Analysing Acids and Bases

Transcription

1 Week 4 Analysing Acids and Bases Acid A substance that donates a hydrogen ion (proton) A proton is donated in the acidbase reaction: HCl (aq) + H H O (aq) + Cl (aq) Strong acids completely ionise in water Weak acids weakly deionise e.g. COOH (l) + H O COO (aq) + H O (aq) At any instance, most ethanoic acid molecules are not ionised. Base A substance that accepts a hydrogen ion (proton) Strong bases readily accept protons e.g. NaOH, KOH. Weak bases accept protons less readily e.g. ammonia (NH ) Base The concentration of H O + ions is referred to as the solution s acidity. Acidity is measured using a logarithmic scale called the ph scale. The definition of ph is: ph = log[h O + ] Where [H O + ] is the concentration of H O + ions measured in mol L 1 Acidic solutions have a ph < 7 Basic solutions have a ph > 7 Indicators Used in acidbase titrations to identify the equivalence point. Acidbase indicator : a substance whose color depends on the concentration of H O + ions in solution. They are weak acids: acid form has one color and conjugate base another. Indicators must be chosen carefully to ensure that color change occurs at the titration end point the equivalence point of the reaction. 1

2 Worked example 4.b 5.00 ml Concentrated HCl Diluted to ml Strong acid with weak base: EP occurs between ph 7; methyl orange Weak acid with strong base: EP occurs between ph 7 11; phenolpthalein Weak acid with weak base: ph change too gradual for indicators 5.00 ml M Na The methyl orange indicator changed permanently from yellow to pink when ml of the diluted cleaner was added. Calculate the concentration of hydrochloric acid in the concrete cleaner. HCl (aq) + Na NaCl (aq) + H O (aq) + C In 0.00 ml of Na solution n(na )= c(na ) X V(Na ) = mol L 1 X 0.00 L = mol From the equation, mol of HCl reacts with 1 mol of Na So the ratio n(hcl) = n(naco) 1 n(hcl) = X n(na ) = X = mol n(hcl) c(hcl) = V(HCl) = mol L = 0.90 mol L 1 The concentration of the diluted cleaner is 0.90 M Since the cleaner had been diluted before titration, the concentration of the concrete cleaner is c(hcl) = 0.90 X = 9.0 M Chapter review Acidbase reactions 10. Write full equations for the acidbase reactions that occur when: a. Nitric acid is added to sodium hydroxide HNO (aq) + NaOH (aq) NaNO (aq) + H b. Sulfuric acid is added to potassium hydroxide H SO 4 (aq) + KOH (aq) K SO 4 (aq) + H c. Hydrochloric acid is added to ammonia solution HCl (aq) + NH NH 4 Cl (aq) d. Ethanoic acid solution is added to potassium hydroxide solution COOH (aq) + KOH (aq) COOK (aq) + H

3 11. Write ionic equations for the reactions in Question 10. a. Nitric acid is added to sodium hydroxide H + (aq) + OH (aq) H b. Sulfuric acid is added to potassium hydroxide H + (aq) + OH (aq) H c. Hydrochloric acid is added to ammonia solution H + (aq) + NH (aq) NH 4+ (aq) d. Ethanoic acid solution is added to potassium hydroxide solution COOH (aq) + OH (aq) COO (aq) + H 1. What mass of sodium sulfate is produced when 5.0 ml of M sulfuric acid is added to 0.0 ml of 0.15 M sodium hydroxide solution? 0.1 g 1. What volume of M sulfuric acid would be required to neutralise a solution containing g of sodium hydroxide and g of potassium hydroxide? 0.14 L 16. The concentration of sodium hydroxide in waste water from an alumina refinery was found by titrating 0.00 ml aliquots of waste water against M hydrochloric acid, using phenolpthalein as indicator. The average titre of several titrations was ml. a. Why is an indicator used? To detect the equivalence point as both reactants are colorless. b. Write an equation for the reaction that occurred. NaOH (aq) + HCl (aq) NaCl (aq) + H c. What was the molarity of sodium hydroxide in the waste water? M c. What mass of sodium hydroxide would be present in 100 L of the waste water? 4 g 17. A 4.7 ml volume of a hydrochloric acid solution is required to react completely with 0.0 ml of 0.61 M sodium carbonate solution. a. Write an equation for the reaction. HCl (aq) + Na (aq) NaCl (aq) + H O (aq) + C b. Calculate the concentration of the hydrochloric acid, in mol L M 18. A 1.0 g antacid tablet contains 80.)% by mass of magnesium hydroxide as the active ingredient. What volume of M hydrochloric acid would the antacid tablet neutralise? 0 ml 0. A 50 ml sample of vinegar was diluted to 50 ml in a volumetric flask. A 0.00 ml aliquot of this solution required the addition of 7.98 ml of 0.14 M sodium hydroxide solution in order to be neutralised. a. Write an equation for the neutralisation equation. COOH (aq) + NaOH (aq) COONa (aq) + H b. What is the molarity of the ethanoic acid in the original vinegar? 0.97 M c. Express your answer to Part B in g L g L 1

