DP Chemistry SL R1.3 Energy from Fuels

DP Chemistry SL R1.3 Energy from Fuels - Full notes

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DP Chemistry SL | R1.3 Energy from Fuels

DP CHEMISTRY · REACTIVITY

R1.3

Energy from Fuels

Full notes for SL · IB-style

Contents

1	Combustion reactions
2	Complete and incomplete combustion
3	Fossil fuels
4	Specific energy and energy density
5	Fuels and climate change
6	Biofuels and renewable alternatives
7	Balancing combustion equations
8	Incomplete combustion
9	Specific energy and energy density
10	Fossil fuels: formation and types
11	Carbon dioxide emissions calculations
12	The greenhouse effect
13	Biofuels
14	Photosynthesis and the carbon cycle
15	Fuel cells
16	Comparing energy sources
17	Efficiency of energy conversion
18	Mixed review: worked problems
19	Additional exam-style practice
20	Catalytic converters
21	Energy and sustainability
22	Common misconceptions and links
23	Exam-style question with markscheme
24	Check your understanding
25	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
write equations for the combustion of hydrocarbons, alcohols and other fuels
distinguish complete and incomplete combustion and their products and hazards
describe the formation and use of fossil fuels: coal, crude oil, natural gas
calculate and compare specific energy and energy density of fuels
explain the role of CO2 and methane in the greenhouse effect
evaluate biofuels and renewable energy sources

This booklet covers the SL content of this subtopic. Additional Higher Level material is not included.

Guiding questions

What are the challenges of using chemical energy to meet society's needs? How do different fuels compare?

How this topic is examined

R1.3 is examined through combustion equations, specific energy calculations and evaluation of fuels, often linked to climate change data.

Balance combustion equations carefully and state the products of incomplete combustion.

Command term	What the examiner wants
State / Identify	a brief answer, no explanation
Outline / Describe	a brief or detailed account, without reasons
Explain	a detailed account with causes, reasons or mechanisms
Calculate / Determine	a numerical answer with working, correct significant figures and units
Deduce / Predict	reach a conclusion from given information
Draw / Sketch	a clear labelled diagram, structure or graph
Evaluate / Discuss	weigh strengths and limitations; reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the chemistry shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 Combustion reactions

Fuels react with oxygen, releasing energy.

Think about it

A gas cooker flame is blue, but a candle flame is yellow and leaves soot on a cold spoon. What is different about these two combustion reactions?

A fuel is a substance that releases useful energy when it reacts, usually by combustion (burning in oxygen). Combustion reactions are exothermic (R1.1) and are redox reactions in which the fuel is oxidized.

Complete combustion of a hydrocarbon or an alcohol in excess oxygen produces carbon dioxide and water: C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l). C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l).

To balance: balance C first (CO2), then H (H2O), then O (O2); fractions may be used, then multiplied through.

WORKED EXAMPLE 1 Balancing a combustion equation

Write a balanced equation for the complete combustion of octane, C8H18.

Solution

C8H18 + O2 → 8CO2 + 9H2OM1

O atoms needed: 16 + 9 = 25, so 12½ O2M1

2C8H18 + 25O2 → 16CO2 + 18H2OA1

2 Complete and incomplete combustion

Limited oxygen produces carbon monoxide and soot.

Incomplete combustion occurs when the oxygen supply is limited. Products include carbon monoxide (CO), carbon (soot) and water. For example: CH4 + 1½O2 → CO + 2H2O; CH4 + O2 → C + 2H2O.

Incomplete combustion releases less energy per mole of fuel. Carbon monoxide is toxic: it binds to haemoglobin more strongly than oxygen, reducing the blood's ability to carry oxygen. It is colourless and odourless, so faulty gas appliances are dangerous. Soot (particulates) causes respiratory problems and blackens buildings. A yellow flame indicates incomplete combustion; a blue flame indicates complete combustion.

Fuels with a higher proportion of carbon (such as coal and long-chain hydrocarbons) are more likely to undergo incomplete combustion.

	Complete combustion	Incomplete combustion
Oxygen	excess	limited
Products	CO2 and H2O	CO, C (soot), H2O
Energy released	maximum	less
Flame	blue	yellow, sooty
Hazards	CO2 (greenhouse gas)	toxic CO, particulates

Quick check

1. Why is carbon monoxide toxic?

2. Write an equation for incomplete combustion of ethane forming CO.

Answers: 1. binds to haemoglobin, reducing oxygen transport 2. C2H6 + 2½O2 → 2CO + 3H2O

3 Fossil fuels

Ancient stored solar energy.

