6.2. Rate of reaction
A subsection of Chemistry, 0620, through 6. Chemical reactions
Listing 10 of 319 questions
Hydrogen peroxide decomposes slowly at room temperature to form water and oxygen. The reaction is catalysed by manganese(oxide. 2H2O2 → 2H2O + O2 A student used the apparatus shown below to study how changing the concentration of hydrogen peroxide affects the speed of this reaction. hydrogen peroxide manganese(oxide gas syringe oxygen collects here Apart from the volume of hydrogen peroxide, state two things that the student must keep the same in each experiment. 1. 2. The student measured the volume of oxygen produced using three different concentrations of hydrogen peroxide. The results are shown on the graph below. time / s volume of oxygen / cm3 concentration of hydrogen peroxide in g / dm3 A B C Describe how the speed of the reaction varies with the concentration of hydrogen peroxide. Explain why the fi nal volume of oxygen given off is less for graph B than for graph A. From the graph, determine the time taken for the reaction to be completed when 3 g / dm3 hydrogen peroxide (line A) was used. the volume of oxygen produced by 2 g / dm3 hydrogen peroxide (line B) in the fi rst 15 seconds. The student then tested various compounds to see how well they catalysed the reaction. He used the same concentration of hydrogen peroxide in each experiment. The table shows the time taken to produce 20 cm3 of oxygen using each compound as a catalyst. compound time taken to produce 20 cm3 of oxygen / s copper(oxide lead(oxide magnesium oxide did not produce any oxygen manganese(oxide Put these compounds in order of their effectiveness as catalysts. worst catalyst best catalyst
0620_w11_qp_22
THEORY
2011
Paper 2, Variant 2
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Hydrogen peroxide decomposes slowly at room temperature to form water and oxygen. The reaction is catalysed by manganese(oxide. 2H2O2 → 2H2O + O2 A student used the apparatus shown below to study how changing the concentration of hydrogen peroxide affects the speed of this reaction. hydrogen peroxide manganese(oxide gas syringe oxygen collects here Apart from the volume of hydrogen peroxide, state two things that the student must keep the same in each experiment. 1. 2. The student measured the volume of oxygen produced using three different concentrations of hydrogen peroxide. The results are shown on the graph below. time / s volume of oxygen / cm3 concentration of hydrogen peroxide in g / dm3 A B C Describe how the speed of the reaction varies with the concentration of hydrogen peroxide. Explain why the fi nal volume of oxygen given off is less for graph B than for graph A. From the graph, determine the time taken for the reaction to be completed when 3 g / dm3 hydrogen peroxide (line A) was used. the volume of oxygen produced by 2 g / dm3 hydrogen peroxide (line B) in the fi rst 15 seconds. The student then tested various compounds to see how well they catalysed the reaction. He used the same concentration of hydrogen peroxide in each experiment. The table shows the time taken to produce 20 cm3 of oxygen using each compound as a catalyst. compound time taken to produce 20 cm3 of oxygen / s copper(oxide lead(oxide magnesium oxide did not produce any oxygen manganese(oxide Put these compounds in order of their effectiveness as catalysts. worst catalyst best catalyst
0620_w11_qp_23
THEORY
2011
Paper 2, Variant 3
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The rate of the reaction between iron and aqueous bromine can be investigated using the apparatus shown below. piece of metal solution of bromine attached to stirrer piece of metal rotates A piece of iron was weighed and placed in the apparatus. It was removed at regular intervals and the clock was paused. The piece of iron was washed, dried, weighed and replaced. The clock was restarted. This was continued until the solution was colourless. The mass of iron was plotted against time. The graph shows the results obtained. mass of iron time (iron was in solution of bromine) Suggest an explanation for the shape of the graph. Predict the shape of the graph if a similar piece of iron with a much rougher surface had been used. Explain your answer. Describe how you could fi nd out if the rate of this reaction depended on the speed of stirring. Iron has two oxidation states +2 and +3. There are two possible equations for the redox reaction between iron and bromine. Fe + Br2 → Fe2+ + 2Br – 2Fe + 3Br2 → 2Fe3+ + 6Br – Indicate, on the fi rst equation, the change which is oxidation. Give a reason for your choice. Which substance in the fi rst equation is the reductant (reducing agent)? Describe how you could test the solution to fi nd out which ion, Fe2+ or Fe3+, is present.
0620_w11_qp_31
THEORY
2011
Paper 3, Variant 1
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Starch, a complex carbohydrate, is a natural macromolecule or polymer. It can be formed from its monomer by condensation polymerisation. Explain the terms: monomer condensation polymerisation Draw the structural formula of starch to include three monomer units. Glucose, the monomer, can be represented as . Starch can be hydrolysed to simple sugars by heating with dilute sulfuric acid or by warming with a dilute solution of saliva. The reaction can be catalysed by H+ ions from the acid or by the enzymes in saliva. What is an enzyme? Explain why, if the saliva / starch mixture is heated above 70 °C, the hydrolysis stops. The complete acid-catalysed hydrolysis of starch forms only glucose. The partial acid-catalysed hydrolysis of starch forms a mixture of sugars which includes glucose. Describe how you could identify the different sugars in this mixture. HO OH
0620_w11_qp_33
THEORY
2011
Paper 3, Variant 3
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The speed of a chemical reaction depends on a number of factors which include temperature and the presence of a catalyst. Reaction speed increases as the temperature increases. Explain why reaction speed increases with temperature. Reactions involving enzymes do not follow the above pattern. The following graph shows how the speed of such a reaction varies with temperature. speed temperature Suggest an explanation why initially the reaction speed increases then above a certain temperature the speed decreases. An organic compound decomposes to give off nitrogen. C6H5N2Cl → C6H5Cl + N2The speed of this reaction can be determined by measuring the volume of nitrogen formed at regular intervals. Typical results are shown in the graph below. time volume of nitrogen The reaction is catalysed by copper. Sketch the graph for the catalysed reaction on the diagram above. How does the speed of this reaction vary with time? Why does the speed of reaction vary with time? Catalytic converters reduce the pollution from motor vehicles. oxides of nitrogen and carbon monoxide less harmful gases to atmosphere catalysts: rhodium, platinum, palladium Describe how carbon monoxide and the oxides of nitrogen are formed in car engines. Describe the reactioninside the catalytic converter which change these pollutants into less harmful gases. Include at least one equation in your description.
0620_w12_qp_31
THEORY
2012
Paper 3, Variant 1
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