galvanic and electrolytic cells. redox reactions oxidation reduction a reaction in which a substance...
TRANSCRIPT
GALVANIC AND ELECTROLYTIC CELLS
REDOX REACTIONS
OXIDATION REDUCTION
A reaction in which a substance loses electrons
A reaction in which a substance gains electrons
Mg → Mg2+ + 2e- oxidation reaction
reducing agent (donates electrons and so can cause reduction)
Cl2 + 2e- → 2Cl- reduction reaction
oxidising agent (accepts electrons and so can cause oxidation)
Mg → Mg2+ + 2e- Mg is oxidised (1)
Cl2 + 2e- → 2Cl- Cl2 is reduced (2)
Mg + Cl2 → MgCl2 Redox reaction
The electrons cancel each other out.
MgCl2 is an ionic compound (Mg2+ 2Cl-)
The equation shows 2 half reactions (1 and 2) that add to give the full redox reaction.
A coil of copper was placed in a silver nitrate solution
The solution became blue because copper ions were formed.
Solid silver deposited on the copper wire.
DIRECT ELECTRON TRANSFER
From the observations we can infer that:
Cu → Cu2+ + 2e-
Ag+ + e- → Ag
Electrons are transferred from the copper atoms on the piece of copper, to the silver ions in the silver nitrate solution.
This is a redox reaction.
This is a spontaneous reaction.
A GALVANIC CELL
THE ZINC-COPPER CELL
The following observations were made:
• the zinc plate decreases in mass.
• the copper plate increases in mass.
• the voltmeter reading indicates that electrons flow from the zinc plate to the copper plate.
From these observations we can infer that
The salt bridge:
• is a gel solution that connects the two electrolyte solutions that the metals dip into.
• acts as a transfer medium that allows ions to flow through but prevents the two solutions mixing.
• completes the circuit allowing electrons to flow through the connecting wire.
Zn(s) → Zn(aq)2+ + 2e-
Cu(aq)2+ + 2e- → Cu(s)
Zn(s) + Cu(aq)2+ → Zn (aq)
2+ + Cu(s)
The redox reaction for the cell:
The zinc dissolves – the mass of the zinc plate decreases.
The copper plate increases in mass because copper ions deposit as copper metal.
The electrodes:
Each metal strip is an electrode.
Each electrode with its associated electrolyte solution, is called a half cell.
The electrode where oxidation occurs is the ANODE. It has a negative charge.
The electrode where reduction occurs is the CATHODE. It has a positive charge.
Cell notation:
Zn / Zn2+ // Cu2+ / Cu
Zinc atoms are oxidised to Zn2+ ions (anode).
Cu2+ ions are reduced to copper atoms (cathode). Reduction written on the Right
Write the anode half reaction first – salt bridge (//) – then the
cathode half reaction
ELECTROLYTIC CELLS
Source of energy
Positive electrode Negative electrode
A battery is always required in this type of cell.
Chemical reactions occurring in the cell:
Na+ ions are attracted to the negative electrode, where they gain electrons.
Na+(aq) + e- → Na(s)
Reduction occurs at the negative electrode – the CATHODE.
Cl- ions are attracted to the positive electrode, where they lose electrons.
Cl-(aq) → Cl -(g) + e-
Oxidation occurs at the positive electrode – the ANODE.
Comparison of galvanic and electrolytic cells.
Galvanic cell Electrolytic cell
Produces an emf from a chemical reaction
Uses the emf of a cell to produce a chemical reaction
Chemical potential energy converted to electrical potential energy
Electrical potential energy converted to chemical potential energy
Positive terminal - cathode
Positive terminal - anode
Two containers each with an electrodes
One container with both electrodes dipping in
What should you be able to do?
• distinguish between galvanic and electrolytic cells.
• write equations for the reactions occurring in galvanic and electrolytic cells.
• explain the difference between oxidation and reduction and explain why the two types of reactions go together.
• label diagrams of galvanic and electrolytic cells.