Fossil fuels (coal, crude oil and natural gas) formed over millions of years from the remains of plants and marine organisms, buried and subjected to heat and pressure in the absence of oxygen. They store energy originally captured by photosynthesis. They are non-renewable: they are being used much faster than they form.

Coal is mainly carbon; crude oil is a mixture of hydrocarbons separated by fractional distillation into fractions such as petrol, kerosene and diesel; natural gas is mainly methane.

Advantages: high energy density, readily available, easily transported (oil and gas), established infrastructure. Disadvantages: finite; combustion releases CO2 (climate change); coal and oil release sulfur dioxide (acid rain) and particulates; oil spills; mining damages landscapes.

Fuel	Main component	CO2 per kJ	Other pollutants
Coal	carbon	highest	SO2, particulates
Oil (petrol)	C5–C10 hydrocarbons	medium	NOx, CO
Natural gas	methane	lowest	few

4 Specific energy and energy density

Energy per kilogram and per litre.

Fuels are compared using two measures. Specific energy is the energy released per unit mass of fuel (kJ kg−1 or MJ kg−1). Energy density is the energy released per unit volume (kJ dm−3 or MJ dm−3). Both are calculated from the enthalpy of combustion.

Specific energy = |ΔHc| ÷ molar mass (converting units); energy density = specific energy × density. Hydrogen has a very high specific energy but a very low energy density (it is a gas), which makes storage difficult; petrol has high values of both, which is why it is convenient for vehicles.

WORKED EXAMPLE 2 Specific energy

ΔHc of methane is −890 kJ mol−1. Calculate its specific energy in MJ kg−1.

Solution

1 kg = 1000 g; n = 1000 ÷ 16.05 = 62.3 molM1

Energy = 62.3 × 890 = 5.55 × 104 kJ = 55.5 MJ kg−1A1

Figure 1. Specific energies of common fuels.

Quick check

1. Define specific energy.

2. Why is hydrogen hard to use in cars despite its high specific energy?

Answers: 1. energy released per unit mass 2. low energy density; storage under high pressure

5 Fuels and climate change

Combustion releases greenhouse gases.

Burning fossil fuels releases carbon dioxide, a greenhouse gas. Greenhouse gases absorb long-wave infrared radiation emitted by the Earth's surface, because the radiation causes their bonds to vibrate (bending and asymmetric stretching change the dipole moment). They then re-emit radiation in all directions, warming the atmosphere. Water vapour, methane and nitrous oxide are also greenhouse gases.

Atmospheric CO2 has risen from about 280 ppm before 1800 to over 420 ppm today, and global average temperature has risen by about 1.2 °C. Consequences include rising sea levels, more extreme weather, and ocean acidification (CO2 dissolving to form carbonic acid).

Methane is a more powerful greenhouse gas per molecule but is present in much lower concentrations; sources include natural gas leaks, livestock, rice paddies and landfill.

Quick check

1. Why do CO2 molecules absorb infrared?

2. Name two greenhouse gases other than CO2.

Answers: 1. IR causes bond vibrations that change the dipole moment 2. methane, water vapour, nitrous oxide

6 Biofuels and renewable alternatives

Fuels made from recent biomass.

Biofuels are produced from recently living biological material (biomass). Bioethanol is made by fermentation of sugars from crops such as sugar cane or maize: C6H12O6(aq) → 2C2H5OH(aq) + 2CO2(g), catalysed by yeast enzymes in anaerobic conditions. Biodiesel is made by transesterification of vegetable oils. Biogas (mainly methane) is produced by anaerobic digestion of waste.

Biofuels are described as carbon neutral in principle, because the CO2 released on burning was recently absorbed by photosynthesis. In practice they are not fully carbon neutral: energy is used in farming, fertilizer production, harvesting, processing and transport. Growing fuel crops uses land that could produce food and may involve deforestation.

Other alternatives to fossil fuels include solar, wind, hydroelectric, geothermal and nuclear power, which do not produce CO2 during operation.

	Advantages	Disadvantages
Fossil fuels	high energy density; reliable	non-renewable; CO2; pollution
Biofuels	renewable; closer to carbon neutral	land use; food competition; lower energy density
Hydrogen	high specific energy; water only product	storage; mostly made from fossil fuels

Quick check

1. Write the equation for fermentation of glucose.

2. Why are biofuels not completely carbon neutral?

Answers: 1. C6H12O6 → 2C2H5OH + 2CO2 2. energy for farming, processing and transport comes from fossil fuels

7 Balancing combustion equations

Carbon, then hydrogen, then oxygen.

