7.2 Addition, elimination and substitution reactions

Substitution reactions

A substitution reaction occurs when an exchange of elements in the reactants takes place. The initial reactants are transformed or 'swopped around' to give a final product. A simple example of a reaction like this is shown below:

Some simple examples of substitution reactions are shown below:

In this example, a chlorine atom and a hydrogen atom are exchanged to create a new product.

In this example, four waters and four chlorines are exchanged to create a new product.

Addition, substitution and elimination reactions

1. Refer to the diagram below and then answer the questions that follow:

   

   1. Is this reaction an example of substitution, elimination or addition?
   2. Give a reason for your answer above.
2. The following diagram shows the reactants in an addition reaction.

   

   1. Draw the final product in this reaction.
   2. What is the chemical formula of the product?
3. The following reaction takes place:

   

   Is this reaction a substitution, addition or dehydration reaction? Give a reason for your answer.
4. Consider the following reaction: $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)+2{\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{s}\right)\to {\mathrm{CaCl}}_2\left(\mathrm{s}\right)+2{\mathrm{NH}}_3\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$ Which one of the following best describes the type of reaction which takes place?
   1. Redox reaction
   2. Acid-base reaction
   3. Dehydration reaction

Summary

• There are many different types of reactions that can take place. These include acid-base, acid-carbonate, redox, addition, substitution and elimination reactions.
• The Arrhenius definition of acids and bases defines an acid as a substance that increases the concentration of hydrogen ions (H ${}^{+}$ or H ${}_3$ O ${}^{+}$ ) in a solution. A base is a substance that increases the concentration of hydroxide ions (OH ${}^{-}$ ) in a solution. However this definition only applies to substances that are in water.
• The Bronsted-Lowry definition is a much broader one. An acid is a substance that donates protons and a base is a substance that accepts protons .
• In different reactions, certain substances can act as both an acid and a base. These substances are called ampholytes and are said to be amphoteric . Water is an example of an amphoteric substance.
• A conjugate acid-base pair refers to two compounds in a reaction (one reactant and one product) that transform or change into the other through the loss or gain of a proton.
• When an acid and a base react, they form a salt and water. The salt is made up of a cation from the base and an anion from the acid. An example of a salt is sodium chloride (NaCl), which is the product of the reaction between sodium hydroxide (NaOH) and hydrochloric acid (HCl).
• The reaction between an acid and a base is a neutralisation reaction.
• Titrations are reactions between an acid and a base that are used to calculate the concentration of one of the reacting substances. The concentration of the other reacting substance must be known.
• In an acid-carbonate reaction , an acid and a carbonate react to form a salt, carbon dioxide and water.
• A redox reaction is one where there is always a change in the oxidation numbers of the elements that are involved in the reaction.
• Oxidation is the loss of electrons and reduction is the gain of electrons.
• When two or more reactants combine to form a product that contains all the atoms that were in the reactants, then this is an addition reaction . Examples of addition reactions include the reaction between ethene and bromine, polymerisation reactions and hydrogenation reactions.
• A reaction where the reactant is broken down into one or more product, is called an elimination reaction . Alcohol dehydration and ethane cracking are examples of elimination reactions.
• A substitution reaction is one where the reactants are transformed or swopped around to form the final product.

Summary exercise

1. Give one word/term for each of the following descriptions:
   1. A chemical reaction during which electrons are transferred
   2. The addition of hydrogen across a double bond
   3. The removal of hydrogen and halogen atoms from an alkane to form an alkene
2. For each of the following, say whether the statement is true or false. If the statement is false, re-write the statement correctly.
   1. The conjugate base of NH ${}_4^{+}$ is NH ${}_3$ .
   2. The reactions $\mathrm{C}+{\mathrm{O}}_2\to {\mathrm{CO}}_2$ and $2{\mathrm{KClO}}_3\to 2\mathrm{KCl}+3{\mathrm{O}}_2$ are examples of redox reactions.
3. For each of the following questions, choose the one correct statement from the list provided.
   1. The following chemical equation represents the formation of the hydronium ion: ${\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{H}}_3{\mathrm{O}}^{+}\left(\mathrm{aq}\right)$ In this reaction, water acts as a Lewis base because it...
      1. accepts protons
      2. donates protons
      3. accepts electrons
      4. donates electrons
      (IEB Paper 2, 2005)
   2. When chlorine water (Cl ${}_2$ dissolved in water) is added to a solution of potassium bromide, bromine is produced. Which one of the following statements concerning this reaction is correct?
      1. Br ${}^{-}$ is oxidised
      2. Cl ${}_2$ is oxidised
      3. Br ${}^{-}$ is the oxidising agent
      4. Cl ${}^{-}$ is the oxidising agent
      (IEB Paper 2, 2005)
4. The stomach secretes gastric juice, which contains hydrochloric acid. The gastric juice helps with digestion. Sometimes there is an overproduction of acid, leading to heartburn or indigestion. Antacids, such as milk of magnesia, can be taken to neutralise the excess acid. Milk of magnesia is only slightly soluble in water and has the chemical formula Mg(OH) ${}_2$ .
   1. Write a balanced chemical equation to show how the antacid reacts with the acid.
   2. The directions on the bottle recommend that children under the age of 12 years take one teaspoon of milk of magnesia, whereas adults can take two teaspoons of the antacid. Briefly explain why the dosages are different.
   3. Why is it not advisable to take an overdose of the antacid in the stomach? Refer to the hydrochloric acid concentration in the stomach in your answer. In an experiment, 25.0 cm ${}^3$ of a standard solution of sodium carbonate of concentration 0.1 mol.dm ${}^{-3}$ was used to neutralise 35.0 cm ${}^3$ of a solution of hydrochloric acid.
   4. Write a balanced chemical equation for the reaction.
   5. Calculate the concentration of the acid.
   (DoE Grade 11 Exemplar, 2007)