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Reactivity 2: how much, how fast, how far About this practice Stoichiometric calculations, reaction rates and chemical equilibrium.
Chemistry SL Reactivity 2: how much, how fast, how far
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1 25.0 cm³ of 0.250 mol dm⁻³ hydrochloric acid exactly neutralises 20.0 cm³ of sodium hydroxide solution. Calculate the concentration of sodium hydroxide, using HCl + NaOH → NaCl + H₂O. Medium 4 marks Calculator + 2 Marking analysis: A learner attempts the following task: “25.0 cm³ of 0.250 mol dm⁻³ hydrochloric acid exactly neutralises 20.0 cm³ of sodium hydroxide solution. Calculate the concentration of sodium hydroxide, using HCl + NaOH → NaCl + H₂O.” Their response addresses only this point: “Calculates moles of HCl: 0.250 × 0.0250 = 0.00625 mol.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks. Marking analysis Medium 4 marks Calculator + 3 Magnesium reacts with hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂. When 0.10 mol Mg reacts with 0.15 mol HCl, identify the limiting reactant and calculate the amount of hydrogen formed. Medium 4 marks Calculator + 4 Marking analysis: A learner attempts the following task: “Magnesium reacts with hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂. When 0.10 mol Mg reacts with 0.15 mol HCl, identify the limiting reactant and calculate the amount of hydrogen formed.” Their response addresses only this point: “Uses the equation ratio 1 mol Mg : 2 mol HCl.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks. Marking analysis Medium 4 marks Calculator + 5 State the collision theory requirements for a chemical reaction to occur between two particles. Easy 2 marks No calculator + 6 Marking analysis: A learner attempts the following task: “State the collision theory requirements for a chemical reaction to occur between two particles.” Their response addresses only this point: “States that particles must collide with each other.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks. Marking analysis Easy 2 marks No calculator + 7 Explain, using collision theory, why increasing temperature increases the rate of a chemical reaction. Easy 3 marks No calculator + 8 Marking analysis: A learner attempts the following task: “Explain, using collision theory, why increasing temperature increases the rate of a chemical reaction.” Their response addresses only this point: “States that increasing temperature increases the average kinetic energy of the particles.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks. Marking analysis Easy 3 marks No calculator + 9 Sketch (in words) how a catalyst affects the activation energy of a reaction, and explain why a catalyst increases reaction rate without being consumed. Easy 3 marks No calculator + 10 Marking analysis: A learner attempts the following task: “Sketch (in words) how a catalyst affects the activation energy of a reaction, and explain why a catalyst increases reaction rate without being consumed.” Their response addresses only this point: “States that a catalyst provides an alternative reaction pathway (mechanism) with a lower activation energy.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks. Marking analysis Easy 3 marks No calculator + 11 Write the equilibrium constant expression, Kc, for the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g). Easy 2 marks No calculator + 12 Marking analysis: A learner attempts the following task: “Write the equilibrium constant expression, Kc, for the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g).” Their response addresses only this point: “Places products (raised to their coefficients) in the numerator and reactants (raised to their coefficients) in the denominator.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks. Marking analysis Easy 2 marks No calculator + 13 State Le Chatelier's principle, and use it to predict the effect of increasing pressure on the position of equilibrium for N₂(g) + 3H₂(g) ⇌ 2NH₃(g). Easy 3 marks No calculator + 14 Marking analysis: A learner attempts the following task: “State Le Chatelier's principle, and use it to predict the effect of increasing pressure on the position of equilibrium for N₂(g) + 3H₂(g) ⇌ 2NH₃(g).” Their response addresses only this point: “States Le Chatelier's principle: if a system at equilibrium is subjected to a change, the equilibrium shifts to partially counteract that change.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks. Marking analysis Easy 3 marks No calculator + 15 The forward reaction A(g) + B(g) ⇌ C(g) is exothermic. Predict, with a reason, the effect of increasing temperature on the value of Kc. Easy 3 marks No calculator + 16 Marking analysis: A learner attempts the following task: “The forward reaction A(g) + B(g) ⇌ C(g) is exothermic. Predict, with a reason, the effect of increasing temperature on the value of Kc.” Their response addresses only this point: “States that increasing temperature favours the endothermic direction of the reaction (by Le Chatelier's principle).” