# DF knu: Combustion; pollutants; fuels & sustainability Flashcards

1
Q

What is formed during complete and incomplete combustion?

A
• Complete: CO2, H2O
• Incomplete: H2​O, CO and/or C (ratios are determined by proportions on RHS of equation - in other words, O2 supply)
2
Q

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

C7H16 + 10O2

A

C7H16 + 10O2 → 5CO2 + 8H2O + 2CO

3
Q

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

A
4
Q

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.

A

S(s) + O2(g) → SO2(g)

5
Q

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

A
• 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)

6
Q

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

A

SO2 + H2O → H2SO3

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

SO2 + 1/2O2 → SO3

SO3 + H2O → H2SO4

= Sulfuric(VI) acid - strong (worse)

7
Q

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

A

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

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

8
Q

What problems are caused by the formation of SOx and NOx?

A
• Cause acid rain
• Contribute to photochemical smog
9
Q

What problems arise from acid rain?

A
• Health problems (especially for asthmatics)
• Corrosion of limestone buildings
• Damage plants/animals in forests + water
10
Q

What are the origins of CO emissions?

A

Incomplete combustion of hydrocarbons in fossil fuels + biofuels.

11
Q

What problems arise from CO production?

A
• Toxic (odourless + colourless)
• Oxidised to CO2 in atmosphere (greenhouse gas)
12
Q

What are the origins of particulate emissions?

A
• Burning fuels
• Volcanoes
13
Q

What problems arise from the emission of particulates?

A
• Irritation / cancer of lungs
• Heart attacks
14
Q

What are secondary pollutants?

A

Ones which are formed from reactions of primary pollutants.

15
Q

What are the 2 major sources of unburnt hydrocarbons?

A
• Chemical plants
• Unburnt fuel from vehicle engines
16
Q

Name an issue associated with the emission of unburnt hydrocarbons.

A

Contribute to photochemical smog.

17
Q

What is photochemical smog and how is it formed?

A
• Mixture of primary + secondary pollutants which clouds up troposphere
• Formed when sunlight acts on primary pollutants to produce secondary ones
18
Q

What issues arise from photochemical smogs?

A
• Haziness + reduced visibility in ground-level air
• Eye + nose irritation
• Breathing difficulties (especially for asthmatics, children + older people)
19
Q

How is ozone formed?

A

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

20
Q

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

A
• 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)
21
Q

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 SOx emission are best prevented.
A

CO + 0.5O2 CO2

C7H16 + 11O2 7CO2 + 8H2O

NO + CO CO2 + 1/2N2

Preemptively removing sulfur impurities from fuels.

22
Q

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

A

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

23
Q

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

A
• Diesel engines have a high concentration of O2
• Reducing agent would be oxidised by O2 before getting a chance to reduce NOx
24
Q

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

A
• Particulate filters (often ceramic)
• Filters regenerated by increasing temperature, which increases fuel consumption
25
Q

Diesel NOx 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.
A
• Lowers temperature + therefore quantity of NOx formed
• 4NO + 4NH3 + O2 → 4N2 + 6H2O
26
Q

What are biofuels?

A

Ones derived from renewable plant and animal materials.

27
Q

What are the advantages of hydrogen as a fuel?

A
• Renewable
• Combustion [directly] produces only water
• Can be sent down pipelines
• Can be used to generate electricity in engines/cells
28
Q

What are the disadvantages of hydrogen as a fuel?

A
• Poor energy density
• Compression to store at higher density makes it volatile
• High heat of combustion causes higher NOx emissions
• Produced from electrolysis of water; electricity may not be clean
• Derived from natural gas; defeats the object
29
Q

List the advantages of biofuels over fossil fuels.

A
• 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
30
Q

List the disadvantages of biofuels compared to fossil fuels.

A
• 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 NOx 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
31
Q
A

A

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

H2 contains no carbon or sulfur

32
Q

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

A

Diesel has a higher boiling point / is less volatile.