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Energetics of Reactions - Reaction Profiles

Grade 9IB

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

🔑Concepts

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Exothermic Reactions: Reactions that release energy to the surroundings, usually in the form of heat. The temperature of the surroundings increases. For these reactions, the enthalpy change is negative (ΔH<0)(\Delta H < 0).

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Endothermic Reactions: Reactions that absorb energy from the surroundings. The temperature of the surroundings decreases. For these reactions, the enthalpy change is positive (ΔH>0)(\Delta H > 0).

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Activation Energy (Ea)(E_{a}): The minimum amount of energy required for a chemical reaction to occur. On a reaction profile, it is the energy difference between the reactants and the peak of the curve.

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Enthalpy Change (ΔH)(\Delta H): The difference between the energy of the products and the energy of the reactants. It represents the overall energy change in the system.

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Reaction Profile Diagrams: Visual representations showing the energy levels of reactants and products over the course of a reaction. In an exothermic profile, the product level is lower than the reactant level. In an endothermic profile, the product level is higher.

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Bond Energetics: Breaking chemical bonds is an endothermic process (requires energy, +Value+\text{Value}), while forming new bonds is an exothermic process (releases energy, −Value-\text{Value}).

📐Formulae

ΔH=Eproducts−Ereactants\Delta H = E_{\text{products}} - E_{\text{reactants}}

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

Ea=Etransition state−EreactantsE_{a} = E_{\text{transition state}} - E_{\text{reactants}}

💡Examples

Problem 1:

A reaction has a total energy of reactants equal to 250 kJ/mol250 \text{ kJ/mol} and the total energy of the products is 100 kJ/mol100 \text{ kJ/mol}. Calculate the enthalpy change (ΔH)(\Delta H) and determine if the reaction is exothermic or endothermic.

Solution:

ΔH=Eproducts−EreactantsΔH=100−250ΔH=−150 kJ/mol\begin{array}{r} \Delta H = E_{\text{products}} - E_{\text{reactants}} \\ \Delta H = 100 - 250 \\ \Delta H = -150 \text{ kJ/mol} \end{array}

Explanation:

The enthalpy change is negative (−150 kJ/mol-150 \text{ kJ/mol}), which indicates that energy has been released to the surroundings. Therefore, the reaction is exothermic.

Problem 2:

In a specific reaction, the activation energy is 80 kJ/mol80 \text{ kJ/mol} and the enthalpy change is +30 kJ/mol+30 \text{ kJ/mol}. If the reactants start at 120 kJ/mol120 \text{ kJ/mol}, calculate the energy level of the products and the peak energy of the transition state.

Solution:

Product Energy=120+30=150 kJ/mol\text{Product Energy} = 120 + 30 = 150 \text{ kJ/mol} Peak Energy=120+80=200 kJ/mol\text{Peak Energy} = 120 + 80 = 200 \text{ kJ/mol}

Explanation:

Since ΔH\Delta H is positive, we add it to the reactant energy to find the product level (150 kJ/mol150 \text{ kJ/mol}). The activation energy is the height from the reactants to the peak, so we add 8080 to the reactant energy to find the peak at 200 kJ/mol200 \text{ kJ/mol}.