Two reactions occurring in the manufacture of sulphuric acid are shown below: Reaction I: S s + O 2 g ⇌ SO 2 g ; ∆ H ∘ = - 297 kJ , Reaction II: SO 2 g + 1 2 O 2 g ⇌ SO 3 g ; ∆ H ∘ = - 92 kJ . State the name of the term ∆ H ∘ . State with a reason, whether reaction (I) would be accompanied by a decrease or increase in temperature?

Consider the reaction (I), in the manufacture of sulphuric acid, S s + O 2 g ⇌ SO 2 g ; ∆ H ∘ = - 297 kJ , The term ∆ H ∘ is stated as standard enthalpy change of the reaction. This can be defined as the change in enthalpy that occurs when the number of moles of the reactants indicated by the balanced chemical equation have completely reacted at standard conditions. The sign of ∆ H ∘ in the reaction is negative, this indicates that, the reaction is exothermic, and the heat is liberated. Therefore, the reaction is accompanied by an increase in temperature.

Two reactions occurring in the manufacture of sulphuric acid are shown below: Reaction I: S s + O 2 g ⇌ SO 2 g ; ∆ H ∘ = - 297 kJ , Reaction II: SO 2 g + 1 2 O 2 g ⇌ SO 3 g ; ∆ H ∘ = - 92 kJ . Deduce the ∆ H ∘ value of the reaction, S s + 1 1 2 O 2 g → SO 3 g .

Consider the reactions, in the manufacture of sulphuric acid, S s + O 2 g ⇌ SO 2 g ; ∆ H ∘ = - 297 kJ ------(i) SO 2 g + 1 2 O 2 g ⇌ SO 3 g ; ∆ H ∘ = - 92 kJ ------(ii) On adding equations (i) and (ii) we get, S s + 1 1 2 O 2 g ⇌ SO 3 g ; ∆ H ∘ = - 297 + - 92 = - 389 kJ Therefore, ∆ H ∘ = - 389 kJ .

But- 1 -ene gas burns in oxygen to produce carbon dioxide and water vapour according to the following equation: C 4 H 8 + 6 O 2 → 4 CO 2 + 4 H 2 O Use the given data table to calculate the value of ∆ H ∘ for the combustion of But- 1 -ene. Bond C - C C = C C - H O = O C = O O - H Average bond enthalpy ( kJ mol - 1 ) 348 612 412 496 743 463

Consider, the combustion reaction of But- 1 -ene, C 4 H 8 + 6 O 2 → 4 CO 2 + 4 H 2 O . The amount of energy required to break bonds of reactants: 8 × BE C - H + 2 × BE C - c + 1 × BE C = C + 6 × BE O = O = 8 × 412 + 2 × 348 + 1 × 612 + 6 × 496 = 7580 kJ The amount of energy released during bond formation: 4 × 2 × BE C = O + 4 × 2 × BE O - H = 4 × 2 × 743 + 4 × 2 × 463 = 9648 kJ The enthalpy change of the reaction, ∆ H ∘ = ∑ (Bond Enthalpies of bonds broken) - ∑ (Bond Enthalpies of bonds formed) Therefore, ∆ H ∘ = 7580 - 9648 = - 2068 kJ

Given the following data: C s + 2 F 2 g → CF 4 g ; ∆ H 1 = - 680 kJ mol - 1 , F 2 g → 2 F g ; ∆ H 2 = + 158 kJ mol - 1 and C s → C g ; ∆ H 3 = + 715 kJ mol - 1 . Calculate te average bond enthalpy (in kJ mol - 1 ) for the C - F bond.

Consider the given data, C s + 2 F 2 g → CF 4 g ; ∆ H 1 = - 680 kJ mol - 1 -------(i) F 2 g → 2 F g ; ∆ H 2 = + 158 kJ mol - 1 -------(ii) C s → C g ; ∆ H 3 = + 715 kJ mol - 1 --------(iii) On multiplying equation (ii) by 2 , and reversing, we get, 4 F g → 2 F 2 g ; ∆ H 2 = - 2 × 158 kJ mol - 1 --------(iv) C g → C s ; ∆ H 3 = - 715 kJ mol - 1 ----------(v) (On reversing the equation (iii)) Then, after adding equations (i), (iv) and (v), we get the required equation, C g + 4 F g → CF 4 g ; ∆ H = - 680 + - 2 × 158 + - 715 = - 1711 kJ mol - 1 Therefore, average bond enthalpy of C - F bond = - 1711 4 = - 427 . 75 = 428 kJ mol - 1 .

