CGP EXAM QUESTIONS: DF

Question 1

10g of calcium carbonate react with excess dilute hydrochloric acid at RTP to produce calcium chloride, water and carbon dioxide: CaCO_3(s) + 2HCl_(aq) –> CaCl_2(s) + H₂O_(l) + CO_2(g)

What volume of carbon dioxide gas is given off?

Answer 1

M_r of CaCO₃ = 40.1 + 12.0 + (3 x 16.0) = 100.1
So 10g of calcium carbonate is (100.1/10) = 0.10 mol
From the equation, 1 mol of CaCO₃, reacting with excess HCl, will produce 1 mol of CO₂, so 0.10 mol of CaCO₃ will produce 0.10 mol of CO₂

At RTP, 0.10 mol of gas has a volume of 0.10 x 24.0 = 2.4 dm³

Question 2

4 dm³ of ethane (C₂H₆) are burned in 20 dm³ of oxygen to produce carbon dioxide and water vapour.

Write a balanced equation for the reaction.

Answer 2

2C₂H₆ + 7O₂ –> 4CO₂ + 6H₂O

or C₂H₆ + 3½O₂ –> 2CO₂ + 3H₂O

Question 3

4 dm₃ of ethane (C₂H₆) are burned in 20 dm³ of oxygen to produce carbon dioxide and water vapour.
What is the total volume (at constant temperature and pressure) of all species present once the reaction is complete?

A) 20 dm³

B) 24 dm³

C) 26 dm³

D) 30 dm³

Answer 3

C) 26 dm³

6dm³ of unreacted O₂, 8 dm³ of CO₂ and 12 dm³ of H₂O

Question 4

Magnesium carbonate (MgCO₃) thermally decomposes to produce magnesium oxide (MgO) and carbon dioxide. What mass of magnesium carbonate is needed to produce 6.00 dm³ of carbon dioxide at RTP?

Answer 4

MgCO₃ –> MgO + CO₂

1 mole of MgCO₃ produces 1 mole of CO₂
At RTP, 6.00 dm³ of CO₂ = 6.00/24.0 = 0.250 mol

So, 0.250 mol of CO₂ is produced by 0.250 mol of MgCO₃
M_r of MgCO₃ = 84.3 x 0.250 = 21.1g

Question 5

At what temperature will 1.28g of chlorine gas occupy 98.6 dm³, at a pressure of 175 Pa?

Answer 5

Moles of Cl₂ = (1.28/(35.5 x 2)) = 0.0180 mol
Rearranging pV=nRT to find T gives T = (pV)/nR

So, T = (175 x (98.6 x 10^-3))/(0.0180 x 8.314) = 115 K

Question 6

Explain why the bond enthalpy of C=O in ketones is greater than the bond enthalpy of C-O in alcohols

Answer 6

The C=O bond consists of four shared electrons. The C-O bond is only made up of two shared electrons. This means there is a greater electron density between the positive nuclei in the C=O bond, so the nuclei are more strongly attracted to the shared electrons

Question 7

Hydrogen peroxide has the structure: H-O-O-H
Using values from the table, calculate the overall enthalpy change for the reaction:

H₂O_2(aq) –> H₂O_(l) + ½O_2(g)

Answer 7

Overall enthalpy change = total energy absorbed - total energy released

= [(2 x 463) + (1 x 146)] - [(2 x 463) + (½ x 498)]
= 1072 - 1175 = 103 kJ mol^-1

Question 8

If the standard enthalpy change of formation of Al₂O_3(s) is -1676 kJ mol^-1 and the standard enthalpy change of formation of MgO_(s) is -602 kJ mol^-1, calculate the enthalpy change of the following reaction:

Al₂O_3(s) + 3Mg_(s) –> 2Al_(s) + 3MgO_(s)

Answer 8

Δ_rH^ϴ​ = sum of Δ_fH^ϴ​ (products) - sum of Δ_fH^ϴ​ (reactants)
= [0 + (3 x -602)] - [-1676 + 0]
= 130 kJ mol^-1

Question 9

Write a balanced equation, including state symbols, for the complete combustion of propane gas.

