DM.4 Flashcards
Electrochemical cells
What do redox reactions involve?
Electron transfer.
What is an equation called when spectator ions are removed from the overall equation of the reaction?
It is now an ionic equation.
What happens in terms of electrons in the oxidation half equation?
Electrons are produced.
What happens in terms of electrons in the reduction half equation?
Electrons are accepted/used up.
When is something an oxidising agent?
When it accepts electrons to aid the oxidation of another substance. It is reduced itself.
When is something a reducing agent?
When it produces electrons to aid the reduction of another substance. It is oxidised itself.
What are two parts of an electrochemical cell, where one is accepting electrons and one is producing electrons, individually known as?
Half cells.
Are individual half equations reversible?
Yes, electrons can be lost or gained, it just depends on what they’re reacting with?
When two half cells are combined, what is it called?
A full electrochemical cell.
What does the voltage measure?
The potential difference between the negative and positive terminals.
What is E cell the value of?
The potential difference of the cell when no current flows.
What happens to the voltage when a higher current is drawn?
The voltage would decrease.
What type of voltmeter is used when measuring E cell?
A high-resistance voltmeter. This will record the maximum potential difference between the electrodes of the half cells.
How does a half cell have its own potential difference? Using zinc as an example.
Zinc atoms in the solid zinc strip release electrons and form Zn2+ ions. This makes the zinc strip negatively charged relative to the solution of Zn2+ ions. Zn2+ ions in the solution accept electrons and reform Zn atoms. This sets up a dynamic equilibrium, and the position of the equilibrium determines the size of the potential difference.
What is the general equilibrium equation for a metal half cell?
M2+(aq) + 2e- —-> M(s)
What does the electrode potential of a half cell represent?
Its tendency to be reduced or oxidised.
What does a more positive electrode potential mean about the half cell?
It has a greater tendency to be reduced. It is a better oxidising agent.
What does a more negative electrode potentail mean about the half cell?
It has a greater tendency to be oxidised. It is a better reducing agent.
When two half cells are connected to a make full cell, which terminal is the one with the more positive electrode potential?
Positive terminal, as this half cell has a higher tendency reduced and electrons are gained.
When two half cells are connected to make a full cell, which terminal is the one with the more negative electrode potential?
Negative terminal, as this half cell has a higher tendency to be oxidised and electrons are lost.
What is used to complete the circuit when the half cells are set up in separate beakers?
A salt bridge between beakers and a metal wire connecting electrode to a voltmeter (high-resistance).
What is a salt bridge made of?
It is a strip of filter paper soaked in a saturated potassium nitrate(V) solution.
How does the salt bridge complete the circuit?
By the movement of ions, not electrons. The potassium ions and nitrate ions carry the current to allow electrical contact. A salt bridge is sometimes also known as an ion bridge.
What half-cell is used as a reference half-cell that all other half cells are measured against?
The standard hydrogen half cell. Also known as the standard hydrogen electrode.
What is the standard conditions for the hydrogen half cell?
Concentration of hydrogen [H+(aq)] = 1 moldm-3
Pressure of hydrogen gas 10^5 Pa( 1 atm) and temperature of 298K (which is rtp)
What is the half equation of the standard hydrogen half cell?
2H+(aq) + 2e- —> H2(g)
What factors will vary the electrode potential of a half cell?
Temperature, concentration/pressure.
What are the standard conditions for measuring the standard electrode potential of a half cell?
1 moldm-3 solution.
298K and 10^5 Pa (1 atm) (rtp).
What voltage is chosen to define the standard hydrogen half cell’s potential?
0V.
What symbol represents the standard electrode potential of a half cell?
E⦵
What is the definition of the standard electrode potential of a half cell, E⦵?
The potential difference between the half cell and a standard hydrogen half cell.
What electrode can be used when the half cell does not contain metal?
An inert metal electrode like platinum.
What is the equation to work out the E cell (the potential difference of the cell as a whole)?
E cell = E⦵reduced - E⦵oxidised
What can you use electrode potentials for?
To make predictions about the feasibility of redox reactions.
When is a redox reaction feasible?
When the E cell value is positive.
If the redox reaction is predicted to be feasible, but nothing seems to happen, what might be the reason for this?
The rate of the redox reaction cannot be predicted, therefore although it may be feasible, the rate may be so slow that it essential doesn’t happen at all.
If the reaction is feasible but the rate is too slow, what could this mean about the reaction?
It must have a very high activation enthalpy. Adding a catalyst could increase the rate.
How can you make a redox reaction that is not feasible under standard condition happen?
Change the conditions.