Methanol is made in large quantities as it is used in the production of polymers and in fuels. The enthalpy of combustion of methanol can be determined theoretically or experimentally CH 3 OH l + 1 1 2 O 2 g → CO 2 g + 2 H 2 O g . Using the data from the given table, and determine the theoretical enthalpy of combustion of methanol. Bond C - H C - O O = O C = O O - H Average bond enthalpy ( kJ mol - 1 ) 412 358 496 743 463

Consider, the combustion reaction of methanol, CH 3 OH l + 1 1 2 O 2 g → CO 2 g + 2 H 2 O g . The amount of energy required to break bonds of reactants: 3 × BE C - H + 1 × BE C - O + 1 × BE O - H + 3 2 × BE O = O = 3 × 412 + 1 × 358 + 1 × 463 + 1 . 5 × 496 = 2801 kJ mol - 1 7580 kJ The amount of energy released during bond formation: 2 × BE C = O + 2 × 2 × BE O - H = 2 × 743 + 4 × 463 = 3338 kJ mol - 1 The enthalpy change of the reaction, ∆ H = ∑ (Bond Enthalpies of bonds broken) - ∑ (Bond Enthalpies of bonds formed) Therefore, ∆ H = 2801 - 3338 = - 537 kJ mol - 1

The enthalpy of combustion of methanol can also be determined experimentally in a school laboratory. A burner containing methanol was weighed and used to heat water in a test tube as illustrated in figure. The data shown in table were collected. Initial mass of burner and methanol / g 80 . 557 Final mass of burner and methanol / g 80 . 034 Mass of water in test tube / g 20 . 000 Initial temperature of water / ° C 21 . 5 Final temperature of water ° C 26 . 4 Calculate the amount, in mol , of methanol burnt.

Mass of methanol burnt = Initial mass of burner and methanol - Final mass of burner and methanol = 80 . 557 - 80 . 034 = 0 . 523 g The amount of methanol burnt in mol = Mass of methanol burnt Molecular mass of methanol g mol - 1 = 0 . 523 32 = 0 . 0163 mol .

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The reaction between ethene and hydrogen gas is exothermic. Explain by referring to the bonds in molecule, why the reaction is exothermic?

Important Questions on Energetics and Thermochemistry

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Diploma

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Oxford IB Diploma Programme Chemistry Course Companion>Chapter 5 - Energetics and Thermochemistry>Questions>Q 11

Two reactions occurring in the manufacture of sulphuric acid are shown below:

Reaction I: $S (s) + O_{2} (g) \overset{}{⇌} {SO}_{2} (g); ∆ H^{\circ} = - 297 kJ$ , Reaction II: ${SO}_{2} (g) + \frac{1}{2} O_{2} (g) \overset{}{⇌} {SO}_{3} (g); ∆ H^{\circ} = - 92 kJ$ . State the name of the term $∆ H^{\circ}$ . State with a reason, whether reaction (I) would be accompanied by a decrease or increase in temperature?

EASY

Diploma

IMPORTANT

Oxford IB Diploma Programme Chemistry Course Companion>Chapter 5 - Energetics and Thermochemistry>Questions>Q 11

Two reactions occurring in the manufacture of sulphuric acid are shown below:

Reaction I: $S (s) + O_{2} (g) \overset{}{⇌} {SO}_{2} (g); ∆ H^{\circ} = - 297 kJ$ , Reaction II: ${SO}_{2} (g) + \frac{1}{2} O_{2} (g) \overset{}{⇌} {SO}_{3} (g); ∆ H^{\circ} = - 92 kJ$ . At room temperature, sulphur trioxide, ${SO}_{3}$ is a solid. Deduce with a reason, whether the $∆ H^{\circ}$ value would be more negative or less negative if ${SO}_{3} (s)$ instead of ${SO}_{3} (g)$ were formed in reaction (II).

EASY

Diploma

IMPORTANT

Oxford IB Diploma Programme Chemistry Course Companion>Chapter 5 - Energetics and Thermochemistry>Questions>Q 11

Two reactions occurring in the manufacture of sulphuric acid are shown below:

Reaction I: $S (s) + O_{2} (g) \overset{}{⇌} {SO}_{2} (g); ∆ H^{\circ} = - 297 kJ$ , Reaction II: ${SO}_{2} (g) + \frac{1}{2} O_{2} (g) \overset{}{⇌} {SO}_{3} (g); ∆ H^{\circ} = - 92 kJ$ . Deduce the $∆ H^{\circ}$ value of the reaction, $S (s) + 1 \frac{1}{2} O_{2} (g) \overset{}{\to} {SO}_{3} (g)$ .

