The Reactivity Series
Not all elements have the same reactivity strength.
- For metals the bigger the atom the easier it would be to lose an electron, and thus the lower down in a group the more reactive the metal. This would mean that Potassium is more reactive than Sodium and Calcium more reactive than Magnesium.
- For non-metals the smaller the atom the easier it would be to gain an electron, and thus the higher up in the group, the more reactive the non-metal. This would mean that Fluorine is more reactive than Chlorine and Oxygen more reactive than Sulfur.
The reactivity series for metals is as follows:
Potasssium>Sodium>Calcium>Magnesium>Zinc>Iron>Hydrogen>Iron>Silver>Gold
K>Na>Ca>Mg>Zn>Fe>H2>Fe>Ag>Au
The reactivity series for non-metals can be summarised up by the reactivity of the halogens:
Fluorine>Chlorine>Bromine>Iodine
Displacement Reactions
Displacement reaction is a chemical reaction in which a more reactive element displaces a less reactive element from its compound. Both metals and non-metals take part in displacement reactions.
Metal Displacement:
Mg(s) + CuSO4(aq) → MgSO4(aq) + Cu(s)
Non-metal Displacement:
Cl2(aq) + 2NaBr(aq) → 2NaClaq + Br2(aq)
Definitions for Electrochemistry
Conductors: Materials that allow electrons to flow through them.
Non-cpnductors: Materials that do not allow electrons to flow through them.
Electrolytes: A chemical compound that dissociates into ions and hence is capable of transporting electric charge.
Non-electrolytes: A chemical compound that does not dissociates into ions and hence is not capable of transporting electric charge.
Anode: The positive electrode at which the anions are attracted to.
Cathode: The negative electrode at which the cations are attracted to.
Effect of electricity on materials
Solids: Electricity either passes through a solid, making the solid a conductor, or else it does not pass through the solid making it a non-conductor.
Liquids;
- Solutions: Any liquid that has dissolved ions can allow electrons to flow through it.
- Molten: Unlike solid ionic compounds, molten ionic compounds can allow electrons to flow through.
Electrolytic decomposition reactions (Electrolysis)
Electrolytic decomposition may result when electric current is passed through an aqueous solution of a compound. A good example is the electrolysis of water.
Electrolysis of water: Electrolysis of water is the decomposition of water into hydrogen and oxygen due to the passage of electric current through it.
Decomposition of sodium chloride: On passing electricity through molten sodium chloride, it decomposes into sodium and chlorine.
What happens if more than 1 ion is present?
In some cases, multiple cations or anions might be present, and these would be chosen according to the following two preferences:
- Reactivity Series: The cation of the metal lower down in the reactivity will be the first one to decompose. For anions, the hydroxide (OH–) is the ion that normally decomposes.
- Concentration: The higher the concentration of the ion the bigger the chance of it decomposing when electrons are passed through it. Whilst this does not really effect cations, it affects anions.
Copper Sulfate Solution
Brine Solution
Copper electrodes
Quantitative aspects of electrolytic cells
In electrolysis, electrons are used to decompose a cation or an anion. The charge associated with passing 1 mole of electrons is equal to 96, 500Coulombs.
This can be worked out using the equation:
(coulombs, C) = (amperes, A) * (seconds, s)
Example:
Calculate the amount of charge transferred when a 5 A current is used for 2 minutes during electrolysis.
2 minutes = 2 × 60 = 120 s
Charge = current × time
Charge = 5 × 120 = 600 C