For complete combustion of CxHyOz: balance carbon (CO2), then hydrogen (H2O), then oxygen last. Fractions such as 7/2 O2 are allowed, or multiply through by 2.

WORKED EXAMPLE 3 Hydrocarbon (basic)

Balance the complete combustion of butane.

Solution

C4H10 + 13/2O2 → 4CO2 + 5H2OA1

or 2C4H10 + 13O2 → 8CO2 + 10H2OA1

WORKED EXAMPLE 4 Alcohol (standard)

Balance the complete combustion of propan-1-ol.

Solution

C3H7OH + 9/2O2 → 3CO2 + 4H2OA2

Quick check

1. Balance the combustion of pentane.

2. Balance the combustion of methanol.

Answers: 1. C5H12 + 8O2 → 5CO2 + 6H2O 2. 2CH3OH + 3O2 → 2CO2 + 4H2O

8 Incomplete combustion

Carbon monoxide and soot.

With limited oxygen, hydrocarbons burn incompletely to form carbon monoxide (CO) and/or carbon (soot), as well as water. Less energy is released. CO is toxic because it binds strongly to haemoglobin; soot causes respiratory problems and global dimming.

WORKED EXAMPLE 5 Equations (standard)

Write equations for the incomplete combustion of methane forming (a) CO and (b) C.

Solution

(a) 2CH4 + 3O2 → 2CO + 4H2OA1

(b) CH4 + O2 → C + 2H2OA1

In the real world

Carbon monoxide detectors are recommended in homes with gas boilers, because faulty appliances with poor ventilation can produce CO, which is colourless and odourless.

Quick check

1. Why does incomplete combustion release less energy?

2. Explain why CO is toxic.

Answers: 1. fewer bonds to oxygen formed (C=O in CO2 not completely formed) 2. binds to haemoglobin, preventing oxygen transport

9 Specific energy and energy density

Per gram and per volume.

Specific energy = energy released per unit mass (kJ g−1); energy density = energy per unit volume (kJ cm−3). Hydrogen has a very high specific energy but a low energy density as a gas; petrol has a high energy density, which is convenient for vehicles.

Fuel	ΔHc / kJ mol−1	Specific energy / kJ g−1
hydrogen	−286	142
methane	−891	55.5
octane	−5470	47.9
ethanol	−1367	29.7
coal (carbon)	−394	32.8

WORKED EXAMPLE 6 Specific energy (standard)

Calculate the specific energy of octane (M = 114.26).

Solution

5470/114.26M1

= 47.9 kJ g−1A1

Quick check

1. Calculate the energy released by burning 1.00 kg of methane.

2. Which fuel releases most energy per gram?

Answers: 1. 55 500 kJ (55.5 MJ) 2. hydrogen

10 Fossil fuels: formation and types

Coal, oil and natural gas.

Fossil fuels formed over millions of years from the remains of plants and marine organisms subjected to heat and pressure. Coal is mainly carbon; crude oil is a mixture of hydrocarbons separated by fractional distillation; natural gas is mainly methane. They are non-renewable.

WORKED EXAMPLE 7 Comparing fuels (standard)

Compare coal and natural gas in terms of CO2 produced per kJ of energy.

Solution

Coal (C): 1 mol CO2 per 394 kJ = 2.54 mmol per kJM1

Methane: 1 mol CO2 per 891 kJ = 1.12 mmol per kJ: coal emits moreA1

Quick check

1. Why are fossil fuels called non-renewable?

Answers: 1. they form over millions of years, much more slowly than we use them

11 Carbon dioxide emissions calculations

Mass of CO2 per mass of fuel.

From the balanced equation, find the moles of CO2 per mole of fuel, then convert to masses. For example, octane: C8H18 → 8CO2, so 114 g of octane produces 352 g of CO2.

WORKED EXAMPLE 8 Petrol car (exam level)

A car burns 5.0 kg of octane. Calculate the mass of CO2 produced.

Solution

n(octane) = 5000/114.26 = 43.8 molA1

n(CO2) = 350 molA1

m = 350 × 44.01 = 15.4 kgA1

Quick check

1. Find the mass of CO2 from burning 1.00 kg of methane.

Answers: 1. 2.74 kg

12 The greenhouse effect

Infrared absorption.

The Earth's surface absorbs visible and UV radiation from the Sun and re-emits infrared. Greenhouse gases (CO2, CH4, H2O, N2O) absorb infrared because their bonds vibrate in ways that change the dipole moment, and re-emit it in all directions, warming the lower atmosphere.

WORKED EXAMPLE 9 Mechanism (exam level)

Explain why CO2 is a greenhouse gas but N2 is not.

