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Chemistry - Chemical Reactions and Equations

Grade 7IGCSE

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

🔑Concepts

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A chemical reaction is a process where substances, known as reactants, are transformed into new substances called products. This involves the breaking and forming of chemical bonds.

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The Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction. Therefore, the total mass of reactants must equal the total mass of products, which is why equations must be balanced.

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Word equations use the names of the chemicals (e.g., Iron+Oxygen→IronOxideIron + Oxygen \rightarrow Iron Oxide), while symbol equations use chemical formulae (e.g., 4Fe+3O2→2Fe2O34Fe + 3O_2 \rightarrow 2Fe_2O_3).

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State symbols are used to show the physical state of substances: (s)(s) for solid, (l)(l) for liquid, (g)(g) for gas, and (aq)(aq) for aqueous (dissolved in water).

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Oxidation is a reaction where a substance gains oxygen, such as the rusting of iron or the combustion of magnesium: 2Mg+O2→2MgO2Mg + O_2 \rightarrow 2MgO.

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Thermal Decomposition is a reaction where a single compound breaks down into two or more simpler substances when heated, such as CaCO3→CaO+CO2CaCO_3 \rightarrow CaO + CO_2.

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Exothermic reactions release energy to the surroundings (temperature increases), while Endothermic reactions take in energy from the surroundings (temperature decreases).

📐Formulae

Reactants→ProductsReactants \rightarrow Products

2H2(g)+O2(g)→2H2O(l)2H_2(g) + O_2(g) \rightarrow 2H_2O(l)

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

Acid+Alkali→Salt+WaterAcid + Alkali \rightarrow Salt + Water

6CO2+6H2O→lightC6H12O6+6O26CO_2 + 6H_2O \xrightarrow{light} C_6H_{12}O_6 + 6O_2

Zn+H2SO4→ZnSO4+H2Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2

💡Examples

Problem 1:

Balance the following chemical equation for the combustion of propane: C3H8+O2→CO2+H2OC_3H_8 + O_2 \rightarrow CO_2 + H_2O

Solution:

C3H8+5O2→3CO2+4H2OC_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O

Explanation:

To balance the equation, we first look at Carbon: there are 3 on the left, so we need 3CO23CO_2. Next, Hydrogen: there are 8 on the left, so we need 4H2O4H_2O. Finally, we count the Oxygen atoms on the right (3×2+4=103 \times 2 + 4 = 10) and balance the left side with 5O25O_2.

Problem 2:

Write the balanced symbol equation for the reaction between Hydrochloric acid (HClHCl) and Sodium Hydroxide (NaOHNaOH).

Solution:

HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)

Explanation:

This is a neutralization reaction. One mole of acid reacts with one mole of base to produce one mole of salt (Sodium Chloride) and one mole of water. The equation is already balanced as the number of atoms for each element is equal on both sides.

Problem 3:

If 10g10g of Calcium Carbonate (CaCO3CaCO_3) is heated and decomposes to produce 5.6g5.6g of Calcium Oxide (CaOCaO), what mass of Carbon Dioxide (CO2CO_2) is released?

Solution:

4.4g4.4g of CO2CO_2

Explanation:

According to the Law of Conservation of Mass: Massreactants=MassproductsMass_{reactants} = Mass_{products}. Therefore, 10g(CaCO3)=5.6g(CaO)+Mass(CO2)10g (CaCO_3) = 5.6g (CaO) + Mass (CO_2). Subtracting 5.6g5.6g from 10g10g gives 4.4g4.4g.

Chemical Reactions and Equations - Revision Notes & Key Formulas | IGCSE Grade 7 Science