05 May O Level Chemistry Tuition Question

3 .        A simplified diagram of the nitrogen cycle is shown below.

(a)        Although certain bacteria in the soil convert nitrogen gas into nitrates, other bacteria convert nitrogen into ammonium salts. The ionic equation for this second reaction is

N₂ + 8H⁺ + 6e^– → 2NH₄⁺

Explain why this is a reduction reaction.

(b)        In the presence of hydrogen ions, a different type of bacterium converts nitrate ions into nitrogen gas and water. Give the ionic equation, with state symbols, for this reaction.

(c)        Ammonia is synthesized by the Haber process.

N₂ + 3H₂ → 2NH₃

(i)         State the sources of both the nitrogen and hydrogen needed for the Haber process.

(ii)        State the essential conditions for the Haber process.

(d)        Fertilisers are added to the soil to improve crop yields.

A farmer has the choice of two fertilisers, ammonium nitrate, NH₄NO₃, or diammonium hydrogen phosphate, (NH₄)₂HPO₄.

Show by calculation which of these fertilisers contains the greater percentage of nitrogen by mass. You must show your working.

(e)        State one major problem caused when the nitrates from fertilisers leach from the soil into streams and rivers.

Miracle Learning Centre (O Level Chemistry Tuition/ IP Chemistry Tuition)

Did you answer the Secondary Chemistry Tuition Question correctly?

3.a.  nitrogen has gained electrons/oxidation number of nitrogen has

decreased;

1. 2NO₃ ^– + 12H⁺ + 10e^– → N₂ + 6H₂O

c.i. nitrogen from the air/atmosphere;

hydrogen from methane/natural gas/water/cracking hydrocarbons;

IF: (nitrogen and hydrogen) from the air = 1

1. Any two of the following specified conditions: range 380-450^oC/

any pressure in range between 180-220 atm;

221. NH₄NO₃ 35% > (NH₄)₂HPO₄ 221.2%

1. eutrophication/increase in algal growth (on surface of water)/algal

bloom/reduction of dissolved oxygen in water/water plants die

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