7. Equilibria
A section of Chemistry, 9701
Listing 10 of 344 questions
Hydrogen is the most abundant element in the Universe, although on Earth only very small quantities of molecular hydrogen have been found to occur naturally. Hydrogen is manufactured on a large scale for use in the chemical industry and is also regarded as a possible fuel to replace fossil fuels in internal combustion engines. State one large scale use of hydrogen in the chemical industry. One common way of producing hydrogen on a large scale for use in the chemical industry is by the steam ‘reforming’ of methane (natural gas), in which steam and methane are passed over a catalyst at 1000–1400 K to produce carbon monoxide and hydrogen. CH4+ H2OCO+ 3H2∆H = +206 kJ mol–1 Use the information above to state and explain the effect on the equilibrium position of the following changes. increasing the pressure applied to the equilibrium decreasing the temperature of the equilibrium What will be the effect on the rate of the reaction of increasing the pressure at which it is carried out? Explain your answer. Further hydrogen can be obtained by the ‘water-gas shift’ reaction in which the carbon monoxide produced is reacted with steam. CO+ H2OCO2+ H2Kc = 6.40 × 10–1 at 1100 K A mixture containing 0.40 mol of CO, 0.40 mol of H2O, 0.20 mol of CO2 and 0.20 mol of H2 was placed in a 1 dm3 fl ask and allowed to come to equilibrium at 1100 K Give an expression for Kc for this reaction. Calculate the amount, in moles, of each substance present in the equilibrium mixture at 1100 K. CO+ H2OCO2+ H2initial moles 0.40 0.40 0.20 0.20
9701_w12_qp_23
THEORY
2012
Paper 2, Variant 3
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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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The enthalpy change of solution, , of the Group2 sulfates becomes more endothermic down the group. State and explain the trend in the solubility of the Group 2 sulfates down the group. Write the expression for Kw , the ionic product of water. Kw =  The numerical value of Kw increases with increasing temperature. Place a tick () in the appropriate column in each row to show the effect of increasing the temperature of water on the pH and on the ratio [H+] : [OH–]. effect of increasing temperature of water decrease stay the same increase pH ratio [H+] : [OH–]  An aqueous solution of sodium hydroxide has a pH of 13.25 at 298 K. Calculate the concentration of this sodium hydroxide solution.  concentration = mol dm–3 Buffer solutions are used to regulate the pH of a solution to keep its pH value within a narrow range. Write two equations to describe how hydrogencarbonate ions, HCO3 –, and carbonic acid, H2CO3, control the pH of blood. The Ka for ethanoic acid is 1.75×10–5 mol dm–3 at 298 K. When ethanoic acid is dissolved in water, an equilibrium mixture containing two acid‑base pairs is formed. Write an equation for this equilibrium. In the boxes label each species acidic or basic to show its behaviour in this equilibrium. CH3CO2H + +  A buffer solution was prepared by adding 30.0 cm3 of 0.25 mol dm–3 ethanoic acid, an excess, to 20.0 cm3 of 0.15 mol dm–3 sodiumhydroxide. Calculate the pH of the buffer solution formed at 298 K. Give your answer to one decimal place.  pH = Titration curves for two different acid-base reactions, M and N, are shown. pH volume of acid added / cm3 pH volume of acid added / cm3 reaction M reaction N Use the titration curve for reaction M to deduce the volume of acid added at the end‑point for this titration.  volume of acid added at the end-point = cm3 The table shows some acid-base indicators. name of indicator pH range of colour change malachite green 0.2–1.8 bromocresol green 3.8–5.4 bromothymol blue 6.0–7.6 thymolphthalein 9.3–10.6 Name a suitable indicator for each of the acid-base titrations M and N. Explain your answers. reaction M reaction N explanation  
9701_w18_qp_41
THEORY
2018
Paper 4, Variant 1
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The enthalpy change of solution, , of the Group2 sulfates becomes more endothermic down the group. State and explain the trend in the solubility of the Group 2 sulfates down the group. Write the expression for Kw , the ionic product of water. Kw =  The numerical value of Kw increases with increasing temperature. Place a tick () in the appropriate column in each row to show the effect of increasing the temperature of water on the pH and on the ratio [H+] : [OH–]. effect of increasing temperature of water decrease stay the same increase pH ratio [H+] : [OH–]  An aqueous solution of sodium hydroxide has a pH of 13.25 at 298 K. Calculate the concentration of this sodium hydroxide solution.  concentration = mol dm–3 Buffer solutions are used to regulate the pH of a solution to keep its pH value within a narrow range. Write two equations to describe how hydrogencarbonate ions, HCO3 –, and carbonic acid, H2CO3, control the pH of blood. The Ka for ethanoic acid is 1.75×10–5 mol dm–3 at 298 K. When ethanoic acid is dissolved in water, an equilibrium mixture containing two acid‑base pairs is formed. Write an equation for this equilibrium. In the boxes label each species acidic or basic to show its behaviour in this equilibrium. CH3CO2H + +  A buffer solution was prepared by adding 30.0 cm3 of 0.25 mol dm–3 ethanoic acid, an excess, to 20.0 cm3 of 0.15 mol dm–3 sodiumhydroxide. Calculate the pH of the buffer solution formed at 298 K. Give your answer to one decimal place.  pH = Titration curves for two different acid-base reactions, M and N, are shown. pH volume of acid added / cm3 pH volume of acid added / cm3 reaction M reaction N Use the titration curve for reaction M to deduce the volume of acid added at the end‑point for this titration.  volume of acid added at the end-point = cm3 The table shows some acid-base indicators. name of indicator pH range of colour change malachite green 0.2–1.8 bromocresol green 3.8–5.4 bromothymol blue 6.0–7.6 thymolphthalein 9.3–10.6 Name a suitable indicator for each of the acid-base titrations M and N. Explain your answers. reaction M reaction N explanation  
9701_w18_qp_43
THEORY
2018
Paper 4, Variant 3
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