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Chemical Reactions - Types of Reactions (Synthesis, Decomposition, Combustion)

Grade 9IB

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

🔑Concepts

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Synthesis (Combination) reactions involve two or more reactants joining together to form a single product, represented by the general form A+B→ABA + B \rightarrow AB.

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Decomposition reactions occur when a single compound breaks down into two or more simpler substances (AB→A+BAB \rightarrow A + B), usually requiring energy input such as heat (Δ\Delta), light, or electricity.

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Combustion is an exothermic reaction where a fuel reacts with an oxidant (usually Oxygen, O2O_2) to produce heat and oxidized products.

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Complete combustion of hydrocarbons always produces Carbon Dioxide (CO2CO_2) and Water (H2OH_2O), whereas incomplete combustion may produce Carbon Monoxide (COCO) or Carbon (soot, CC).

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The Law of Conservation of Mass must be followed in all chemical equations, meaning the number of atoms of each element must be equal on both the reactant and product sides.

📐Formulae

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

AB→ΔA+B (Decomposition)AB \xrightarrow{\Delta} A + B \text{ (Decomposition)}

CxHy+(x+y4)O2→xCO2+y2H2O (Hydrocarbon Combustion)C_xH_y + (x + \frac{y}{4})O_2 \rightarrow xCO_2 + \frac{y}{2}H_2O \text{ (Hydrocarbon Combustion)}

2Mg(s)+O2(g)→2MgO(s) (Metal Synthesis)2Mg(s) + O_2(g) \rightarrow 2MgO(s) \text{ (Metal Synthesis)}

💡Examples

Problem 1:

Identify the type of reaction and balance the following equation: HgO(s)→Hg(l)+O2(g)HgO(s) \rightarrow Hg(l) + O_2(g)

Solution:

2HgO(s)→Δ2Hg(l)+O2(g)2HgO(s) \xrightarrow{\Delta} 2Hg(l) + O_2(g)

Explanation:

This is a Decomposition reaction. A single reactant, Mercury(II) Oxide, breaks down into its constituent elements, Mercury and Oxygen, upon heating.

Problem 2:

Write the balanced chemical equation for the synthesis of Ammonia (NH3NH_3) from Nitrogen gas (N2N_2) and Hydrogen gas (H2H_2).

Solution:

N2(g)+3H2(g)→2NH3(g)N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)

Explanation:

This is a Synthesis reaction where two elemental gases combine to form a single compound. Coefficients are added to satisfy the Law of Conservation of Mass.

Problem 3:

Predict the products and balance the equation for the complete combustion of Methane (CH4CH_4).

Solution:

CH4(g)+2O2(g)→CO2(g)+2H2O(g)CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)

Explanation:

In complete combustion of a hydrocarbon, the Carbon atoms form CO2CO_2 and the Hydrogen atoms form H2OH_2O. We balance the Carbon first, then Hydrogen, and finally Oxygen.