Download midterm 2 exam for section 3 from 2015

CAMOSUN COLLEGE
Chemistry 121 Section 03
Instructor: H. M. Cartwright
Midterm 2 Examination, March 25th, 2015, 6.30 p.m.
Time: 1 ½ hours
Name
____________________________________
Student Number ____________________
Answer all questions on the examination paper itself. If you need extra space, write on the
reverse of the paper, indicating clearly within your answer to the question that you have
done so.
Attempt all the questions. The numbers in square brackets indicate the approximate number
of marks available for each part of the question.
Do not open the examination paper until you are instructed to do so.
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1.
(a) What is an exothermic reaction?
[1]
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(b) What sign does H have for an exothermic reaction?
2.
[1]
Explain the difference in thermodynamics between the system and the surroundings.
[3]
3.
(a) What is a thermochemical equation?
(b) What is Hess's law?
[1]
[2]
(c) Calculate H for the formation of acetylene, C2H2, as shown in the reaction
2C(s) + H2(g)  C2H2(g)
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using the following data:
[5]
C2H2(g) + 2 ½ O2(g)  2CO2 (g) + H2O(l)
H = ─ 1299.6kJ
C(s) + O2(g)  CO2(g)
H = ─ 393.5 kJ
H2(g) + ½ O2(g)  H2O(l)
H = ─ 285.8 kJ
(d) Acetylene is burnt in oxy-acetylene torches to give a very high temperature. Taking
note of your answer to part (c) above, and the equations given in the question, give
reasons why an oxy-acetylene flame reaches such a high temperature.
[3]
4.
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Explain what is meant by the following terms which are used in thermochemistry:
[3]
(i) Open system
(ii) Closed system
(iii) Isolated system
5.
(a) What is a state function?
[2]
(b) For each of the following quantities place a tick to indicate whether the quantity is,
or is not, a State Function:
[3]
Quantity
6.
Is a State Function
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Is not a State Function
Temperature


Work


Internal energy


Entropy


Heat


Enthalpy


(a) Define the standard enthalpy of formation,Hof, of a compound.
[2]
(b) Carbon nanotubes are an elemental form of carbon produced by an electrical
discharge between carbon electrodes. Which ONE of the following statements about
the standard enthalpy of formation of carbon nanotubes is true?
[2]
(i) Hof is zero because nanotubes are just another elemental form of carbon.
(ii) Hof is zero because changing the subscripts of an elemental formula does not change the
standard heat of formation.
(iii) Hof is not zero because carbon nanotubes are not the most stable form of the element
under standard conditions.
(iv) Hof is not zero because there is a temperature change when nanotubes are formed.
7.
For EITHER the 1st law OR the 2nd law of thermodynamics:
(a) State the chosen law, indicating which law it is.
[2]
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(b) Write the relevant equation for the law.
[1]



