Practical 7 by Seah Shun Zhou (29), Eddie Lim (21), Chen Ming (5) and Hong Jin xiang (13)

Observation

	Sodium Hydroxide + ammonium chloride	Carbon hydroxide + ammonium nitrate
Colour of gas	Colourless	Colourless
Odour of gas	Smelly	Odourless
Effect of gas on damp red litmus paper	Turns blue	No change observed
Effect of gas on damp blue litmus paper	No change observed	Turns red
Name of gas formed	Ammonia gas	Ammonia gas

1. What can you conclude about the reaction between an alkali and an ammonium salt?

Alkali reacts with ammonium salt to produce: Salt +water + ammonia

2. Write the general equation for the reaction:

Alkali + Ammonium salt à Salt + water + ammonia gas

3. Why is it necessary to warm the mixture when we need to test for ammonia gas?

The reaction requires heat energy to break down the bonds in the compound formed before ammonia gas is released.

4. From the effect of gas in damp litmus paper, what can you conclude about the nature of ammonia gas?

It is alkaline

5. When pieces of dry litmus paper are placed above a bottle of aqueous ammonia, there is no visible observation. However, then the experiment is repeated using damp red litmus paper, the damp litmus paper turned blue. What can you conclude from the above observations?

The litmus paper can only show its alkaline properties when it is placed in water. The alkaline properties cannot be observed when it is dry.

6. Substance X is a white solid and is soluble in water. Describe a chemical test to confirm X is an ammonium salt.

1. Dissolve substance X in the water till it is completely dissolved.

2. Heat the solution.

3. Place damp litmus papers (blue and red) over the heated solution.

4. If X is an ammonium salt, the damp read litmus paper will turn blue while the blue litmus paper would remain blue. This is caused by ammonia gas, which is only released when ammonium salt solution is heated.

7.Farmers have to add slaked lime, solid calcium hydroxide to soil to remove excess acid in the soil if the pH is too low for plants to grow well. However, it is not advisable to add nitrogenous fertilizers like ammonium nitrate together with slaked lime. Can you suggest a reason to this?

Fertilizers that contain ammonium are used commercially to provide crops with nitrate, an important source of nitrogen for plants. The ammonium reacts with water to form nitrate, but the reaction slightly acidifies the soil. Soil that is too acidic presents two different problems for crops. First, the amount of necessary minerals in the soil such as potassium and calcium decreases. Second, aluminum and manganese levels increase to potentially toxic levels below a pH of roughly 5.5.


Exercise

1 (a) sodium hydroxide +ammonium chloride

->

sodium chloride + water + ammonia gas

(b) NaOH(aq) + NH₄Cl

->

NaCl +H₂O(l) + NH₃(g)

2 (a) sodium hydroxide + ammonium sulfate

->

sodium sulfate + water + ammonium

(b) 2NaOH(l) + (NH₄)₂SO₄(aq)

->

Na₂SO₄ (aq) + 2H₂O + 2NH₃ (l)

3 (a) sodium hydroxide + ammonium nitrate

->

sodium nitrate + water + ammonia gas

(b) NaOH(s) + NH₄NO₃

->

NaNO₃(s) + H₂O (l) + NH₃

4

(a) calcium hydroxide + ammonium chloride -> Calcium chloride + water + ammonia gas

(b) Ca(OH)₂(aq) +2NH₄Cl(s) -> CaCl₂(aq) + 2H₂O (l) + 2NH₃(g)

5 (a) calcium hydroxide + ammonium sulfate -> calcium sulfate + water + ammonia gas

(b) Ca(OH)₂(aq) + (NH₄)₂SO₄(aq) -> CaSO₄ + 2H₂O (l) + 2NH₃(g)

6 (a) calcium hydroxide + ammonium nitrate -> Calcium nitrate + water + ammonia gas

(b) Ca(OH)₂(aq) + NH₄NO₃ -> Ca(NO₃)₂ + 2H₂O + 2NH₃(g)