Presentation on theme: "HIGHER CHEMISTRY REVISION. Unit 3 :- Acids and Bases 1. A student added 50 cm 3 of 4.0 mol l -1 hydrochloric acid to 4.0 g of magnesium ribbon. (a) The."— Presentation transcript:
HIGHER CHEMISTRY REVISION. Unit 3 :- Acids and Bases 1. A student added 50 cm 3 of 4.0 mol l -1 hydrochloric acid to 4.0 g of magnesium ribbon. (a) The balanced equation for the reaction is: Mg(s) + 2HCl(aq) MgCl 2 (aq) + H 2 (g) Show by calculation which reactant is in excess. (b) The experiment was repeated under the same conditions using ethanoic acid instead of hydrochloric acid. Circle the correct words in the table to show the results for ethanoic acid. Ethanoic acid Rate of reactionfaster / same / slower Volume of gas produced more / same/ less (a) 4 g of Mg = 4 / 24.3 = mol. 50 cm 3 of 4.0 mol l -1 HCL = 50 / 1000 x 0.4 = 0.2 mol From equation mol of Mg would need 2 x = 0.33 mol of HCl. We only have 0.2 mol HCl so the Mg is in excess. slower same
2. Potassium cyanide, KCN, can be made by the reaction of an acid with an alkali. A solution of the salt has a pH of 8. (a) What is the concentration of H + (aq), in mol l -1, in the solution? (b) What can be concluded about the strengths of the acid and the alkali used in the reaction? (c) Write the formula for the acid used in the reaction. (a)1 x mol l -1. (b)Weak acid and strong base. (c) KCN 3. Compounds can have different structures and properties. NH 4 NO 3 BaSO 4 Na 2 CO 3 SiO 2 K 2 O P 2 O 5 Identify the salt which dissolves in water to form an alkaline solution. Na 2 CO 3
3. Two flasks contained equal volumes of 0.1 mol l -1 hydrochloric acid and 0.1 mol l -1 ethanoic acid. Identify the two true statements about both solutions. AThey give the same colour with Universal Indicator solution.. BThey have a pH less than 7. CThey conduct electricity equally well. DThe have equal concentrations of hydrogen ions. EThey react at the same rate with magnesium. FThey neutralise the same number of moles of sodium hydroxide. B and F 4. A solution of mol l -1 hydrochloric acid can be described as a dilute solution of a strong acid. (a)What is meant by a strong acid? (b) What is the pH of the mol l -1 hydrochloric acid used in the experiment? (a) A strong acid is fully ionised in aqueous solution. (b)[H + (aq)] = 1 x mol l -1. pH = 3.
5. Calcium hydroxyapatite makes up 95% of tooth enamel. Tooth decay is caused when tooth enamel is attacked by acid in the mouth. (a) One of the acids which attacks tooth enamel is 2-hydroxypropanoic acid, which has the molecular formula C 3 H 6 O 3. Draw the structural formula for this acid. (b) Calcium hydroxyapatite reacts with acid in the mouth as shown by the following balanced equation. Ca 10 (PO 4 ) 6 (OH) 2 + 8H + 10Ca H 2 O + 6HPO 4 x- calcium hydroxyapatite What is the value of x? (c) The pH of a solution in the mouth is 5. What is the concentration of hydrogen ions, in mol l -1, in this solution? (a)H H OH H-C –C - C =O H H (b)x = 2 (c) 1 x mol l Ammonium chloride (NH 4 Cl) is soluble in water. How does the pH of a solution of ammonium chloride compare with the pH of water? Ammonium chloride has a lower pH as it is an acidic solution – salt of strong acid and weak base.