4.1 Tests for positive ions
Some positive ions can be identified using a flame test or by using sodium hydroxide solution.
Some metal ions produce colours when put into a flame.
- Lithium (Li+) - crimson
- Sodium (Na+) - yellow
- Potassium (K+) - lilac
- Calcium (Ca2+) - red
- Barium (Ba2+) - green
The hydroxide of most metals that have ions with 2+ and 3+ charges are insoluble in water.
When sodium hydroxide is added to solutions of these ions a precipitate of the metal hydroxide forms.
Fe3+ (aq) + 3OH- (aq) --> Fe (OH)3 (s)
4.2 Tests for negative ions
We identify carbonates by adding dilute acid, which produces carbon dioxide gas. The gas turns limewater cloudy.
e.g. 2HCl (aq) + CaCO3 (s) --> CaCl2 (aq) + H2O (l) + CO2 (g)
We identify halides by adding nitric acid, then silver nitrate solution. This produces a precipitate of silver halide solution:
Chloride ions - white Bromide ions - cream Iodide ions - yellow
e.g. AgNO3 (aq) + NaCl (aq) --> Ag Cl (s) + NaNO3 (aq)
We identify sulfates by adding hydrochloric acid, then barium chloride solution. This produces a white precipitate of barium sulfate.
BaCl2 (aq) + MgSO4 (aq) --> BaSO4 (s) + MgCl2 (aq)
Titration is used to measure accurately how much acid and alkali react together completely.
To do a titration:
- a pipette is used to measure the volume of alkali that is put into a conical flask
- an indicator is added to the alkali
- a burette is filled with acid which is then added gradually to the flask
- when the indicator changes colour the end point has been reached
- the volume of acid used is found from the initial and final burette readings
pipette - measure a fixed volume of solution
burette - measure the volume of the solution added
4.4 Titration calculations
To calculate the concentration of a solution, given the mass of a solute in a certain volume:
- calculate the mass (in grams) of solute in 1 cm3 of solution
- calculate the mass (in grams) of solute in 1000 cm3 of solution
- convert the mass (in grams) to moles
To calculate the mass of solute in a certain volume of solution of known concentration:
- calculate the mass (in grams) of the solute there is in 1 dm3 (1000 cm3) of solution
- calculate the mass (in grams) of the solute in 1 cm3 of solution
- calculate the mass (in grams) of solute there is in the given volume of the solution
number of moles = mass in grams/relative formula mass
4.6 Chemical equilibrium
In a reversible reaction, the products of the reaction can react to re-form the original reactants.
A + B C + D
In a closed system, equilibrium is achieved when the rates of the forward and reverse reactions are equal.
Changing the reaction conditions can change the amounts of products and reactants in a reaction mixture at equilibrium.
Increasing the concentration of a reactant will cause more products to be formed as the system tries to achieve equilibrium.
If a product is removed, more reactants will react to try and achieve equilibrium and so more product is formed.
4.7 Altering conditions
If we change the conditions of a system at equilibrium, the position of equilibrium shifts as if to try and cancel out the change.
If the forward reaction produces more molecules of gas:
- an increase in pressure decreases the amount of products formed
- a decrease in pressure increases the amount of products formed
If the forward reaction produces fewer molecules of gas:
- an increase in pressure increases the amount of products formed
- a decrease in pressure decreases the amount of products formed
4.7 Altering conditions 2
Changing temperature: more products, less of the reactants.
If the forward reaction is exothermic:
- an increase in temperature decreases the amount of products formed
- a decrease in temperature increases the amount of products formed
If the forward reaction is exothermic:
- an increase in temperature increases the amount of products formed
- a decrease in temperature decreases the amount of products formed
The Haber Process
Ammonia is an important chemical for making other products, including fertilisers.
Ammonia is made from nitrogen and hydrogen in the Haber Process.
We carry out the Haber Process under conditions which are chosen to give a reasonable yield of ammonia as quickly as possible.
Nitrogen from the air and hydrogen (usually obtained from natural gas) are purified and mixed in the correct proportions.
The gases are passed over an iron catalyst at a temperature of about 450 C and a pressure of 200 atmospheres.
Any unreacted nitrogen and hydrogen are recycled in the Haber Process.