Solution

CO2 bonds are polar; asymmetric stretching and bending change the dipole momentR1

N2 is non-polar; its vibration does not change the dipole, so it does not absorb IRR1

In the real world

Atmospheric CO2 has risen from about 280 ppm before the industrial revolution to over 420 ppm today.

Quick check

1. Name two greenhouse gases other than CO2.

2. Why is methane a more potent greenhouse gas per molecule than CO2?

Answers: 1. CH4, N2O (or H2O) 2. absorbs IR more strongly and at wavelengths CO2 does not

13 Biofuels

Renewable carbon.

Biofuels such as bioethanol (from fermentation of sugars) and biodiesel (from vegetable oils) are made from recently living biomass. They are considered approximately carbon-neutral because the CO2 released on burning was recently absorbed by photosynthesis, although farming, processing and transport release additional CO2.

WORKED EXAMPLE 10 Evaluating biofuels (exam level)

Give one advantage and two disadvantages of bioethanol.

Solution

Advantage: renewable; lower net CO2R1

Disadvantages: uses land needed for food; lower specific energy than petrolR1 R1

Quick check

1. Write the equation for fermentation of glucose.

2. Why is biodiesel not completely carbon-neutral?

Answers: 1. C6H12O6 → 2C2H5OH + 2CO2 2. fossil fuels used in farming, processing and transport

14 Photosynthesis and the carbon cycle

Natural energy capture.

Photosynthesis stores solar energy in glucose: 6CO2 + 6H2O → C6H12O6 + 6O2 (ΔH = +2803 kJ mol−1). Respiration and combustion release it. The carbon cycle balances these processes, but burning fossil fuels adds carbon stored for millions of years.

WORKED EXAMPLE 11 Energy stored (standard)

How much energy is stored when 1.00 kg of glucose is formed?

Solution

n = 1000/180.18 = 5.55 molA1

× 2803 = 15 600 kJA1

15 Fuel cells

Chemical energy directly to electricity.

A hydrogen fuel cell combines hydrogen and oxygen to produce electricity, with water as the only product: anode H2 → 2H+ + 2e−; cathode O2 + 4H+ + 4e− → 2H2O. Fuel cells are more efficient than combustion engines, but hydrogen production and storage are challenges.

WORKED EXAMPLE 12 Evaluating (exam level)

Discuss the use of hydrogen fuel cells in cars.

Solution

Only water emitted; high efficiencyR1

Hydrogen is often made from fossil fuels; storage needs high pressure; few refuelling stationsR1 R1

Quick check

1. Write the overall equation for a hydrogen fuel cell.

Answers: 1. 2H2 + O2 → 2H2O

16 Comparing energy sources

Beyond combustion.

Source	Renewable?	CO2 in use?	Issue
coal	no	high	air pollution
natural gas	no	medium	methane leaks
biofuels	yes	low net	land use
solar	yes	none	intermittent
wind	yes	none	intermittent
nuclear	no (but long-lasting)	none	radioactive waste

WORKED EXAMPLE 13 Choosing (standard)

Suggest why natural gas is described as a 'transition fuel'.

Solution

Lower CO2 per kJ than coalR1

Can be used while renewable capacity expandsR1

17 Efficiency of energy conversion

Useful energy out.

Efficiency = (useful energy output/total energy input) × 100. Car engines convert only about 25–30% of the fuel's chemical energy into motion; the rest is lost as heat. Fuel cells can reach 50–60%.

WORKED EXAMPLE 14 Efficiency (standard)

A car engine produces 12 MJ of useful work from fuel releasing 40 MJ. Find its efficiency.

Solution

12/40 × 100M1

= 30%A1

Quick check

1. A power station is 38% efficient. How much fuel energy is needed to produce 1.0 MJ of electricity?

Answers: 1. 2.6 MJ

18 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 15 Mixed problem (exam level)

Compare ethanol and octane as fuels: (a) specific energy; (b) mass of CO2 per kJ released.

Solution

(a) ethanol 29.7, octane 47.9 kJ g−1A1

(b) ethanol: 2 × 44.01/1367 = 0.0644 g kJ−1; octane: 8 × 44.01/5470 = 0.0644 g kJ−1M1 A1

Similar CO2 per kJ, but bioethanol's carbon was recently absorbedR1

Quick check

1. Find the volume of CO2 at STP from burning 1.00 mol of propane.

Answers: 1. 3 mol: 68.1 dm3

19 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 16 Paper 1 question (standard)

Which gas is not a greenhouse gas? A. CH4 B. N2O C. O2 D. H2O

Solution

CA1

WORKED EXAMPLE 17 Paper 2 question (exam level)

(a) Write an equation for the incomplete combustion of octane forming CO. (b) Explain one health effect of CO. (c) Suggest one way to reduce CO emissions from cars.

