Galvanic and Electrolytic Cells

Based on Lucarelli's Essential Chemistry Chapter 7

Galvanic Cell

An electrochemical cell where the two halves of a spontaneous redox reaction occur in different locations connected by an external circuit to allow electrons to flow, transforming chemical potential energy into electrical energy. (This can be thought of as pushing a boulder down a hill)

What is the purpose of a salt bridge?

It allows ions to flow between anode and cathode, preventing the buildup of charge on the electrodes (if this is not prevented the cell will stop working)

What is happening at the anode in a galvanic cell?

The anode is in contact with the reducing agent, which is oxidised in the reaction, producing electrons that flow through the external circuit. Anions flow through the salt bridge to it. It is labelled - as electrons flow out of it.

What is happening at the cathode in a galvanic cell?

The cathode is in contact with the oxidising agent, which is reduced when electrons from the anode flow into it. Cations flow through the salt bridge to it. It is labelled + as electrons flow into it.

What are the electrodes submerged in?

Electrolyte solutions

What determines the voltage (potential difference) produced by a galvanic cell?

The combined strength of the reducing and oxidising agents involved

The oxidising agent with the greatest standard reduction potential (E_⁰red)…

Has the greatest oxidising agent strength and the weakest reducing agent strength in its reduced form

The oxidising agent with the lowest standard reduction potential (E_⁰red)…

Has the lowest oxidising agent strength and the strongest reducing agent strength in its reduced form

What are the strongest and weakest Oxidising agents in the SRP table?

Strongest: F_2(g)

Weakest: K⁺_(aq)

What are the strongest and weakest Reducing agents in the SRP table?

Strongest: K_(s)

Weakest: 2F^-_(aq)

How can you determine if a redox reaction will be spontaneous from the SRP table?

If the specie to the reduced (oxidising agent, cathode) is higher than the specie to be oxidised (reducing agent, anode), so that the reaction makes a clockwise circle

How can you determine if a redox reaction will be spontaneous mathematically?

E⁰_cell = E⁰_red + E⁰_ox*, if E⁰_cell is positive, the reaction will be spontaneous

*Invert the E⁰_red value of a half-reaction to get E⁰_ox

If a redox reaction is spontaneous in one direction, will it be spontaneous in reverse?

No, see above

What would you observe in each half-cell of a galvanic cell?

The anode half-cell’s electrode will lose mass, and if coloured, its electrolyte solution will intensify in colour

The cathode half-cell’s electrode will gain mass, and if coloured, its electrolyte solution will fade

What is an electrolytic cell?

A pair of electrodes connected to a DC power supply, that are dipped into a molten or aqueous electrolyte. When a current is applied to the electrodes, an otherwise non-spontaneous redox reaction occurs, converting electrical energy into chemical potential energy. (This can be thought of as rolling a boulder up a hill - you need to add energy into the system to do it).

What happens at the anode (+) of an electrolytic cell?

Anions move to the anode, where they are oxidised.

What happens at the cathode (-) of an electrolytic cell?

Cations move to the anode, where they are reduced.

With what sign are the anodes and cathodes labelled in each type of cell?

Galvanic cell:

Cathode +

Anode -

Electrolytic cell:

Cathode -

Anode +

What are some uses of electrolysis (the process done by an electrolytic cell)?

Producing pure substances e.g. metals, electroplating

What type of electrode should be used for electrolysis of a molten salt?

Inert, e.g. Pt

What is an advantage and a disadvantage of electrolysing a molten salt instead of aqueous?

Pro: No competing reactions

Con: Requires high temperature

What will happen if water is oxidised during electrolysis of an aqueous solution?

The solution’s pH will fall in the vicinity of the anode as colourless oxygen gas and hydrogen ions are produced

What will happen if water is reduced during electrolysis of an aqueous solution?

The solution’s pH will fall in the vicinity of the anode as colourless hydrogen gas and hydroxide ions are produced

What will happen if a metallic electrode is oxidised during electrolysis of an aqueous solution?

The anode will dissolve and lose mass

What are the most probable reactions in an electrolytic cell under standard conditions?

The one with the highest standard reduction potential (E⁰_red) will be the reduction reaction, while the one with the lowest E⁰_red (aka the highest E⁰_ox) will be the oxidation reaction

What does OLEA stand for?

Oxidation (Loss of Electrons) occurs at the Anode

What does GERC stand for?

Gaining electrons (Reduction) occurs at the Cathode

How are the anode and cathode different in a Galvanic and electrolytic cell?

In a galvanic cell, the spontaneous redox reaction determines the anode (oxidation site) and the cathode (reduction site).

In an electrolytic cell, the direction of the applied DC voltage determines the flow of electrons, with electrons flowing out of the anode and into the cathode.

Which direction to electrons flow in an electrochemical cell?

From anode to cathode

How do you calculate the minimum potential difference required to operate an electolytic cell? (assuming no energy lost to the surroundings, no resistance in the circuit, etc)

Add the standard reduction potential of the reductant to the standard oxidation potential of the oxidant (your result should be negative, the required voltage is that multiplied by -1)