Use the following energy profile diagram to answer Questions (a) to (c) below. (a) From the diagram, determine the value of the; (i) enthalpy of the reactio...

Answer Details

Reading the diagram. The vertical axis is Energy in \(kJ\,mol^{-1}\), gridded at \(0, 100, 200, 300, 400\). From the plotted levels:

• energy of the reactants (A + B) \(\approx 100\ kJ\,mol^{-1}\);
• energy at the top of the curve (the peak / transition state) \(\approx 330\ kJ\,mol^{-1}\);
• energy of the products (C + D) \(\approx 50\ kJ\,mol^{-1}\).

(a)(i) Enthalpy of the reaction. The enthalpy change is the difference between the energy of the products and that of the reactants:

\[\Delta H = H_{products} - H_{reactants} = 50 - 100 = -50\ kJ\,mol^{-1}\]

The negative sign shows that energy is given out.

(a)(ii) Activation energy of the reaction. The activation energy is measured from the reactant level up to the top of the energy barrier:

\[E_a = E_{peak} - H_{reactants} = 330 - 100 = 230\ kJ\,mol^{-1}\]

(b) Type of reaction. The products lie at a lower energy than the reactants and \(\Delta H\) is negative, so the profile is for an exothermic reaction (energy is released to the surroundings).

(c) Substances providing an alternative pathway. Catalysts. A catalyst provides an alternative reaction pathway of lower activation energy, so the reaction proceeds faster without the catalyst itself being used up.