(c) Explain the meaning of the symbols in the equation.
8.
(a) Explain what is meant by the specific heat of a material.
[2]
[2]
(b) 26 ml of 0.2 M HCl reacted with 8 ml of 0.6 M NaOH in a calorimeter. When the
reaction was complete it was found that the temperature had risen by 1.83 K. You may assume
that the calorimeter and liquids were all at the same temperature initially.
(i) Determine which, if either, of the reagents is present in excess.
[2]
(ii) Assuming that the density of the solutions is 1 g ml-1, the specific heat of the solutions is
4.18 J K-1 g-1 and the heat capacity of the calorimeter is 41 J K-1, calculate the total heat released
by the neutralisation.
[2]
(iii) Calculate the enthalpy of the neutralisation reaction per mole of acid.
[2]
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(iv) In a second experiment the HCl was replaced by 26 ml of 0.2M HNO3 and the experiment
repeated. What should the temperature rise be? Justify your answer.
[2]
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(v) In a final experiment the HCl was replaced by 26 mL of 0.2M CH3COOH (acetic acid). Should
the temperature rise be the same as that in part (iv) or different? Justify your answer.
[2]
9.
For each of the following processes predict whether the entropy of the system will
increase or decrease; briefly explain your reasoning for each one:
[3 x 2]
O3(g)  1 ½ O2(g)
Liquid sulphur (108oC)  Solid sulphur (108oC)
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2HBr(g)  Br2(g) + H2(g)
10.
(a) What is a spontaneous process in thermodynamics?
(b) What thermodynamic parameter is used in thermodynamics to predict the
spontaneity of the reaction?
[2]
[1]
(c) Write down an equation for the change in Gibb's Free Energy, G, in terms of
changes in entropy and enthalpy.
[1]
(d) Write a balanced equation for the combustion of liquid dimethyl ether (C2H6O) in
oxygen.
[2]
(e) What sign should S have for the combustion in part (d)? Justify your answer.
[2]
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(f) What sign should H have for the combustion in part (d)? Justify your answer.
[2]
(g) Taking note of your answers to parts (c), (e) and (f), over what temperature range
would this reaction be spontaneous?
[2]
11.
Halogens can react with one another to form "interhalogen" compounds; the reaction
between chlorine and brome is typical:
Cl2(g) + Br2(g)  2ClBr(g)
(a) Which ONE of the following statements is true when the reaction has reached
equilibrium at a temperature of 80oC when all species are gases?
[2]
(i) All reaction has come to a halt.
(ii) The amounts of reactant and products are equal.
(iii) The amount of reactant is zero.
(iv) The concentration of chlorine and bromine is equal.
(v) The rates of the forward and back reaction are equal.
(b) The normal boiling point of ClBr is 5oC, of Cl2 is -34oC, and of Br2 is 59oC. Suppose
that the reaction above is carried out at 25oC; would your conclusion about which of
options (i)-(v) is true be different? Explain.
12.
(a) State Le Chatelier's Principle.
[3]
(b) Write down the expression for the equilibrium constant Kc for each of the following
reactions:
[4]
2NH3 (g)  N2 (g) + 3 H2 (g)
CH3COOH (aq)  CH3COO  (aq) + H+ (aq)
CH3COOH (aq) + H2O (l)  CH3COO  (aq) + H3O+ (aq)
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CaCO3(s)  CaO (s) + CO2 (g)
(c) Predict whether the reaction given below is more likely to be exothermic or more
likely to be endothermic (hint: consider the reverse reaction). Justify your choice.
[2]
2CO2(g) + 3H2O(l)  C2H5OH (l) + 3 O2(g)
(d) The reaction shown in part (c) above is allowed to run to equilibrium. In which
direction would the equilibrium shift if each of the following changes were made:
[4 x 2]
(i) An increase in the concentration of CO2?
(ii) An increase in the amount of liquid methanol?
(iii) An increase in the temperature?
(iv) A decrease in the total pressure?
(v) The addition of a catalyst?
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13.
Phosphorus trichloride reacts with chlorine in an equilibrium reaction to generate
phosphorus pentachloride:
PCl3 (g) + Cl2 (g)  PCl5 (g)
(a) Write down the expression for the equilibrium constant for the reaction, Kc. [1]
(b) At a certain temperature when the reaction had reached equilibrium, the
concentrations of the three species were:
[PCl3] = 0.42 M
[Cl2] = 0.37 M
[PCl5] = 0.39M
Calculate the value of the equilibrium constant.
[1]
(c) In a second experiment at the same temperature, the initial concentrations were:
[PCl3] = 0.53M
[Cl2] = 0.53M
[PCl5] = 1.1M.
Determine the concentrations of all three species in this second experiment once
equilibrium has been achieved.
[4]
Note: the solutions of the quadratic equation ax2 + bx + c = 0 are 𝑥 =
−𝑏±√(𝑏 2 −4𝑎𝑐)
2𝑎
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14.
Explain the difference between Arrhenius and Bronsted-Lowry acids. Give one example
of each type.
[4]
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