8. Reaction kinetics
A section of Chemistry, 9701
Listing 10 of 220 questions
Acetals are compounds formed when aldehydes are reacted with an alcohol and an acid catalyst. The reaction between ethanal and methanol was studied in the inert solvent dioxan. H+ CH3CHO + 2CH3OH CH3CH(OCH3)2 + H2O ethanal methanol acetal A When the initial rate of this reaction was measured at various starting concentrations of the three reactants, the following results were obtained. experiment number / mol dm–3 / mol dm–3 [H+] / mol dm–3 relative rate 0.20 0.10 0.05 1.00 0.25 0.10 0.05 1.25 0.25 0.16 0.05 2.00 0.20 0.16 0.10 3.20 Use the data in the table to determine the order with respect to each reactant. order with respect to order with respect to order with respect to [H+] Use your results from part to write the rate equation for the reaction. State the units of the rate constant in the rate equation Calculate the relative rate of reaction for a mixture in which the starting concentrations of all three reactants are 0.20 mol dm–3. relative rate = The concentration of the acetal product was measured when experiment number 1 was allowed to reach equilibrium. The result is included in the following table. / mol dm–3 / mol dm–3 [H+] / mol dm–3 [acetal A] / mol dm–3 / mol dm–3 at start 0.20 0.10 0.05 0.00 0.00 at equilibrium (0.20–x at equilibrium 0.025 Complete the second row of the table in terms of x, the concentration of acetal A at equilibrium. You may wish to consult the chemical equation opposite. Using the [acetal A] as given, 0.025 mol dm–3, calculate the equilibrium concentrations of the other reactants and products and write them in the third row of the table. Write the expression for the equilibrium constant for this reaction, Kc, stating its units. Kc = units = Use your values in the third row of the table to calculate the value of Kc. Kc =
9701_w11_qp_41
THEORY
2011
Paper 4, Variant 1
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Acetals are compounds formed when aldehydes are reacted with an alcohol and an acid catalyst. The reaction between ethanal and methanol was studied in the inert solvent dioxan. H+ CH3CHO + 2CH3OH CH3CH(OCH3)2 + H2O ethanal methanol acetal A When the initial rate of this reaction was measured at various starting concentrations of the three reactants, the following results were obtained. experiment number / mol dm–3 / mol dm–3 [H+] / mol dm–3 relative rate 0.20 0.10 0.05 1.00 0.25 0.10 0.05 1.25 0.25 0.16 0.05 2.00 0.20 0.16 0.10 3.20 Use the data in the table to determine the order with respect to each reactant. order with respect to order with respect to order with respect to [H+] Use your results from part to write the rate equation for the reaction. State the units of the rate constant in the rate equation Calculate the relative rate of reaction for a mixture in which the starting concentrations of all three reactants are 0.20 mol dm–3. relative rate = The concentration of the acetal product was measured when experiment number 1 was allowed to reach equilibrium. The result is included in the following table. / mol dm–3 / mol dm–3 [H+] / mol dm–3 [acetal A] / mol dm–3 / mol dm–3 at start 0.20 0.10 0.05 0.00 0.00 at equilibrium (0.20–x at equilibrium 0.025 Complete the second row of the table in terms of x, the concentration of acetal A at equilibrium. You may wish to consult the chemical equation opposite. Using the [acetal A] as given, 0.025 mol dm–3, calculate the equilibrium concentrations of the other reactants and products and write them in the third row of the table. Write the expression for the equilibrium constant for this reaction, Kc, stating its units. Kc = units = Use your values in the third row of the table to calculate the value of Kc. Kc =
9701_w11_qp_42
THEORY
2011
Paper 4, Variant 2
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What is meant by the term ligand as applied to the chemistry of the transition elements? Describe the type of bonding that occurs between a ligand and a transition element. Chromium hexacarbonyl undergoes the following ligand replacement reaction. Cr(CO)6 + PR3 Cr(CO)5PR3 + CO Two separate experiments were carried out to study the rate of this reaction. In the first experiment, the ligand PR3 was in a large excess and [Cr(CO)6] was measured with time. The results are shown on the graph below. 0.00000 0.00200 0.00400 0.00600 concentration / mol dm–3 0.00800 0.01000 time/s In the second experiment, Cr(CO)6 was in a large excess, and was measured with time. The following results were obtained. time / s / mol dm–3 0.0100 0.0076 0.0060 0.0028 Plot the data in the table on the graph above, using the same axis scales, and draw the best-fit line through your points. Use the graphs to determine the order of reaction with respect to Cr(CO)6 and PR3. In each case explain how you arrived at your answer. Cr(CO)6 PR3 Write the rate equation for the reaction, and calculate a value for the rate constant, using the method of initial rates, or any other method you prefer. State the units of the rate constant. Four possible mechanisms for this reaction are given below. Draw a circle around the letter next to the one mechanism which is consistent with the rate equation you have written in . A Cr(CO)6 Cr(CO)5 + CO fast Cr(CO)5 + PR3 Cr(CO)5PR3 slow B Cr(CO)6 Cr(CO)5 + CO slow Cr(CO)5 + PR3 Cr(CO)5PR3 fast C Cr(CO)6 + PR3 [OC- - -Cr(CO)4- - -PR3] Cr(CO)5PR3 + CO (transition state) D Cr(CO)6 + PR3 Cr(CO)6PR3 slow Cr(CO)6PR3 Cr(CO)5PR3 + CO fast Explain your answer.
