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Chemical Energetics - Energy level diagrams

Grade 11A LevelChemistry

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

🔑Concepts

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Energy Level Diagrams (Enthalpy Profile Diagrams) visually represent the relative energy levels of reactants and products during a chemical reaction. The y-axis represents enthalpy (HH) and the x-axis represents the progress of the reaction.

Basic axes for an energy level diagram showing Enthalpy vs Progress of Reaction.
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In an Exothermic reaction, the enthalpy of the products is lower than the enthalpy of the reactants (Hproducts<HreactantsH_{products} < H_{reactants}). Energy is released to the surroundings, resulting in a negative enthalpy change (ΔH<0\Delta H < 0).

Exothermic energy level diagram showing products lower than reactants.
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In an Endothermic reaction, the enthalpy of the products is higher than the enthalpy of the reactants (Hproducts>HreactantsH_{products} > H_{reactants}). Energy is absorbed from the surroundings, resulting in a positive enthalpy change (ΔH>0\Delta H > 0).

Endothermic energy level diagram showing products higher than reactants.
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Activation Energy (EaE_a) is the minimum energy required for a reaction to occur. On an energy level diagram, it is represented by the 'hump' or the vertical distance from the energy of the reactants to the peak of the curve.

Diagram highlighting activation energy (Ea) as the height from reactant level to peak.

📐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.

Problem 3:

Draw the energy level diagram for the thermal decomposition of calcium carbonate (CaCO3→CaO+CO2CaCO_3 \rightarrow CaO + CO_2), which is an endothermic reaction with ΔH=+178 kJ/mol\Delta H = +178\text{ kJ/mol}. Label the reactants, products, and ΔH\Delta H.

Endothermic energy level diagram for Calcium Carbonate decomposition.

Solution:

  1. Identify that the reaction is endothermic, so the product level must be higher than the reactant level.
  2. Draw the reactant line (CaCO3CaCO_3) at a lower energy level.
  3. Draw the product line (CaO+CO2CaO + CO_2) at a higher energy level.
  4. Connect them with a curve showing the activation energy.
  5. Mark the difference between the levels as ΔH=+178 kJ/mol\Delta H = +178\text{ kJ/mol}.

Explanation:

Because heat is absorbed from the surroundings to break the bonds in calcium carbonate, the internal energy (enthalpy) of the system increases.

Problem 4:

The reaction between hydrogen and oxygen to form water (2H2+O2→2H2O2H_2 + O_2 \rightarrow 2H_2O) releases 484 kJ484\text{ kJ} of energy. Represent this as an energy level diagram showing the enthalpy change.

Exothermic energy level diagram for the formation of water.

Solution:

  1. Identify that 'releasing energy' means the reaction is exothermic.
  2. Draw the reactant line (2H2+O22H_2 + O_2) at a higher energy level.
  3. Draw the product line (2H2O2H_2O) at a lower energy level.
  4. Represent the enthalpy change as ΔH=−484 kJ/mol\Delta H = -484\text{ kJ/mol}.

Explanation:

Energy is released when the new bonds in water are formed, resulting in products that have less chemical potential energy than the reactants.

Energy level diagrams Grade 11 Notes & Examples