6.3. Reversible reactions and equilibrium
A subsection of Chemistry, 0620, through 6. Chemical reactions
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The halogens are a group of non-metals in Group VII of the Periodic Table. The reactivity of the halogens decreases down the group. Describe an experiment which shows that chlorine is more reactive than iodine. Include an equation in your answer. The halogens form interhalogen compounds. These are compounds which contain two different halogens. Deduce the formula of the compound which has the composition 0.013 moles of iodine atoms and 0.065 moles of fl uorine atoms. Iodine reacts with chlorine to form a dark brown liquid, iodine monochloride. chlorine supplied crystals of iodine I2+ Cl 2→ 2ICl dark brown When more chlorine is added and the tube is sealed, a reversible reaction occurs and the reaction comes to equilibrium. ICl + Cl 2ICl 3dark brown yellow Give another example of a reversible reaction. Explain the term equilibrium. Chlorine is removed from the tube and a new equilibrium is formed. Explain why there is less of the yellow solid and more dark brown liquid in the new equilibrium mixture. A sealed tube containing the equilibrium mixture is placed in ice-cold water. There is an increase in the amount of yellow solid in the equilibrium mixture. What can you deduce about the forward reaction in this equilibrium? ICl + Cl 2ICl 3Explain your deduction.
0620_s15_qp_31
THEORY
2015
Paper 3, Variant 1
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The Atacama desert in Chile has deposits of the salt sodium nitrate. Very large amounts of this salt were exported to Europe for use as a fertiliser. After the introduction of the Haber process in 1913, this trade rapidly diminished. Explain why the introduction of the Haber process reduced the demand for sodium nitrate. Suggest why surface deposits of sodium nitrate only occur in areas with very low rainfall such as desert areas. The desert has smaller surface deposits of potassium nitrate. Suggest why potassium nitrate is a better fertiliser than the sodium salt. All nitrates decompose when heated. The extent to which a nitrate decomposes is determined by the metal in the salt. Sodium nitrate decomposes to form sodium nitrite, NaNO2. Write the equation for decomposition of sodium nitrate. Sodium nitrite is a reducing agent. What would be observed if an excess of sodium nitrite solution was added to a solution of acidifi ed potassium manganate(? Copper(nitrate decomposes to form copper(oxide, nitrogen dioxide and oxygen. What is the relationship between the extent of decomposition and the reactivity of the metal in the nitrate? The equation for the decomposition of copper(nitrate is given below. 2Cu(NO3)2 → 2CuO + 4NO2 + O2 Predict what you would observe when copper(nitrate is heated. Copper(nitrate forms a series of hydrates with the formula Cu(NO3)2.xH2O. All these hydrates decompose to form copper(oxide. 1 mole of Cu(NO3)2.xH2O forms 1 mole of CuO. What is meant by 1 mole of a substance? 7.26 g of a hydrate, Cu(NO3)2.xH2O, formed 2.4 g copper(oxide. number of moles of CuO formed = number of moles of Cu(NO3)2.xH2O in 7.26 g = mass of 1 mole of Cu(NO3)2.xH2O = g mass of 1 mole of Cu(NO3)2 is 188 g the value of x in this hydrate =
0620_s15_qp_32
THEORY
2015
Paper 3, Variant 2
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Sulfur dioxide reacts with excess oxygen to form sulfur trioxide. 2SO3⇌ O2+ 2SO2What is the meaning of the symbol ⇌? The energy level diagram for the reaction is shown. energy 2SO2+ O22SO3Is this reaction exothermic or endothermic? Give a reason for your answer. 1 12 The graph shows how the percentage yield of sulfur trioxide changes with temperature when the pressure is kept constant. temperature / °C % yield of sulfur trioxide Describe how the percentage yield of sulfur trioxide changes with temperature. Determine the percentage yield of sulfur trioxide when the temperature is 500ºC. Describe a test for sulfur dioxide. test result Give one use of sulfur dioxide. [ 1 12 Sulfur dioxide reacts with magnesium. 2Mg + SO2 →2MgO + S Which substance is reduced in this reaction? Explain your answer. Sulfur dioxide reacts with water to form sulfurous acid, H2SO3. Sulfurous acid reacts with hydrogen sulfide to form water and sulfur. Complete the chemical equation for this reaction. H2SO3 + 2H2S → H2O + 3S 1 12
0620_s16_qp_32
THEORY
2016
Paper 3, Variant 2
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