Answer 9

C₃H_8(g) + 5O_2(g) –> 3CO_2(g) + 4H₂O_(l)

Question 10

Using the values of Δ_fH^ϴ​ given in the table, draw an enthalpy cycle to determine the standard enthalpy of combustion of propane. Hence, calculate a value for Δ_cH^ϴ [propane]

Answer 10

Δ_cH^ϴ​ = sum of Δ_fH^ϴ​ (products) - sum of Δ_fH^ϴ​ (reactants)
= [(3 x -394.0) + (4 x -285.0)] - [(-104.7) + 0]
= -2221 kJ mol^-1

Question 11

The equation below shows the fermentation of glucose
C₆H₁₂O_6(s) –> 2C₂H₅OH_(l) + 2CO_2(g)

Calculate the enthalpy change for the reaction.
Use the enthalpy cycle and the standard enthalpies of combustion shown below in your calculations

Answer 11

Δ_rH^ϴ = Δ_cH^ϴ (glucose) - 2Δ_cH^ϴ​ (ethanol)
= [-2820] - [2 x -1367]
= -86 kJ mol^-1

Question 12

The initial temperature of 25.0 cm³ of 1.00 mol dm^-3 hydrochloric acid in a polystyrene cup was measured at 19.0 ^oC. This acid was exactly neutralised by 25.0 cm³ of 1.00 mol dm^-3 sodium hydroxide solution.
The maximum temperature of the resulting solution was measure as 25.8^oC

Calculate the standard enthalpy change of neutralisation for the reaction. (You many assume the neutral solution formed has a specific heat capacity of 4.18 J g^-1 K^-1, and a density of 1.00g cm^-3.)

Answer 12

Δ*T* = 25.8 - 19.0 = 6.80<sup>o</sup>C = 6.80 K
m = 25.0 + 25.0 = 50.0 cm<sup>3</sup> of solution, which has a mass of 50.0g

Assume density to be 1.00g cm^-3
Heat produced by reaction = mcΔT
= 50.0 x 4.18 x 6.80 = 1421.2 J

No. of moles of HCl = 1 x (25.0/1000) = 0.0250
No. of moles of NaOH = 1 x (25.0/1000) = 0.0250
Therefore, no. of moles of water = 0.0250
Producing 0.0250 mol of water takes 1421.2 J of heat, therefore producing 1 mol of water takes 1421.2/0.0250 = 56 848 J
= 58.6 kJ

So enthalpy change is -56.8 kJ mol^-1 (3.s.f)
negative - because exothermic

Question 13

A 50.0 cm³ sample of 0.200 mol dm^-3 copper(II) sulfate solution placed in a polystyrene beaker gave a temperature increase of 2.00 K when excess zinc powder was added and stirred. (Ignore the increase in volume due to the zinc.)

Calculate the enthalpy change when 1 mol of zinc reacts. Assume the solution’s specific heat capacity is 4.18 J g^-1 K^-1. The equation for the reaction is: Zn_(s) + CuSO_4(aq) –> Cu_(s) + ZnSO_4(aq)

Answer 13

No. of moles of CuSO₄ = 0.200 x (50.0/1000) = 0.0100 mol
From the equation, 1 mole of CuSO₄ reacts with 1 mole of Zn
So, 0.0100 mol of CuSO₄ reacts with 0.0100 mol of Zn
Heat produced by reaction = mcΔT
= 50.0 x 4.18 x 2.00 = 418 J
0.0100 mol of zinc produces 418 J of heat, therefore 1 mol of zinc produces 418/0.0100 = 41800 J = 41.8 kJ
So the enthalpy change is -41.8 kJ

Question 14

A 50.0 cm³ sample of 0.200 mol dm^-3 copper(II) sulfate solution placed in a polystyrene beaker gave a temperature increase of 2.00 K when excess zinc powder was added and stirred. (Ignore the increase in volume due to the zinc.)
The equation for the reaction is: Zn_(s) + CuSO_4(aq) –> Cu_(s) + ZnSO_4(aq)

Draw a labelled enthalpy profile diagram for this reaction, showing E_a and Δ_rH

Answer 14

Question 15

The following reaction represents an important stage in the industrial manufacture of sulfuric acid

2SO_2(g) + O_2(g) –> 2SO_3(g)

The catalyst used is V₂O_5(s). Explain why this is considered a heterogeneous catalyst.

Answer 15

V₂O_5(s) is classed as a heterogeneous catalyst, as it is in a different state/phase from the reactants
Vanadium(V) oxide’s a solid, but the reactants are gases

Question 16

The following reaction represents an important stage in the industrial manufacture of sulfuric acid

2SO_2(g) + O_2(g) –> 2SO_3(g)

The catalyst used is V₂O_5(s). How could you show, experimentally, that V₂O_5(s) is a catalyst and not a reactant?

