Module 3  

Lesson 3.4  Chemical Potential Energy Diagrams



Key Concepts


In Lesson 1, you learned about energy pathway diagrams. However, when communicating enthalpy change, simpler diagrams are often used. These diagrams, called chemical potential energy diagrams, do not show the activation energy and neither axis is labelled with numbers. Rather, these diagrams show the relative potential energy of both the reactants and products. In an endothermic reaction, the products possess more potential energy than the reactants have; in an exothermic reaction, the products possess less potential energy than the reactants have.

Chemical potential energy diagrams indicate the sign and magnitude of the enthalpy change. The enthalpy change of a reaction is the difference between the chemical potential energy of the reactants and the chemical potential energy of the products and is represented by ∆rH , or simply  ∆H . The basic format of chemical potential energy diagrams is shown below.


Fig. 1


Fig. 2

Learning Tip

When asked to determine whether a reaction is exothermic or endothermic, think of the energy change in a reaction from the perspective of the reactants. Are the reactants releasing or absorbing energy? Read carefully! The question may also ask you about the reverse reaction!

Watch



  Read the section in your textbook entitled "Method 4: Chemical Potential Energy Diagrams" (pages 498-499 in your text). 

Check Your Understanding


Complete Section 11.3 Questions 1 to 8 on page 501 of the textbook. Check your work by clicking the banner below.

Page 501 Section 11.3 Question 1
  1. The symbols used represent the following:
    • Δ = change
    • C = combustion
    • H = enthalpy
    • M = molar
    • ° = standard conditions

    Therefore, the symbols represent the molar enthalpy change for a combustion reaction involving methane at standard conditions.

  2. The symbols used represent the following:
    • n = amount
    • Δ = change
    • H = enthalpy
    • M = molar
    • ° = standard conditions

    Therefore, the symbols represent the moles of propane involved in a chemical process multiplied by its molar enthalpy of reaction.

  3. The thermal energy change for water

Page 501 Section 11.3 Question 2

  1. \( \mathrm { \Delta_fH_m^{\circ} = +227.4 \frac{kJ}{mol} } \)

    \( \mathrm { 2C(s) + H_2(g) \rightarrow C_2H_2(g) } \)
    \( \mathrm { \Delta_fH_m^{\circ} = +227.4 \frac{kJ}{mol} } \)

    \( \mathrm { 2C(s) + H_2(g) + 227.4 kJ \rightarrow C_2H_2(g) } \)





  2. \( \mathrm { \Delta_{sd}H_m^{\circ} = +1675.7 kJ/mol } \)

    \( \mathrm { Al_2O_3(s) \rightarrow 2Al(s) + \frac{3}{2}O_2(g) } \)
    \( \mathrm { \Delta_{sd}H_m^{\circ} = +1675.7 kJ/mol } \)

    \( \mathrm { Al_2O_3(s)+ 1675.7~kJ \rightarrow 2Al(s) + \frac{3}{2}O_2(g) } \)





  3. \( \mathrm { \Delta_cH_m^{\circ} = -393.5 \frac{kJ}{mol} } \)

    \( \mathrm { C(s) + O_2(g) \rightarrow CO_2(g) } \)
    \( \mathrm { \Delta_cH_m^{\circ} = -393.5 \frac{kJ}{mol} } \)

    \( \mathrm { C(s) + O_2(g) \rightarrow CO_2(g) + 393.5 kJ } \)





Page 501 Section 11.3 Question 3

  1. \( \mathrm { \Delta_cH_m^{\circ} = -241.8 \frac{kJ}{mol}~for~hydrogen } \)

  2. \( \mathrm { \Delta_cH_m^{\circ} = -318.0 \frac{kJ}{mol}~for~ammonia } \)

  3. \( \mathrm { \Delta_cH_m^{\circ} = +81.6\frac{kJ}{mol}~for~nitrogen } \)

  4. \( \mathrm { \Delta_cH_m^{\circ} = -372.8\frac{kJ}{mol}~for~iron } \)


Page 501 Section 11.3 Question 4

  1. \( \mathrm { \Delta_rH = -114~kJ } \)

  2. \( \mathrm { H_2SO_4(aq) + 2NaOH(aq) \rightarrow Na_2SO_4(aq) + 2H_2O(l)~~~~~\Delta_rH = -114~kJ } \)

  3. \( \mathrm { \Delta_rH = \frac{-114~kJ}{1~mol} = \frac{-114~kJ}{mol~of~sulfuric~acid} } \)

  4. \( \mathrm { \Delta_rH = \frac{-114~kJ}{2~mol} = \frac{-57~kJ}{mol~of~aqueous~sodium~hydroxide} } \)


Page 501 Section 11.3 Question 5

  1. \( \mathrm { CH_3OH(l) + \frac{3}{2}O_2(g) \rightarrow CO_2(g) + 2H_2O(g) + 725.9 \frac{kJ}{mol} } \)

  2. \( \mathrm { C(s) + \frac{1}{4}S_8(s) + 89.0~kJ \rightarrow CS_2(l) } \)

  3. \( \mathrm { ZnS(s) + \frac{3}{2}O_2(g) \rightarrow ZnO(s) + SO_2(g) + 441.3 kJ } \)

  4. \( \mathrm { Fe_2O_3(s) + 824.2 kJ \rightarrow 2Fe(s) + \frac{3}{2}O_2(g) } \)


Page 501 Section 11.3 Question 6
  1. \( \mathrm { C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(g)~~~~~\Delta_cH = -2043.9~kJ } \)

    \( \mathrm { C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(g) + 2043.9~kJ } \)

  2. \( \mathrm { \frac{1}{2}N_2(g) + \frac{1}{2}O_2(g) \rightarrow NO(g)~~~~~~\Delta_fH = +91.3~kJ } \)

    \( \mathrm { \frac{1}{2}N_2(g) + \frac{1}{2}O_2(g) + 91.3~kJ \rightarrow NO(g) } \)

  3. \( \mathrm { C_2H_5OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(g)~~~~~\Delta_cH = -1234.8~kJ } \)

    \( \mathrm { C_2H_5OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(g) + 1234.8~kJ } \)


Page 501 Section 11.3 Question 7
  1. \( \mathrm { H_2(g) + \frac{1}{2}O_2(g) \rightarrow H_2O(g)~~~~~\Delta_cH = -285.8 kJ } \)

    \( \mathrm { H_2O(g) \rightarrow H_2(g) + \frac{1}{2}O_2(g)~~~~~\Delta_{sd}H = +285.8 kJ } \)

  2. The enthalpy changes are identical, except that the combustion reaction is exothermic and the decomposition reaction is endothermic.

\( \mathrm { \Delta_rH_{(reverse)} = -\Delta_rH_{(forward)} } \)