Question

physical science/control test 7 nw/september 2020
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 the of 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 amount of energy that reactant molecules must possess in order to undergo a chemical reaction.

4.2

Step1: Analyze heat of reaction value

The heat of the reaction $\Delta H=- 7.28\ kJ\ mol^{-1}$. Since $\Delta H < 0$, the reaction is exothermic.

Step2: State the reason

In an exothermic reaction, the products have less energy than the reactants, and heat is released to the surroundings.

4.3

Step1: Recall the formula

The formula for the heat of the reaction is $\Delta H=H_{products}-H_{reactants}$.

Step2: Rearrange the formula to solve for $H_{products}$

$H_{products}=\Delta H + H_{reactants}$.

Step3: Substitute the given values

$H_{reactants}=26.6\ kJ\ mol^{-1}$ and $\Delta H=-7.28\ kJ\ mol^{-1}$. So $H_{products}=-7.28 + 26.6=19.32\ kJ\ mol^{-1}$.

4.4

When drawing the potential - energy versus course of reaction graph:

• Mark the heat of reactants ($H_{reactants}=26.6\ kJ\ mol^{-1}$) on the y - axis at the start of the reaction.
• The heat of products ($H_{products}=19.32\ kJ\ mol^{-1}$) is marked on the y - axis at the end of the reaction.
• The activation energy ($E_a = 6.73\ kJ\ mol^{-1}$) is the energy difference between the reactants and the activated complex. The energy of the activated complex is $26.6 + 6.73=33.33\ kJ\ mol^{-1}$. Draw a curve starting from the reactants, rising to the activated complex and then falling to the products.

4.5

When a catalyst is added:

• A catalyst lowers the activation energy of the reaction. On the same graph, draw a dotted - line curve that starts and ends at the same points as the original curve (representing the reactants and products), but has a lower peak (lower activation energy).

Answer:

4.1: Activation energy is the minimum energy required for reactant molecules to react.
4.2: Exothermic. Reason: $\Delta H=-7.28\ kJ\ mol^{-1}<0$, heat is released.
4.3: $19.32\ kJ\ mol^{-1}$
4.4: Graph with $H_{reactants}=26.6\ kJ\ mol^{-1}$, $H_{products}=19.32\ kJ\ mol^{-1}$, energy of activated complex $33.33\ kJ\ mol^{-1}$.
4.5: Dotted - line curve on the same graph with lower activation energy.