Enthalpy changes that are difficult to measure directly can often be determined using Hess’ Law to construct an enthalpy cycle.
Which enthalpy change is indicated by X in the enthalpy cycle shown?
+ 1 x Enthalpy of formation of water
- 1 x Enthalpy of formation of water
+ 3 x Enthalpy of formation of water
- 3 x Enthalpy of formation of water
A student drew a Hess cycle to calculate the enthalpy of reaction to produce ethane from ethene and hydrogen.
The student used the following enthalpy of combustion data
|
|
C2H4 (g) |
H2 (g) |
C2H6 (g) |
|
ΔHϴc / kJ mol-1 |
- 1411 |
- 286 |
- 1560 |
What are the correct labels for the arrows for the student’s Hess cycle?
|
|
Arrow 1 |
Arrow 2 |
Arrow 3 |
Arrow 4 |
|
A |
ΔHϴr |
- 1411 |
- 286 |
- 1560 |
|
B |
ΔHϴc |
- 1411 |
+ 286 |
- 1560 |
|
C |
ΔHϴc |
+ 1411 |
- 286 |
+ 1560 |
|
D |
ΔHϴr |
- 1411 |
- 286 |
+ 1560 |
Which is the correct equation to calculate the enthalpy of reaction using enthalpy of formation data?
ΔHr = ΣΔHf products + ΣΔHf reactants
ΔHr = ΣΔHf products - ΣΔHf reactants
ΔHr = ΣΔHf reactants - ΣΔHf products
ΔHr = ΣΔHf reactants + ΣΔHf products
Enthalpy of formation data is often used to calculate the enthalpy of reaction. Which of the following does not have a standard enthalpy of formation of 0 kJ mol-1?
C (s)
N2 (l)
O2 (g)
F2 (g)
Iron(II,III) oxide, Fe3O4, is reduced by graphite according to the following equation:
Fe3O4 (s) + 2C (s) → 3Fe (s) + 2CO2 (g)
The standard enthalpy of formation values for iron(II,III) oxide and carbon dioxide are -1118 kJ mol-1 and -394 kJ mol-1 respectively.
The standard enthalpy of reaction can be determined using which of the following calculations?
-1118 - (2 x -394)
-394 + 1118
(2 x -394) - 1118
(2 x -394) + 1118
Hydrogen peroxide slowly decomposes to form water and oxygen:
2H2O2 (l) → 2H2O (l) + O2 (g)
|
|
ΔHӨf, kJ mol-1 |
|
H2O2 (l) |
-188 |
|
H2O (l) |
-286 |
Which calculation gives the correct enthalpy of reaction for the decomposition of hydrogen peroxide?
-286 + 188
(2 x -286) - (2 x -188)
-286 + (2 x -188)
(2 x -188) - (2 x -286)
The diagram shows two possible reaction pathways for the reaction of A → D:
Which of the following statements are correct?
I and II only
I and III only
II and III only
I, II and III
The standard enthalpy of formation of sulfur dioxide cannot be measured directly.
Which equation(s) provide a suitable alternative pathway to determine the standard enthalpy of formation of sulfur dioxide?
S (s) + O2 (g) → SO2 (g)
|
S (s) + O (g) → SO (g) |
SO (g) + O (g) → SO2 (g) |
|
S (s) + 1.5O2 (g) → SO3 (g) |
SO3 (g) → SO2 (g) + 0.5O2 (g) |
|
S (s) + 0.5O2 (g) → SO (g) |
SO (g) + O2 (g) → SO3 (g) |
The thermal decomposition of calcium carbonate is very slow and requires a high temperature to go to completion. This makes it impractical to measure the enthalpy change for the direct reaction.
The enthalpy change for the thermal decomposition of calcium carbonate can be determined by two chemical reactions with dilute hydrochloric acid.
Which set of chemicals correctly completes the Hess cycle diagram?
CaCO3 + CaO + HCl
CaCl + H2O + CO2
CaCl2 + H2O
CaCl2 + H2O + CO2
Hess’s Law can be used to calculate the enthalpy change for reactions that are difficult to measure experimentally, such as the conversion of graphite to diamond.
Which equation shows the correct application of Hess’s law to calculate the enthalpy change for the conversion of graphite to diamond?
ΔHr = ΔH1 + ΔH2
ΔHr = ΔH1 - ΔH2
ΔHr = ΔH2 - ΔH1
ΔHr = ΔH1 x ΔH2
