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Types of chemical reactions

Physical Sciences - Grade 11 · Chemical Change

Types of Chemical Reactions

Chemical reactions are processes where substances, known as reactants, are transformed into new substances called products. Understanding the different types of chemical reactions is essential in chemistry, as it helps you predict the outcomes of reactions and understand the behaviour of substances.

1. Combination Reactions

In a combination reaction, two or more reactants combine to form a single product. The general form of a combination reaction can be written as:

A + B → AB

For example, when hydrogen gas (H2) combines with oxygen gas (O2) to form water (H2O), the reaction is:

2 H2(g) + O2(g) → 2 H2O(l)

Remember: In combination reactions, the number of products is always one, no matter how many reactants are involved.

2. Decomposition Reactions

Decomposition reactions occur when a single compound breaks down into two or more simpler substances. The general form is:

AB → A + B

An example of a decomposition reaction is the breakdown of calcium carbonate (CaCO3) when heated:

CaCO3(s) → CaO(s) + CO2(g)

Watch out: Students often confuse decomposition reactions with combination reactions. Remember that decomposition reactions start with one compound and end with multiple products.

3. Single Replacement Reactions

In a single replacement reaction, one element replaces another element in a compound. The general form is:

A + BC → AC + B

For example, when zinc (Zn) reacts with hydrochloric acid (HCl), zinc replaces hydrogen in the acid:

Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g)

Tip: To predict the products of single replacement reactions, use the activity series of metals. A more reactive metal will replace a less reactive metal in a compound.

4. Double Replacement Reactions

Double replacement reactions occur when the ions of two compounds exchange places in an aqueous solution. The general form is:

AB + CD → AD + CB

An example is the reaction between silver nitrate (AgNO3) and sodium chloride (NaCl):

AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)

Remember: In double replacement reactions, one product is often a precipitate, a gas, or water.

5. Combustion Reactions

Combustion reactions involve the reaction of a substance with oxygen, releasing energy in the form of light or heat. The general form is:

Fuel + O2 → CO2 + H2O + energy

A common example is the combustion of methane (CH4):

CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g) + energy

Watch out: Not all combustion reactions produce carbon dioxide and water. Incomplete combustion can produce carbon monoxide (CO) or soot.

6. Redox Reactions

Redox reactions, or reduction-oxidation reactions, involve the transfer of electrons between substances. These reactions are essential in many processes, including respiration and photosynthesis. In a redox reaction, one substance is oxidised (loses electrons) while another is reduced (gains electrons).

For example, in the reaction between iron (Fe) and copper(II) sulfate (CuSO4), iron is oxidised and copper is reduced:

Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

Remember: In redox reactions, always identify the oxidising agent (the substance that gains electrons) and the reducing agent (the substance that loses electrons).

Summary

  • Combination reactions involve two or more reactants forming one product.
  • Decomposition reactions involve one compound breaking down into simpler substances.
  • Single replacement reactions involve one element replacing another in a compound.
  • Double replacement reactions involve the exchange of ions between two compounds.
  • Combustion reactions involve a substance reacting with oxygen, producing energy.
  • Redox reactions involve the transfer of electrons between substances.

Check your understanding

  1. What is the general form of a combination reaction?
  2. Give an example of a decomposition reaction and write the balanced equation.
  3. In a single replacement reaction, what determines whether a metal can replace another metal in a compound?
  4. What type of reaction is occurring when silver nitrate and sodium chloride react?