Chemical Energetics: practice questions

Question 1

A student determines the enthalpy change of combustion of propan-1-ol, CH3CH2CH2OH (molar mass 60.0 g mol^-1), using a simple calorimetry experiment. A spirit burner containing propan-1-ol is used to heat some water held in a metal can, and is then reweighed. Quantity Value Volu…

Accepted Answer

((a)) The enthalpy change when one mole of a substance is completely burned in excess oxygen, with all reactants and products in their standard states at a stated temperature (usually 298 K) and 100 kPa pressure.  ((b)) q = 10 784 J = 10.8 kJ (3 s.f.)  ((c)) Amount burned = 0.0080 mol; Hc = -1350 kJ mol^-1 (3 s.f.)  ((d)) Heat is lost to the surroundings and to the apparatus (can, burner, air) rather than all of it heating the water, and combustion may be incomplete, so the experiment transfers less energy than the true enthalpy of combustion.

Question 2

Calcium oxide reacts with carbon dioxide gas to re-form calcium carbonate: CaO(s) + CO2(g) → CaCO3(s) The table gives the standard enthalpy change of formation of each compound. Substance Hf^ / kJ mol^-1 CaO(s) -635 CO2(g) -394 CaCO3(s) -1207 (a) State Hess's law. [1] (b) Constr…

Accepted Answer

((a)) Hess's law states that the total enthalpy change for a reaction is independent of the route taken between the same initial and final states, provided the starting and finishing conditions are the same.  ((b)) H^ = -178 kJ mol^-1  ((c)) Exothermic, since H^ is negative. Energy is released to the surroundings.  ((d)) 0.0500 mol of CaO releases 8.90 kJ of heat energy.

Question 3

Methane reacts with chlorine to form chloromethane:

$\text{CH}_4\text{(g)} + \text{Cl}_2\text{(g)} \rightarrow \text{CH}_3\text{Cl(g)} + \text{HCl(g)}$

The table gives some mean bond enthalpies.

Bond	Mean bond enthalpy / $\text{kJ mol}^{-1}$
C–H	$412$
Cl–Cl	$244$
C–Cl	$338$
H–Cl	$428$

Using these mean bond enthalpies, what is the enthalpy change, $\Delta H$ , for this reaction?

Accepted Answer

A

Question 4

The table gives enthalpy data needed to construct a Born-Haber cycle for strontium chloride, SrCl2. Enthalpy change Value / kJ mol^-1 Standard enthalpy change of formation of SrCl2(s) -828 Standard enthalpy change of atomisation of Sr(s) +166 First ionisation energy of Sr +548 S…

Accepted Answer

((a)) The enthalpy change when one mole of gaseous chlorine atoms each gain one electron to form one mole of gaseous 1- ions: Cl(g) + e^- → Cl^-(g).  ((b)) The second electron is removed from the Sr^+ ion, not a neutral atom: the same nuclear charge now attracts one fewer electron, so each remaining electron is held more strongly (greater effective nuclear charge per electron), requiring more energy to remove a second electron.  ((c)) Lattice energy of SrCl2 = -2146 kJ mol^-1  ((d)) Real ionic compounds usually show some covalent character (the small, doubly-charged Sr^2+ ion polarises the Cl^- ion), so the Born-Haber (experimental) lattice energy is typically more exothermic than a value calculated from a purely ionic electrostatic model.

Question 5

Magnesium carbonate decomposes on heating: MgCO3(s) → MgO(s) + CO2(g) For this reaction, H^ = +117 kJ mol^-1. The table gives the standard entropy of each substance. Substance S^ / J K^-1mol^-1 MgCO3(s) 65.1 MgO(s) 26.9 CO2(g) 213.6 (a) Calculate the standard entropy change, S^,…

Accepted Answer

((a)) S^ = +175.4 J K^-1mol^-1  ((b)) Positive, because one mole of solid reactant forms one mole of gas among the products; gas particles are far more disordered (many more accessible microstates) than solid particles, so the total entropy of the system increases.  ((c)) T = 667 K (3 s.f.)  ((d)) At 293 K, G^ = +65.6 kJ mol^-1 (positive), so the reaction is not thermodynamically feasible at this temperature.

Question 6

A student measures the enthalpy change of neutralisation between hydrochloric acid and sodium hydroxide by mixing the two solutions in an insulated cup and recording the temperature. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) Quantity Value Volume of 1.00 mol dm^-3 HCl(aq) used 50.0…

Accepted Answer

((a)) The enthalpy change when one mole of water is formed by the neutralisation of an acid by an alkali, under standard conditions, using dilute aqueous solutions.  ((b)) q = 2884.2 J = 2.88 kJ (3 s.f.)  ((c)) n(H2O) = 0.0500 mol; Hneut = -57.7 kJ mol^-1 (3 s.f.)  ((d)) Heat is lost from the mixture to the surroundings, the cup and the thermometer during the experiment, so less heat is measured than is actually released by the reaction.

