Question

physical science/control test 7 nw/september 2022
question 4
consider the decomposition reaction of dinitrogen pentoxide:
2 n₂o₅(g) → 2 n₂o₄(g) + o₂(g)
the table below shows the different energies for the above reaction.

heat of the reactants (hr)	26.6 kj·mol⁻¹
activation energy (ea)	6.73 kj·mol⁻¹
heat of the reaction (δh)	-7.28 kj·mol⁻¹

4.1 define the term activation energy.
4.2 is the above reaction endothermic or exothermic? give a reason for the answer.
4.3 calculate the heat of the products.
4.4 draw the potential energy versus course of reaction graph for the above reaction. on the graph indicate values for: heat of reactants (h reactants), heat of products (h products), energy at the activated complex, heat of the reaction (δh)
4.5 on the same graph drawn in question 4.4, use a dotted line and draw the shape of the graph when a catalyst is added to the original reaction.

Explanation:

4.1 Activation energy is the minimum energy that reacting particles must have in order to undergo a chemical reaction.
4.2 The reaction is exothermic. The heat of reaction ($\Delta H$) is -7.28 kJ/mol. A negative $\Delta H$ value indicates that heat is released during the reaction.
4.3 We know that $\Delta H=H_{products}-H_{reactants}$. Rearranging for $H_{products}$, we get $H_{products}=\Delta H + H_{reactants}$. Substituting the given values: $H_{products}=-7.28\ kJ/mol+ 26.6\ kJ/mol = 19.32\ kJ/mol$.
4.4 To draw the potential - energy versus course of reaction graph:

• Mark the heat of reactants ($H_{reactants}=26.6\ kJ/mol$) on the y - axis.
• Since $\Delta H=-7.28\ kJ/mol$, the heat of products ($H_{products}=19.32\ kJ/mol$) is lower than the heat of reactants.
• The activation energy ($E_a = 6.73\ kJ/mol$) is the energy difference between the reactants and the energy at the activated complex. Draw a curve starting from the reactants, rising to the activated complex (with the activation - energy height) and then dropping to the products.

4.5 When a catalyst is added, it lowers the activation energy. On the same graph, draw a dotted line with a lower peak (lower activation energy) than the original solid - line curve, while the heat of reactants and products remain the same.

Answer:

4.1 Activation energy is the minimum energy required for reacting particles to react.
4.2 Exothermic. Reason: $\Delta H=-7.28\ kJ/mol$ (negative value means heat is released).
4.3 $19.32\ kJ/mol$
4.4 Graph should show reactants at 26.6 kJ/mol, products at 19.32 kJ/mol, and activation - energy of 6.73 kJ/mol with a curve connecting them.
4.5 Draw a dotted line on the graph with a lower activation energy peak while reactants and products energies are unchanged.