8. Reaction kinetics
A section of Chemistry, 9701
Listing 10 of 220 questions
Enzymes are protein molecules that are highly efficient in catalysing specific chemical reactions in living organisms. To work in tissues, enzyme molecules generally need to be water-soluble. What does this tell you about the nature of the side-chains on the exterior of the molecules? Enzymes function by a substrate molecule interacting with a particular part of the enzyme known as the ‘active site’. The substrate is converted into products that are then released, to be replaced by another substrate molecule. Describe briefly the primary, secondary and tertiary structures of an enzyme. The activity of an enzyme depends upon the tertiary structure of the protein molecule. Explain how the tertiary structure produces an effective active site. Give two conditions that can reduce the activity of an enzyme, explaining the reason in each case. I II An individual enzyme operates best at a specific pH. Different enzymes operate best under conditions of different pH. Three enzymes involved in the digestion of food are amylase, pepsin and trypsin. • Amylase, found in saliva, hydrolyses starch to a mixture of glucose and maltose under approximately neutral conditions. • Pepsin hydrolyses proteins to peptides in the acid conditions of the stomach. • Trypsin continues the hydrolysis of peptides to amino acids in the mildly alkaline conditions of the small intestine. The graph below shows the activity of two of the three enzymes mentioned above. pH activity Label each peak shown with the name of the enzyme responsible, either amylase, pepsin or trypsin. On the axes above, sketch the graph that the third enzyme would produce, and label it with the name of that enzyme.
9701_s11_qp_42
THEORY
2011
Paper 4, Variant 2
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Enzymes are protein molecules that are highly efficient in catalysing specific chemical reactions in living organisms. To work in tissues, enzyme molecules generally need to be water-soluble. What does this tell you about the nature of the side-chains on the exterior of the molecules? Enzymes function by a substrate molecule interacting with a particular part of the enzyme known as the ‘active site’. The substrate is converted into products that are then released, to be replaced by another substrate molecule. Describe briefly the primary, secondary and tertiary structures of an enzyme. The activity of an enzyme depends upon the tertiary structure of the protein molecule. Explain how the tertiary structure produces an effective active site. Give two conditions that can reduce the activity of an enzyme, explaining the reason in each case. I II An individual enzyme operates best at a specific pH. Different enzymes operate best under conditions of different pH. Three enzymes involved in the digestion of food are amylase, pepsin and trypsin. • Amylase, found in saliva, hydrolyses starch to a mixture of glucose and maltose under approximately neutral conditions. • Pepsin hydrolyses proteins to peptides in the acid conditions of the stomach. • Trypsin continues the hydrolysis of peptides to amino acids in the mildly alkaline conditions of the small intestine. The graph below shows the activity of two of the three enzymes mentioned above. pH activity Label each peak shown with the name of the enzyme responsible, either amylase, pepsin or trypsin. On the axes above, sketch the graph that the third enzyme would produce, and label it with the name of that enzyme.
9701_s11_qp_43
THEORY
2011
Paper 4, Variant 3
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Carbon monoxide, CO, occurs in the exhaust gases of internal combustion engines. Suggest a dot-and-cross diagram for CO. Suggest one reason why CO is produced in addition to CO2 in some internal combustion engines. Carbon monoxide can be removed from the exhaust gases by a catalytic converter. Write an equation for a reaction that occurs in a catalytic converter that removes CO. The standard enthalpy change of formation, , of CO is –111 kJ mol–1, and that of CO2 is –394 kJ mol–1. Calculate the standard enthalpy change of the following reaction. C+ CO2→ 2CO∆H o = kJ mol–1 Carbon monoxide reacts with a ruthenium(chloride complex according to the equation [Ru(H2O)2Cl 4]2– + CO → [Ru(H2O)(CO)Cl 4]2– + H2O. Describe the type of reaction that is occurring here. During the reaction, the colour of the solution changes from deep blue to green. Explain the origin of colour in transition element complexes, and why different complexes often have different colours. The following table shows how the initial rate of this reaction varies with different concentrations of reactants. [[Ru(H2O)2Cl 4]2–] / mol dm–3 / mol dm–3 rate / mol dm–3 s–1 1.1 × 10–2 1.7 × 10–3 1.6 × 10–7 1.6 × 10–2 3.6 × 10–3 2.3 × 10–7 2.2 × 10–2 2.7 × 10–3 3.2 × 10–7 Use these data to determine the order of reaction with respect to each reagent, and write the rate equation for the reaction. There are three possible mechanisms for this reaction, which are described below. mechanism 1 [Ru(H2O)2Cl 4]2– + CO [Ru(H2O)(CO)Cl 4]2– + H2O [Ru(H2O)Cl 4]2– + CO [Ru(H2O)(CO)Cl 4]2– mechanism 2 [Ru(H2O)2Cl 4]2– [Ru(H2O)Cl 4]2– + H2O slow slow fast [Ru(H2O)2(CO)Cl 4]2– [Ru(H2O)(CO)Cl 4]2– + H2O mechanism 3 [Ru(H2O)2Cl 4]2– + CO [Ru(H2O)2(CO)Cl 4]2– slow fast Deduce which of these three mechanisms is consistent with the rate equation you suggested in part . Explain your answer.
9701_s12_qp_42
THEORY
2012
Paper 4, Variant 2
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Questions Discovered
220