IGCSE Chemistry 0620: Oxides of Nitrogen and Catalytic Converter Mole Calculations

At the Ardmoor Motor Testing Facility, engineers investigate the exhaust of a delivery van both before and after it passes through the vehicle's catalytic converter.

(a) Explain, in terms of the gases present and the conditions inside the engine, how oxides of nitrogen form in a car engine. [2]

(b) Inside the catalytic converter, carbon monoxide reacts with nitrogen monoxide according to the equation: $2\text{CO} + 2\text{NO} \rightarrow 2\text{CO}_2 + \text{N}_2$ A sample of the van's exhaust gas passing over the catalyst contains $0.60\ \text{mol}$ of carbon monoxide and $0.60\ \text{mol}$ of nitrogen monoxide.

(i) Calculate the maximum amount, in mol, of nitrogen gas that could be produced. [2]

(ii) State why converting the carbon monoxide and nitrogen monoxide into carbon dioxide and nitrogen, as in this reaction, reduces the harm caused by the van's exhaust. Even though carbon dioxide is still released. [2]

Part (a): How oxides of nitrogen form in a car engine

The air drawn into a car engine is mostly nitrogen and oxygen. Nitrogen is normally very unreactive at ordinary temperatures. However, inside the engine’s cylinders, combustion produces very high temperatures.

At these high temperatures, nitrogen and oxygen from the air have enough energy to react directly with each other:

$\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$

This is why oxides of nitrogen form as a by-product of running the engine, even though the fuel itself contains no nitrogen.

Part (b)(i): Maximum moles of nitrogen produced

The balanced equation is:

$2\text{CO} + 2\text{NO} \rightarrow 2\text{CO}_2 + \text{N}_2$

The mole ratio of $\text{NO} : \text{N}_2$ is $2 : 1$ .

Since carbon monoxide and nitrogen monoxide are both present in a $1:1$ ratio ( $0.60\ \text{mol}$ each), which exactly matches the $2\text{CO}:2\text{NO}$ ratio in the equation, both are used up together with none left over.

Using the moles of nitrogen monoxide:

$n(\text{N}_2) = \frac{0.60}{2} = 0.30\ \text{mol}$

So the maximum amount of nitrogen gas produced is $\boxed{0.30}\ \text{mol}$ .

Part (b)(ii): Why this reaction reduces the harm from the exhaust

Before the catalytic converter, the exhaust contains carbon monoxide (a toxic gas) and nitrogen monoxide (which contributes to acid rain, photochemical smog and respiratory problems).

The reaction converts these two pollutants into:

Carbon dioxide, not directly toxic and does not cause acid rain or smog (though it remains a greenhouse gas contributing to global warming).
Nitrogen, the main, harmless component of the air itself.

So although carbon dioxide is still released, the specific dangers of carbon monoxide poisoning and the acid rain/smog/respiratory effects of nitrogen monoxide are greatly reduced, because these gases are no longer being released in their original harmful forms.

Final answers

(a) High engine temperatures allow unreactive nitrogen from the air to react directly with oxygen, forming oxides of nitrogen.
(b)(i) $0.30\ \text{mol}$ of nitrogen gas
(b)(ii) The reaction removes the directly toxic carbon monoxide and the acid-rain/smog-causing nitrogen monoxide, converting them into harmless nitrogen and (still a greenhouse gas, but not directly toxic or acid-rain-forming) carbon dioxide.