IGCSE Chemistry Paper 4 (Theory / Extended)

Topic 5: Chemical Energetics -- Mock Exam 1

Duration1 hour 15 minutes

Total Marks80

Questions7

Time Remaining75:00

Syllabus0620

Instructions

Answer all questions in the spaces provided.
Show all working for calculations.
Use appropriate scientific terminology.
Your answers will be automatically graded when you submit.

Question Navigation

Question 1 -- Exothermic and Endothermic Reactions

Total: 12 marks

A student in Mumbai carries out experiments to investigate energy changes in chemical reactions. She measures temperature changes during different reactions.

(a) [4]

(i) State what is meant by an exothermic reaction. [2]

(ii) State what is meant by an endothermic reaction. [2]

Model Answer -- 1(a)

An exothermic reaction is one that releases / gives out heat energy to the surroundings [1]

The temperature of the surroundings increases [1]

An endothermic reaction is one that absorbs / takes in heat energy from the surroundings [1]

The temperature of the surroundings decreases [1]

⚠ If you missed marks here: 'An endothermic reaction needs heating' is a common confusion – dissolving ammonium nitrate needs no heating at all; the definition is that thermal energy is TAKEN IN from the surroundings, so the temperature of the surroundings goes DOWN. Each definition needs both halves for both marks: which way the thermal energy moves AND what happens to the temperature of the surroundings.

1 mark -- exothermic releases gives out heat energy surroundings 1 mark -- temperature surroundings increases 1 mark -- endothermic absorbs takes heat energy surroundings 1 mark -- temperature surroundings decreases

(b) [4]

The student records the following temperature changes. Classify each reaction as exothermic or endothermic.

Reaction	Start temp / °C	End temp / °C
Hydrochloric acid + sodium hydroxide	22	35
Citric acid + sodium hydrogencarbonate	21	14
Magnesium + hydrochloric acid	20	42
Ammonium nitrate dissolved in water	23	16

Model Answer -- 1(b)

Hydrochloric acid + sodium hydroxide: exothermic (temperature increased from 22 to 35 °C) [1]

Citric acid + sodium hydrogencarbonate: endothermic (temperature decreased from 21 to 14 °C) [1]

Magnesium + hydrochloric acid: exothermic (temperature increased from 20 to 42 °C) [1]

Ammonium nitrate dissolved in water: endothermic (temperature decreased from 23 to 16 °C) [1]

⚠ If you missed marks here: Students often reverse the logic for the two that cool: a falling thermometer does NOT mean heat was given out – the reaction is taking thermal energy from the solution, so citric acid + sodium hydrogencarbonate (21 → 14 °C) and dissolving ammonium nitrate (23 → 16 °C) are endothermic. The two that warm up (22 → 35 °C and 20 → 42 °C) are exothermic.

1 mark -- hydrochloric acid sodium hydroxide exothermic 1 mark -- citric acid sodium hydrogencarbonate endothermic 1 mark -- magnesium hydrochloric acid exothermic 1 mark -- ammonium nitrate endothermic

(i) Give two examples of exothermic processes that occur in everyday life. [2]

(ii) Give two examples of endothermic processes that occur in everyday life. [2]

Model Answer -- 1(c)

Exothermic example 1: combustion of fuels / burning gas for cooking [1]

Exothermic example 2: respiration / neutralisation / hand warmers [1]

Endothermic example 1: photosynthesis [1]

Endothermic example 2: thermal decomposition / instant cold packs / dissolving ammonium nitrate [1]

⚠ If you missed marks here: Watch the pair students swap: respiration RELEASES energy (exothermic) while photosynthesis TAKES IN light energy (endothermic). Name the reaction, not the activity – 'burning gas for cooking' (combustion) scores but plain 'cooking' does not, because the food itself takes in heat; and 'burning wood' plus 'burning gas' are both combustion, so they count only once.

1 mark -- exothermic example combustion burning respiration neutralisation hand warmers 1 mark -- second exothermic example different valid 1 mark -- endothermic example photosynthesis thermal decomposition cold packs 1 mark -- second endothermic example different valid

Question 2 -- Energy Level Diagrams

Total: 12 marks

A chemistry teacher in London explains energy level diagrams to her class. She uses the diagrams to illustrate exothermic and endothermic reactions.

(a) [4]

The energy level diagram below represents a reaction.

