7.1. Chemical equilibria: reversible reactions, dynamic equilibrium
A subsection of Chemistry, 9701, through 7. Equilibria
Listing 10 of 211 questions
Dinitrogen tetraoxide, N2O4, and nitrogen dioxide, NO2, exist in dynamic equilibrium with each other. N2O42NO2∆H = +54 kJ mol–1 The energy profile for this reaction is shown. energy reaction pathway N2O42NO2Add labelled arrows to the energy profile to indicate • the enthalpy change of the reaction, ∆H, • the activation energy of the forward reaction, Ea. 0.0500 mol of N2O4 was placed in a sealed vessel of volume 1.00 dm3, at a temperature of 50 °C and a pressure of 1.68 × 105 Pa. The mass of the resulting equilibrium mixture was 4.606 g. Calculate the average molecular mass, Mr, of the resulting equilibrium mixture. Give your answer to three significant figures. Mr = The number of moles of N2O4 that dissociated can be represented by n. State, in terms of n, the amount, in moles, of NO2 in the equilibrium mixture. moles of NO2 = The number of moles of N2O4 remaining at equilibrium is (0.05 – n). State, in terms of n, the total amount, in moles, of gas in the equilibrium mixture. State, in terms of n, the mole fraction of NO2 in the equilibrium mixture. In this equilibrium mixture, the mole fraction of NO2 is 0.400. Use your answers to and to calculate the amount in moles of each gas in the equilibrium mixture. Give your answers to three significant figures. amount of N2O4 = mol amount of NO2 = mol Write the expression for the equilibrium constant, Kp, for this equilibrium. Kp = Use the total pressure of the mixture, 1.68×105 Pa, to calculate the value of the equilibrium constant, Kp, and give its units. Kp = units =
9701_w16_qp_22
THEORY
2016
Paper 2, Variant 2
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At 450 K phosphorus(chloride, PCl 5, decomposes to form phosphorus(chloride, PCl 3, and chlorine, Cl 2. A dynamic equilibrium is established as shown. PCl 5PCl 3+ Cl 2ΔH = +124 kJ mol–1 The enthalpy change of formation of PCl 3under these conditions is given. ΔHf PCl 3= –320 kJ mol–1 Calculate the enthalpy change of formation of PCl 5under these conditions. Include a sign with your answer. enthalpy change = kJ mol–1 State and explain the effect of increasing temperature on the rate of decomposition of PCl 5. State and explain the effect of increasing temperature on the percentage of PCl 5that decomposes. Explain the meaning of the term dynamic equilibrium and the conditions necessary for it to become established. When 2.00 mol of PCl 5are decomposed at 450 K and 1.00×105 Pa the resulting equilibrium mixture contains 0.800 mol of Cl 2. Calculate the partial pressure of phosphorus(chloride, pPCl 5, in this equilibrium mixture. pPCl 5 = Pa Write the expression for the equilibrium constant, Kp, for the decomposition of PCl 5. Kp = The partial pressures of PCl 3and of Cl 2in this equilibrium mixture are both 2.86×104 Pa. Calculate the value of Kp and state its units. Kp = units =
9701_w17_qp_22
THEORY
2017
Paper 2, Variant 2
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When Cl NO2 reacts with NO an equilibrium is established. Cl NO2+ NONO2+ Cl NOIn each Cl NO2 molecule the nitrogen atom is bonded to the chlorine atom and bonded to each of the oxygen atoms separately. Draw a ‘dot-and-cross’ diagram for the Cl NO2 molecule.  The reaction between Cl NO2 and NO is first order with respect to each reactant. Write the rate equation for this reaction. rate = Deduce the units of the rate constant, k, when the concentrations of both gases are measured in mol dm–3 and the rate is measured in mol dm–3 s–1. State and explain whether or not the reaction could take place in a single step. An experiment is carried out in which the initial [Cl NO2] is 2.0×10–4 mol dm–3. A large excess of NO is used. The initial rate of reaction is 1.0×10–4 mol dm–3 s–1. The rate of the reaction is assumed to be constant for the first 0.20seconds. Draw a graph on the grid to show how the concentration of Cl NO2 varies for the first 0.20seconds. 3.0 × 10–4 2.5 × 10–4 2.0 × 10–4 1.5 × 10–4 1.0 × 10–4 0.5 × 10–4 0.05 0.10 0.15 [Cl NO2] / mol dm–3 time / s 0.20 0.25  Deduce the concentration of the NO2 product at 0.20seconds. After 20seconds the concentration of Cl NO2 remains constant. Explain this observation. 
9701_w19_qp_41
THEORY
2019
Paper 4, Variant 1
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When Cl NO2 reacts with NO an equilibrium is established. Cl NO2+ NONO2+ Cl NOIn each Cl NO2 molecule the nitrogen atom is bonded to the chlorine atom and bonded to each of the oxygen atoms separately. Draw a ‘dot-and-cross’ diagram for the Cl NO2 molecule.  The reaction between Cl NO2 and NO is first order with respect to each reactant. Write the rate equation for this reaction. rate = Deduce the units of the rate constant, k, when the concentrations of both gases are measured in mol dm–3 and the rate is measured in mol dm–3 s–1. State and explain whether or not the reaction could take place in a single step. An experiment is carried out in which the initial [Cl NO2] is 2.0×10–4 mol dm–3. A large excess of NO is used. The initial rate of reaction is 1.0×10–4 mol dm–3 s–1. The rate of the reaction is assumed to be constant for the first 0.20seconds. Draw a graph on the grid to show how the concentration of Cl NO2 varies for the first 0.20seconds. 3.0 × 10–4 2.5 × 10–4 2.0 × 10–4 1.5 × 10–4 1.0 × 10–4 0.5 × 10–4 0.05 0.10 0.15 [Cl NO2] / mol dm–3 time / s 0.20 0.25  Deduce the concentration of the NO2 product at 0.20seconds. After 20seconds the concentration of Cl NO2 remains constant. Explain this observation. 
9701_w19_qp_43
THEORY
2019
Paper 4, Variant 3
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