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Chemistry - Chemical Reactions

Grade 10IGCSE

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

🔑Concepts

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Chemical reactions involve the breaking of bonds in reactants and the formation of new bonds in products, characterized by an enthalpy change ΔH\Delta H.

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Exothermic reactions release thermal energy to the surroundings, resulting in a temperature increase and a negative enthalpy change (ΔH<0\Delta H < 0).

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Endothermic reactions absorb thermal energy from the surroundings, resulting in a temperature decrease and a positive enthalpy change (ΔH>0\Delta H > 0).

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The Rate of Reaction is defined as the change in concentration of a reactant or product per unit time. It is influenced by temperature, concentration/pressure, surface area, and the presence of a catalyst.

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Collision Theory states that for a reaction to occur, particles must collide with sufficient energy (greater than or equal to the Activation Energy, EaE_a) and in the correct orientation.

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Redox reactions involve the transfer of electrons. Oxidation is the loss of electrons (e−e^-) or the gain of oxygen, while Reduction is the gain of electrons or the loss of oxygen (OIL RIG).

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Reversible reactions can proceed in both directions, indicated by the ⇌\rightleftharpoons symbol. Dynamic equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction in a closed system.

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Le Chatelier’s Principle states that if a change is made to the conditions of a system at equilibrium, the system will shift its position to counteract that change.

📐Formulae

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

Rate of Reaction=Amount of reactant used or product formedTime taken\text{Rate of Reaction} = \frac{\text{Amount of reactant used or product formed}}{\text{Time taken}}

Zn(s)+CuSO4(aq)→ZnSO4(aq)+Cu(s) (Displacement)Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s) \text{ (Displacement)}

2H2O2(aq)→MnO22H2O(l)+O2(g) (Catalytic Decomposition)2H_2O_2(aq) \xrightarrow{MnO_2} 2H_2O(l) + O_2(g) \text{ (Catalytic Decomposition)}

N2(g)+3H2(g)⇌2NH3(g) (Haber Process)N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \text{ (Haber Process)}

💡Examples

Problem 1:

Identify the oxidizing agent and the reducing agent in the following reaction: Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g).

Solution:

MgMg is the reducing agent; H+H^+ (from HClHCl) is the oxidizing agent.

Explanation:

In this reaction, MgMg atoms lose electrons to become Mg2+Mg^{2+} ions (Mg→Mg2++2e−Mg \rightarrow Mg^{2+} + 2e^-), so MgMg is oxidized and acts as the reducing agent. H+H^+ ions gain electrons to become H2H_2 gas (2H++2e−→H22H^+ + 2e^- \rightarrow H_2), so H+H^+ is reduced and acts as the oxidizing agent.

Problem 2:

Calculate the average rate of reaction if 50 cm350\text{ cm}^3 of H2H_2 gas is collected in 2020 seconds.

Solution:

Rate=2.5 cm3/sRate = 2.5\text{ cm}^3/s

Explanation:

Using the formula Rate=Volume of gasTimeRate = \frac{\text{Volume of gas}}{\text{Time}}, we calculate 50 cm320 s=2.5 cm3/s\frac{50\text{ cm}^3}{20\text{ s}} = 2.5\text{ cm}^3/s.

Problem 3:

Predict the shift in equilibrium for the exothermic reaction 2SO2(g)+O2(g)⇌2SO3(g)2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g) if the temperature is increased.

Solution:

The equilibrium shifts to the left (towards reactants).

Explanation:

Since the forward reaction is exothermic (releases heat), increasing the temperature provides more energy to the system. According to Le Chatelier’s Principle, the equilibrium will shift to the left (the endothermic direction) to absorb the excess heat, decreasing the yield of SO3SO_3.