Question

1. (a) (i) name the ore from which iron is obtained.

(ii) write a balanced chemical equation for the reaction of iron oxide with carbon monoxide.
(iii) describe how slag is formed during the extraction of iron.
(b) explain why water stored in metal iron containers usually has a brown colour.
(c) iron oxide is reduced by carbon as shown in the equation: $2fe_2o_3 + 3c
ightarrow 4fe + 3co_2$
(i) calculate the mass of iron oxide needed to produce 14 000 kg of iron.
(ii) calculate the amount of ore required to produce 14 000 kg of iron given that 40 % of the ore are impurities.
(d) (i) describe any two human activities that increase soil acidity.
(ii) state how the high levels of acidity can be reduced.
(iii) explain the environmental effect of using too much nitrogenous fertilisers.

Explanation:

(a)(i)

Hematite is a common ore from which iron is obtained.

(a)(ii)

The balanced chemical equation for the reaction of iron(III) oxide ($Fe_2O_3$) with carbon - monoxide ($CO$) is:

$$Fe_2O_3 + 3CO ightarrow 2Fe+3CO_2$$

(a)(iii)

During iron extraction, limestone ($CaCO_3$) is added to the blast - furnace. Limestone decomposes at high temperatures to calcium oxide ($CaO$) and carbon dioxide ($CO_2$). The calcium oxide reacts with silica (an impurity in the iron ore) to form calcium silicate ($CaSiO_3$), which is the slag. The reaction is $CaO + SiO_2
ightarrow CaSiO_3$.

(b)

Water stored in metal iron containers has a brown color because iron reacts with oxygen and water in the air to form hydrated iron(III) oxide (rust). The chemical reaction is $4Fe + 3O_2+6H_2O
ightarrow 4Fe(OH)_3$, and as it dehydrates, it forms the brown - colored $Fe_2O_3\cdot xH_2O$.

(c)(i)

1. First, find the molar mass of $Fe_2O_3$ and $Fe$:

• The molar mass of $Fe_2O_3$ ($M_{Fe_2O_3}$): $M_{Fe_2O_3}=2\times55.85 + 3\times16=159.7\ g/mol$.
• The molar mass of $Fe$ ($M_{Fe}$): $M_{Fe}=55.85\ g/mol$.
• From the balanced equation $2Fe_2O_3 + 3C

ightarrow 4Fe+3CO_2$, the mole ratio of $Fe_2O_3$ to $Fe$ is $\frac{n_{Fe_2O_3}}{n_{Fe}}=\frac{2}{4}=\frac{1}{2}$.

• The number of moles of $Fe$ in $14000\ kg$ (or $14000000\ g$) of iron is $n_{Fe}=\frac{m_{Fe}}{M_{Fe}}=\frac{14000000}{55.85}\ mol$.
• The number of moles of $Fe_2O_3$ required, $n_{Fe_2O_3}=\frac{1}{2}\times n_{Fe}=\frac{1}{2}\times\frac{14000000}{55.85}\ mol$.
• The mass of $Fe_2O_3$ required, $m_{Fe_2O_3}=n_{Fe_2O_3}\times M_{Fe_2O_3}=\frac{1}{2}\times\frac{14000000}{55.85}\times159.7\ g$.
• $m_{Fe_2O_3}=\frac{14000000\times159.7}{2\times55.85}\ g = 20000000\ g=20000\ kg$.

(c)(ii)

If $40\%$ of the ore is impurities, then $60\%$ of the ore is the iron - containing compound. Let the mass of the ore be $m_{ore}$.
We know that the mass of pure iron - containing compound (which we assume is equivalent to the mass of $Fe_2O_3$ calculated above) is $20000\ kg$.
If $0.6m_{ore}=20000\ kg$, then $m_{ore}=\frac{20000}{0.6}\ kg\approx33333.3\ kg$.

(d)(i)

1. Acid rain: Burning of fossil fuels such as coal and oil releases sulfur dioxide ($SO_2$) and nitrogen oxides ($NO_x$). These gases react with water vapor in the atmosphere to form sulfuric acid ($H_2SO_4$) and nitric acid ($HNO_3$), which fall as acid rain and increase soil acidity.
2. Over - use of chemical fertilizers: Some fertilizers, such as ammonium - based fertilizers, release hydrogen ions ($H^+$) into the soil as they are used by plants, increasing soil acidity.

(d)(ii)

High levels of soil acidity can be reduced by adding lime (calcium oxide, $CaO$) or limestone (calcium carbonate, $CaCO_3$). These substances react with the hydrogen ions in the soil to neutralize the acidity. For example, $CaCO_3 + 2H^+
ightarrow Ca^{2 +}+H_2O + CO_2$.

(d)(iii)

Using too much nitrogenous fertilizers can lead to eutrophication in water bodies. When it rains, the excess nitrogen from the soil leaches into nearby water sources. This excess nitrogen causes an over - growth of algae and other aquatic plants. When these plants die and decompose, the oxygen in the water is depleted, leading to the death of fish and other aquatic organisms.

Answer:

(a)(i) Hematite
(a)(ii) $Fe_2O_3 + 3CO
ightarrow 2Fe+3CO_2$
(a)(iii) Limestone decomposes to $CaO$, which reacts with $SiO_2$ (an impurity) to form $CaSiO_3$ (slag).
(b) Iron reacts with oxygen and water to form hydrated iron(III) oxide (rust).
(c)(i) $20000\ kg$
(c)(ii) Approximately $33333.3\ kg$
(d)(i) Acid rain from fossil - fuel burning; over - use of chemical fertilizers.
(d)(ii) Add lime or limestone.
(d)(iii) Causes eutrophication in water bodies, leading to oxygen depletion and death of aquatic organisms.