(i) State whether the reaction shown is exothermic or endothermic. Explain your answer. [2]

(ii) On the diagram, what does the symbol ΔH represent? [1]

(iii) State the sign of ΔH for this reaction. [1]

Model Answer -- 2(a)

The reaction is exothermic [1]

Because the products are at a lower energy level than the reactants / energy is released to the surroundings [1]

ΔH represents the overall energy change / enthalpy change of the reaction [1]

ΔH is negative for this reaction [1]

⚠ If you missed marks here: The hump does not decide it – what matters is that the PRODUCTS line is below the REACTANTS line, so energy has been released to the surroundings and the reaction is exothermic. ΔH is the gap between those two lines (the overall enthalpy change), not the height of the hump (that is the activation energy), and for an exothermic reaction its sign is negative.

1 mark -- reaction exothermic 1 mark -- products lower energy level reactants energy released 1 mark -- delta enthalpy change overall energy change reaction 1 mark -- delta negative

(b) [4]

Draw a labelled energy level diagram for the thermal decomposition of calcium carbonate. This is an endothermic reaction.

Your diagram should show:
-- the energy levels of reactants and products
-- the activation energy (E_a)
-- the enthalpy change (ΔH)
-- labels on both axes

Model Answer -- 2(b)

Reactants (CaCO₃) drawn at a lower energy level than products (CaO + CO₂) [1]

Products drawn at a higher energy level [1]

Activation energy (Eₐ) labelled -- from reactant level to top of the curve [1]

ΔH labelled as positive -- from reactant level to product level, with axes labelled [1]

⚠ If you missed marks here: A common slip is drawing the products BELOW the reactants – for this endothermic reaction CaO + CO₂ must sit ABOVE CaCO₃, with the ΔH arrow pointing UP and labelled positive. Measure E_a from the REACTANT line to the top of the hump (not from the products or the axis), and label the axes 'Energy' and 'Progress of reaction'.

1 mark -- reactants drawn lower energy level 1 mark -- products drawn higher energy level 1 mark -- activation energy labelled reactant level top curve 1 mark -- delta labelled positive axes labelled

(i) Define the term activation energy. [2]

(ii) Explain why a match must be used to light a Bunsen burner, even though the combustion of methane is exothermic. [2]

Model Answer -- 2(c)

Activation energy is the minimum energy that colliding particles must have in order to react [1]

It is the energy needed to start / initiate the reaction [1]

The match provides the activation energy needed to start the combustion reaction [1]

Once started, the exothermic reaction releases enough energy to keep the reaction going / sustain itself [1]

⚠ If you missed marks here: 'The energy needed to start a reaction' is only half of (i) – the full definition is the MINIMUM energy that COLLIDING particles must have in order to react. In (ii), the match does not make the reaction exothermic: it supplies the activation energy to start it, and after that the heat released by the burning methane supplies the activation energy for the next molecules, so the flame keeps going.

1 mark -- activation energy minimum energy colliding particles react 1 mark -- energy needed start initiate reaction 1 mark -- match provides activation energy start combustion 1 mark -- exothermic releases enough energy keep reaction going sustain

Question 3 -- Catalysts and Energy Diagrams

Total: 12 marks

In a laboratory in Birmingham, students investigate the effect of manganese dioxide (MnO₂) on the decomposition of hydrogen peroxide.

(a) [3]

(i) Define the term catalyst. [2]

(ii) State the role of MnO₂ in the decomposition of hydrogen peroxide. [1]

Model Answer -- 3(a)

A catalyst is a substance that increases the rate of a chemical reaction [1]

without being used up / chemically unchanged at the end of the reaction [1]

MnO₂ acts as a catalyst -- it speeds up the decomposition of hydrogen peroxide into water and oxygen [1]

⚠ If you missed marks here: 'A catalyst makes a reaction happen' or 'starts the reaction' loses (i) – hydrogen peroxide decomposes slowly anyway; a catalyst only INCREASES THE RATE and is chemically unchanged at the end. For (ii), MnO₂ is not a reactant and the oxygen does not come from it: it speeds up the breakdown of hydrogen peroxide into water and oxygen.

1 mark -- catalyst substance increases rate chemical reaction 1 mark -- without used up chemically unchanged end reaction 1 mark -- manganese dioxide catalyst speeds decomposition hydrogen peroxide

(b) [4]

The energy level diagram below shows an exothermic reaction without a catalyst.

