Rate Of Reaction

Anglican High School Sec 4 Chemistry/Science.Created by Tan Siok HiangConfidential

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Topic :Speed of reaction.(J.G.Briggs) 21.1. Definition of the speed of reaction. It refers to rate of reaction..This can be obtained by a.Measuring the amount of reactants which disappear in a unit time. b.Measuring the amount of products which are formed in a unit time. c.Measuring the times for a reaction to complete. 21.2 Ways of measuring reaction rates. 1. Measuring a change in mass during the reaction 2. Measuring the volume of a gaseous product. 3. Measuring the time for a colour to appear or disappear 4. Measuring the changes in pH. 5. Measuring the changes in temperature. 6. Measuring the time for a precipitate to form. The values obtained are then plotted on a graph . X-axis :Time taken Y axis : Volume of the product or the mass of the reactant. Analysis of reaction curve. 1. Study the graphs carefully below and take note of the y axis.In all cases the graph has decreasing gradient.This means that a. the rate of reaction is greatest at the beginning but will always decreases with time(concentration factor) and stops as there are no more reactants. b. The rate is not the same throughout. c. The steeper the curve at that point ,the faster the rate.

d. We can determine the amount of reactants used from the volume of the product which was produced. Example.

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Anglican High School Sec 4 Chemistry/Science.Created by Tan Siok HiangConfidential

Page 2

Calcium carbonate reacts with dilute hydrochloricacid.The volume of carbon dioxide can be read from the graph below.

Example.

24dm3

24dm3 of carbon dioxide contains 1 mole.From the equation below CaCO3 + 2HCL------CaCL2 +H2O + CO2 So 1 mole of carbondioxide is produced from 2mole of acid and 1 mole of CaC03.So we can determine the mass of calcium carbonate and hydrochloric acid.Calculate the concentration of acid used if 25cm3 of acid was used. Ans. 1. No of moles of acid =2mole Vol of acid =25/1000dm3 Molarity of acid=Mole/Vol=2/(0.025)=____Mole/dm3 2.Mass of calcium carbonate=Mole*Molar mass=1x100=100g. From the graph, The difference in initial mass and final mass will give the mass of gaseous product which is formed. The mass of the gaseous product will enable us to determine the mass of reactants because we can convert the mass of product into mole and use the mole ratio to determine the mole and therefore the mass of reactants. 2

Anglican High School Sec 4 Chemistry/Science.Created by Tan Siok HiangConfidential

Page 3

Theory of rate of reaction. 1.Collision theory. a. It states that particles move and collide with each other. b.Some collisons would produce a reaction which we classify as “successful collisions”. c.Some collisions would not produce reactions which we call “unsuccessful collisions”. d. For every reaction there is an energy requirement which we called “activation energy”.(Treat this as entrance requirement for entrance into college) e. Reactions which occur easily has low activation energy .hence most collisions would be successful.(Sodium reacts with water) f. Reactions which do not occur easily have high activation energy.,hence collisions would not be successful.(Iron reacts with water). g.We can use the above concept to explain the factors which influence the rate of reaction

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Anglican High School Sec 4 Chemistry/Science.Created by Tan Siok HiangConfidential

Page 4

Factors

How it affect

Reason

1.Temperatur

High temp=>faster

High temp means more kinetic energy

e

low temp=>slower

and move faster,so more chances of

Graphical representation.

collisions hence greater chances of achieving successful collisions 2.there is an increase in the number of particles having the activation energy. 3.the total number of collisions per 2.Concentrati

High

unit volume increase High concentrations mean greater

on

concentration=>faster

number of particles per unit volume so

Low

high chances of collisions

concentration=>slower Presence of

Certain catalyst will lower the orginal

catalyst=>faster

activation energy of the specific

Absence=>slower

reaction hence more particles will

Only affect gas,

collide successfully. High pressure means more particles

3.Catalyst

4.Pressure

per unit volume,hence greater collisions per unit volume=> faster 5.Surface

Only affect solid.

rate. Powdered means there is greater

area.

Powdered (greater

suface area of contact hence higher

surface area)means faster

chances of collisions and no of

rate.

partices per unit volume for reaction 4

Anglican High School Sec 4 Chemistry/Science.Created by Tan Siok HiangConfidential

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increase,so chances of collision are greater and chances of success will be higher.

a b

temp catalyst Surface area pressure Concentration

(a) High temp yes powdered High High

(b) Low temp No Big pieces low low

(no of moles of reactants used the same) Catalyst. Concentration of a solution is measured by Molarity.The greater the concentration 1.Definition : A catalyst is a substance which increases the rate of a chemical reaction without itself being chemically changed at the end of the reaction.(To check this is so,weigh the ,the faster the reaction,hence the steeper the graph..The amount of products is affected catalyst before and after the reaction.There is no change.in mass ) 2.Biological catalyst are called enzymes.Yeast is an example. by the no.of moles.(No of moles=MxV) We must assume that acid is the limiting 3.Examples of industrial catalyst: Iron: Haber process to make ammonia reagents.Since the number of mole of acid control the volume of gas evolved Vanadium (V) oxide used to make sulphuric acid in Contact process. Concentrated sulphuric acid to make ester. Graphical representation when different no of moles of reactants were used Nickel is used to make margarine from vegetable oils. 4. Characteristics. 2M1dm3 a. they are specific. 1M2dm3 b. Only a small amount is required c. Many catalyst are tansition metals or its compounds. Experiments often amount used to investigate the rate of reaction d. Small of 3 impurities may “poison” catalyst. (Car exhaust has platinum to 1M1dm 1.Reaction between carbonate diluteoxide hydrochloric acid. catalysecalcium the conversion ofand nitrogen and carbon monoxide to harmless nitrogen 2.Reaction ofand magnesium and dilute sulphuric acid. carbon dioxide. 2CO +2 NO------ N2 +2 CO2 3. Decomposition hydrogen manganese oxide as catalyst. 3 5 Howeverof this catalyticperoxide.using reaction will be ineffective(IV) if lead is present. Lead will “poison” 2M 0.5dm 4. Reaction between dilute sulphuric acid and sodium thiosulphate.(we observe the the catalyst and make it ineffective formation of sulphur as a yellow solid