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# Reaction rates (Examville.com)

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### Reaction rates (Examville.com)

1. 1. Rate of Reaction I Mind Map Definition What is rate of reaction? 1. Rate of reaction is defined as the change in the amount of reactants or products per unit time. 2. It means ‘how fast is the reaction’ or 'the speed of the reaction'. • Fast Reaction  High rate • Slow Reaction  Low rate 3. Therefore, Quantity change of reactants/products Rates of reaction = Total time for the reaction Example 1 In a chemical reaction, 2.5g of calcium carbonate react completely with excess hydrochloric acid to produce 600cm3 of carbon dioxide gas in 1.5 minutes. Find the rate of reaction in term of a. decreasing mass of calcium carbonate b. increasing volume of carbon dioxide gas produced [a. -0.0278g/s; b. 6.67cm3/s] 1
2. 2. Example of fast reaction Type of Reaction Example Combustion Combustion of magnesium in oxygen Combustion of ethane (C2H6) Reaction between Reaction between potassium and water reactive metal and water Reaction between metal Reaction between limestone/ marble and sulphuric acid carbonate and acid Ionic precipitation Precipitation of silver(I) chloride (Double decomposition) AgNO3 + HCl → AgCl + HNO3 Example of slow reaction Type of Reaction Example Photosynthesis 6CO2 + 6 H2O → C6H12O6 + 6O2 Rusting 4Fe + 3O2 + 2H2O → 2Fe2O3•H2O Fermentation C6H12O6 → 2C2H5OH + 2CO2 Measurement and Calculation Quantity Measure and Average Rate Measurable quantity During a chemical reaction, two things happen 2. The easily measured quantity changes include The quantities of reactants reduce. a. Mass The quantities of products increase. b. Concentration (Conductivity) 1. Therefore, the rate of the reaction can be c. Volume of gas determined by a. measuring the decrease of the amount of the reactants over time. b. measuring the increase of the amount of the products over time. 2
3. 3. Example Example 2 Reaction between calcium carbonate and In a reaction, 5 g of calcium carbonate takes 250 hydrochloric acid. seconds to completely react with solution of hydrochloric acid. Calculate the average rate for CaCO3 + 2HCl → CaCl2 + CO2 + H2O this reaction in units The rate of the reaction can be determined by (a) g s-1 and a. Measuring the reduces of the mass of the (b) mol s-1 calcium carbonate (reactants) over time [ Relative atomic mass: C 12; 0, 16; Ca, 40] Mass of CaCO3 reacted Rate of Reaction = Time taken b. Measuring the increases of the volume of the carbon dioxide gas produced over time. Vlolume of CO 2 produced Rate of Reaction = Time taken [a. 0.02g/s; b. 0.0002mol/s] Immeasurable quantity and average rate 1. In some reaction, some changes are observable Example 4 but difficult to be measured. For example When the aqueous of ethanadioic is mixed with a. Change in colour acided potassium manganate(VII) , the reaction b. Precipitation happen slowly at room temperature. The purple 2. The time taken for the colour of a reactant to colour of the solution is bleached after 40 seconds. change or certain amount of precipitate to Calculate the average rate of reaction. form can be used to measure the rate of Answer reaction. Rate of reaction = If the quantity change is immeasurable 1 Rates of reaction = Total time for the reaction [0.025s-1] Example 5 Example 3 Cr2O72-(aq) + 14H+ + C2O42- → 6CO2(g) + 7H2O(l) + 2Cr3+ (aq) 2HCl(ak) + Na2S2O3(ak) 2NaCl(ak) + S(p) + SO2(g) + H2O(ce) The chemical equation above represents a reaction between acidic potassium dichromate(VI) and In a reaction between dilute hydrochloric acid and ethanedioic. Which of the following can be used sodium thiosulphate, sulphur precipitate was to determine the rate of reaction? produced after 2 minute . What is the rate of the I. Volume of carbon dioxide collected over time. reaction? II. Time for chromium ion to form. Answer III. Time for the colour of the solution to change from orange to green. Rate of reaction = IV. Time for the pH change. A. I and II B. I and III C. I II and III [0.00833s-1] D. II, III and IV 3
4. 4. The graph of quantity of chemical over time Example 6 The reaction between dilute hydrochloric acid c) the concentration of hydrochloric acid against and excess marble will produce calcium chloride time. and gas of carbon dioxide. Sketch the graph of d) the concentration of calcium chloride against a) the mass of the marble against time. time. b) the volume of carbon dioxide against time. Answer a) b) c) d) Calculate the Average Rate from the graph The rate of reaction is equal to the slope of the graph of quantity against time. Example 7 In a reaction between calcium carbonate and liquid hydrochloric acid, carbon dioxide gas that is released is collected in a burette. The graph below shows the volume of carbon dioxide collected over time. Find the average rate of reaction in the first 60s. [0.0417cm3/s] 4
