SESSION 13: CHEMICAL REACTIONS Key Concepts In this session we will focus on summarising what you need to know about: Redox reactions Direct transfer of electrons Rules for assigning oxidation numbers Acids and Bases theories Common acids and bases General chemical equations of acids with different compounds Indicators X-planation REDOX REACTIONS Oxidation: loss of electrons Reduction: gain of electrons Redox reaction: oxidation-reduction reaction. A reaction that involves a transfer of electrons In some redox reactions the electrons are transferred directly; in other reactions there is an indirect transfer of electrons DIRECT TRANSFER OF ELECTRONS Example: Zn + CuSO 4 ZnSO 4 + Cu Blue clear brown Observations: a small Zn plate placed in a blue copper sulphate solution gets covered with a brown precipitate of copper and eventually the solution turns clear. The clear solution is zinc sulphate. Reaction in ionic form: Zn + Cu 2+ 2- + SO 4 Zn 2+ 2- + SO 4 Oxidation half: Zn Zn 2+ + 2e - Reduction half: Cu 2+ e - Cu Nett equation: Zn + Cu 2+ Zn 2+ + Cu + Cu RULES FOR ASSIGNING OXIDATION NUMBERS 1. All elements have an oxidation number equal to zero. 2. The total oxidation number of a compound is zero. 3. The oxidation number of an ion or polyatomic ion is equal to the charge on the ion. 4. In a compound, the unit oxidation number of hydrogen is +1 and oxygen is -2. Brought to you by Page 1
ACIDS AND BASES Arrhenius definitions: An acid is a compound that contains hydrogen and releases hydrogen ions (H + ) in water A base is a compound that produces hydroxide (OH - ) ions in water Bronsted-Lowry definitons: An acid is a proton donor A base is a proton acceptor An ampholyte is a substance that can act as either an acid or a base. In the presence of a strong base it will act as an acid while in the presence of a strong acid it will act as a base. A strong acid is an acid which will ionise almost completely in water. A strong base is a base which will dissociate almost completely in water. Common acids Common bases Hydrochloric acid HCl Sodium carbonate NaCO 3 Nitric acid HNO 3 Sodium hydrogen carbonate NaHCO 3 Sulphuric acid H 2 SO 4 Sodium hydroxide NaOH Acetic acid CH 3 COOH When an acid donates a proton, a conjugate base is formed. When a base accepts a proton, a conjugate acid is formed. General reactions with acids Acid + metal salt + hydrogen Acid + metal oxide salt + water Acid + Hydroxide salt + water Acid + metal carbonate salt + water + carbon dioxide Brought to you by Page 2
X-ample Questions QUESTION 1 Magnesium ribbon is reacted with hydrochloric acid. 1.1 Complete the following reaction: Mg + HCl + 1.2 Write the oxidation half reaction. 1.3 Write the reduction half reaction. 1.4 Name the spectator ion. 1.5 Name the reducing agent. 1.6 Name the oxidising agent. QUESTION 2 Use oxidation numbers to identify which if any, redox reaction takes place in each of the following reactions: A: S + O 2 SO 2 B 2SO 2 + O 2 2SO 3 C SO 3 + H 2 SO 4 2H 2 S 2 O 7 Where a redox reaction takes place, identify the reducing agent present. QUESTION 3 Copper reacts with silver nitrate. 3.1 Write the half reactions. 3.2 Identify the oxidising and reducing agents. Chlorine is added to a solution of sodium bromide 3.3 Give the half reactions for the reaction that happens. 3.4 Give the balanced net ionic equation for the reaction. QUESTION 4 4.1. Give the properties of an acid. 4.2. Give the properties of a base. QUESTION 5 Identify the conjugate acid base pairs in the following reactions 5.1. 5.2. 5.3. Brought to you by Page 3
QUESTION 6 20g of calcium carbonate reacts with excess dilute sulphuric acid. 6.1. Sulphuric acid is a strong acid. What does this mean? 6.2. Distinguish between a weak acid and a dilute acid. 6.3. Provide a balanced equation for this reaction. 6.4. Calculate the volume of carbon dioxide produced at STP. 6.5. What is a standard solution? 6.6. What is a Bronsted-Lowry acid? QUESTION 7 The stomach secretes gastric juice which contains hydrochloric acid. The gastric juice helps with digestion. Sometimes there is an over production of acid leading to heartburn and 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 a formula Mg(OH) 2. 7.1. Write a balanced chemical reaction to show how the antacid reacts with the acid. 7.2. The directions on the bottle recommend that children under the age of 12 years take one teaspoon of milk of magnesia, where as adults can take two teaspoons. Briefly explain why the dosages are different. 7.3. Why is it not recommended taking an overdose of antacid in the stomach? Refer to the hydrochloric acid concentration in the stomach. In an experiment, 25 cm 3 of a standard solution of sodium carbonate of concentration 0,1 mol.dm -3 was used to neutralise 35 cm 3 of a solution of hydrochloric acid. 7.4. Write a balanced equation for this reaction. 7.5. Calculate the concentration of the acid. Brought to you by Page 4
X-ercise Question 1 Consider the following reaction Al + Cl 2 AlCl 3 1.1 Write the oxidation half reaction. (2) 1.2 Write the reduction half reaction. (2) 1.3 Write the nett redox reaction. (2) 1.4 Identify the oxidising agent and the reducing agent. (2) Question 2 Explain the difference between redox and acid-base reactions. (2) Question 3 Provide the oxidation numbers of the underlined atoms in these compounds? 3.1 CO 3.2 BCl 3 3.3 Na 2 SO 4 3.4 CaS (4) Question 4 4.1 What element is found in every acid? (1) Provide: 4.2 two examples of weak acids. (2) 4.3 two examples of strong acids. (2) 4.4 one example of a weak base. (2) 4.5 one example of a strong base. (2) Question 5 12,3 cm 3 of a 0,03 mol.dm -3 solution of sulphuric acid reacts with 36,9 cm 3 of a sodium hydroxide solution. 5.1. Provide a suitable indicator for this reaction. (1) 5.2. Write a balanced equation for this reaction. (4) 5.3. Calculate the concentration of the sodium hydroxide solution. (5) Brought to you by Page 5