(i) Describe how a catalyst affects the activation energy of a reaction. [1]

(ii) Explain how a catalyst provides an alternative reaction pathway. [1]

(iii) State whether a catalyst changes the overall energy change (ΔH) of the reaction. Explain your answer. [2]

Model Answer -- 3(b)

A catalyst lowers / reduces the activation energy [1]

The catalyst provides an alternative reaction pathway that has a lower activation energy / requires less energy to start the reaction [1]

A catalyst does not change ΔH [1]

Because the energy levels of the reactants and products remain the same / the overall energy difference is unchanged [1]

⚠ If you missed marks here: 'A catalyst gives the particles more energy' is a classic wrong answer – it adds no energy; it provides a different route with a LOWER activation energy, so more collisions have enough energy to react. ΔH is unchanged because the reactants and products are the same substances at the same energy levels, so 'the catalyst makes the reaction more exothermic' loses both marks in (iii).

1 mark -- catalyst lowers reduces activation energy 1 mark -- alternative reaction pathway lower activation energy 1 mark -- catalyst does not change delta 1 mark -- energy levels reactants products remain same overall energy difference unchanged

(i) Name the catalyst used in the Haber process for making ammonia. [1]

(ii) Name the catalyst used in the Contact process for making sulfuric acid. [1]

(iii) Explain why catalysts are important in industrial processes. Give three reasons. [3]

Model Answer -- 3(c)

Haber process catalyst: iron [1]

Contact process catalyst: vanadium(V) oxide / V₂O₅ [1]

Catalysts allow reactions to occur at lower temperatures, reducing energy / fuel costs [1]

Catalysts increase the rate of production / speed up the reaction [1]

Catalysts are not used up, so they do not need to be replaced frequently / they are cost-effective [1]

⚠ If you missed marks here: Swapping the two catalysts, or writing just 'vanadium', costs marks: iron for the Haber process and vanadium(V) oxide, V₂O₅, for the Contact process. Never give 'catalysts increase the yield' as a reason – a catalyst makes the product FASTER, not MORE; the credited reasons are faster production, lower temperatures (so lower energy costs) and not being used up.

1 mark -- haber process iron catalyst 1 mark -- contact process vanadium oxide catalyst 1 mark -- lower temperatures reducing energy fuel costs 1 mark -- increase rate production speed reaction 1 mark -- not used up do not need replaced cost-effective

Question 4 -- Bond Energy Calculations

Total: 12 marks

A student in Delhi uses bond energy data to calculate the energy change for the combustion of hydrogen.

(a) [4]

(i) State whether energy is required or released when bonds are broken. [1]

(ii) State whether energy is required or released when bonds are formed. [1]

(iii) Explain, in terms of bond breaking and bond making, why a reaction is exothermic. [2]

Model Answer -- 4(a)

Energy is required / absorbed / taken in when bonds are broken (endothermic process) [1]

Energy is released / given out when new bonds are formed (exothermic process) [1]

A reaction is exothermic when the energy released by forming new bonds is greater than the energy required to break the existing bonds [1]

The overall energy change is negative / there is a net release of energy [1]

⚠ If you missed marks here: A very common misconception is 'breaking bonds releases energy' – it is the reverse: breaking bonds always TAKES IN energy and making bonds always GIVES OUT energy. For (iii), 'more bonds are made than broken' is not the reason; compare the ENERGIES: the reaction is exothermic when the energy released making new bonds is greater than the energy needed to break the old ones.

1 mark -- energy required absorbed taken bonds broken endothermic 1 mark -- energy released given out bonds formed exothermic 1 mark -- exothermic energy released forming bonds greater energy required breaking bonds 1 mark -- overall energy change negative net release energy

(b) [5]

The equation for the combustion of hydrogen is:

2H₂(g) + O₂(g) → 2H₂O(g)

Use the bond energy data below to calculate the enthalpy change (ΔH) for this reaction.

Bond	Bond energy / kJ mol⁻¹
H–H	436
O=O	498
O–H	464

Show all your working.