5. 5. Instantaneous Rate 1. The rate of reaction changes from time to time as the reaction happens. 2. The rate of reaction at a particular time is called the instantaneous rate. 3. The instantaneous rate of a reaction is equal to the gradient of tangent at a particular time. ∆(product) Rate of reaction = ∆(Time) δ (product) ∆(product) Rate of reaction = Rate of reaction = δ (Time) ∆(Time) δ(Product) = Small change of the amount of product ∆ (Product) = Change of the amount of product δ(Time) = Small change of the time ∆ (Time) = Change of the time This method is not practical because δ(Product) and δ(Time) is too small to be measured. Simple Exercise 1 Find the reaction rate at t = 40s. [1.4cm3/s] 5
6. 6. Example 8 a. Find the rate of reaction at 50 second. b. Find the rate of reaction at 200 second. 6
7. 7. Example 9 Time/s Mass lost/g In a reaction between sodium and water, the mass 30 1.10 lost of sodium is recorded every 30s, as shown in 60 1.75 the table below. By using a graph, 90 2.20 Find the rate of reaction at 1 minute. 120 2.50 [0.019 g s-1] 150 2.58 Find the average rate of reaction at the third 180 2.60 minute. 210 2.60 [0.0016g s-1] 240 2.60 270 2.60 7
8. 8. Graph analysis Example 10 Example 11 8
9. 9. Problems Solving Sketching a Graf 1. To sketch a graph, we need to determine • the total quantity of the product that formed • relative reaction rate ( higher or lower ). Revision Notes Mole of reactant/product For solution For gas (When volume is given) MV Volume of gas n= n= Molar volume of gas (22.4dm3 at stp / 24dm3 at rtp) 1000 For solid, liquid or gas ( When mass is given ) mass n= Molar mass (RAM/RMM) Example 12 a. Calculate the number of moles of In an experiment, a quantity of excess zinc hydrogen that is produced when the powder is added to 25 cm3 of liquid reaction is completed. [Molar volume of hydrochloric acid. Figure below shows the gas: 24 dm3 mol-1 at room temperature]. curve obtained when hydrogen that is released is plotted against time. b. Calculate the concentration of hydrochloric acid that is used in the experiment. Structure Question 1. The rate of reaction between calcium carbonate with liquid hydrochloric acid is determined by the following method: 6 g chips of calcium carbonate are mixed with 100 cm3 of solution of hydrochloric acid 1.0 mol dm-3 in a conical flask. The flask is place on to a balance and the decrease in mass of its contents is recorded at certain time intervals. A graph of the decrease in mass against time is drawn and the results are as shown below: 9
10. 10. a. Write an equation for the reaction between calcium carbonate and liquid hydrochloric acid. b. State the change in rate of reaction in the first 240 seconds. Why? c. Why is it that the decrease in mass of the contents of the conical flask does not change after 120 seconds? d. The experiment is repeated using 25 cm3 of hydrochloric acid 2.0 mol dm-3, instead of 100 cm3 hydrochloric acid 1.0 mol dm-3 and other factors are maintained. Sketch the change of the shape of the curve that will be obtained on the graph above. e. Calculate the volume of carbon dioxide that is released in the experiment that uses 100 cm3 hydrochloric acid 1.0 mol dm-3 [Molar volume of gas:24 dm3mol-1 at room temperature. The relative atomic mass: C, 12; O, 16] 10
11. 11. f. Sketch the shape of the graph that will be obtained if the readings of the balance against time are plotted. 2. A conical flask that contains 50 cm3 of solution of sodium thiosulphate is left on top of a sheet of white paper which has been marked with an "X". The stopwatch is started as soon as 5 cm3 of hydrochloric acid 2 mol dm-3 is mixed into the conical flask. The contents of the conical flask, is stirred. The reaction is left to occur until the mark "X" is not visible and then the stopwatch is stopped. This experiment is repeated four times, each time using a different concentration of sodium thiosulphate while all other conditions are kept constant. a. State the variable that is Manipulated dependent b. Write one equation for the reaction that occurs in the experiment c. What causes the mark "X" to become not visible? d. Sketch one graph for the concentration of sodium thiosulphate solution against time. e. If a bigger conical flask is used in this experiment, sketch on the same axes, the expected graph to be obtained using a dashed line. 11
12. 12. f. What conclusion can be drawn about the graph of the concentration of sodium thiosulphate solution against time? g. Sketch one graph of concentration of sodium thiosulphate solution against 1 /time. h. What is the unit for the rate of reaction calculated from this graph? 12