MEDIUM

Diploma

IMPORTANT

Oxford IB Diploma Programme Chemistry Course Companion>Chapter 5 - Energetics and Thermochemistry>Questions>Q 12

But- $1$ -ene gas burns in oxygen to produce carbon dioxide and water vapour according to the following equation:

$C_{4} H_{8} + 6 O_{2} \overset{}{\to} 4 {CO}_{2} + 4 H_{2} O$

Use the given data table to calculate the value of $∆ H^{\circ}$ for the combustion of But- $1$ -ene.

Bond	$C - C$	$C = C$	$C - H$	$O = O$	$C = O$	$O - H$
Average bond enthalpy ( $kJ {mol}^{- 1}$ )	$348$	$612$	$412$	$496$	$743$	$463$

EASY

Diploma

IMPORTANT

Oxford IB Diploma Programme Chemistry Course Companion>Chapter 5 - Energetics and Thermochemistry>Questions>Q 12

But- $1$ -ene gas burns in oxygen to produce carbon dioxide and water vapour according to the following equation:

$C_{4} H_{8} + 6 O_{2} \overset{}{\to} 4 {CO}_{2} + 4 H_{2} O$

The value of $∆ H^{\circ}$ for the combustion of But- $1$ -ene is calculated using the given data table, and is found to be $- 2068 kJ$ . State and explain whether the given reaction is endothermic or exothermic?

Bond	$C - C$	$C = C$	$C - H$	$O = O$	$C = O$	$O - H$
Average bond enthalpy ( $kJ {mol}^{- 1}$ )	$348$	$612$	$412$	$496$	$743$	$463$

MEDIUM

Diploma

IMPORTANT

Oxford IB Diploma Programme Chemistry Course Companion>Chapter 5 - Energetics and Thermochemistry>Questions>Q 13

Given the following data:

$C (s) + 2 F_{2} (g) \overset{}{\to} {CF}_{4} (g); ∆ H_{1} = - 680 kJ {mol}^{- 1}$ , $F_{2} (g) \overset{}{\to} 2 F (g); ∆ H_{2} = + 158 kJ {mol}^{- 1}$ and $C (s) \overset{}{\to} C (g); ∆ H_{3} = + 715 kJ {mol}^{- 1}$ . Calculate te average bond enthalpy (in $kJ {mol}^{- 1}$ ) for the $C - F$ bond.

MEDIUM

Diploma

IMPORTANT

Oxford IB Diploma Programme Chemistry Course Companion>Chapter 5 - Energetics and Thermochemistry>Questions>Q 14

Methanol is made in large quantities as it is used in the production of polymers and in fuels. The enthalpy of combustion of methanol can be determined theoretically or experimentally ${CH}_{3} OH (l) + 1 \frac{1}{2} O_{2} (g) \overset{}{\to} {CO}_{2} (g) + 2 H_{2} O (g)$ .

Using the data from the given table, and determine the theoretical enthalpy of combustion of methanol.

Bond	$C - H$	$C - O$		$O = O$	$C = O$	$O - H$
Average bond enthalpy ( $kJ {mol}^{- 1}$ )	$412$	$358$		$496$	$743$	$463$

EASY

Diploma

IMPORTANT

Oxford IB Diploma Programme Chemistry Course Companion>Chapter 5 - Energetics and Thermochemistry>Questions>Q 14

The enthalpy of combustion of methanol can also be determined experimentally in a school laboratory. A burner containing methanol was weighed and used to heat water in a test tube as illustrated in figure.

Question Image

The data shown in table were collected.

Initial mass of burner and methanol / $g$	$80.557$
Final mass of burner and methanol / $g$	$80.034$
Mass of water in test tube / $g$	$20.000$
Initial temperature of water / $° C$	$21.5$
Final temperature of water $° C$	$26.4$

Calculate the amount, in

mol

, of methanol burnt.

The reaction between ethene and hydrogen gas is exothermic. Explain by referring to the bonds in molecule, why the reaction is exothermic?

Important Questions on Energetics and Thermochemistry

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