Reactivity 1: What drives chemical reactions?IB Chemistry SL: Topic test

20 questions, 54 marks

IB Chemistry SL

Reactivity 1: What drives chemical reactions? topic test

Total 54 marks

Name

Class

Date

1
A student adds 3.20 g of ammonium chloride, NH4Cl (M = 53.5 g mol⁻¹), to 100.0 cm³ of water in an insulated cup and stirs until it dissolves. The temperature falls from 21.4 °C to 16.8 °C. The specific heat capacity of the solution is taken as 4.18 J g⁻¹ K⁻¹ and its density as 1.00 g cm⁻³.
(a)
Is the dissolving of NH4Cl exothermic or endothermic, and why?
[1 mark]
Aendothermic, because the temperature of the solution decreases
Bexothermic, because the temperature of the solution increases
Cexothermic, because the temperature of the solution decreases
Dendothermic, because the temperature of the solution increases
(b)
What is q, the heat absorbed by the dissolving process from the solution, to 3 significant figures?
[1 mark]
A0.418 kJ
B1.92 kJ
C19.2 kJ
D192 kJ
(c)
Calculate ΔH for the dissolving process, in kJ mol⁻¹, showing it is consistent with an endothermic process.
[2 marks]
Total for question 1: 4 marks
2
Average bond enthalpies, in kJ mol⁻¹, are: C–H 414, C=C 614, C–C 346, C–Cl 324, H–Cl 431. Consider the addition reaction: CH2=CH2(g) + HCl(g) → CH3CH2Cl(g).
(a)
What is the total bond enthalpy of the bonds broken in this reaction?
[1 mark]
A1084 kJ mol⁻¹
B839 kJ mol⁻¹
C1045 kJ mol⁻¹
D1477 kJ mol⁻¹
(b)
What is the enthalpy change, ΔH, for this reaction, in kJ mol⁻¹?
[1 mark]
A+39 kJ mol⁻¹
B+1123 kJ mol⁻¹
C−1123 kJ mol⁻¹
D−39 kJ mol⁻¹
(c)
State whether this reaction is exothermic or endothermic, and explain how the sign of ΔH shows this in terms of the relative strength of the bonds broken and formed.
[2 marks]
Total for question 2: 4 marks
3
Propane, C3H8, is burned as a fuel in a camping stove. In a limited supply of air, incomplete combustion can occur, producing carbon monoxide and/or carbon (soot) instead of only carbon dioxide.
(a)
Deduce a balanced equation for the complete combustion of propane in excess oxygen, and a balanced equation for its incomplete combustion producing only carbon monoxide and water.
[3 marks]
(b)
A camper burns 440 g of propane (M = 44.0 g mol⁻¹) completely in excess oxygen. Calculate the mass of CO2 released, and outline one advantage and one disadvantage of propane compared with a renewable biofuel as an energy source.
[4 marks]
Total for question 3: 7 marks
4
A student wants to determine the enthalpy change for the reaction Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g), ΔH1, which is too vigorous to measure safely with a simple calorimeter. Instead, the student applies Hess's law using two other reactions. Reaction 2: MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l), ΔH2. Reaction 3: Mg(s) + ½O2(g) → MgO(s), ΔH3 = −602 kJ mol⁻¹ (from a data source). The enthalpy change of H2(g) + ½O2(g) → H2O(l) is ΔH4 = −286 kJ mol⁻¹ (also from a data source). To find ΔH2 experimentally, the student adds 1.00 g of MgO (M = 40.3 g mol⁻¹) to 50.0 cm³ of excess 2.00 mol dm⁻³ hydrochloric acid in an insulated polystyrene cup. The temperature rises from 19.8 °C to 32.6 °C. Assume the density and specific heat capacity of the resulting solution are 1.00 g cm⁻³ and 4.18 J g⁻¹ K⁻¹.
(a)
Calculate the enthalpy change, ΔH2, for Reaction 2, in kJ mol⁻¹, showing your working and including the correct sign.
[6 marks]
(b)
Using Hess's law, construct an energy cycle linking Reactions 1, 2 and 3 and the formation of water from hydrogen and oxygen, and use it to calculate ΔH1, the enthalpy change for Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g). Use your value of ΔH2 from part (a); if you were unable to calculate it, use −108 kJ mol⁻¹.