Solution

(a) 2C8H18 + 17O2 → 16CO + 18H2OA1

(b) binds haemoglobin, reducing oxygen transportR1

20 Catalytic converters

Cleaning exhaust gases.

Catalytic converters use platinum, palladium and rhodium on a ceramic honeycomb to convert CO, NO and unburned hydrocarbons into CO2, N2 and H2O: 2CO + 2NO → 2CO2 + N2.

WORKED EXAMPLE 18 Equation (standard)

Write the equation for octane being oxidized in a catalytic converter.

Solution

2C8H18 + 25O2 → 16CO2 + 18H2OA1

Quick check

1. Why is a honeycomb structure used?

Answers: 1. large surface area for the catalyst

21 Energy and sustainability

Global perspective.

Meeting global energy demand while reducing emissions requires improved efficiency, more renewable sources, carbon capture and storage, and changes in consumption. Chemistry contributes through better batteries, catalysts, solar cells and fuels.

Quick check

1. Describe carbon capture and storage in one sentence.

Answers: 1. CO2 is captured from power stations and stored underground in rock formations

22 Common misconceptions and links

Students often think…	Actually…
incomplete combustion releases more energy	it releases less
biofuels are fully carbon neutral	farming and processing add CO2
specific energy and energy density are the same	per mass vs per volume

How R1.3 connects to the rest of the course

Topic	Connection
R1.1 Enthalpy	enthalpy of combustion
R3.2 Electron transfer	redox and cells
S3.2 Functional groups	alkanes and alcohols
S3.1 Periodic table	acidic oxides and acid rain

23 Exam-style question with markscheme

Try this under exam conditions (about 15 minutes) before reading the markscheme.

Figure 2. Specific energies of fuels.

(a) Write a balanced equation for the complete combustion of butane. [2]

(b) Calculate the specific energy of butane (ΔHc = −2878 kJ mol−1) in MJ kg−1. [2]

(d) Evaluate bioethanol as an alternative to petrol. [4]

Markscheme

Part	Answer	Marks
(a)	2C4H10 + 13O2 → 8CO2 + 10H2O	A2
(b)	1000 ÷ 58.14 = 17.2 mol	M1
	17.2 × 2878 = 49.5 MJ kg−1	A1
(c)	carbon monoxide formed	R1
	binds haemoglobin; colourless, odourless	R1
(d)	renewable; closer to carbon neutral	R1 R1
	lower specific energy; land use / food competition	R1 R1

Examiner's comments. In (d), the strongest answers balanced environmental and practical considerations.

24 Check your understanding

Section A: multiple choice

1. Complete combustion of a hydrocarbon produces

A. CO2 and H2O B. CO and H2O C. C and H2 D. CO2 and H2

2. A yellow sooty flame indicates

A. complete combustion B. no combustion C. incomplete combustion D. hydrogen burning

3. Specific energy is energy per unit

A. volume B. mass C. mole D. time

4. Bioethanol is made by

A. cracking B. distillation of crude oil C. electrolysis D. fermentation

5. Greenhouse gases absorb

A. ultraviolet only B. infrared radiation C. visible light D. radio waves

6. Natural gas is mainly

A. methane B. ethane C. propane D. hydrogen

Section B: structured

7. Compare complete and incomplete combustion. [3]

8. Explain how burning fossil fuels contributes to climate change. [3]

9. Explain why biofuels are said to be carbon neutral and why this is not fully true. [3]

Answers

1. A 2. C 3. B 4. D 5. B 6. A

7. Complete: excess O2, CO2 + H2O, more energy; incomplete: limited O2, CO and C, less energy, toxic.

8. CO2 released; absorbs and re-emits IR; enhanced greenhouse effect; warming.

9. CO2 released was absorbed in photosynthesis; but farming, processing, transport emissions.

25 Summary and topic map

Idea	Remember
Combustion	exothermic oxidation
Complete / incomplete	CO2 + H2O / CO, C
Fossil fuels	coal, oil, gas; non-renewable
Specific energy	per mass; energy density per volume
Climate	CO2 and CH4 absorb IR
Biofuels	fermentation; not fully carbon neutral

Before the exam, make sure you…

•balance combustion equations in order C, H, O
•name products of incomplete combustion
•calculate specific energy with unit conversions
•evaluate fuels both ways

Energy from Fuels on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 3. Topic map for Energy from Fuels.

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