4 . In order to standardise a solution of hydrochloric acid, a student titrated the solution against 0.00 ml aliquots of a standard solution of sodium carbonate. Methyl orange indicator was used to identify the end point of the reaction: HCl (aq) + Na (aq) NaCl (aq) + H + C The sodium carbonate solution had been prepared by dissolving 1.6 g of anhydrous Na in water and making the solution up to 50.0 ml in a volumetric flask. The titres recorded were 1.56 ml, 0.98 ml, 0.96 ml and 1.0 ml. What is a redox reaction? Involves the transfer of electrons One of the reactants loses electrons: oxidation One of the reactants gains these electrons: reduction Oxidation and reduction occur simultaneously a. What value for the titre of sodium carbonate solution should the student use in the calculation of the acid concentration? Explain your answer ml b. What is the molarity of the sodium carbonate solution? M c. Calculate the concentration of the hydrochloric acid in mol L mol L 1 Consider Oxidant The fate of magnesium in fireworks causes gains undergoes Oxidation part of the overall reaction Mg Mg + + e Oxidation Is loss of Electrons Is gain of Reduction Reduction part of the overall reaction + 4 e undergoes loses Reductant causes The number of electrons produced during the oxidation is the same as the number consumed in the reduction 4

5 1. Name the chemicals that undergo oxidation in the following reactions a. Zn (s) + ZnO (s) b. Ca (s) + Cl CaCl c. AgBr (s) Ag (s) + Br. Indentify the oxidants and reductants in each of the reactions above. Oxidation state A measure of the degree of oxidation of an atom in a substance. It is defined as the charge an atom might be imagined to have when electrons are counted according to an agreedupon set of rules Species Free elements Oxidation number 0 Rules for determining oxidation numbers Examples Cl; Mg; C; ; H Ionic compounds Oxygen in compounds Equal to the charge on ion Defined as in its compounds. H is an exception, where it is Na + + Cl Al Ca + Cl O O = in H O, C, Na O Hydrogen in compounds Molecular ions and molecules Defined as +1 in compounds with nonmetals The sum of the oxidation numbers equals the charge on the molecular ion (0 in the case of neutral molecules). The most electronegative element has the negative oxidation number H = +1 in HCl, H S, CH 4 For Mno 4 oxygen is defined as. Because there are 4 oxygen atoms, to have an overall charge of 1, Mn must have an oxidation number of +7 Worked example 5.b For, the sum of the oxidation numbers equals the ionic charge of. The oxidation number of O is fixed as. Find the oxidation number of carbon. Variable oxidation number The oxidation number of the transition metals vary depending upon the compound. The different oxidation states often have characteristic colors e.g. as in vanadium compounds (oxidation number of C) + X (oxidation number of O) = If, (oxidation number of C) + () = then the oxidation number of C must = The oxidation numbers are C O s of vanadium compounds that have various oxidation numbers Using oxidation numbers to name chemicals e.g. there are iron chlorides. Roman numerals are used to represent the oxidation state: FeCl is iron (II) chloride FeCl is iron (III) chloride 5