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks. Marking analysis Easy 3 marks No calculator + 17 At a certain temperature, the equilibrium concentrations for the reaction A(g) ⇌ B(g) are [A] = 0.20 mol dm⁻³ and [B] = 0.80 mol dm⁻³. Calculate the value of Kc at this temperature. Easy 2 marks Calculator + 18 Marking analysis: A learner attempts the following task: “At a certain temperature, the equilibrium concentrations for the reaction A(g) ⇌ B(g) are [A] = 0.20 mol dm⁻³ and [B] = 0.80 mol dm⁻³. Calculate the value of Kc at this temperature.” Their response addresses only this point: “Uses Kc = [B]/[A].” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks. Marking analysis Easy 2 marks Calculator + 19 Explain why a catalyst does not affect the position of equilibrium or the value of Kc for a reversible reaction, even though it increases the rate at which equilibrium is reached. Easy 2 marks No calculator + 20 Marking analysis: A learner attempts the following task: “Explain why a catalyst does not affect the position of equilibrium or the value of Kc for a reversible reaction, even though it increases the rate at which equilibrium is reached.” Their response addresses only this point: “States that a catalyst lowers the activation energy of both the forward and reverse reactions by exactly the same amount.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks. Marking analysis Easy 2 marks No calculator + 21 Explain, using collision theory, why increasing the concentration of a reactant in solution increases the rate of a chemical reaction. Easy 3 marks No calculator + 22 Marking analysis: A learner attempts the following task: “Explain, using collision theory, why increasing the concentration of a reactant in solution increases the rate of a chemical reaction.” Their response addresses only this point: “States that increasing concentration increases the number of reactant particles present in a given volume.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks. Marking analysis Easy 3 marks No calculator + 23 Explain, using collision theory, why increasing the surface area of a solid reactant, e.g. using powder instead of a single lump, increases the rate of reaction. Easy 3 marks No calculator + 24 Marking analysis: A learner attempts the following task: “Explain, using collision theory, why increasing the surface area of a solid reactant, e.g. using powder instead of a single lump, increases the rate of reaction.” Their response addresses only this point: “States that increasing surface area exposes more reactant particles at the surface to collisions with particles of the other reactant.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks. Marking analysis Easy 3 marks No calculator + 25 For the equilibrium Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq), predict and explain, using Le Chatelier's principle, the effect on the position of equilibrium of adding more Fe³⁺(aq) ions. Easy 3 marks No calculator + 26 Marking analysis: A learner attempts the following task: “For the equilibrium Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq), predict and explain, using Le Chatelier's principle, the effect on the position of equilibrium of adding more Fe³⁺(aq) ions.” Their response addresses only this point: “States that adding more Fe³⁺(aq) increases the concentration of a reactant.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks. Marking analysis Easy 3 marks No calculator + 27 The rate equation for a reaction is rate = k[A][B]². State the order of reaction with respect to A, with respect to B, and the overall order of reaction. Easy 3 marks No calculator + 28 Marking analysis: A learner attempts the following task: “The rate equation for a reaction is rate = k[A][B]². State the order of reaction with respect to A, with respect to B, and the overall order of reaction.” Their response addresses only this point: “States that the reaction is first order with respect to A.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks. Marking analysis Easy 3 marks No calculator + 29 Distinguish between percentage yield and atom economy, and state one reason a reaction might have a high atom economy but a low percentage yield. Easy 3 marks No calculator + 30 Marking analysis: A learner attempts the following task: “Distinguish between percentage yield and atom economy, and state one reason a reaction might have a high atom economy but a low percentage yield.” Their response addresses only this point: “States that percentage yield = (actual yield ÷ theoretical yield) × 100, comparing the mass of product actually obtained to the maximum possible mass.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks. Marking analysis Easy 3 marks No calculator + Self-assessed Not marked yet
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