Answer 16

Find the mass of the V₂O_5(s) before and after the reaction - if it is acting as a catalyst, they will be the same

Question 17

The following reaction represents an important stage in the industrial manufacture of sulfuric acid

2SO_2(g) + O_2(g) –> 2SO_3(g)

The catalyst used is V₂O_5(s). Platinum catalysts are more efficient than vandium catalysts but are seldom used because they are susceptibel to poisoning by arsenic. Suggest an explanation for how the poisoning happens

Answer 17

The arsenic probably clings to the surface of the platinum and stops it getting involved in the reaction

This is how heterogeneous catalysts are normally poisoned

Question 18

Crude oil is a source of fuels and petrochemicals. It’s vaporised and separated into fractions using fractional distillation. Some heavier fractions are processed using cracking

Give one economic reason why a catalyst is used in the cracking process

Answer 18

Cracking can be carried out at lower temperatures and pressures using a catalyst

Question 19

Crude oil is a source of fuels and petrochemicals. It’s vaporised and separated into fractions using fractional distillation. Some heavier fractions are processed using cracking

Write an equation for the thermal cracking of dodecane, C₁₂H₂₆

Answer 19

E.g. C₁₂H₂₆ –> C₂H₄ + C₁₀H₂₂

Many answers - make sure C’s and H’s balance and there’s an alkane and an alkene

Question 20

There are a number of different molecules with the molecular formula C₅H₁₀

Draw an unbranched cycloalkane with this fomula

Answer 20

Question 21

There are a number of different molecules with the molecular formula C₅H₁₀

Draw a branched alkene with this fomula

Answer 21

Question 22

There are a number of different molecules with the molecular formula C₅H₁₀

Draw an unbranched alkene with this fomula

Answer 22

Question 23

Draw the structure of the molecule 3-methylbutan-2-ol

Answer 23

Question 24

Draw the three-dimensional structure of propene. Mark two different bond angles on your diagram. Label each of these angles with its size.

Answer 24

Question 25

Propene contains sigma bonds and one pi bond. Draw a labelled diagram showing where the pi bond is in the molecule and the electron arrangement in the bond.

Answer 25

Question 26

Below is the structural formula of propa-1,2-diene. Mark two different bond angles on your diagram. Label each of these angles with its size.

Answer 26

Question 27

Draw the structural formula for methanol

Answer 27

Question 28

Draw the three-dimensional structure of methanol. On your diagram, mark one tetrahedral bond angle

Explain why the molecule exists in this shape

Answer 28

Electron pairs repel each other. This shape allows the electron pairs to be as far apart as possible

Question 29

Draw the two structural isomers of chloropropane (C₃H₇Cl)

Answer 29

Question 30

Draw the structural and skeletal formula of a cyclic alkane with the formula C₆H₁₂

Answer 30

Question 31

Draw the structural and skeletal formula of a branched alkene with the formula C₄H₈

Answer 31

Question 32

Name the molecule below

Answer 32

propan-1-ol

Question 33

Name the molecule below

Answer 33

2-methylbutane

Question 34

Which three of the following molecules are structural isomers?

Answer 34

A, B and C

A, B and C have the molecular formula C₁₀H₁₂O₂
D has the molecular formula C₁₁H₁₄O₂

Question 35

Draw and name the E/Z isomers of pent-2-ene

Answer 35

Question 36

Explain why alkenes can have E/Z isomers but alkanes cannot

Answer 36

E/Z isomers occur because atoms can’t rotate about C=C double bonds like they can around single bonds
Alkenes contain C=C double bonds and alkanes do not, so alkenes can form E/Z isomers and alkanes can’t

Question 37

Which of the following molecules is an E isomer?

Answer 37

B

Question 38

Name the molecule below

Answer 38

trans-2-chlorobut-2-ene

Question 39

Name the molecule below

Answer 39

Z-hex-3-ene

Question 40

Chloroethene CH₂=CHCl forms the polymer poly(chloroethene), commonly known as PVC.

Write an equation to show the polymerisation of chloroethene. Your equation should show clearly the structure of the repeating unit in poly(chloroethene).

