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Chemical Energetics - Energy level diagrams

Grade 11IGCSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Chemical Energetics involves the study of energy changes during chemical reactions, primarily as heat energy.

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In an Exothermic reaction, heat energy is released to the surroundings. The enthalpy of the products is lower than the enthalpy of the reactants, resulting in a negative enthalpy change: ΔH<0\Delta H < 0.

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In an Endothermic reaction, heat energy is absorbed from the surroundings. The enthalpy of the products is higher than the enthalpy of the reactants, resulting in a positive enthalpy change: ΔH>0\Delta H > 0.

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The Activation Energy (EaE_a) is the minimum energy required for a collision between particles to result in a chemical reaction. On a diagram, it is the energy gap between the reactants and the peak of the energy curve.

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Bond Breaking is an endothermic process (requires energy, +ΔH+\Delta H), while Bond Making is an exothermic process (releases energy, −ΔH-\Delta H).

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An Energy Level Diagram (or Reaction Profile) shows the relative enthalpies of reactants and products and the energy barrier (EaE_a) that must be overcome.

📐Formulae

ΔH=Hproducts−Hreactants\Delta H = H_{products} - H_{reactants}

ΔH=Energy absorbed to break bonds−Energy released to form bonds\Delta H = \text{Energy absorbed to break bonds} - \text{Energy released to form bonds}

Q=mcΔTQ = mc\Delta T

💡Examples

Problem 1:

The combustion of methane is represented by the equation: CH4(g)+2O2(g)→CO2(g)+2H2O(l)CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l) with ΔH=−890 kJ/mol\Delta H = -890\text{ kJ/mol}. Describe the energy level diagram for this reaction.

Solution:

The reactants (CH4+2O2CH_4 + 2O_2) are placed at a higher energy level than the products (CO2+2H2OCO_2 + 2H_2O). A curve rises from the reactant level to a peak (representing EaE_a) and then drops significantly to the product level. The downward arrow from reactants to products indicates ΔH=−890 kJ/mol\Delta H = -890\text{ kJ/mol}.

Explanation:

Because ΔH\Delta H is negative, the reaction is exothermic. Energy is lost to the surroundings, meaning the chemical system loses potential energy, placing products 'lower' on the graph than reactants.

Problem 2:

Calculate the enthalpy change (ΔH\Delta H) for the reaction H2+Cl2→2HClH_2 + Cl_2 \rightarrow 2HCl given the bond energies: H−H=436 kJ/molH-H = 436\text{ kJ/mol}, Cl−Cl=242 kJ/molCl-Cl = 242\text{ kJ/mol}, and H−Cl=431 kJ/molH-Cl = 431\text{ kJ/mol}. Determine if it is endothermic or exothermic.

Solution:

Energy to break bonds=436+242=678 kJ/mol\text{Energy to break bonds} = 436 + 242 = 678\text{ kJ/mol} Energy released making bonds=2×431=862 kJ/mol\text{Energy released making bonds} = 2 \times 431 = 862\text{ kJ/mol} ΔH=678−862=−184 kJ/mol\Delta H = 678 - 862 = -184\text{ kJ/mol}

Explanation:

Since ΔH\Delta H is negative (−184 kJ/mol-184\text{ kJ/mol}), the reaction is exothermic. More energy is released during the formation of H−ClH-Cl bonds than is required to break the H−HH-H and Cl−ClCl-Cl bonds.

Energy level diagrams - Revision Notes & Key Formulas | IGCSE Grade 11 Chemistry