1000 Questions Hess Cycle

Hess cycle

Notes on Hess’s Cycle

Question 1

$S_{(rhombic)}+O_2\rightarrow SO_2\: \: \: \: \: \: \Delta H: -297 kJ\: mol^-^1$

$S_{(monoclinic)}+O_2\rightarrow SO_2\: \: \: \: \: \: \Delta H: -293.3 kJ\: mol^-^1$

Find the enthalpy change for the allotropic change of sulfur, as express in the following reaction.

$S_{(rhombic)}\rightarrow S_{(monoclinic)}\: \: \: \: \: \: \Delta H: ?\: mol^-^1$

Answer: 3.7kJ (Working)

Question 2

$H_{2(g)}+I_{2(g)}\rightarrow 2HI_{(g)}\: \: \: \: \Delta H=51.9kJ$

$H_{2(g)}+I_{2(s)}\rightarrow 2HI_{(g)}\: \: \: \: \Delta H=-9.2kJ$

Find the heat of sublimation for iodine, as described in the below equation.

$I_{2(s)}\rightarrow I_{2(g)}\: \: \: \: \Delta H=?kJ$

Answer:-60.4kJ (Working)

Question 3

$C_{(s)}+O_2_{(g)}\rightarrow CO_2_{(g)}\: \: \: \: \Delta H: -393.5kJ$

$2CO_{(g)}+O_2_{(g)}\rightarrow 2CO_{2(g)}\: \: \: \: \Delta H: -565.9kJ$

Find the enthalpy of reaction for the following reaction:

$2C_{(s)}+O_2_{(g)}\rightarrow 2CO_{(g)}\: \: \: \: \Delta H: \: ?kJ$

Answer:-221.1kJ (Working)

Question 4

The ΔH_fof AgCl is -127 kJ mol^-1.

If $Ag^+_{(aq)}+Cl^-_{(aq)}\rightarrow AgCl_{(s)} \: \: \: \Delta H:-127kJ \: mol^-^1$ , find the enthalpy change for the below equation:

$Ag_{(s)}+\frac{1}{2}Cl_{2(aq)}+H_2O\rightarrow Ag^+_{(aq)}+Cl^-_{(aq)} \: \: \: \Delta H:\: ?kJ \: mol^-^1$

Answer:7kJ (Working)

Question 5

2-methylpropan-1-ol can also be converted to produce jet and diesel fuel. The first step is the preparation of 2-methylpropene as seen in the following equation:

$C_4H_1_0O_{(l)}\rightarrow C_4H_8_{(g)}+H_2O_{(g)}$

If ΔH_fof C₄H₁₀O is -335 kJ mol^-1, ΔH_fof C₄H₈ is -17 kJ mol^-1and ΔH_fof H₂O is -242 kJ mol^-1 find the ΔH of the reaction of the conversion of 2-methylpropan-1-ol into 2-methylpropene.

Answer:74kJ (Working)

Question 6

The ΔH_c of silane gives SiO₂ and H₂O and releases -1507 kJ mol^-1.

The ΔH_c of silicon releases -911 kJ mol^-1.

The ΔH_c of hydrogen releases -286 kJ mol^-1.

Using the above information, find the ΔH_fof SiH₄.

Answer:24kJ (Working)

Question 7

Some reactions, such as the formation of KClO₃ cannot be measured directly. Using the following equations, find the ΔH_fof KClO₃:

$KClO_3_{(s)}+3Mg_{(s)}\rightarrow KCl_{(s)}+3MgO_{(s)}\: \: \: \Delta H:-1852 kJ\: mol^-^1$

$K_{(s)}+\frac{1}{2}Cl_{2(s)}\rightarrow KCl_{(s)}\: \: \: \Delta H:-437 kJ\: mol^-^1$

$Mg_{(s)}+\frac{1}{2}O_{2(g)}\rightarrow MgO_{(s)}\: \: \: \Delta H:-602 kJ\: mol^-^1$

Answer:-391kJ (Working)

Question 8

B₂H₆ is used as a rocket fuel. Calculate the ΔH_cof B₂H₆if ΔH_fof B₂H₆is -36 kJ mol^-1, ΔH_fof H₂Ois -286 kJ mol^-1and ΔH_fof B₂O₃is -1274 kJ mol^-1

Answer:-2096kJ (Working)

Question 9

The first step in the preparation of methanol is the reaction of methane with water as seen below:

$CH_4_{(g)}+H_2O_{(g)}\rightarrow CO_{(g)}+3H_{2(g)}$

Use the dara below to calculate the entahlpy change of the reaction of methane with water:

$CO_{(g)}+\frac{1}{2}O_{2(g)}\rightarrow CO_{2(g)}\: \: \: \Delta H:-283kJ\: mol^-^1$

$H_{2(g)}+\frac{1}{2}O_{2(g)}\rightarrow H_2O_{(g)}\: \: \: \Delta H:-242kJ\: mol^-^1$

$CH_4{(g)}+2O_{2(g)}\rightarrow CO_{2(g)}+2H_2O_{(g)}\: \: \: \Delta H:-803kJ\: mol^-^1$

Hint: The enthalpy of combustion of water is 0 kJ mol^-1

Answer:206kJ (Working)

Question 10 – Challenging!!

The bombardier beetle can fire a hot, toxic mixture of chemicals when they feel threatened. The mixture containes C₆H₄O₂, a compound hat can be formed using the following equation:

$C_6H_4(OH)_{2(aq)}+H_2O_{2(aq)}\rightarrow C_6H_4O_{2(aq)}+2H_2O_{(l)}$

Use the following data to calculate the enthalpy change for the above reaction:

$C_6H_4(OH)_{2(aq)}\rightarrow C_6H_4O_{2(aq)}+H_2{(g)}\: \: \: \Delta H:\: 177.4kJ\: mol^-^1$

$H_{2(g)}+O_{2(g)}\rightarrow H_2O_{2(aq)}\: \: \: \Delta H:-191.2kJ\: mol^-^1$

$H_{2(g)}+\frac{1}{2}O_{2(g)}\rightarrow H_2O_{(l)}\: \: \: \Delta H:-286kJ\: mol^-^1$

Answer:-203.4kJ (Working)