Lesson test: The heat of a reaction

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The heat of a reaction

1.[2p]
Why is the heat of a reaction in an open flask a property of the reaction rather than of the apparatus?

At constant pressure the heat equals the change in enthalpy, which is a state function

Correct

Heat is always a state function, whatever the constraint

The answer is: At constant pressure the heat equals the change in enthalpy, which is a state function

An open flask does no work on its surroundings

The answer is: At constant pressure the heat equals the change in enthalpy, which is a state function

The atmosphere supplies whatever heat is missing

The answer is: At constant pressure the heat equals the change in enthalpy, which is a state function

2.[3p]
Using $Δ_{f} H^{ominus} = - 824.2$ kJ mol⁻¹ for $F$ and $- 1675.7$ for $A$ , what is $Δ_{r} H^{ominus}$ in kJ mol⁻¹ for $F + 2 Al \to A + 2 Fe$ ?

CorrectNot quite: -851.5

3.[3p]
Bond enthalpies are C to C double $614$ , H to H $436$ , C to C single $347$ , C to H $413$ kJ mol⁻¹. Estimate $Δ_{r} H$ in kJ mol⁻¹ for $+ \to$ .

CorrectNot quite: -123

4.[1p]
A bomb calorimeter measures $Δ H$ directly, because the reaction happens at the pressure of the laboratory.

True

The answer is: False

False

Correct

5.[2p]
Bond enthalpies predict the hydrogenation of benzene to be about $150$ kJ mol⁻¹ more exothermic than it is. Why?

The ring is delocalised and so more stable than three isolated double bonds

Correct

Mean bond enthalpies are measured only for liquids

The answer is: The ring is delocalised and so more stable than three isolated double bonds

Benzene has more hydrogen atoms than the table assumes

The answer is: The ring is delocalised and so more stable than three isolated double bonds

Hess's law fails for cyclic molecules

The answer is: The ring is delocalised and so more stable than three isolated double bonds

6.[3p]
Mixing $50.0$ mL of $1.00$ mol dm⁻³ acid with $50.0$ mL of $1.00$ mol dm⁻³ base gives $100.0$ g of solution whose temperature rises by $6.8$ K. Taking $c = 4.18$ J g⁻¹ K⁻¹, what is $Δ H$ of neutralisation in kJ per mole of water formed?

CorrectNot quite: -56.8

7.[3p]
For $+ 3 \to 2 N$ , $Δ_{r} H^{ominus} (298) = - 91.8$ kJ mol⁻¹ and $Δ_{r} C_{p} = - 45.5$ J K⁻¹. Using Kirchhoff's law, what is $Δ_{r} H$ at $500$ K, in kJ mol⁻¹?

CorrectNot quite: -101.0

8.[3p]
Which of these processes are endothermic and happen anyway?

Select all that apply

Ammonium nitrate dissolving in water

Correct

Ice melting in a room at 10 degrees Celsius

Correct

Barium hydroxide octahydrate reacting with ammonium thiocyanate

Correct

Methane burning in air

The answer is: Ammonium nitrate dissolving in water, Ice melting in a room at 10 degrees Celsius, Barium hydroxide octahydrate reacting with ammonium thiocyanate

9.[2p]
Match each quantity to what it is for.

Formation enthalpies
Mean bond enthalpies
Kirchhoff's law
Bomb calorimeter

the same reaction at another temperature
a gas phase estimate and its reason
the internal energy change
the accurate reaction enthalpy

Show the answer

Formation enthalpies: the accurate reaction enthalpy Mean bond enthalpies: a gas phase estimate and its reason Kirchhoff's law: the same reaction at another temperature Bomb calorimeter: the internal energy change

1.[2p] Why is the heat of a reaction in an open flask a property of the reaction rather than of the apparatus?

2.[3p] Using ΔfHominus=-824.2 kJ mol⁻¹ for F and -1675.7 for A, what is ΔrHominus in kJ mol⁻¹ for F+2Al→A+2Fe?

3.[3p] Bond enthalpies are C to C double 614, H to H 436, C to C single 347, C to H 413 kJ mol⁻¹. Estimate ΔrH in kJ mol⁻¹ for +→.

4.[1p] A bomb calorimeter measures ΔH directly, because the reaction happens at the pressure of the laboratory.

5.[2p] Bond enthalpies predict the hydrogenation of benzene to be about 150 kJ mol⁻¹ more exothermic than it is. Why?

6.[3p] Mixing 50.0 mL of 1.00 mol dm⁻³ acid with 50.0 mL of 1.00 mol dm⁻³ base gives 100.0 g of solution whose temperature rises by 6.8 K. Taking c=4.18 J g⁻¹ K⁻¹, what is ΔH of neutralisation in kJ per mole of water formed?

7.[3p] For +3→2N, ΔrHominus(298)=-91.8 kJ mol⁻¹ and ΔrCp=-45.5 J K⁻¹. Using Kirchhoff's law, what is ΔrH at 500 K, in kJ mol⁻¹?

8.[3p] Which of these processes are endothermic and happen anyway?

9.[2p] Match each quantity to what it is for.

1.[2p]
Why is the heat of a reaction in an open flask a property of the reaction rather than of the apparatus?

2.[3p]
Using $Δ_{f} H^{ominus} = - 824.2$ kJ mol⁻¹ for $F$ and $- 1675.7$ for $A$ , what is $Δ_{r} H^{ominus}$ in kJ mol⁻¹ for $F + 2 Al \to A + 2 Fe$ ?

3.[3p]
Bond enthalpies are C to C double $614$ , H to H $436$ , C to C single $347$ , C to H $413$ kJ mol⁻¹. Estimate $Δ_{r} H$ in kJ mol⁻¹ for $+ \to$ .

4.[1p]
A bomb calorimeter measures $Δ H$ directly, because the reaction happens at the pressure of the laboratory.

5.[2p]
Bond enthalpies predict the hydrogenation of benzene to be about $150$ kJ mol⁻¹ more exothermic than it is. Why?

6.[3p]
Mixing $50.0$ mL of $1.00$ mol dm⁻³ acid with $50.0$ mL of $1.00$ mol dm⁻³ base gives $100.0$ g of solution whose temperature rises by $6.8$ K. Taking $c = 4.18$ J g⁻¹ K⁻¹, what is $Δ H$ of neutralisation in kJ per mole of water formed?

7.[3p]
For $+ 3 \to 2 N$ , $Δ_{r} H^{ominus} (298) = - 91.8$ kJ mol⁻¹ and $Δ_{r} C_{p} = - 45.5$ J K⁻¹. Using Kirchhoff's law, what is $Δ_{r} H$ at $500$ K, in kJ mol⁻¹?

8.[3p]
Which of these processes are endothermic and happen anyway?

9.[2p]
Match each quantity to what it is for.