Question 7

Ethyne, C2H2(g), cannot be prepared cleanly by direct reaction of its elements, so its standard enthalpy change of formation must be found indirectly. The table gives standard enthalpies of combustion. Substance Hc^ / kJ mol^-1 C(graphite) -394 H2(g) -286 C2H2(g) -1300 The forma…

Accepted Answer

((a)) Carbon and hydrogen do not react directly and cleanly to form ethyne under standard conditions (the reaction does not go to completion, and other products would also form), so a single accurate value cannot be measured directly and must instead be found indirectly using Hess's l…  ((b)) Hf^ [C2H2] = +226 kJ mol^-1  ((c)) Endothermic, since Hf^ is positive. Energy is absorbed from the surroundings when ethyne forms from its elements.

Question 8

Methane burns completely in oxygen, with all species treated as gases:

$\text{CH}_4\text{(g)} + 2\text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} + 2\text{H}_2\text{O(g)}$

The table gives some mean bond enthalpies.

Bond	Mean bond enthalpy / $\text{kJ mol}^{-1}$
C–H	$412$
O=O	$496$
C=O (in $\text{CO}_2$ )	$805$
O–H	$463$

Using these mean bond enthalpies, what is the enthalpy change, $\Delta H$ , for this reaction?

Accepted Answer

A

Question 9

Powdered aluminium reacts with iron(III) oxide in the thermite reaction: 2Al(s) + Fe2O3(s) → Al2O3(s) + 2Fe(s) The table gives standard enthalpy changes of formation. Substance Hf^ / kJ mol^-1 Al2O3(s) -1676 Fe2O3(s) -824 (a) Define the term standard enthalpy change of formation…

Accepted Answer

((a)) The enthalpy change when one mole of a compound is formed from its constituent elements, with all reactants and products in their standard states at a stated temperature (usually 298 K) and 100 kPa pressure.  ((b)) H^ = -852 kJ mol^-1  ((c)) Exothermic, since H^ is negative. Energy is released to the surroundings.  ((d)) Al2O3 and Fe2O3 (and the metals Al and Fe) are giant ionic or metallic lattice structures, not simple discrete covalent molecules, so there are no individual covalent bonds between specific pairs of atoms to assign a mean bond enthalpy to; the bond-enthalpy method only applies t…

Question 10

A student adds 0.120 g of magnesium ribbon (molar mass 24.0 g mol^-1) to 50.0 cm^3 of 2.00 mol dm^-3 hydrochloric acid, which is in excess, in an insulated cup: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) The temperature of the solution rises from 18.0 °C to 29.2 °C. Assume the mass of…

Accepted Answer

A

Quantity	Value
Volume of water heated	$150\ \text{cm}^3$ (density $1.00\ \text{g cm}^{-3}$ )
Initial temperature of the water	$19.5\,^\circ\text{C}$
Final temperature of the water	$36.7\,^\circ\text{C}$
Mass of spirit burner + fuel before burning	$82.46\ \text{g}$
Mass of spirit burner + fuel after burning	$81.98\ \text{g}$
Specific heat capacity of water, $c$	$4.18\ \text{J g}^{-1}\text{ K}^{-1}$

Substance	$\Delta H_f^{\ominus}$ / $\text{kJ mol}^{-1}$
$\text{CaO(s)}$	$-635$
$\text{CO}_2\text{(g)}$	$-394$
$\text{CaCO}_3\text{(s)}$	$-1207$

Enthalpy change	Value / $\text{kJ mol}^{-1}$
Standard enthalpy change of formation of $\text{SrCl}_2\text{(s)}$	$-828$
Standard enthalpy change of atomisation of $\text{Sr(s)}$	$+166$
First ionisation energy of $\text{Sr}$	$+548$
Second ionisation energy of $\text{Sr}$	$+1060$
Standard enthalpy change of atomisation of $\text{Cl}_2\text{(g)}$ (per mole of Cl atoms)	$+121$
First electron affinity of $\text{Cl}$	$-349$

Substance	$S^{\ominus}$ / $\text{J K}^{-1}\text{mol}^{-1}$
$\text{MgCO}_3\text{(s)}$	$65.1$
$\text{MgO(s)}$	$26.9$
$\text{CO}_2\text{(g)}$	$213.6$

Quantity	Value
Volume of $1.00\ \text{mol dm}^{-3}$ HCl(aq) used	$50.0\ \text{cm}^3$
Volume of $1.00\ \text{mol dm}^{-3}$ NaOH(aq) used	$50.0\ \text{cm}^3$
Initial temperature (both solutions, before mixing)	$19.0\,^\circ\text{C}$
Highest temperature reached after mixing	$25.9\,^\circ\text{C}$
Density of the resulting mixture	$1.00\ \text{g cm}^{-3}$
Specific heat capacity of the mixture, $c$	$4.18\ \text{J g}^{-1}\text{ K}^{-1}$

Chemical Energetics: Chemistry 9701 (Cambridge International AS & A Level)

Syllabus coverage

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Substance	$\Delta H_c^{\ominus}$ / $\text{kJ mol}^{-1}$
$\text{C(graphite)}$	$-394$
$\text{H}_2\text{(g)}$	$-286$
$\text{C}_2\text{H}_2\text{(g)}$	$-1300$

Substance	$\Delta H_f^{\ominus}$ / $\text{kJ mol}^{-1}$
$\text{Al}_2\text{O}_3\text{(s)}$	$-1676$
$\text{Fe}_2\text{O}_3\text{(s)}$	$-824$