Model Answer -- 4(b)

Bonds broken: 2 × H–H + 1 × O=O = (2 × 436) + (1 × 498) = 872 + 498 = 1370 kJ [1]

Bonds formed: 4 × O–H (two water molecules, each with 2 O–H bonds) = 4 × 464 = 1856 kJ [1]

ΔH = energy of bonds broken − energy of bonds formed [1]

ΔH = 1370 − 1856 = −486 kJ mol⁻¹ [1]

The negative sign indicates the reaction is exothermic [1]

⚠ If you missed marks here: If you got +442 you counted only two O–H bonds – each H₂O has TWO O–H bonds and there are two water molecules, so 4 × 464 = 1856 kJ is released. If you got +486 you did formed − broken; it is always ΔH = bonds broken − bonds formed = 1370 − 1856 = −486 kJ mol⁻¹.

1 mark -- bonds broken correctly calculated 1370 1 mark -- bonds formed correctly calculated 1856 four bonds 1 mark -- correct formula bonds broken minus bonds formed 1 mark -- answer negative 486 1 mark -- negative sign indicates exothermic

(i) Explain, using your answer to part (b), why the combustion of hydrogen is exothermic. [2]

(ii) Suggest why the actual enthalpy change measured experimentally might differ from the calculated value. [1]

Model Answer -- 4(c)

More energy is released when bonds are formed (1856 kJ) than is required to break bonds (1370 kJ) [1]

So there is a net release of energy / surplus energy is released to the surroundings [1]

Bond energies are average values measured across many different compounds, so the actual bond energy in a specific molecule may be slightly different [1]

⚠ If you missed marks here: For (i) the question says 'using your answer to part (b)', so 'because ΔH is negative' is not enough – quote the numbers: the 1856 kJ released making bonds is more than the 1370 kJ taken in breaking them. For (ii) the mark is for bond energies being AVERAGE values taken from many different compounds, so the real bonds in these particular molecules may be slightly stronger or weaker.

1 mark -- more energy released forming bonds than required breaking bonds 1 mark -- net release energy surplus energy released surroundings 1 mark -- bond energies average values different compounds actual may differ

Question 5 -- Comparing Fuels Using Bond Energies

Total: 10 marks

Scientists in Pune compare the energy released by burning methane (CH₄) and ethanol (C₂H₅OH) as fuels.

(a) [5]

The equation for the complete combustion of methane is:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

Use the bond energy data to calculate ΔH for this reaction.

Bond	Bond energy / kJ mol⁻¹
C–H	413
O=O	498
C=O	805
O–H	464

Model Answer -- 5(a)

Bonds broken: 4 × C–H + 2 × O=O = (4 × 413) + (2 × 498) = 1652 + 996 = 2648 kJ [1]

Bonds formed: 2 × C=O + 4 × O–H = (2 × 805) + (4 × 464) = 1610 + 1856 = 3466 kJ [1]

ΔH = bonds broken − bonds formed [1]

ΔH = 2648 − 3466 = −818 kJ mol⁻¹ [1]

The reaction is exothermic (negative ΔH) [1]

⚠ If you missed marks here: Check your bond counts against the equation: −13 means you gave CO₂ only one C=O bond (it has TWO, so 2 × 805), and −1316 means you broke only one O=O when the equation has 2O₂. The correct totals are 2648 kJ broken and 3466 kJ formed, so ΔH = 2648 − 3466 = −818 kJ mol⁻¹.

1 mark -- bonds broken 2648 correctly four C-H two O=O 1 mark -- bonds formed 3466 correctly two C=O four O-H 1 mark -- correct formula used bonds broken minus bonds formed 1 mark -- answer negative 818 1 mark -- reaction exothermic negative

(b) [5]

(i) Using your answer from Question 4(b) (ΔH = −486 kJ mol⁻¹ for hydrogen) and your answer to part (a), state which fuel releases more energy per mole of fuel burned. [1]

(ii) Explain why methane releases more energy per mole than hydrogen, in terms of the number of bonds broken and formed. [2]

(iii) Despite releasing less energy per mole, hydrogen is considered a "clean fuel". Explain why. [2]

Model Answer -- 5(b)

Methane releases more energy per mole (−818 kJ mol⁻¹) compared to hydrogen (−486 kJ mol⁻¹) [1]

Methane has more bonds to break and more bonds are formed in the products [1]

The difference between bonds formed and broken is larger for methane, resulting in a greater net energy release [1]

Hydrogen produces only water when it burns -- no carbon dioxide or pollutants [1]

This means hydrogen combustion does not contribute to global warming / climate change / greenhouse effect [1]

⚠ If you missed marks here: For (ii), 'methane has stronger bonds' is wrong with this data (C–H is 413 kJ, weaker than H–H at 436 kJ) – the reason is that methane breaks and makes MORE bonds, so the surplus is bigger: 3466 − 2648 = 818 kJ against 1856 − 1370 = 486 kJ for hydrogen. For (iii), 'hydrogen is clean' is too vague: say it produces only water, so no carbon dioxide, and so it does not add to global warming or climate change.

