Which equation below can represent both an enthalpy change of formation and combustion?
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
4Na (s) + O2 (g) → 2Na2O (s)
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
CO (g) + C (s) → CO2 (g)
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The reaction pathway for a reversible reaction is shown below:
Which statement is correct?
The activation energy of the reverse reaction is +90 kJ mol–1
The activation energy of the forward reaction is +20 kJ mol–1
The activation of the reverse reaction is +20 kJ mol–1
The enthalpy change of forwards reaction is - 70 kJ mol–1
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The reaction pathway for a reversible reaction is shown below.
Which statements are correct?
1 only
1 and 2
2 and 3
1, 2 and 3
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A student carried out an experiment to determine the enthalpy change for the combustion of ethanol.
The following results were obtained by the student. The specific heat capacity of water is 4.18 J g−1 ˚C−1.
|
Start temperature of the water |
21 °C |
|
Final temperature of the water |
54 °C |
|
Mass of alcohol burner before burning |
259.75 g |
|
Mass of alcohol burner after burning |
259.18 g |
|
Mass of glass beaker plus water |
150.00 g |
|
Mass of glass beaker |
50.0 g |
How much of the heat energy produced by the burning of ethanol went into the water?
6897 J
20691 J
22572 J
13794 J
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PCl5 (g) dissociates as follows.
PCl5 (g) → PCl3 (g) + Cl2 (g)
The yield of the products increases as the temperature is raised from 80oC to 110oC.
Which row correctly describes the formation of PCl3 (g)?
|
Shape of PCl3 molecule |
The reaction is |
|
|
A |
Pyramidal |
Endothermic |
|
B |
Pyramidal |
Exothermic |
|
C |
Trigonal |
Endothermic |
|
D |
Trigonal |
Exothermic |
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Hydrogen atoms bond covalently to Iodine atoms to form hydrogen iodide as shown in the equation below:
H2 (g) + I2 (g) → 2HI (g)
Which statement best describes what is meant by the average HI bond enthalpy?
The energy stored in a covalent bond.
The energy required to break one covalent bond in the gas phase.
The energy required to break one mole of the HI bonds in the gas phase.
The energy released when two atoms form a covalent bond.
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A student mixed 30.0 cm3 of 0.0250 mol dm-3 potassium hydroxide solution with 30.0 cm3 of 0.0250 mol dm-3 nitric acid. The temperature rose by 0.50 oC. Assume no heat was lost to the surroundings.
The final mixture had a specific heat capacity of 4.18 J cm-3 K-1.
What is the molar enthalpy change for the reaction?
-84.0 kJ mol-1
-83.6 kJ mol-1
167.2 kJ mol-1
-167.2 kJ mol-1
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An experiment was carried out to determine the approximate value for the molar enthalpy change of neutralisation.
75 cm3 of 3.00 mol dm–3 hydrochloric acid was placed in a polystyrene beaker of negligible heat capacity. Its temperature was recorded, and then 75 cm3 of 3.00 mol dm−3 potassium hydroxide at the same temperature was quickly added, and the solution was stirred.
The temperature rose by 14 °C. The resulting solution may be considered to have a specific heat capacity of 4.18 J g-1 K-1.
Calculate the molar enthalpy change of neutralisation.
-4876.7 J mol-1
-39 013.3 J mol-1
-39.0 J mol-1
-11 843.3 J mol-1
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The standard enthalpy change ∆Hθ for the below reaction is -1776 kJ.
2N (g) + 6F (g) → 2NF3 (g)
What is the bond energy of the N–F bond?
- 592
+ 592
+ 296
- 296
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The complete combustion of ethyne, C2H2, is shown in the equation below.
H−C≡C−H + 2½ O=O → O−H−H + 2O=C=O
Using the average bond enthalpies given in the table, what is the enthalpy change of combustion of ethyne?
Bond |
Average bond enthalpy |
|
C−H |
390 |
|
C≡C |
870 |
|
O=O |
489 |
|
C=O |
790 |
|
O−H |
510 |
|
O−C |
290 |
+ 1307.5 kJ mol–1
- 1307.5 kJ mol–1
+ 1390 kJ mol–1
- 1390 kJ mol–1
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Compound R into compound T, it was found that the reaction proceeded by way of compound S, which could be isolated. The following steps were involved.
R → S; ΔH is positive
S → T; ΔH is negative
Which reaction profile fits these data?
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The table below discusses three types of enthalpy change:
‘+’ means that this type of standard enthalpy change can only have positive values.
‘–’ means that this type of standard enthalpy change can only have negative values.
‘+ / –’ means that either positive or negative values are possible.
Which row is correct?
|
Formation |
Combustion |
Neutralisation |
|
|
A |
+ |
+ |
+ / – |
|
B |
+ / – |
+ |
+ / – |
|
C |
+ / – |
– |
– |
|
D |
– |
– |
+ |
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In the gas phase, phosphorus pentachloride can be thermally decomposed into gaseous phosphorus trichloride and chlorine.
PCl5 → PCl3 + Cl2
The table below gives the relevant bond energies found in these compounds.
|
Bond |
Bond energy / kJ mol–1 |
|
P – Cl (in both chlorides) Cl – Cl |
328 241 |
What is the enthalpy change in the decomposition of the reaction?
– 415 kJ mol–1
+ 415 kJ mol–1
+ 95 kJ mol–1
– 95 kJ mol–1
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In a calorimetric experiment 2.50 g of a fuel is burnt in oxygen. 30 % of the energy released during the combustion is absorbed by 500 g of water, the temperature of which rises from 25 °C to 68 °C.
The specific heat capacity of water is 4.18 J g–1 K–1.
What is the total energy released per gram of fuel burnt?
25 284 J
63 210 J
119 827 J
301 000 J
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Which equation correctly shows how the bond energy for the covalent bond Y-Z can be calculated by dividing ΔH by n?
n YZ (g) → n Y (g) + 
Z2 (g)
Z (g) + Y Zn-1 (g) → YZn (g)
2 YZn (g) → 2 YZn-1 (g) + Y2 (g)
YZn (g) → Y (g) + nZ (g)
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The diagram shows the skeletal formula of cyclobutane.
The enthalpy change of formation of cyclobutane is +75.1 kJ mol–1, and the enthalpy change of atomisation of graphite is +712 kJ mol–1.
The bond enthalpy of C–H is 390 kJ mol–1 and of H–H is 429 kJ mol–1.
What is the average bond enthalpy of the C–C bond in cyclobutane, rounded to the nearest whole number?
236 kJ mol–1
315 kJ mol–1
342 kJ mol–1
700 kJ mol–1
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Some bond energy values are listed below.
|
Bond |
Bond energy / kJ mol-1 |
|
Br – Br Cl – Cl C – H C – Cl |
194 247 412 338 |
These bond energy values relate to the following four reactions:
What is the correct order of enthalpy changes of the above reactions from most negative to most positive?
Y → Z → W → X
Z → W → X → Y
Y → X → W → Z
X → Y → Z → W
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