Lesson test: Enthalpy and heat capacity

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Enthalpy and heat capacity

1.[2p]
Why is $Δ H = Q_{p}$ a useful result rather than a trivial one?

Heat is normally path-dependent, but at constant pressure it equals the change in a state function

Correct

Enthalpy is a form of energy stored in the $p V$ term

The answer is: Heat is normally path-dependent, but at constant pressure it equals the change in a state function

It shows that heat and work are interchangeable

The answer is: Heat is normally path-dependent, but at constant pressure it equals the change in a state function

It proves that no work is done at constant pressure

The answer is: Heat is normally path-dependent, but at constant pressure it equals the change in a state function

2.[2p]
A bomb calorimeter measures which quantity directly, and why?

$Δ U$ , because the fixed volume means no expansion work is done

Correct

$Δ H$ , because the vessel is at the pressure of the laboratory

The answer is: $\Delta U$, because the fixed volume means no expansion work is done

$Δ U$ , because the steel vessel absorbs the work

The answer is: $\Delta U$, because the fixed volume means no expansion work is done

$Δ H$ , because the water bath is open to the atmosphere

The answer is: $\Delta U$, because the fixed volume means no expansion work is done

3.[2p]
For an ideal diatomic gas at room temperature, what is $C_{p, m}$ in J mol⁻¹ K⁻¹?

CorrectNot quite: 29.10

4.[3p]
Using $Δ_{f} H^{ominus}$ values of $- 393.5$ kJ mol⁻¹ for CO $_2$ (g), $- 285.8$ for H $_2$ O(l) and $- 277.6$ for ethanol C $_2$ H $_5$ OH(l), what is $Δ_{r} H^{ominus}$ in kJ mol⁻¹ for the combustion of ethanol to CO $_2$ (g) and H $_2$ O(l)?

CorrectNot quite: -1367.0

5.[3p]
The combustion of methane has $Δ_{c} H^{ominus} = - 890.5$ kJ mol⁻¹ with liquid water as product. Taking $R T = 2.479$ kJ mol⁻¹, what is $Δ_{c} U^{ominus}$ in kJ mol⁻¹?

CorrectNot quite: -885.5

6.[2p]
Carbon dioxide has a larger molar $C_{V}$ than nitrogen at $298$ K. What accounts for the difference?

Its bending vibration lies low enough in energy to be partly excited at room temperature

Correct

It is a triatomic molecule and so has three rotational degrees of freedom

The answer is: Its bending vibration lies low enough in energy to be partly excited at room temperature

It is heavier, so each molecule carries more kinetic energy

The answer is: Its bending vibration lies low enough in energy to be partly excited at room temperature

It is not an ideal gas, so equipartition does not apply

The answer is: Its bending vibration lies low enough in energy to be partly excited at room temperature

7.[1p]
A reaction with a negative $Δ H$ is guaranteed to be spontaneous.

True

The answer is: False

False

Correct

8.[3p]
Which statements about heat capacities are correct?

Select all that apply

$C_{p}$ exceeds $C_{V}$ because some heat supplied at constant pressure leaves as expansion work

Correct

For an ideal gas, $C_{p, m} - C_{V, m} = R$

Correct

$C_{p}$ and $C_{V}$ differ substantially for solids at room temperature

The answer is: $C_p$ exceeds $C_V$ because some heat supplied at constant pressure leaves as expansion work, For an ideal gas, $C_{p,m} - C_{V,m} = R$, A heat capacity is undefined until the constraint on the path is stated

A heat capacity is undefined until the constraint on the path is stated

Correct

9.[2p]
Match each gas or process to the value it is associated with.

Argon, $γ$ at 298 K
Nitrogen, $γ$ at 298 K
Ice melting, $Δ H$ per mole
Ammonium nitrate dissolving

+25.7 kJ per mole
+6.01 kJ
1.40
1.67

Show the answer

Argon, $γ$ at 298 K: 1.67 Nitrogen, $γ$ at 298 K: 1.40 Ice melting, $Δ H$ per mole: +6.01 kJ Ammonium nitrate dissolving: +25.7 kJ per mole

1.[2p] Why is ΔH=Qp a useful result rather than a trivial one?

2.[2p] A bomb calorimeter measures which quantity directly, and why?

3.[2p] For an ideal diatomic gas at room temperature, what is Cp,m in J mol⁻¹ K⁻¹?

4.[3p] Using ΔfHominus values of -393.5 kJ mol⁻¹ for CO_2(g), -285.8 for H_2O(l) and -277.6 for ethanol C_2H_5OH(l), what is ΔrHominus in kJ mol⁻¹ for the combustion of ethanol to CO_2(g) and H_2O(l)?

5.[3p] The combustion of methane has ΔcHominus=-890.5 kJ mol⁻¹ with liquid water as product. Taking RT=2.479 kJ mol⁻¹, what is ΔcUominus in kJ mol⁻¹?

6.[2p] Carbon dioxide has a larger molar CV than nitrogen at 298 K. What accounts for the difference?

7.[1p] A reaction with a negative ΔH is guaranteed to be spontaneous.

8.[3p] Which statements about heat capacities are correct?

9.[2p] Match each gas or process to the value it is associated with.

1.[2p]
Why is $Δ H = Q_{p}$ a useful result rather than a trivial one?

2.[2p]
A bomb calorimeter measures which quantity directly, and why?

3.[2p]
For an ideal diatomic gas at room temperature, what is $C_{p, m}$ in J mol⁻¹ K⁻¹?

4.[3p]
Using $Δ_{f} H^{ominus}$ values of $- 393.5$ kJ mol⁻¹ for CO $_2$ (g), $- 285.8$ for H $_2$ O(l) and $- 277.6$ for ethanol C $_2$ H $_5$ OH(l), what is $Δ_{r} H^{ominus}$ in kJ mol⁻¹ for the combustion of ethanol to CO $_2$ (g) and H $_2$ O(l)?

5.[3p]
The combustion of methane has $Δ_{c} H^{ominus} = - 890.5$ kJ mol⁻¹ with liquid water as product. Taking $R T = 2.479$ kJ mol⁻¹, what is $Δ_{c} U^{ominus}$ in kJ mol⁻¹?

6.[2p]
Carbon dioxide has a larger molar $C_{V}$ than nitrogen at $298$ K. What accounts for the difference?

7.[1p]
A reaction with a negative $Δ H$ is guaranteed to be spontaneous.

8.[3p]
Which statements about heat capacities are correct?

9.[2p]
Match each gas or process to the value it is associated with.