QUESTION 2 Ethanoic acid (acetic acid) is a weak acid and hydrolyses in water according to the following reaction:

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1 QUESTION 1 When a small marble chip is placed into a test tube containing 2.0M hydrochloric acid, bubbles are noticed around the solid and after some time the marble chip is no longer visible. The above reaction is considered to go to completion because: One of the reactants appears to have been used up completely. Reactions involving acids always go to completion. It is an equilibrium reaction. It involves the release of a gas. QUESTION 2 Ethanoic acid (acetic acid) is a weak acid and hydrolyses in water according to the following reaction: H 3OOH aq) H O( l) H OO ( aq) H O ( aq) ( M solution of this acid is observed to contain acetate ions (H 3OO - ), hydronium ions (H 3O + ) and acetic acid molecules (H 3OOH). The hydrolysis of ethanoic acid is considered to be a reversible reaction because: The acetic acid molecule is too large to hydrolyse completely. The solution has a low ph. oth the reactants and the products are found in the solution. Not all the products are found in the solution. QUESTION 3 Some theoretical chemists claim that, strictly speaking all chemical reactions are reversible but some reactions are considered to be irreversible because: They never reach equilibrium. The extent of the reverse reaction is extremely small or negligible compared to the forward reaction. There is no reverse reaction. There is no forward reaction. The School For Excellence 2018 Unit 3 & 4 hemistry Resources Test Page 1

2 QUESTION 4 Equilibrium is said to be achieved when: oth the forward and reverse reactions stop. The concentration of products equals the concentration of reactants. ll the products have reacted. The rate of the forward reaction equals the rate of the reverse reaction. QUESTION 5 Equilibrium is said to be dynamic because: Equilibrium reactions always occur in a fluid medium (Water or gas) high amount of energy is transferred during the reaction oth the forward and reverse reactions continue to occur, but at the same speed None of the above. QUESTION 6 Which of the following statements does not describe a feature of equilibrium? The reaction can start either at the reactant end or the product end. Temperature and pressure must be kept constant if equilibrium is to be reached. The reaction never stops. Only the reactants are present in the equilibrium mixture. QUESTION 7 Equilibrium is reached because: It produces the maximum amount of product. It is a state of maximum stability. It is a state of maximum energy content. None of the above. The School For Excellence 2018 Unit 3 & 4 hemistry Resources Test Page 2

3 QUESTION 8 The following is a rate-time graph for a chemical reaction: Forward Reverse Reaction Rate Time The above reaction: For practical reasons may be considered irreversible. Has not reached equilibrium. Has stopped. Will never reach equilibrium. QUESTION 9 The following is a concentration-time graph for a chemical reaction: oncentration Reactants Products Time mong other things, the graph is telling us that: The system has not yet reached equilibrium. The reaction hasn t started. Equilibrium was reached within a relatively short time. The concentrations of products and reactants are equal. The School For Excellence 2018 Unit 3 & 4 hemistry Resources Test Page 3

4 QUESTION 10 One way of preventing a system from reaching equilibrium would be to: hange the pressure. hange the temperature. ll of the above. None of the above. QUESTION 1 onsider the following 2 reactions: I. Zn s) 2Hl( aq) Znl ( aq) H ( ) ( 2 2 g ( aq) H 2O( l) NH 4 ( aq) OH II. NH 3 ( aq) Indicate which of the above reactions is considered reversible. State one important condition required for the reversible reaction to reach equilibrium. Explain how chemists can tell that a reversible reaction is at equilibrium by examining the substances found in the reaction mixture. (d) What evidence would you require to show that one of the reactions above goes to completion? ( =5 Marks) The School For Excellence 2018 Unit 3 & 4 hemistry Resources Test Page 4

5 QUESTION 2 Offer your own definition of ynamic Equilibrium. (3 Marks) QUESTION 3 The following is a sketch of a concentration-time graph for an equilibrium reaction. oncnetration in arbitrary units Time (minutes) Indicate which line represents the reactants and which represents the products. pproximately, after how many minutes was equilibrium reached? Sketch a rate-time graph for the same reaction, showing when equilibrium was reached (1+1+3=5 Marks) The School For Excellence 2018 Unit 3 & 4 hemistry Resources Test Page 5

6 QUESTION 4 Offer a simple explanation as to why some reactions reach equilibrium. (2 Marks) QUESTION 5 Weak acids do not hydrolyse completely in water therefore, the hydrolysis reaction is considered to be reversible. One such acid is methanoic (formic) acid and its hydrolysis reaction is shown below: HOOH aq) H 2O( l) HOO ( aq) H O ( aq) ( 3 What is the rate of the reverse reaction at the start of the hydrolysis? t equilibrium, would you expect the concentration of H 3O + to be higher or lower than HOOH? If the ph of the solution is monitored with a ph meter throughout the reaction, how can you tell when the system has reached equilibrium? (3x1 = 3 Marks) Total = 28 Marks The School For Excellence 2018 Unit 3 & 4 hemistry Resources Test Page 6

7 QUESTION 1 QUESTION 2 QUESTION 3 QUESTION 4 QUESTION 5 QUESTION 6 QUESTION 7 QUESTION 8 QUESTION 9 QUESTION 10 nswer is nswer is nswer is nswer is nswer is nswer is nswer is nswer is nswer is nswer is QUESTION 1 (d) Reaction II. Temperature and pressure must be kept constant. The equilibrium mixture must contain both the products and the reactants and their concentration must remain constant. The solid zinc eventually disappears from the reaction mixture. QUESTION 2 The definition must make the following points: Reaction has to be reversible. The reaction does not stop. The rates of the forward and reverse reactions become equal and no change in concentrations is observed. The School For Excellence 2018 Unit 3 & 4 hemistry Resources Test Page 7

8 QUESTION 3 Reactants = red line; Products = blue line. bout 15 minutes. Reaction rate in arbitrary units Time (minutes) QUESTION 4 reaction reaches equilibrium because that is considered to be a state of maximum stability in terms of energy within the particles and the overall equilibrium system. QUESTION 5 The rate is zero. Lower. Equilibrium is reached when the concentrations of all species remain unchanged, including H 3O +. constant ph would indicate no change in hydronium ion concentration and therefore, a state of equilibrium. The School For Excellence 2018 Unit 3 & 4 hemistry Resources Test Page 8