9701_w11_qp_43
THEORY
2011
Paper 4, Variant 3
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Enzymes are particular types of proteins that catalyse chemical reactions. The effi ciency of enzymes can be reduced by the presence of other substances known as inhibitors. State one example of a substance that can act as a non-competitive inhibitor in enzyme reactions. For the inhibitor you have identifi ed, explain why it is a non-competitive inhibitor. The graph shows the rate of an enzyme-catalysed reaction against the substrate concentration in the absence of an inhibitor. rate of reaction concentration of substrate On the same axes, sketch a graph showing the rate of this reaction if a competitive inhibitor was present. DNA is responsible for encoding the amino acid sequence to produce proteins. Ribosome, tRNA and mRNA are all involved in the process of protein synthesis. Write ribosome, tRNA and mRNA in the boxes below to show the correct sequence in which they are involved. DNA protein Sequences of three bases code for specifi c amino acids. The code UGA however does not usually code for an amino acid. Suggest its use. Much of the energy used in biochemical reactions is provided by the hydrolysis of the molecule ATP. What are the breakdown products of the hydrolysis of ATP? Give two uses for the energy released by ATP hydrolysis in cells. 1. 2.
9701_w13_qp_43
THEORY
2013
Paper 4, Variant 3
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The oxidation of nitrogen(oxide is shown in the equation. 2NO+ O2→ 2NO2The initial rate of this reaction was measured, starting with different concentrations of the two reactants. The following results were obtained. experiment number / mol dm–3 / mol dm–3 initial rate / mol dm–3 s–1 0.032 0.012 4.08 × 10–3 0.032 0.024 8.15 × 10–3 0.064 0.024 3.28 × 10–2 0.096 0.036 Use the data in the table to determine the order with respect to each reactant. Show your reasoning. Calculate the initial rate in experiment 4. Give your answer to two signifi cant fi gures. initial rate = mol dm–3 s–1 Write the rate equation for this reaction. Use the results of experiment 1 to calculate the rate constant, k, for this reaction. Include the units of k. rate constant, k = units On the following axes ● draw two Boltzmann distribution curves, at two different temperatures, T1 and T2 (T2 > T1), ● label the curves and the axes. State and explain, using your diagram, the effect of increasing temperature on the rate of reaction. The compound nitrosyl fl uoride, NOF, can be formed by the following reaction. 2NO+ F22NOFThe rate is fi rst order with respect to NO and F2. The reaction mechanism has two steps. Suggest equations for the two steps of this mechanism, stating which is the rate determining slower step.
9701_w14_qp_41
THEORY
2014
Paper 4, Variant 1
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The oxidation of nitrogen(oxide is shown in the equation. 2NO+ O2→ 2NO2The initial rate of this reaction was measured, starting with different concentrations of the two reactants. The following results were obtained. experiment number / mol dm–3 / mol dm–3 initial rate / mol dm–3 s–1 0.032 0.012 4.08 × 10–3 0.032 0.024 8.15 × 10–3 0.064 0.024 3.28 × 10–2 0.096 0.036 Use the data in the table to determine the order with respect to each reactant. Show your reasoning. Calculate the initial rate in experiment 4. Give your answer to two signifi cant fi gures. initial rate = mol dm–3 s–1 Write the rate equation for this reaction. Use the results of experiment 1 to calculate the rate constant, k, for this reaction. Include the units of k. rate constant, k = units On the following axes ● draw two Boltzmann distribution curves, at two different temperatures, T1 and T2 (T2 > T1), ● label the curves and the axes. State and explain, using your diagram, the effect of increasing temperature on the rate of reaction. The compound nitrosyl fl uoride, NOF, can be formed by the following reaction. 2NO+ F22NOFThe rate is fi rst order with respect to NO and F2. The reaction mechanism has two steps. Suggest equations for the two steps of this mechanism, stating which is the rate determining slower step.
9701_w14_qp_42
THEORY
2014
Paper 4, Variant 2
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Questions Discovered
220