Answer 40

Question 41

Write an equation using shortened structural formulas for the reaction of pent-2-ene with hydrogen gas

Give details of a chemical test that would enable you to distinguish between the product and the reactant. You should include details of how your results should be interpreted

Answer 41

CH₃CH₂CH=CHCH₃ + H₂ –> CH₃CH₂CH₂CH₂CH₃

Add bromine water to samples of each substance and shake mixture. With pentane, the solution will remain orange. With pent-2-ene, the solution will become colourless

Question 42

Ethene can undergo a hydrolysis reaction with cold water in the presence of a catalyst

Name the catalyst that can be used in the reaction

Answer 42

sulfuric acid

Question 43

Ethene can undergo a hydrolysis reaction with cold water in the presence of a catalyst

Describe what happens in the two stages of this reaction

Answer 43

The ethene reacts with the sulfuric acid to form ethyl hydrogen sulfate. The ethyl hydrogen sulfate then reacts with water/is hydrolysed to form ethanol

Question 44

Write a balanced equation for the complete combustion of hexane

Answer 44

C₆H₁₄ + 9½O₂ –> 6CO₂ + 7H₂O
or 2C₆H₁₄ + 19O₂ –> 12CO₂ + 14H₂O

Question 45

Write a balanced equation for the incomplete combustion of pentanol

Answer 45

C₅H₁₁OH + 5O₂ –> 5CO + 6H₂O

Question 46

Explain why burning an alkane in a limited supply of oxygen is potentially dangerous

Answer 46

if there is not enough oxygen to allow the fuel to burn fully, then incomplete combustion takes place. One of the products of incomplete combustion is carbon monoxide (CO), which is poisionous

Question 47

Explain how the burning of fossil fuels in a car engine can lead to photochemical smog

Answer 47

Oxides of nitrogen (NO_x) are produced when nitrogen and oxygen atoms in the air react together as a result of the high temperature and pressure in an engine. Unburnt hydrocarbons and these nitrogen oxides react with in the presence of sunlight to form ground-level ozone (O₃), which is a major component of smog

Question 48

State how the levels of the pollutants that lead to photochemical smog can be reduced

Answer 48

E.g. catalytic converters on cars can remove unburnt hydrocarbons and oxides of nitrogen from the exhaust

Question 49

A problem caused by pollution is acid rain.

Name two pollutants which lead to acid rain, and explain how this problem can be reduced.

Answer 49

Sulfur dioxide and nitrogen dioxide escape into the atmosphere. The problem can be reduced by using calcium oxide to remove sulfur dioxide from power station flue gases
Catalytic converters can remove both from vehicle emissions

Question 50

Describe two types of measure that governments can take to try to reduce pollution

Answer 50

change the law/tax pollution more highly/encourage people to change their behaviour/fund the development of new fuels that cause less pollution

Question 51

Describe two advantages associated with the continued large-scale use of fossil fuels as an energy source

Answer 51

Continued use of fossil fuels means increasing amounts of carbon dioxide being emitted, which could lead to problems with climate change.
Fossil fuels are also non-renewable, and so they will eventually run out, leading to problems if we do not have alternatives

Question 52

Give two reasons why the demand for energy is rising

Answer 52

E.g. The world’s population is rising so there are more people demanding energy
Developing countries are getting richer, meaning their industry is growing and more energy-consuming products are being bought

Question 53

Describe one advantage and one disadvantage of using wind power as a fossil fuel alternative

Answer 53

Advantage - wind power does not emit carbon dioxide into the atmosphere (although CO₂ will be emitted during the manufacture of wind turbines)
Disadvantage - wind power is not completely reliable

Question 54

Describe one advantage and one disadvantage of using hydrogen as a fossil fuel alternative

Answer 54

Advantage - hydrogen is a ‘clean’ fuel whose only waste product is water
Disadvantage - it takes energy to extract from seawater

Question 55

In a classroom debate on the future of fuels, Martin argues that bioethanol is not a solution to the Earth’s ‘greenhouse effect’ problem because “the combustion of bioethanol still produces carbon dioxide”
However, Samantha maintains that bioethanol is “carbon neutral”

To what extent are Martin and Samantha’s arguments right or wrong?

Answer 55

Burning biofuels does produce carbon dioxide, although it is CO₂ that the plants, that go into making biofuels, absorbed while they were growing. This is why biofuels are often considered to be carbon neutral. However CO₂ is also given out while refining and transporting the fuel so the overall production is not carbon neutral

Question 56

In a classroom debate on the future of fuels, Martin argues that bioethanol is not a solution to the Earth’s ‘greenhouse effect’ problem because “the combustion of bioethanol still produces carbon dioxide”
However, Samantha maintains that bioethanol is “carbon neutral”

Describe two disadvantages of replacing fossil fuels with biofuel

Answer 56

E.g. Petrol engines in cars would have to be modified to use biofuels. Land currently used for farming might be used to grow the crops used to produce biofuel, which might cause food shortages in developing countries