Electrode Potentials Flashcards by Fatimah Akhtar

• Describe the setup of a simple electrochemical cell and how it produces a voltage

• An electrochemical 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 as 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 (ie become a battery or a cell)

How well did you know this?

Not at all

Perfectly

• Why does a voltage form?

• In the cell pictured below, 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

How well did you know this?

Not at all

Perfectly

• 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

How well did you know this?

Not at all

Perfectly

• What is the purpose of a salt bridge?

• To complete the circuit
• To allows ions to move between half cells
• Allow movement of ions between electrodes
• To maintain electrical neutrality

How well did you know this?

Not at all

Perfectly

• What is a salt bridge usually made from?

• A salt bridge is usually made from a piece of filter paper (or material) soaked in a salt solution, usually potassium nitrate

How well did you know this?

Not at all

Perfectly

• What are the requirements of a salt bridge?

• Must not react with the electrolyte / ions in solution
• The salt should be unreactive with the electrodes and electrode solutions

How well did you know this?

Not at all

Perfectly

• Why is a wire not used as a salt bridge?

• A wire is not used because the wire would set up its own electrode system with the solutions

How well did you know this?

Not at all

Perfectly

• What happens if a current is allowed to flow?

• If the voltmeter is removed and replaced with a bulb or if the circuit is short circuited, a current flows
• The reactions will then occur separately at each electrode
• The voltage will fall to zero as the reactants are used up
• The most positive electrode will always undergo reduction (positive as electrons are used up)
• The most negative electrode will always undergo oxidation (negative as electrons are given off)

How well did you know this?

Not at all

Perfectly

• When constructing a cell practically, how can electrical contact between the electrodes and solutions be improved?

• Rubbing electrode with sandpaper to remove any oxide layer on the surface of the electrode
• Wiping the electrodes with propanone to remove any grease on the surface of the metal

How well did you know this?

Not at all

Perfectly

• State the meaning of the term electrochemical series

• List of electrode potentials / E0 in numerical order
• Half cells / equations in numerical order of electrode potential / E0

How well did you know this?

Not at all

Perfectly

• Diluting the magnesium chloride decreases the concentration of Mg2+
• This shifts the equilibrium to favour the oxidation of magnesium
• The magnesium electrode potential becomes more negative
• So the EMF of the cell increases

How well did you know this?

Not at all

Perfectly

• Describe the relationship between voltage and equilibrium

• If there is a large voltage, the equilibrium is to the right
• If there is a small voltage, the equilibrium is to the left

How well did you know this?

Not at all

Perfectly

• Why is KNO3 a suitable solution for a salt bridge?

• KNO3 is unreactive with the electrodes and the ions are free to move

How well did you know this?

Not at all

Perfectly

• Why might the current produced by a cell fall to zero after some time?

• All the reactants are used up

How well did you know this?

Not at all

Perfectly

• What will happen to a cell once the reactants are used up?

• Stops working or starts to leak

How well did you know this?

Not at all

Perfectly

• When can a platinum electrode be used?

• When there is no solid metal in the reaction, such as when there are metal ions of two different charges in the same solution
• Eg a solution containing Fe2+ and Fe3+ ions
• This solution could be made by dissolving FeSO4 (which contains Fe2+) and Fe2(SO4)3 (which contains Fe3+)

How well did you know this?

Not at all

Perfectly

• Why is platinum a suitable electrode?

Study These Flashcards

• It is unreactive and conducts electricity

• What are the conditions needed for the standard hydrogen electrode?

Study These Flashcards

• H2 gas is pumped in at a pressure of 100kPa
• The electrolyte contains H+ ions of concentration 1mol dm-3 eg HCl of conc 1 mol dm-3 / H2SO4 of 0.5 mol dm-3
• There must be a platinum electrode
• The whole system must be at a temperature of 298K
• The voltage of the standard hydrogen half cell is defined as zero

• Draw and label the standard hydrogen electrode

Study These Flashcards

Study These Flashcards

• The concentration of the CuSO4 solution is not 1 mol dm-3

• Describe the IUPAC cell representation

Study These Flashcards

• Vertical solid lines indicate a phase boundary (ie between the solid and aqueous phases)
• The species with the highest oxidation state should be written closest to the salt bridge
• A double vertical line in the middle represents the salt bridge

• Draw the cell representation of the standard hydrogen half cell

Study These Flashcards

• Which is the strongest oxidising agent and which is the weakest reducing agent?

Study These Flashcards

• The strongest oxidising agent (on the left) will have the most positive standard electrode potential
• In this table it is Ag+ (aq)
• The weakest reducing agent (on the right) will have the most positive standard electrode potential, in this table it is Ag (s)

• Which is the weakest oxidising agent and which is the strongest reducing agent?

Study These Flashcards

• The weakest oxidising agent (on the left) will have the least positive standard electrode potential, in this table it is Li+ (aq)
• The strongest reducing agent on the right will have the least positive standard electrode potential, in this table it is Li (s)

• What would happen if the concentration of CuSO4 was increased?

• The equilibrium would shift to the right • This means there would be more Cu and less Cu2+ and electrons • As there are fewer electrons the electrode potential would become more positive

• What would happen if the concentration of CuSO4 was decreased?

• The equilibrium would shift to the left • This means that there would be less Cu and more Cu2+ and electrons • As there are more electrons present the electrode potential would become more negative

• The Mg equilibrium shifts to the left • More electrons donated by Mg • EMF increases for Mg

•When will the current fall to zero?

• When the reactants are used up

• From right to left • The concentration of Cu2+ is greater on the left, so reduction on Cu2+ is more likely to happen on the left (Cu2+ + 2e- -> Cu) • The left electrode is the positive electrode, so the right electrode is the negative electrode

• When will the current fall to zero?

• When the concentration of Cu2+ in both half cells is equal

• H2 gas at 100kPa • [H+] = 1 mol dm-3 • Temperature of 298K • Platinum electrode

• Stage 1: preparing solution • Weigh 7.995g TiOSO4 • Dissolve in 0.50moldm-3 sulfuric acid • Transfer to volumetric flask and make up to the mark • Stage 2: set up cell • Piece of Ti immersed in 1.0 moldm-3 acidified TiO2+ (aq) • Connect solutions with salt bridge • Connect metals through high resistance voltmeter • Stage 3: measurements and calculation • Record voltage/potential difference of the cell • Ecell = ERHS - ELHS • ELHS = ERHS - Ecell • Ecell should be +1.22V (or -1.22 if Cu electrode on LHS)