Energy Profiles and Bond Theory

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Energy Profiles and Bond Theory — Free MYP5 Chemistry Practice Questions

1QuestionInterpreting energy diagrams (peaks, energy levels)Concept Practice

4 marks~6 minCriterion A

The energy profile below represents a chemical reaction.

Diagram description: Reactants at

50 \text{ kJ}

, transition state (peak) at

120 \text{ kJ}

, products at

-30 \text{ kJ}

. The x-axis is labelled "Reaction progress" and the y-axis "Potential energy (kJ)". Arrow A runs from reactants to the peak; Arrow B runs from the peak to products; Arrow C runs from reactants to products.

Identify which arrow (A, B, or C) represents the activation energy of this reaction. [1]

Deduce the overall energy change of the reaction, in kJ, and state whether energy is released or absorbed. [1]

The products of this reaction are described as thermodynamically more stable than the reactants. Using values from the diagram, explain how the energy profile supports this claim. [2]

Solutions

2QuestionInterpreting energy diagrams (peaks, energy levels)Assessment Practice

5 marks~8 minCriterion C

An energy diagram for a reaction shows reactant energy at

+50\ \text{kJ}

, product energy at

+20\ \text{kJ}

, and a peak energy at

+110\ \text{kJ}

. A student proposes: "Catalyst concentration inversely correlates with activation energy."

Experimental data collected at varying catalyst concentrations:

Catalyst concentration (M)	0.0	0.1	0.2
Measured $E_a$ (kJ)	60	45	35

Calculate the activation energy

E_a

from the energy diagram. [1]

Deduce what the energy diagram represents in terms of the experimental conditions, using your calculated

E_a

and the data table. [2]

Evaluate whether the experimental data supports or refutes the student's hypothesis, justifying your answer with specific values. [2]

Solutions

3QuestionEnergy profile diagrams (reactants → products)Assessment Practice

8 marks~12 minCriterion D

The catalytic decomposition of hydrogen peroxide,

\text{H}_2\text{O}_2 \rightarrow \text{H}_2\text{O} + \tfrac{1}{2}\text{O}_2

, has an enthalpy change of

\Delta H = -98 \text{ kJ mol}^{-1}

and an activation energy of

E_a = 75 \text{ kJ mol}^{-1}

. Without a catalyst, the reaction requires heating to approximately

150^\circ\text{C}

before products form at a measurable rate, despite the overall energy change being negative.

Explain the roles of

\Delta H

and

E_a

as shown on an energy profile diagram for this reaction. [2]

Apply collision theory to explain why heating to

150^\circ\text{C}

is necessary for the reaction to proceed at a measurable rate. [2]

Analyse the thermodynamic feasibility of this reaction and evaluate whether thermodynamic feasibility alone is sufficient to predict whether the reaction will occur under standard laboratory conditions. [4]

Solutions

4QuestionEnergy profile diagrams (reactants → products)Assessment Practice

3 marks~5 minCriterion B

The table below shows activation energy (

E_a

) and enthalpy change (

\Delta H

) for four reactions.

Reaction	$E_a$ (kJ/mol)	$\Delta H$ (kJ/mol)
Combustion of methane	265	−890
Decomposition of calcium carbonate	175	+178
Neutralisation of HCl with NaOH	10	−57
Photosynthesis	500	+2800

Deduce whether each reaction is endothermic or exothermic. [1]

Explain how the sign of

\Delta H

determines the classification of a reaction as endothermic or exothermic. [1]

Analyse the relationship between

E_a

and reaction type (endothermic or exothermic) using the data provided. [1]

Solutions

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