Electrochemistry A - level Chemistry
What happens if the oxidising agent < IM
Favours oxidation, more negative E
What happens if the oxidising agent > 1M
Favours reduction, more positive E
Describe fuel cells.
Fuel must constantly be supplied (for example hydrogen) to react with oxygen and produce electrical energy. The cell e.m.f. is constant because the supply of reactants is constantly fed in.
What is the reaction at the positive electrode in a lithium cell?
Li⁺ + CoO₂ + e⁻ → Li⁺[CoO₂]°
What is the reaction at the positive electrode in an alkaline hydrogen-oxygen fuel cell?
O₂(g) +2H₂O(l) + 4e⁻ → 4OH⁻(aq)
Describe non-rechargeable cells.
The chemicals are stored within the cell, these react and produce electrical energy until the reactants are used up. The reaction cannot be reversed. The cell emf will drop as the concentration of reactants decreases.
Describe rechargeable cells.
The chemicals are stored within the cell, these react and produce electrical energy until the reactants are used up. The cell can be recharged by reversing the reaction. The cell emf will drop as the concentration of reactants decreases. For the cell to be recharged the products must stick to the electrodes and are not dispersed in the electrolyte. It can be connected to another power supply with a larger EMF, therefore, e-s and ions are forced around the circuit in the opposite direction
What happens if the solution is very concentrated/dilute?
The electrode potentials are not equal to the standard electrode potentials
What do electrochemical cells consist of?
Two half-cells that have to be connected with a salt bridge and which have electrodes that can conduct electricity when connected by wires.
Why is water not used as a solvent in the lithium ion cell?
Water reacts very violently with lithium.
What can electrochemical cells be used as? A commercial source of electrical energy.
What can electrochemical cells be used as? A commercial source of electrical energy. Cells can be ∙ non-rechargeable ∙ rechargeable ∙ fuel cells
What is the standard electrode potential?
When an electrode system is connected to the hydrogen electrode system, and standard conditions apply the potential difference that is measured is called the standard electrode potential.
How is the hydrogen electrode represented in a cell diagram?
Pt(s) | H₂ (g) | H⁺(aq)
Oxidising agent
Accepts electrons and gets reduced.
Reducing agent
Donates electrons and becomes oxidized.
How do you calculate the EMF of a cell?
E cell = Erhs - Elhs E cell = Ered - Eox The more negative half cell will always oxidise The more positive half cell will always reduce The more positive the standard electrode potential... ...the stronger the oxidising agent The more negative the standard electrode potential... ...the stronger the reducing agent
Disproportionation
A redox reaction in which the same species is both oxidized and reduced.
What are secondary standards?
A standard electrode which has been calibrated against the primary standard which is the standard hydrogen electrode.
What is the reaction at the negative electrode in a lithium cell?
Li → Li⁺ + e⁻
What are the standard conditions?
- All ion solutions at 1M - Temperature 298K - Gases at 100kPa pressure - No current flowing
What are the components of a standard hydrogen electrode?
1. Hydrogen gas at a pressure of 100kPa 2. Solution containing the hydrogen ion at 1M 3. Temperature at 298K
Electrode potential depends on?
1. Nature of ions in the solution 2. Conc. of ions in the solution 3. Type of electrode 4. Temperature
What is the overall equation of the alkaline hydrogen-oxygen fuel cell?
2H₂(g) + O₂(g) → 2H₂O(l)
What is the reaction at the negative electrode in an alkaline hydrogen-oxygen fuel cell?
H₂(g) + 2OH⁻(aq) → 2H₂O(l) + 2e⁻
Why is the Ecell value significant?
If the Ecell value is positive then the reaction is feasible in the forwards direction.
What happens if the current is allowed to flow between the two half cells
If there was a bulb in the circuit instead of a high resistance voltmeter a current would flow as because of the potential difference chemical energy is converted to electrical energy. The reactions will then occur separately at each electrode. The voltage will fall to zero as the reactants are used up.
How would you measure the electrode potential of a half cell?
It is not possible to measure the absolute potential of a half electrode on its own. To measure the potential electrode potential of a half-cell, it has to be connected to another halfcell of a known potential, and the potential difference between the two half-cells must be measured. By convention we can assign a relative potential to each electrode by linking it to a reference electrode: the hydrogen electrode which is given a potential of zero volts.
What is current?
It is the flow of electricity which results from the ordered directional movement of electrically charged particles
What is the effect of concentration on the Ecell value?
Le Chatelier's principle can be applied if the concentration of the reactants increased so would the Ecell value and if the concentration of the reactants decreased Ecell would also decrease.
What is the effect of temperature on the Ecell value?
Le Chatelier's principle can be applied, most cells are exothermic in the spontaneous direction and therefore a temperature rise would result in a decrease of the Ecell value because the equilibrium would shift backwards.
When using cell notation which half cell goes on the left?
The one with the most negative standard electrode potential.
Why might some E be positive but no reaction occurs?
The reactants may be kinetically stable; have a very high activation energy, and be very slow at room temp
Why do electrochemical cells require a salt bridge?
The salt bridge allows ions to flow between the solutions to complete the circuit, keeping the balance of positive and negative ions in each half cell without them mixing together. The salt bridge is often a piece of filter paper or agar gel soaked in potassium nitrate which is will not react with either of the solutions in the half cells).
Why can't most electrode potentials be calculated experimentally?
The species tend to react with H20 and therefore its calculated using a born-haber cycle
Why must a high resistance voltmeter used in electrochemical cells to measure the potential difference?
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 high resistance voltmeter stops the current from flowing.
What happens when a system does not include a metal that can act as an electrode?
Then a platinum electrode must be used and included in the cell diagram. It provides a conducting surface for electron transfer.
What is electromotive force?
There is a potential difference between the two electrodes. If there was a bulb in the circuit instead of a high resistance voltmeter a current would flow as because of the potential difference chemical energy is converted to electrical energy. The voltmeter takes measurements when there is zero-current which means the measurements is the maximum possible potential difference between the two half cells and this is called the e.m.f.
What happens if 2 gases are want to measured.
They are bubbled through the solution so it it in contact with the electrode
Why is the platinum electrode used?
This is because the platinum electrode conducts electricity and it is inert (very unreactive).
What are the risks of a hydrogen-oxygen fuel cell?
∙ Hydrogen is highly flammable so it needs to be handled carefully when stored or transported. ∙ Energy is required to produce the hydrogen and oxygen. Hydrogen is readily available by the electrolysis of water but this is expensive. To be a green fuel the electricity required would need to be produced from renewable resources. ∙ These cells have limited lifetimes (they require regular replacement and disposal) and high production costs.
What are the benefits of a hydrogen-oxygen fuel cell?
∙ They do not need recharging, unlike other storage cells: as long as hydrogen and oxygen are supplied the cell will produce energy. ∙ The only waste product is water: there are no toxic chemicals to dispose of and no CO₂ or NOx emissions from the cell itself. ∙It is more efficient.
