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Rate of Reaction I

Mind Map

Definition

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]

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/productsRates of reaction =

Total time for the reaction

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Example of fast reaction

Type of Reaction Example Combustion Combustion of magnesium in oxygen

Combustion of ethane (C2H6)

Reaction between reactive metal and water

Reaction between potassium and water

Reaction between metal carbonate and acid

Reaction between limestone/ marble and sulphuric acid

Ionic precipitation (Double decomposition)

Precipitation of silver(I) chloride 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 The quantities of reactants reduce. The quantities of products increase. 1. Therefore, the rate of the reaction can be

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. The easily measured quantity changes include a. Mass b. Concentration (Conductivity) c. Volume of gas

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Example Reaction between calcium carbonate and hydrochloric acid.

CaCO3 + 2HCl → CaCl2 + CO2 + H2O

The rate of the reaction can be determined by a. Measuring the reduces of the mass of the

calcium carbonate (reactants) over time

3Mass of CaCO reactedRate of Reaction =

Time taken

b. Measuring the increases of the volume of the carbon dioxide gas produced over time.

2Vlolume of CO producedRate of Reaction =

Time taken

Example 2 In a reaction, 5 g of calcium carbonate takes 250 seconds to completely react with solution of hydrochloric acid. Calculate the average rate for this reaction in units (a) g s-1 and (b) mol s-1

[ Relative atomic mass: C 12; 0, 16; Ca, 40]

[a. 0.02g/s; b. 0.0002mol/s]

Immeasurable quantity and average rate 1. In some reaction, some changes are observable

but difficult to be measured. For example a. Change in colour b. Precipitation

2. The time taken for the colour of a reactant to change or certain amount of precipitate to form can be used to measure the rate of reaction.

Example 3 2HCl(ak) + Na2S2O3(ak) 2NaCl(ak) + S(p) + SO2(g) + H2O(ce) In a reaction between dilute hydrochloric acid and sodium thiosulphate, sulphur precipitate was produced after 2 minute . What is the rate of the reaction? Answer Rate of reaction =

[0.00833s-1]

Example 4 When the aqueous of ethanadioic is mixed with acided potassium manganate(VII) , the reaction happen slowly at room temperature. The purple colour of the solution is bleached after 40 seconds. Calculate the average rate of reaction. Answer Rate of reaction =

[0.025s-1] Example 5

Cr2O72-(aq) + 14H+ + C2O4

2- →

6CO2(g) + 7H2O(l) + 2Cr3+ (aq)

The chemical equation above represents a reaction between acidic potassium dichromate(VI) and ethanedioic. Which of the following can be used to determine the rate of reaction? I. Volume of carbon dioxide collected over time. II. Time for chromium ion to form. III. Time for the colour of the solution to change

from orange to green. IV. Time for the pH change. A. I and II B. I and III C. I II and III D. II, III and IV

If the quantity change is immeasurable

1Rates of reaction =

Total time for the reaction

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The graph of quantity of chemical over time Example 6 The reaction between dilute hydrochloric acid and excess marble will produce calcium chloride and gas of carbon dioxide. Sketch the graph of a) the mass of the marble against time. b) the volume of carbon dioxide against time.

c) the concentration of hydrochloric acid against time.

d) the concentration of calcium chloride against time.

Answer a)

b)

c)

d)

Calculate the Average Rate from the graph

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]

The rate of reaction is equal to the slope of the graph of quantity against time.

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Instantaneous Rate

(product)

Rate of reaction = (Time)

δδ

δ(Product) = Small change of the amount of product δ(Time) = Small change of the time This method is not practical because δ(Product) and δ(Time) is too small to be measured.

(product)Rate of reaction =

(Time)

∆∆

(Product) = Change of the amount of product∆ (Time) = Change of the time∆

Simple Exercise 1

Find the reaction rate at t = 40s.

[1.4cm3/s]

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)

∆∆

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Example 8

a. Find the rate of reaction at 50 second. b. Find the rate of reaction at 200 second.

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Example 9 In a reaction between sodium and water, the mass lost of sodium is recorded every 30s, as shown in the table below. By using a graph, Find the rate of reaction at 1 minute.

[0.019 g s-1] Find the average rate of reaction at the third minute.

[0.0016g s-1]

Time/s Mass lost/g 30 1.10 60 1.75 90 2.20 120 2.50 150 2.58 180 2.60 210 2.60 240 2.60 270 2.60

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Graph analysis

Example 10

Example 11

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Problems Solving

Example 12 In an experiment, a quantity of excess zinc powder is added to 25 cm3 of liquid hydrochloric acid. Figure below shows the curve obtained when hydrogen that is released is plotted against time.

a. Calculate the number of moles of hydrogen that is produced when the reaction is completed. [Molar volume of gas: 24 dm3 mol-1 at room temperature].

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:

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

1000

MVn =

For gas (When volume is given)

3 3

Volume of gas

Molar volume of gas (22.4dm at stp / 24dm at rtp)n =

For solid, liquid or gas ( When mass is given ) mass

Molar mass (RAM/RMM)n =

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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]

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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.

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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?