UNIT 6338: SAMPLE ACTIVITY 28 SAMPLE ASSESSMENT ACTIVITIES AND SCHEDULES PROTON TRANSFERS

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1 UNIT 6338: SAMPLE ACTIVITY 28 SAMPLE ASSESSMENT ACTIVITIES AND SCHEDULES CHEMISTRY LEVEL 2 PROTON TRANSFERS This activity assesses: Unit: 6338 Characterise the behaviour of weak and strong acids and bases (Level 2). Element: 1 Compare the behaviour of weak and strong acids and bases. 2 Perform calculations involving acid-base reactions. CONDITIONS: Formal classroom activity. INSTRUCTIONS: Task A For each of the equations below identify a species which is acting as an acid and a species acting as a base. 1. NH H 2 O NH 3 + H 3 O + 3. CH 3 COOH + H 2 O CH 3 COO - + H 3 O + 2. HCO H2 O H 2 CO 3 + OH - 4. CO H 2 O HCO OH - Equation Acid Base ASSESSMENT ACTIVITY: PROTON TRANSFERS PAGE 1/4

2 6.130 SAMPLE ASSESSMENT ACTIVITIES AND SCHEDULES UNIT 6338: SAMPLE ACTIVITY 28 Task B 1. Tests are carried out with 0.1 mol L -1 solutions of two acids A and B. The observations recorded are shown in the following table. Results Test Acid A Acid B ph measured 3 1 Rate of reaction with magnesium slow evolution of gas rapid evolution of gas Electrical conductivity poor good State which of the acids is strong and which is weak. Strong acid = Weak acid = 2. Interpret the differences in the behaviour of strong and weak acids in 1. above by completing the table below. Your answers to (a) and (b) should be used in your explanations of the three observed properties in rows (a) to (e). Strong acid Weak acid (a) Extent of reaction with water (complete/incomplete) (b) Relative [H 3 O + ] (high/low) (c) Explanation of measured ph (d) Explanation of rate of reaction of magnesium with acid (e) Explanation of electrical conductivity 1.2 ASSESSMENT ACTIVITY: PROTON TRANSFERS PAGE 2/4

3 UNIT 6338: SAMPLE ACTIVITY 28 SAMPLE ASSESSMENT ACTIVITIES AND SCHEDULES Task C Vinegar (CH 3 COOH) is a weak acid that reacts with baking soda (NaHCO 3 ) as shown in the balanced equation. CH 3 COOH + NaHCO 3 CH 3 COONa + CO 2 + H 2 O Hydrochloric acid is a strong acid. It also reacts with baking soda as shown below. HCI + NaHCO 3 NaCl + CO 2 + H 2 O 20 ml of 2 mol L -1 vinegar is required to react completely with 0.34g of baking soda. Is the volume of 2 mol L -1 hydrochloric acid required to completely react with 0.34g of the same baking soda going to be: (tick the correct answer) less than 20 ml? 20 ml? greater than 20mL? Justify your answer: 1.3 Task D The following results were obtained from a titration between a solution of sodium carbonate (Na 2 CO 3 ) and standard hydrochloric acid (HCI). Volume of sodium carbonate samples = 25.0 ml Concentration of hydrochloric acid = mol L -1 Average volume of hydrochloric acid used = 23.2 ml Equation: Na 2 CO 3 + 2HCI 2NaCl + CO 2 + H 2 O a) Use this equation to determine the ratio moles Na 2 CO 3 : moles HCl. ASSESSMENT ACTIVITY: PROTON TRANSFERS PAGE 3/4

4 6.132 SAMPLE ASSESSMENT ACTIVITIES AND SCHEDULES UNIT 6338: SAMPLE ACTIVITY 28 b) Calculate moles of HCl used. c) Calculate moles of Na 2 CO 3 used. d) Calculate the concentration of Na 2 CO 3 (in mol L -1 to 3 significant figures.). 2.1 Task E For the following questions, use K w = 1 x Convert the following concentrations in mol L -1 to ph (to three significant figures) a) [H 3 O + ] = 1.0 x 10-7 ph = b) [H 3 O + ] = 3.6 x 10-4 ph = c) [OH - ] = 3.2 x 10-6 ph = d) [OH - ] = 2.5 x 10-9 ph = 2. Determine both the [H 3 O + ] and [OH - ] concentrations (to three significant figures) of solutions with the following ph values. a) ph = 3.0 [H 3 O + ] = [OH - ] = b) ph = 8.6 [H 3 O + ] = [OH - ] = 2.2 ASSESSMENT ACTIVITY: PROTON TRANSFERS PAGE 4/4

5 UNIT 6338: SAMPLE ACTIVITY 28 SAMPLE ASSESSMENT ACTIVITIES AND SCHEDULES Assessment Schedule: Proton transfers Unit Standard 6338 Task Number Element and Performance Criteria Evidence (The answers or performance expected from the students) Judgement (A statement that defines the standard to be achieved) A 1.1 Reaction Acid Base 1. NH + 4 or H 3 O + H 2 O or NH 3 2. H 2 O or H 2 CO 3 HCO - 3 or OH - 3. CH 3 COOH or H 3 O + H 2 O or CH 3 COO H 2 O or HCO 3 CO 2-3 or OH - An acid and a base are correctly identified for three out of four equations. B A is weak, B is strong. Unknown acids correctly identified. 2. Strong acid Weak acid Extent of reaction with water and complete incomplete relative [H 3 O + ] for weak and strong acid correctly described. Extent of reaction with water (complete/ incomplete) Relative [H 3 O + ] (high/low) Explanation of measured ph Explanation of rate of reaction of magnesium with acid Explanation of electrical conductivity high high concentration of H 3 O + faster rate of reaction because more H 3 O + good because higher (H 3 O + ) ion concentration low low concentration of H 3 O + slower rate of reaction because less H 3 O + poor because lower (H 3 O + ) ion concentration Two out of three of the following correctly stated: ph rate of reaction conductivity. C mL. As both acids have the same concentration, then equal volumes have the same number of hydrogen ions available for reaction. Volume correct. Explanation implies that the total number of hydrogen ions available is the same for both acids. ASSESSMENT SCHEDULE: PROTON TRANSFERS PAGE 1/2

6 6.134 SAMPLE ASSESSMENT ACTIVITIES AND SCHEDULES UNIT 6338: SAMPLE ACTIVITY 28 Task Number Element and Performance Criteria Evidence (The answers or performance expected from the students) Judgement (A statement that defines the standard to be achieved) D 2.1 a) 1:2 b) n (HCl) = 3.48 x 10-3 (mol) c) n (Na2CO3) = 1.74 x 10-3 (mol) d) c (Na2CO3) = mol L -1 E a) = 7.0 b) = 3.4 c) = 8.5 d) = a) [H 3 O + ] = 1.0 x 10-3 mol L -1 [OH - ] = 1.0 x mol L -1 b) [H 3 O + ] = 2.5 x 10-9 mol L -1 [OH - ] = 4.0 x 10-6 mol L -1 Calculations leading to the correct answer shown. Units must be included for the final concentration and three significant figures used. Three out of four answers are correct to two significant figures. Three out of four answers are correct to two significant figures. Units not required. ASSESSMENT SCHEDULE: PROTON TRANSFERS PAGE 2/2