6.3. Reversible reactions and equilibrium
A subsection of Chemistry, 0620, through 6. Chemical reactions
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Aqueous ammoniumsulfate, (NH4)2SO4, is warmed with aqueous sodiumhydroxide. The pungent-smelling gas ammonia, NH3, is produced. Balance the equation for this reaction. (NH4)2SO4 + NaOH → NH3 + H2O + Na2SO4 A 2.8 g sample of impure ammoniumsulfate is found to contain 0.7 g of impurities. Calculate the percentage of ammoniumsulfate in this sample.  percentage of ammonium sulfate = % Describe a test for ammonia gas. test result  Ammonia gas is prepared at the front of a laboratory. The pungent smell of ammonia spreads throughout the laboratory slowly. Name the process that occurs when ammonia gas spreads throughout the laboratory. Explain, using ideas about particles, why ammonia gas spreads throughout the laboratory. Explain why carbondioxide gas, CO2, will spread throughout the laboratory at a slower rate than ammonia gas, NH3. Ammonia is produced in the Haber process. The equation for the reaction is shown. N2+ 3H2→ 2NH3In the Haber process, a temperature of 450 °C and a pressure of 200atmospheres are used in the presence of finely‑divided iron. A larger equilibrium yield of ammonia would be produced if a lower temperature and a higher pressure are used. Explain why a lower temperature and a higher pressure are not used. lower temperature higher pressure  State the role of iron in the Haber process. Ammonia is a weak base. Explain the meaning of the term base. Suggest the pH of aqueous ammonia. 
0620_w20_qp_41
THEORY
2020
Paper 4, Variant 1
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This question is about nitrogen and some of its compounds. Complete the dot-and-cross diagram to show the electron arrangement in a molecule of nitrogen, N2. Show the outer shell electrons only. N N  Nitrogen can be converted into ammonia by the Haber process. Describe how nitrogen is obtained for the Haber process. Give the essential reaction conditions and write a chemical equation for the reaction occurring in the Haber process. chemical equation: reaction conditions:  Some of the ammonia made by the Haber process is converted into nitricacid. The first stage of this process is the oxidation of ammonia to make nitrogenmonoxide. 4NH3+ 5O24NO+ 6H2OThe process is carried out at 900 °C and a pressure of 5atmospheres using an alloy of platinum and rhodium as a catalyst. The forward reaction is exothermic. State the meaning of the term catalyst. State the meaning of the term oxidation. Complete the table using the words increase, decrease or no change. effect on the rate of the forward reaction effect on the equilibrium yield of NOincreasing the temperature increasing the pressure  Nitrogenmonoxide, NO, is converted into nitrogen dioxide, NO2. 2NO+ O2→ 2NO2The nitrogendioxide reacts with oxygen and water to produce nitric acid as the only product. Write a chemical equation for this reaction. Ammonium nitrate, NH4NO3, is a fertiliser. Calculate the percentage by mass of nitrogen in ammoniumnitrate.  % 
0620_w20_qp_43
THEORY
2020
Paper 4, Variant 3
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This question is about nitrogen and some of its compounds. Nitrogen is converted into ammonia, NH3, in the Haber process. Nitrogen is obtained from air. State the percentage of nitrogen in clean, dry air. State the source of hydrogen for the Haber process. Complete the dot-and-cross diagram in Fig.3.1 for a molecule of ammonia. Show the outer shell electrons only. N H H H  Write a chemical equation for the reaction occurring in the Haber process and give the typical reaction conditions. Include units where appropriate. chemical equation reaction conditions: temperature pressure catalyst  Ammonia is converted into nitricacid. The first stage of this conversion uses a catalyst and occurs at a temperature of 900 °C and a pressure of 5atmospheres. 4NH3+ 5O24NO+ 6H2OThe forward reaction is exothermic. Suggest which of the following elements is most likely to be used as a catalyst. Draw a circle around your answer. calcium lead platinum sodium sulfur State the oxidation number of nitrogen in: NH3 NO  Use your answer to to explain whether the nitrogen in ammonia undergoes oxidation or reduction. Complete Table3.1 using the words increases, decreases or no change. Table 3.1 effect on the equilibrium yield of NOeffect on the rate of the forward reaction decreasing the pressure decreasing the temperature decreases removing the catalyst decreases  Decreasing the temperature causes the rate of the forward reaction to decrease. Explain, using collision theory, why the rate of the reaction is slower at the decreased temperature. In the second stage, nitric acid is produced. Balance the symbol equation for this reaction. NO + O2 + H2O → HNO3 
0620_w23_qp_43
THEORY
2023
Paper 4, Variant 3
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