Chemistry Module 3 Assignment 1
Redox and Chemical Equilibria
Answer all the following questions ON PAPER and submit to your lecturer with a signed cover sheet. Presentation and the accuracy of your written English will be taken into account in marking. Set all calculations out in full. Numerical answers should be given to the appropriate number of significant figures.
Due: Week 3 beginning 23rd March 2020
1. At 250oC, the value of the equilibrium constant Kc1, for the reaction equation labelled (1) below, is 0.0835
(1) C2H4(g) + H2O(g) ⇌ C2H5OH(g) ΔHoR1 = – 45.0 KJ mol-1
(a) (i) Write the expression of Kc1 as a function of the equilibrium concentrations of the reactants and products of reaction (I) (1 mark)
(ii) give the unit of Kc1. (1 mark)
(b) Kc2 and Kc3 are the equilibrium constants for respectively reaction (2) and reaction (3). (2) C2H5OH(g) ⇌ C2H4(g) + H2O(g) (3) 3 C2H4(g) + 3 H2O(g) ⇌ 3 C2H5OH(g)
ΔHoR2 and ΔHoR3 are the standard enthalpy changes for respectively reaction (2) and reaction (3):
(i) Write the expression of Kc2 as a function of the equilibrium concentrations of the reactants and products of reaction (II)
-Compare the expressions of Kc2 and Kc1 and write Kc2 as a function of Kc1 – Calculate the value of Kc2
– Give the unit of Kc2 – Give the value of ΔHoR2 (5 marks) (ii) Write the expression of Kc3 as a function of the equilibrium concentrations of the reactants and products of reaction (3)
– Compare the expressions of Kc3 and Kc1 and write Kc3 as a function of Kc1 – calculate the value of Kc3 – give the unit of Kc3 – give the value of ΔHoR3 (5 marks)
Chemistry Module 3 Assignment 1
Redox and Chemical Equilibria
(c) When reaction (1) is conducted at a higher temperature than 250oC, the value of Kc1 decreases. Using Le Châtelier’s principle give a clear explanation (in complete sentences) as to why the value of Kc1 decreases. (4 marks)
(d) A student wrote about reaction (1): “When pressure is increased, more products are formed, so the value of Kc1 goes up”
Comment the statement made by the student by stating clearly why you believe that the student is right, or wrong, or partially wrong.
(5 marks)
2. Dinitrogen tetroxide (N2O4) is a colourless gaseous oxidising agent that can be used in conjunction with some reducing agents to form rocket propellants. In a closed system, dinitrogen tetroxide is formed from nitrogen dioxide (NO2), a brown toxic gas, by a reversible reaction. A chemical equilibrium is eventually achieved.
2NO2 (g) N2O4 (g) ΔHo = -57 KJ mol-1 brown colourless
0.100 mol of NO2(g) is placed in a 2.00 dm3 closed flask at 25oC. As time passes, the brown colour of the gas mixture gets lighter. Equilibrium is reached when the shade of brown stops changing. At equilibrium, 40% of the NO2(g) has reacted to form N2O4(g).
(a) (i) Explain why the colour of the gas mixture remains constant at equilibrium (2 marks)
(ii) Is it correct to state “The reaction stops once the state of equilibrium is reached”? Justify your answer. (2 marks) (b)
(i) Write an expression for the constant of equilibrium Kc of the reaction as a function of the equilibrium concentrations of the reactant and product.
(ii) Give the units of Kc
(2 marks)
(c) Reproduce the table below, and complete it: (3 marks)
NO2 N2O4 initial amount (mol) 0.100 mol 0.00 mol change of amount amount at equilibrium (mol) concentration at equilibrium
(d) Hence calculate the value of Kc at 25oC. (2 marks)
Chemistry Module 3 Assignment 1
Redox and Chemical Equilibria
(e) (i) What change of colour would you observe if the flask containing the gas mixture at equilibrium were placed in an ice bath? (1 mark)
(ii) Comment on the speed at which that change of colour would occur compared to the initial change of colour to reach equilibrium. (1 mark)
(iii) Give full explanations to your previous two answers to questions (e).(i) and (e).(ii). (4 marks)
3. A rocket propulsion system uses the very exothermic reaction between the gases dinitrogen tetroxide (N2O4(g)) and hydrazine (N2H4(g)). The reaction forms the gas nitrogen and water vapour. The following equation is not balanced:
N2O4(g) + N2H4(g) → N2(g) + H2O(g)
(a) Assign the oxidation states of each element in the reactants and products
of the equation above. (3 marks)
(b) (i) Which reactant is oxidised? (0.5 mark)
(ii) Identify the element that undergoes oxidation and state how many electrons are lost through oxidation by an atom of that element. (1 mark)
(iii) Which reactant is reduced? (0.5 mark)
(iv) Identify the element that undergoes reduction and state how many electrons are gained through reduction by an atom of that element. (1 mark)
(c) Therefore, in what ratio should N2O4 and N2H4 react with each other? Justify your answer. (2 marks)
(d) Balance the reaction equation above. (1 mark)
(e) (i) Which reagent is the oxidising agent? Justify your answer (2 marks)
(ii) Which reagent is the reducing agent? Justify your answer (2 marks)
4. MnO4-, manganate(VII) ions (or permanganate ions) react under alkaline aqueous conditions with C2O42-, ethanedioate ions (or oxalate ions) to form MnO2, manganese(IV) oxide and CO2(g), carbon dioxide.
(a) Balance step by step the following half-reaction equation (A) under alkaline aqueous conditions: (you must show and explain all the steps followed to balance the half-reaction equation in order to get the marks)
Chemistry Module 3 Assignment 1
Redox and Chemical Equilibria
(A) MnO4- (aq) → MnO2(s) (3 marks)
(b) Balance step by step the following half-reaction equation under alkaline aqueous conditions: (you must show and explain all the steps followed to balance the half-reaction equation in order to get the marks)
(B) C2O42- (aq) → CO2 (g) (1 mark)
(c) (i) state whether half-reaction (A) is a reduction or oxidation step, and justify your answer. (1 mark)
(ii) state whether half-reaction (B) is a reduction or oxidation step, and justify your answer. (1 mark)
(iii) combine the two half-reactions to form a balanced redox reaction, include state symbols. (3 marks)
