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Chemical Changes and Reactions - Types of Chemical Reactions

Grade 7ICSE

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

🔑Concepts

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A chemical reaction is a process where the chemical bonds of reactants are broken and new bonds are formed to create products with entirely different properties.

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Direct Combination (Synthesis) Reaction: A reaction in which two or more substances (elements or compounds) combine to form a single product. General form: A+B→ABA + B \rightarrow AB.

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Decomposition Reaction: A reaction in which a single compound breaks down into two or more simpler substances. This usually requires energy in the form of heat (thermal decomposition), light (photochemical), or electricity (electrolytic). General form: AB→A+BAB \rightarrow A + B.

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Displacement Reaction: A reaction in which a more reactive element displaces a less reactive element from its compound (usually in solution). General form: A+BC→AC+BA + BC \rightarrow AC + B.

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Double Decomposition Reaction: A reaction where two compounds react by exchanging their radicals or ions to form two new compounds. This includes Neutralization (Acid + Base) and Precipitation (formation of an insoluble solid).

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Exothermic Reactions: Chemical changes accompanied by the release of heat energy, such as the combustion of carbon: C+O2→CO2+HeatC + O_2 \rightarrow CO_2 + \text{Heat}.

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Endothermic Reactions: Chemical changes that occur by absorbing heat energy from the surroundings, such as N2+O2+Heat→2NON_2 + O_2 + \text{Heat} \rightarrow 2NO.

📐Formulae

A+B→AB (Combination)A + B \rightarrow AB \text{ (Combination)}

AB→heatA+B (Decomposition)AB \xrightarrow{\text{heat}} A + B \text{ (Decomposition)}

A+BC→AC+B (Displacement)A + BC \rightarrow AC + B \text{ (Displacement)}

AB+CD→AD+CB (Double Decomposition)AB + CD \rightarrow AD + CB \text{ (Double Decomposition)}

NaOH+HCl→NaCl+H2O (Neutralization)NaOH + HCl \rightarrow NaCl + H_2O \text{ (Neutralization)}

💡Examples

Problem 1:

What type of reaction occurs when Magnesium ribbon is burnt in air? Write the balanced chemical equation.

Solution:

2Mg+O2→2MgO2Mg + O_2 \rightarrow 2MgO

Explanation:

This is a Direct Combination (Synthesis) reaction because two elements, Magnesium (MgMg) and Oxygen (O2O_2), combine to form a single product, Magnesium Oxide (MgOMgO).

Problem 2:

Identify the reaction: Fe(s)+CuSO4(aq)→FeSO4(aq)+Cu(s)Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)

Solution:

Displacement Reaction

Explanation:

Iron (FeFe) is more reactive than Copper (CuCu). Therefore, Iron displaces Copper from its salt solution (CuSO4CuSO_4) to form Ferrous Sulphate (FeSO4FeSO_4).

Problem 3:

Explain the thermal decomposition of Calcium Carbonate with an equation.

Solution:

CaCO3(s)→ΔCaO(s)+CO2(g)CaCO_3(s) \xrightarrow{\Delta} CaO(s) + CO_2(g)

Explanation:

When Calcium Carbonate (CaCO3CaCO_3) is heated, it breaks down into Calcium Oxide (CaOCaO) and Carbon Dioxide (CO2CO_2). The symbol Δ\Delta represents the application of heat.

Problem 4:

What happens when Silver Nitrate solution is added to Sodium Chloride solution?

Solution:

AgNO3+NaCl→AgCl↓+NaNO3AgNO_3 + NaCl \rightarrow AgCl\downarrow + NaNO_3

Explanation:

This is a Double Decomposition (Precipitation) reaction. The silver (Ag+Ag^+) and sodium (Na+Na^+) ions exchange partners, resulting in the formation of a white precipitate of Silver Chloride (AgClAgCl).