1 mark -- methane releases more energy per mole than hydrogen 1 mark -- methane more bonds break formed products 1 mark -- difference bonds formed broken larger methane greater net energy 1 mark -- hydrogen produces only water no carbon dioxide pollutants 1 mark -- does not contribute global warming climate change greenhouse

Question 6 -- Real-World Applications of Energy Changes

Total: 12 marks

A pharmacist in Kolkata explains how energy changes in chemical reactions are used in everyday products such as hand warmers and cold packs.

(a) [4]

Reusable hand warmers contain a supersaturated solution of sodium ethanoate (sodium acetate). When a metal disc inside is clicked, the solution crystallises and releases heat.

(i) State the type of energy change (exothermic or endothermic) that occurs when a hand warmer is activated. [1]

(ii) Explain why the hand warmer feels warm to the touch. [1]

(iii) To reuse the hand warmer, it must be placed in boiling water. Explain why this step is necessary, using the term "endothermic". [2]

Model Answer -- 6(a)

Exothermic [1]

The crystallisation reaction releases heat energy to the surroundings / to your hand, increasing the temperature [1]

Placing in boiling water dissolves the crystals back into solution -- this is an endothermic process that absorbs heat energy [1]

The heat energy from the boiling water provides the energy needed to reverse the crystallisation, restoring the supersaturated solution [1]

⚠ If you missed marks here: In (iii) the key idea is that the reverse of an exothermic change is ENDOTHERMIC: the crystals must take in thermal energy to dissolve back into solution, and the boiling water supplies it. 'It resets the hand warmer' or 'it heats it up again' describes what happens without the energy idea and does not score.

1 mark -- exothermic 1 mark -- crystallisation releases heat energy surroundings hand temperature increases 1 mark -- boiling water dissolves crystals endothermic absorbs heat energy 1 mark -- heat energy provides energy reverse crystallisation restoring supersaturated solution

(b) [4]

Instant cold packs used in sports injuries contain ammonium nitrate and water in separate compartments. When the pack is squeezed, the compartments break and the ammonium nitrate dissolves in the water.

(i) State the type of energy change that occurs when ammonium nitrate dissolves in water. [1]

(ii) Explain why the cold pack feels cold when applied to an injury. [1]

(iii) Draw a simple energy level diagram for the dissolving process. Label the reactants, products, and ΔH. [2]

Model Answer -- 6(b)

Endothermic [1]

The dissolving process absorbs heat energy from the surroundings / from the skin, causing the temperature to decrease / making it feel cold [1]

Reactants drawn at a lower energy level, products at a higher energy level [1]

ΔH labelled as positive (upward arrow) [1]

⚠ If you missed marks here: 'The pack gives out cold' loses (ii) – cold is not something that can be released; the dissolving TAKES IN thermal energy from your skin, so the skin's temperature falls. On the diagram the dissolved ions (products) must sit ABOVE NH₄NO₃ + H₂O (reactants), with an upward ΔH arrow labelled positive.

1 mark -- endothermic 1 mark -- absorbs heat energy surroundings skin temperature decrease cold 1 mark -- reactants lower energy products higher energy 1 mark -- delta labelled positive upward

Self-heating food cans, popular with hikers in the Scottish Highlands, use the reaction between calcium oxide and water to heat the food.

