the half-equation for the oxidation of ethanedioate ions (C₂O₄²⁻)?
C₂O₄²⁻ → 2CO₂ + 2e⁻
What is the overall equation for the reaction between MnO₄⁻ and C₂O₄²⁻?
2MnO₄⁻ + 16H⁺ + 5C₂O₄²⁻ → 10CO₂ + 2Mn²⁺ + 8H₂O
Why is the reaction between MnO₄⁻ and C₂O₄²⁻ slow at first?
Both are negative ions, so they repel each other. The rate increases once Mn²⁺ is formed (acts as a catalyst).
How can you speed up the MnO₄⁻ and C₂O₄²⁻ reaction in a titration?
Heat the conical flask to 60°C before titration begins.
What indicator is used in the thiosulfate titration and when is it added?
Starch indicator is added near the end point when iodine fades to pale yellow.
What is the colour change in the thiosulfate titration when starch is added
The colour change is from blue/black to colourless (as iodine is reduced).
Why should starch not be added until nearly all the iodine has reacted in the titration?
because the blue complex of starch with high concentrations of iodine is insoluble and does not re-dissolve as more thiosulfate is added, leading to inaccurate results
what is the colour change in the reaction between I2 and thiosulfate S2O32- before stitch is added
yellow brown solution of starch to colourless solution of starch
how to calculate percentage uncertainty
uncertainty/ measurements made on apparatus x 100
ways to decrease apparatus uncertainties
- use apparatus with greater resolution
- increase the size of the measurements made
when do you multiply by 2 when calculation percentage uncertainty
when using a burette
how to reduce the uncertainty in a burette reading
by making the titre a larger volume either by:
- increasing the volume and concentration of the substance in the conical flask
- by decreasing the concentration of the substance in the burette.
how to reduce the uncertainty of measuring cylinders
- use a measuring cylinder with a greater resolution
- Replacing measuring cylinders with pipettes or burettes which have lower apparatus uncertainty will lower the % uncertainty
how to reduce the uncertainty when measuring the mass
- Using a balance that measures to more decimal places or using a larger mass will reduce the % uncertainty in weighing a solid.
- Weighing sample before and after addition and then calculating difference will ensure a more accurate measurement of the mass added.
What does it mean if the % uncertainty due to the apparatus is less than the percentage difference between the actual and calculated values?
It indicates a discrepancy in the result due to other errors, not the apparatus.
What does it mean if the % uncertainty due to the apparatus is greater than the percentage difference between the actual and calculated values?
There is no discrepancy, and any differences can be explained by the sensitivity of the equipment.
