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Chemistry - Chemical Reactions (Exothermic and Endothermic)

Grade 8Cambridge (IGCSE)

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

🔑Concepts

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Exothermic reactions transfer thermal energy to the surroundings. During these reactions, the temperature of the surroundings increases. The overall energy change ΔH\Delta H is negative.

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Endothermic reactions absorb thermal energy from the surroundings. During these reactions, the temperature of the surroundings decreases. The overall energy change ΔH\Delta H is positive.

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Bond breaking is an endothermic process as it requires energy to overcome the forces of attraction between atoms.

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Bond making is an exothermic process as energy is released when new chemical bonds are formed.

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Activation Energy (EaE_a) is the minimum amount of energy required for a collision between particles to result in a chemical reaction.

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Reaction Profiles: In exothermic reactions, the reactants have more energy than the products (Ereactants>EproductsE_{reactants} > E_{products}). In endothermic reactions, the products have more energy than the reactants (Eproducts>EreactantsE_{products} > E_{reactants}).

📐Formulae

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=m×c×ΔTQ = m \times c \times \Delta T

ΔHreaction=Hproducts−Hreactants\Delta H_{reaction} = H_{products} - H_{reactants}

💡Examples

Problem 1:

When 25 cm325\text{ cm}^3 of NaOHNaOH is added to 25 cm325\text{ cm}^3 of HClHCl, the temperature rises from 21.5∘C21.5^\circ C to 28.0∘C28.0^\circ C. Classify this reaction and explain why.

Solution:

The reaction is Exothermic.

Explanation:

The increase in temperature from 21.5∘C21.5^\circ C to 28.0∘C28.0^\circ C (a ΔT\Delta T of +6.5∘C+6.5^\circ C) indicates that thermal energy has been released from the chemical system into the surroundings (the solution).

Problem 2:

In the reaction H2+Cl2→2HClH_2 + Cl_2 \rightarrow 2HCl, the energy required to break 1 mol1\text{ mol} of H−HH-H bonds is 436 kJ436\text{ kJ} and 1 mol1\text{ mol} of Cl−ClCl-Cl bonds is 243 kJ243\text{ kJ}. The energy released forming 1 mol1\text{ mol} of H−ClH-Cl bonds is 432 kJ432\text{ kJ}. Calculate the total energy change.

Solution:

ΔH=(436+243)−(2×432)=679−864=−185 kJ/mol\Delta H = (436 + 243) - (2 \times 432) = 679 - 864 = -185\text{ kJ/mol}

Explanation:

To calculate the energy change, we subtract the energy released during bond making from the energy taken in during bond breaking. Since the result is −185 kJ/mol-185\text{ kJ/mol}, the reaction is exothermic.

Problem 3:

Describe the thermal decomposition of Calcium Carbonate (CaCO3→CaO+CO2CaCO_3 \rightarrow CaO + CO_2) in terms of energy.

Solution:

Endothermic reaction.

Explanation:

Thermal decomposition requires a constant input of heat energy to break the strong ionic bonds within the CaCO3CaCO_3 lattice. Because energy is absorbed, the products have higher chemical energy than the reactants.