Chemical Energy Hess’s Law

1. 1.           What is the relationship between a, b, c and d?  Answer in the form a =

S_(s)  +  H_2(g)  ®  H₂S_(g)                                 ΔH = a
H_2(g)  + ½ O_{2 (g)} ®  H₂O_(l)                           ΔH = b
S_(s)  +  O_2(g)  ®  SO_2(g)                                 ΔH = c
H₂S_(g) + 1 ½  O_2(g)  ®  H₂O_(l) +SO_2(g)         ΔH = d

2.        The enthalpy changes for the formation of one mole of aluminium oxide and one mole of             iron(III) oxide are shown below.

2Al(s)    +     1½O₂(g)          à       Al₂O₃(s)         ΔH = -1676 kJ mol^-1

2Fe(s)    +     1½O₂(g)         à       Fe₂O₃(s)         ΔH = -825 kJ mol^-1

Use the above information to calculate the enthalpy change for the reaction:

2Al(s)     +      Fe₂O₃(s)        à           Al₂O₃(s)      +       2Fe(s)

3.        The equation for the enthalpy of formation of propanone is:

3C(s)   +   3H₂(g)   +   ½O₂(g)                    C₃H₆O(l)

Use the following information on enthalpies of combustion to calculate the enthalpy of formation of propanone.

            C(s)   +   O₂(g)                 CO₂(g)                                ΔH = -394 kJmol^-1

H₂(g)   +   ½O₂(g)                 H₂O(l)                                ΔH = -286 kJmol^-1

C₃H₆O(l)    +    4O₂(g)               3CO₂(g)   +   3H₂O(l)     ΔH = -1804 kJmol^-1

4.        The equation below represents the hydrogenation of ethene to ethane.

C₂H₄(g)    +    H₂(g)              →       C₂H₆(g)

Use the enthalpies of combustion of ethene, hydrogen and ethane from page 9 of the data booklet to calculate the enthalpy change for the above reaction.

5.        Calculate a value for the enthalpy change involved in the formation of one mole of hydrogen peroxide from water (ΔH₃). The enthalpy change when hydrogen forms hydrogen peroxide is -188 kJ mol^-1  and the enthalpy of combustion of hydrogen to form water is -286 kJ mol^-1.

6.        Calculate a value for the enthalpy change involved in the decomposition of nitrogen dioxide to nitrogen monoxide given the following information.

Equation (a)                         N₂(g) + O₂(g)    à    2NO(g)         ΔH = +181 kJ

Equation (b)                         N₂(g) + 2O₂(g)    à   2NO₂(g)      ΔH = +68 kJ

7. The sulphur-iodine cycle is an industrial process used to manufacture hydrogen. There are three steps in the sulphur-iodine cycle.

Step 1:                      I₂ + SO₂ + 2H₂O → 2HI + H₂SO₄

Step 2:                      2HI → I₂ + H₂

Step 3:                      H₂SO₄ → SO₂ + H₂O + ½O₂

(i) Why does step 3 help to reduce the cost of manufacturing hydrogen?

(ii) What is the overall equation for the sulphur-iodine cycle?

 

8.        The enthalpy of formation of glycerol is the enthalpy change for the reaction:

3C(s) +   4H₂(g)   + 1½O₂(g)   →   C₃H₈O₃(ℓ)

(graphite)

Calculate the enthalpy of formation of glycerol, in kJ mol^–1, using information from the data booklet and the following data.

C₃H₈O₃(ℓ)   +   3½O₂(g) →   3CO₂(g) +   4H₂O(ℓ)                    ΔH = – 1654 kJ mol^–1

Using the data above, calculate the enthalpy change, in kJ mol^–1, for the production of butan-2-ol by hydration of but-2-ene.

 

9.        Enthalpy changes can also be calculated using Hess’s Law.

The enthalpy of formation for pentan-1-ol is shown below.

5C(s) + 6H₂(g) + O₂(g) → C₅H₁₁OH( ℓ )             ΔH = –354 kJ mol^–1

Using this value, and the enthalpies of combustion of carbon and hydrogen from the data booklet, calculate the enthalpy of combustion of pentan-1-ol, in kJ mol^–1.

10. Given that

The enthalphy change for the reaction will be

11. The three equations shown below all involve displacement reactions of metals and metal oxides

What is the relationship between A, B and C according to Hess’s Law

12. Given the equations

then according to Hess’sLaw

13. The equation for the enthapy of formation of ethyne is

Use the values for the enthalphy of combustion of ethyne, carbon and hydrogen given in the data booklet to calculate the enthalphy of formation of ethyne.