Rates. Specification points. Year 10 - Rates of Reaction

Transcription

1 Rates Specification points Year 10 - Rates of Reaction Calculating rates of reactions The rate of a chemical reaction can be found by measuring the quantity of a reactant used or the quantity of product formed over time: mean rate of reaction = quantity of reactant used time taken mean rate of reaction = quantity of product formed time taken The quantity of reactant or product can be measured by the mass in grams or by a volume in cm 3. The units of rate of reaction may be given as g/s or cm 3 /s or mol/dm 3. Students should be able to: calculate the mean rate of a reaction from given information about the quantity of a reactant used or the quantity of a product formed and the time taken draw, and interpret, graphs showing the quantity of product formed or quantity of reactant used up against time draw tangents to the curves and use gradient of the tangent to calculate rate. Factors which affect the rates of chemical reactions Factors which affect the rates of chemical reactions include: the concentrations of reactants in solution, the pressure of reacting gases, the surface area of solid reactants, the temperature and the presence of catalysts. Know how to investigate factors affecting rates by measuring the loss in mass of reactants, the volume of gas produced or the time for a solution to become opaque or coloured. Students should be able to explain the effects on rates of reaction of changes in the size of pieces of a reacting solid in terms of surface area to volume ratio. Collision theory and activation energy Be able to use Collision theory to explain how various factors affect rates of reactions. Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy. Students should be able to explain the effects on rates of reaction of: changes in the size of pieces of a reacting solid in terms of surface area to volume ratio changes in temperature, concentration and pressure in terms of collision theory. Factors that increase the rate of reaction Increasing the concentration of reactants in solution, the pressure of reacting gases, and the surface area of solid reactants increases the frequency of collisions and rate. Increasing the temperature increases the frequency of collisions and makes the collisions more energetic, and so increases the rate of reaction. Students should be able to predict and explain the effects of changing conditions on the rate of a reaction use simple ideas about proportionality when using collision theory to explain the effect of a factor on the rate of a reaction Catalysts Catalysts change the rate of chemical reactions but are not used up during the reaction. Different reactions need different catalysts. Enzymes act as catalysts in biological systems.

2 Catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy be able to show this on a reaction profile diagram. Students should be able to identify catalysts in reactions from their effect on the rate of reaction and because they are not included in the chemical equation for the reaction. explain catalytic action in terms of activation energy. give the names of catalysts other than those specified in the subject content. Independent Study suggestions 1. Look at the specification points above use the textbook pages first edition and second edition, to make a few notes/spider diagram/revision cards 2. Watch the Fuse School short 3-4 minute explanation videos on any area where you need more help. Collision Theory Catalysts Hydrogen peroxide breakdown: Rates of reaction I Rates of reaction II Ionic Equations: 3. Have a go at the rates of reaction quizzes: 4. Now have a go at the past paper questions below which you can then mark

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7 Volume of product (cm 3 ) 2. a) Calculate the mean rate of reaction which produces 23 cm 3 of a gas over 140 s. (2 marks) b) Which one of these is the units for the rate of reaction above? cm 3 / mol mol / s s / cm 3 cm 3 / s (1 mark) 3. A student is investigating how different factors affect the reaction between marble chips and hydrochloric acid. This graph shows the volume of gas produced in the reaction over time Time (s) a) The student performs the reaction with the same mass of marble chips, broken into smaller pieces. On the graph, draw a curve to represent this reaction. Label the curve A. (2 marks) b) The student then performs the reaction with the original marble chips, but half the volume of hydrochloric acid. Draw another curve to represent this reaction. Label the curve B. (2 marks) 4. Hydrochloric acid and sodium thiosulfate are both colourless solutions. When they react, one of the products (sulfur) is a yellow solid which makes the solution go cloudy. hydrochloric acid + sodium thiosulfate sodium chloride + sulfur + water + sulfur dioxide colourless colourless yellow solid

8 Plan a practical method for investigating how a change in concentration of hydrochloric acid affects the rate of a reaction with sodium thiosulfate. Name all key equipment used and two variables you will control (6 marks) 5. a) Which of the following is true about the effect of increasing the temperature of a reaction? Tick one or more boxes. Makes the particles move faster Particles are closer together Particles collide with more energy More frequent successful collisions (2 marks) b) Explain how a decrease in pressure affects the rate of a reaction. (3 marks)

9 energy 6. Catalysts are often used in industrial and biological reactions, to increase the rate of reaction. a) Explain how a catalyst increases the rate of reaction. (2 marks) b) Name the type of catalyst used in biological reactions. (1 mark) c) Draw a second labelled curve on this reaction profile to show how a catalyst can affect a reaction. reactants Activation energy Energy change products progress (3 marks) d) When zinc, copper and sulfuric acid are added together to a flask, a catalysed reaction occurs. The catalysed reaction produces zinc sulfate by the following equation: Zn + H2SO4 ZnSO4 + H2 Identify the catalyst in this reaction, and explain how you know it is a catalyst. (2 marks)

10 mass loss (g) 7. Here is a graph for a chemical reaction which produces a gas time (min) a) Draw a tangent to this curve at 3.3 minutes. (1 mark) b) Explain whether the rate is higher at 1 minute or 3.3 minutes. (1 mark) Higher Tier c) Calculate the rate at 3.3 minutes.... g / min (3 marks)

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12 2. a) (1) = 0.16 (1) b) cm 3 / s (1) 3. a) Curve A: Steeper gradient (1), same end-point (1) b) Curve B: Same gradient (1), end point approximately half as high (1) 4. Level 3 a clear and well-ordered response addressing most of the indicative content and including all key equipment. 5 6 Level 2 a mostly well-ordered response, with some omissions or aspects of the answer in an illogical order. Includes most key equipment. 3 4 Level 1 one or two points included, but answer is not ordered in a logical way. 1 2 No relevant content 0 Indicative content: Key equipment 6 Different concentrations of hydrochloric acid

13 energy Cross drawn on paper Conical flask / beaker Timer Control variables Amount of swirling Point when timer is stopped (how visible the cross is) Temperature Concentration of thiosulfate Procedure Flask of known amount of thiosulfate on top of a cross Add hydrochloric acid to thiosulfate and start timer Measure time until cross disappears Wash flask Repeat with different concentrations of hydrochloric acid 5. a) Makes the particles move faster Particles are closer together Particles collide with more energy More frequent successful collisions all correct (2) one mistake (1) two or more mistakes (0) b) Reactant particles further apart (1) so less frequent successful collisions (1) and, therefore, lower rate (1) 6. a) Provides a different route (1) with a lower activation energy (1) b) Enzyme (1) c) peak of curve lower than original (1) starts at reactants and ends at products (1) activation energy labelled (1) reactants Catalysed activation energy Activation energy products Energy change progress d) Copper (1) It is not in the reaction equation (1) 7. a) Straight line touching only at 3.3 minutes (1)

14 mass loss (g) time (min) b) Higher at 1 minute AND it has a steeper slope / higher gradient (1) c) difference in x measured and difference in y matching difference in x (1) e.g. Change in y = = 1.4; Change in x = = 2.6 Gradient = Change in y Change in x e.g. = (1) Gradient = 0.54 (1) (Allow )