6 . State the oxidation number of carbon in a. CO + b. C +4 c. CH 4 4 d. C 0 e. HCO Which one of the following substances contain manganese in the +6 oxidation state: MnCl, MnCl, Mn, K MnO 4, KMnO 4 K MnO 4 5. Find the oxidation numbers of each element in the following compounds or ions. Hint: For ionic compounds, use the charge on each ion to help you. a. CaO Ca + O b. CaCl Ca + Cl 1 c. HSO 4 H +1 S +6 O d. MnO 4 Mn +7 O e. F f. SO g. NaNO h. K Cr O 7 F 0 S +4 O Na +1 N +5 O K +1 Cr +6 O 6. Assign oxidation numbers to each element in these equations, and hence identify the oxidant and the reductant: a. Mg (s) + Cl MgCl (s) Oxidant: Cl ; Reductant: Mg b. S + SO Oxidant: C ; Reductant: S c. Fe O (s) + CO Fe (s) + C Oxidant: Fe O ; Reductant: CO d. Fe + (aq) + H + H + Fe + (aq) + H ) Oxidant: H ; Reductant: Fe + Half equations (Mg Mg + + e ) Oxidation half equation + 4 e Reduction half equation Mg (s) + MgO (s) Overall equation The number of electrons lost must be equal to the number of electrons gained during the reaction. Rule Balance all elements except hydrogen and oxygen Balance oxygen using H O molecules Balance hydrogen using H + ions Balance charge using electrons Rules for balancing redox equations Example Cr O 7 (aq) Cr + (aq) Cr O 7 (aq) Cr + (aq) + 7H Cr O 7 (aq) + 14 H + (aq) Cr + (aq) + 7H Cr O 7 (aq) + 14 H e (aq) Cr + (aq) + 7H Worked example 5. When a pale green solution containing Fe + ions is mixed with a purplecolored solution of MnO4 ions, the purple color disappears. Fe + and Mn + ions are formed. Write a balanced equation for this reaction. Fe + (aq) Fe + (aq) + e MnO 4 Mn + MnO 4 Mn + + 4H O (add water) MnO 4 + 8H + Mn + + 4H O (add H + ) MnO 4 (aq) + 8H + (aq) + 5e Mn + (aq) + 4H (add e ) To write an overall equation, the half equations are multiplied so that the number of electrons on each side is the same. They are added together and simplified if required: 5 X Fe + (aq) Fe + (aq) + e MnO 4 (aq) + 8H + (aq) + 5e Mn + (aq) + 4H 0 (l) MnO 4 (aq) + 8H + (aq) + 5Fe + (aq) Mn + (aq) + 5Fe + (aq) + 4H 6

7 Redox titrations The concentration of redox reactants can be found by volumetric analysis: redox titrations. Worked example 5.4 A 10 ml sample of white wine was placed in a volumetric flask and water was added to make 100 ml of solution. Then 0.0 ml aliquots of the diluted wine were titrated against M acidified potassium dichromate solution. The mean titre was 4.61 ml. Calculate the concentration of ethanol in the sample of white wine. Cr O7 (aq) + CH OH (aq) + 16H + (aq) 4Cr + (aq) + COOH (aq) + 11H n(cr 7 ) = c(cr 7 X V(Cr 7 ) = mol L1 X L = mol From the equation, mol of Cr 7 reacts with mol of CH OH n(ethanol) So, the ratio is = n(dichromate) n(ethanol) = X = mol The amount of ethanol in the 0.0 ml of diluted wine is mol. Since this volume of wine was taken from a total volume of ml, there would be 100/0 or 5X this amount in the original 10.0 ml sample: X 5 = mol n(ethanol) mol c(ethanol) = = = mol L V(ethanol) L The concentration of alcohol in the wine is 1.85 M 1. Potassium permanganate reacts with hydrogen peroxide: MnO 4 (aq) + 5H (aq) + 6H + (aq) Mn + (aq) + 8H ml of 0.0 M KMnO 4 solution is reduced by 0.0 ml of H solution. What is the concentration of the hydrogen peroxide solution? M 1. An artist uses 10.0 ml of 15.0 M HNO to etch a design into a copper sheet. What mass of copper will have reacted with the acid? Cu (s) + 4HNO (aq) Cu(NO ) (aq) + H + N.8 g Page 58 Questions 14 a, c; 15 a,c,e; 17; 1; ; 4; 5; 7; 8 7