[6 marks]
Total for question 4: 12 marks
5
A reaction between solid citric acid and solid sodium hydrogencarbonate is initiated in an insulated cup by adding a small amount of water; bubbles of gas are produced and the temperature of the mixture falls noticeably below room temperature.
(a)
What can be concluded about the relative stability of the reactants and products from the temperature fall?
[1 mark]
Athe products are more stable (lower in energy) than the reactants
Bthe products are less stable (higher in energy) than the reactants
Cthe reactants and products have equal stability
Dstability cannot be determined from a temperature change
(b)
On a sketched energy profile diagram for this reaction, which feature correctly shows that it is endothermic?
[1 mark]
Athe products are drawn at a lower energy level than the reactants
Bthe activation energy is drawn as zero
Cthe products are drawn at a higher energy level than the reactants
Dthe curve returns to the same energy level it started at
(c)
Explain the difference between heat and temperature, using this reaction as an example.
[2 marks]
Total for question 5: 4 marks
6
Average bond enthalpies, in kJ mol⁻¹: C–H 414, O=O 498, C=O 804, O–H 463. Consider the complete combustion of methane: CH4(g) + 2O2(g) → CO2(g) + 2H2O(g).
(a)
What is the total bond enthalpy of the bonds broken in this reaction?
[1 mark]
A3460 kJ mol⁻¹
B1656 kJ mol⁻¹
C2154 kJ mol⁻¹
D2652 kJ mol⁻¹
(b)
What is the enthalpy change, ΔH, for the complete combustion of one mole of methane, in kJ mol⁻¹?
[1 mark]
A−808 kJ mol⁻¹
B+808 kJ mol⁻¹
C−3460 kJ mol⁻¹
D+2652 kJ mol⁻¹
(c)
Explain why a bond-enthalpy calculation like this gives only an approximate value for ΔH, rather than an exact one.
[2 marks]
Total for question 6: 4 marks
7
A hydrogen fuel cell is used to power a small vehicle. Hydrogen gas is supplied to one electrode and oxygen (from air) to the other; the electrodes are separated by an acidic electrolyte that allows H⁺ ions to pass through. The overall reaction is the same as the combustion of hydrogen, but no flame is produced.
(a)
Deduce half-equations for the reactions occurring at each electrode of this hydrogen fuel cell, and state the overall cell reaction.
[3 marks]
(b)
State one advantage and one disadvantage of a hydrogen fuel cell vehicle compared with a vehicle powered by burning a biofuel, and explain whether hydrogen used in this way should be classified as a renewable or a non-renewable energy source, given that most hydrogen gas is currently produced from natural gas (a fossil fuel).
[4 marks]
Total for question 7: 7 marks
8
A greenhouse is heated either by burning kerosene (a fossil fuel, approximated as C12H26) or by burning biodiesel produced from vegetable oil (approximated as C19H36O2, methyl oleate). Combustion of both fuels is exothermic. 1.00 kg of kerosene releases approximately 46 000 kJ of energy when completely combusted; 1.00 kg of biodiesel releases approximately 40 000 kJ.
(a)
Deduce a balanced equation for the complete combustion of kerosene, C12H26, in excess oxygen. Explain, referring to photosynthesis, why burning biodiesel is generally considered to add no net carbon dioxide to the atmosphere over the medium term, while burning kerosene does, even though both release carbon dioxide when burned.
[6 marks]
(b)
Describe, in words, how the energy profile diagrams for the combustion of 1.00 kg of kerosene and 1.00 kg of biodiesel would differ, given the energy values above, and explain what determines the amount of energy released by a fuel in terms of the relative enthalpy (stability) of its reactants and products.
[6 marks]
Total for question 8: 12 marks

End of questions