DF knu: Combustion; pollutants; fuels & sustainability

Question 1

What is formed during complete and incomplete combustion?

Answer 1

• Complete: CO₂, H₂O
• Incomplete: H₂​O, CO and/or C (ratios are determined by proportions on RHS of equation - in other words, O₂ supply)

Question 2

Complete and balance the equation for the incomplete combustion of heptane:

C₇H₁₆ + 10O₂ →

Answer 2

C₇H₁₆ + 10O₂ → 5CO₂ + 8H₂O + 2CO

Question 3

Complete the equations and determine whether each is complete or incomplete combustion.

Answer 3

Question 4

Sulfur oxides may originate from volcanos, or the combustion of sulfur compounds in vehicles. Write an equation to represent the formation of sulfur dioxide in the former situation.

Answer 4

S(s) + O₂(g) → SO₂(g)

Question 5

Describe, and write an equation to show, how nitrous oxides are formed as a result of vehicles.

Answer 5

• Minor cause: combustion of nitrogen compounds in fuels (present in small proportions)
• Major cause: nitrogen and oxygen in air react in high temperatures of vehicle engines:

N₂(g) + O₂(g) → 2NO(g)

Question 6

Explain how sulfur dioxide can react to produce both a weak and a strong acid. Include relevant equations.

Answer 6

SO₂ + H₂O → H₂SO₃

= Sulfuric(IV) acid - weak (not so bad)

SO₂ + 1/2O₂ → SO₃

SO₃ + H₂O → H₂SO₄

= Sulfuric(VI) acid - strong (worse)

Question 7

Explain how nitrogen oxides react to produce acid. Include relevant equations.

Answer 7

2NO + H₂O + 1.5O₂ → 2HNO₃
2NO₂ + H₂O + 0.5O₂ → 2HNO₃

Balance N + H first then work out how much O₂ needed

Question 8

What problems are caused by the formation of SO_x and NO_x?

Answer 8

• Cause acid rain
• Contribute to photochemical smog

Question 9

What problems arise from acid rain?

Answer 9

• Health problems (especially for asthmatics)
• Corrosion of limestone buildings
• Damage plants/animals in forests + water

Question 10

What are the origins of CO emissions?

Answer 10

Incomplete combustion of hydrocarbons in fossil fuels + biofuels.

Question 11

What problems arise from CO production?

Answer 11

• Toxic (odourless + colourless)
• Oxidised to CO₂ in atmosphere (greenhouse gas)

Question 12

What are the origins of particulate emissions?

Answer 12

• Burning fuels
• Volcanoes

Question 13

What problems arise from the emission of particulates?

Answer 13

• Irritation / cancer of lungs
• Heart attacks

Question 14

What are secondary pollutants?

Answer 14

Ones which are formed from reactions of primary pollutants.

Question 15

What are the 2 major sources of unburnt hydrocarbons?

Answer 15

• Chemical plants
• Unburnt fuel from vehicle engines

Question 16

Name an issue associated with the emission of unburnt hydrocarbons.

Answer 16

Contribute to photochemical smog.

Question 17

What is photochemical smog and how is it formed?

Answer 17

• Mixture of primary + secondary pollutants which clouds up troposphere
• Formed when sunlight acts on primary pollutants to produce secondary ones

Question 18

What issues arise from photochemical smogs?

Answer 18

• Haziness + reduced visibility in ground-level air
• Eye + nose irritation
• Breathing difficulties (especially for asthmatics, children + older people)

Question 19

How is ozone formed?

Answer 19

By chemical reactions which occur when sunlight shines on primary pollutants (NO_x, C_xH_y, O₂, water vapour).

Question 20

What 4 problems are associated with the production of ozone in the troposphere?

Answer 20

• Weakens immune system
• Attacks lung tissue
• Acts as a greenhouse gas
• Highly reactive radical; attacks organic matter (C=C bonds vulnerable: plastic, rubber, textiles, paints)

Question 21

Catalytic converters are called-three way catalysts because they speed up three naturally occurring reactions in car engines which remove carbon monoxide, hydrocarbons and nitrogen monoxide.

1. Give balanced symbol equations for these 3 reactions. Use C₇H₁₆ as an example of a hydrocarbon.
2. Suggest how SO_x emission are best prevented.

Answer 21

CO + 0.5O₂ → CO₂

C₇H₁₆ + 11O₂ → 7CO₂ + 8H₂O

NO + CO → CO₂ + 1/2N₂

Preemptively removing sulfur impurities from fuels.

Question 22

Suggest why it is important that catalytic converters start working at as low a temperature as possible.

Answer 22

So that they remove pollutants as soon as possible after the car starts (and therefore starts producing emissions).

Question 23

Diesel engines have oxidation catalysts, but they cannot have reduction catalysts (which could otherwise be used to reduce NO_x to N₂). Suggest why.

Answer 23

• Diesel engines have a high concentration of O₂
• Reducing agent would be oxidised by O₂ before getting a chance to reduce NO_x

Question 24

How are particulates removed from diesel engines? Why is this partly disadvantageous?

Answer 24

• Particulate filters (often ceramic)
• Filters regenerated by increasing temperature, which increases fuel consumption

Question 25

Diesel NO_x emissions are reduced by the following methods:

• Exhaust gases recycled through cylinder
• NO converted into less harmful products, for example using ammonia as a reagent

1. Suggest why the former is useful.
2. Write a balanced equation for the reaction in the latter.

Answer 25

• Lowers temperature + therefore quantity of NO_x formed
• 4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

Question 26

What are biofuels?

Answer 26

Ones derived from renewable plant and animal materials.

Question 27

What are the advantages of hydrogen as a fuel?

Answer 27

• Renewable
• Combustion [directly] produces only water
• Can be sent down pipelines
• Can be used to generate electricity in engines/cells

Question 28

What are the disadvantages of hydrogen as a fuel?

Answer 28

• Poor energy density
• Compression to store at higher density makes it volatile
• High heat of combustion causes higher NO_x emissions
• Produced from electrolysis of water; electricity may not be clean
• Derived from natural gas; defeats the object

Question 29

List the advantages of biofuels over fossil fuels.

Answer 29

• Renewable
• Sustainable
• Can be considered carbon neutral; less greenhouse gas + global warming
• Can be produced from waste (that would otherwise go to landfill)
• Lower emissions of:
  
  • Particulates, which cause lung damage
  • CO, which is poisonous
  • Hydrocarbons, which cause photochemical smog + respiratory problems
  • Sulfur compounds, which cause acid rain
• Can be used as a straight replacement for diesel
• Biodiesel is biodegradable if spilled

Question 30

List the disadvantages of biofuels compared to fossil fuels.

Answer 30

• Not carbon-neutral if accounting for CO₂ produced during production + transportation
• Land used to grow fuel crops can’t be used to grow food crops
• Higher NO_x emissions, causing more tropospheric ozone + respiratory problems
• Not straight replacement for petrol (ethanol too volatile)
• Car engines must be modified (to use fuels of high ethanol concentration)
• Ethanol absorbs moisture, causing engine corrosion

Question 31

Answer 31

A

NO_x produced by reaction of N₂ + O₂ in air in hot engine

H₂ contains no carbon or sulfur

Question 32

Petrol cars produce less NO_x and particulates than diesel cars, but more CO and hydrocarbons. Suggest why diesel cars produce fewer hydrocarbons.

Answer 32

Diesel has a higher boiling point / is less volatile.