CaO(s) + H₂O(l) → Ca(OH)₂(s)

(i) State whether this reaction is exothermic or endothermic. [1]

(ii) Explain how this reaction heats the food in the can. [1]

(iii) State the sign of ΔH for this reaction. [1]

(iv) Suggest one advantage of using a self-heating can compared to cooking on a camping stove. [1]

Model Answer -- 6(c)

Exothermic [1]

The reaction releases heat energy to the surroundings, which heats up the food in the can [1]

ΔH is negative [1]

No fuel or stove needed / more portable / no flame so safer in windy conditions / can be used anywhere [1]

⚠ If you missed marks here: A common slip is giving ΔH as positive 'because heat is produced' – the sign describes the reacting chemicals: energy LEAVES them and heats the food, so ΔH is NEGATIVE. In (iv), 'it is cheaper' is not a credited advantage; the mark is for needing no fuel, stove or flame, so the can is portable and safe to use in wind.

1 mark -- exothermic 1 mark -- releases heat energy surroundings heats food can 1 mark -- delta negative 1 mark -- no fuel stove needed portable safer no flame anywhere

Question 7 -- Bond Energy Calculation: Combustion of Ethanol

Total: 10 marks

Ethanol is used as a biofuel in countries such as Brazil and India. A student in Jaipur investigates the energy change when ethanol burns.

(a) [3]

The structural formula of ethanol is CH₃CH₂OH.

(i) List all the types of bonds present in one molecule of ethanol. [1]

(ii) State how many of each type of bond is in one molecule of ethanol. [2]

Model Answer -- 7(a)

Bond types present: C–H, C–C, C–O, O–H [1]

5 × C–H bonds, 1 × C–C bond [1]

1 × C–O bond, 1 × O–H bond [1]

⚠ If you missed marks here: Ethanol has six hydrogens but only FIVE are on carbon (3 in CH₃, 2 in CH₂) – the sixth is on oxygen, so the counts are 5 C–H and 1 O–H, not 6 C–H. Also check you wrote a SINGLE C–O bond: ethanol has no C=O.

1 mark -- bond types C-H C-C C-O O-H identified 1 mark -- five C-H bonds one C-C bond 1 mark -- one C-O bond one O-H bond

(b) [5]

The equation for the complete combustion of ethanol is:

C₂H₅OH(g) + 3O₂(g) → 2CO₂(g) + 3H₂O(g)

Use the bond energy data to calculate ΔH for this reaction.

Bond	Bond energy / kJ mol⁻¹
C–H	413
C–C	347
C–O	358
O–H	464
O=O	498
C=O	805

Show all your working.

Model Answer -- 7(b)

Bonds broken in reactants: 5(C–H) + 1(C–C) + 1(C–O) + 1(O–H) + 3(O=O) [1]

= (5×413) + (1×347) + (1×358) + (1×464) + (3×498) = 2065 + 347 + 358 + 464 + 1494 = 4728 kJ [1]

Bonds formed in products: 4(C=O) + 6(O–H) [1]

= (4×805) + (6×464) = 3220 + 2784 = 6004 kJ [1]

ΔH = 4728 − 6004 = −1276 kJ mol⁻¹ [1]

⚠ If you missed marks here: If you got −1740 you forgot that ethanol itself contains an O–H bond that must be broken (464 kJ), and if you got +334 you gave the 2CO₂ only two C=O bonds instead of four. With 4728 kJ broken and 6004 kJ formed, ΔH = 4728 − 6004 = −1276 kJ mol⁻¹ – show both totals, because each one carries a mark.

1 mark -- correct identification bonds broken reactants 1 mark -- bonds broken total 4728 1 mark -- correct identification bonds formed products four C=O six O-H 1 mark -- bonds formed total 6004 1 mark -- answer negative 1276

Using the data from this exam, compare the energy released per mole by methane (−818 kJ mol⁻¹) and ethanol (−1276 kJ mol⁻¹).

Suggest why methane is still the preferred fuel for cooking in most homes, despite ethanol releasing more energy per mole.

Model Answer -- 7(c)

Methane is a gas and is easily piped to homes / readily available from natural gas supplies / burns cleanly [1]

Although ethanol releases more energy per mole, methane may release more energy per gram / per unit mass, and it is cheaper and more convenient for domestic use [1]

⚠ If you missed marks here: Do not argue that methane releases more energy per MOLE – the data say the opposite. The point is energy per GRAM: 818 ÷ 16 = 51 kJ per gram for methane against 1276 ÷ 46 = 28 kJ per gram for ethanol, and methane is a gas that can be piped straight into homes.

1 mark -- methane gas easily piped homes available natural gas burns cleanly 1 mark -- methane more energy per gram cheaper convenient domestic

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