15558933 52 speed of reaction

8/12/2019 15558933 52 Speed of Reaction

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Speed Of Reaction

By the end of the topic, students should be able to:

Describe a suitable method for investigating the effectof a given variable (e.g. concentration) on the speed ofreaction. Interpret the results of a reaction rate experiment (e.g.as graphs or as a table of readings). Describe how concentration, pressure, particle sie and

temperature affect the rate of a reaction. !xplain the above four effects in terms of collisionsbetween the reacting particles. Describe the features of catalysts (including enymes). !xplain the meaning of activation energy and howcatalysed reactions have lower activation energies.

Please tick in the box if you can do any of the above.

Done By Kartini Ishak1


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Speed of Reaction Different chemicals reactions ta"e place at different

speeds.

#able $ below gives some other examples of reactions

that proceed at different speeds.Very fast Moderately fast Slow

!xplosion ofpetrol%airmixture

&recipitationreactions

'eactions ofmetals orcarbonateswith diluteacids.

'usting of ironin air.

'eaction ofmagnesiumwith cold water.

ermentation(conversion of

fruit uice intoalcohol).

Table 1 Different reactions proceed at different speeds.

Measuring the speed of reaction#he speed of a reaction can be found by measuring these*uantities at regular time intervals:a) the mass of the reactant that remainsb) the volume of the products that was produced

#he method chosen depends upon the reaction beingstudied. It is usual to record the mass or total volume atregular intervals and plot a graph. #he readings go on thevertical axis and the time goes on the horiontal axis.

Measuring the speed of reaction from changes inmass

#his method wor"s best for reactions which produce gasessuch as carbon dioxide. or example, the speed of reactionbetween calcium carbonate and hydrochloric acid can bestudied this way.

EXPERIMENT #o study the speed of reaction between calcium carbonateand dilute hydrochloric acid.



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Figure 1 An experiment to study the speed of reaction by measuring the mass at different time

intervals.

Procedure$. #he apparatus is set up as shown in igure $. #he

cotton wool in the mouth of the conical flas" is usedto prevent acid spray i.e. to stop the acid from

splashing out as the reaction ta"es place.+. #he mass of the system is recorded. #his includes the

mass of the marble chips (calcium carbonate), dilutehydrochloric acid, conical flas", small test tube, stringand cotton wool.

. . #he conical flas" is sha"en to mix the marble chipsand acid. #he stopwatch is immediately started.

-. #he mass of the system is recorded at one%minuteintervals.

ResultPlot a graph of mass of system against time from asample set of results from E!periment "



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Measuring speed of the reaction from changes in#olume

#he reaction between a reactive metal and a dilute acid isconsidered fast. or example, magnesium reacts withdilute hydrochloric acid according to the e*uation:

agnesium / dilute hydrochloric acid magnesiumchloride/ hydrogen

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1s the reaction proceeds, the total volume of hydrogengas produced increases. #he speed of the reaction cantherefore be determined by collecting and measuring thevolume of hydrogen produced at regular time intervals.

EXPERIMENT $#o study the speed of reaction between dilute hydrochloricacid and magnesium.

Figure 2 An experiment to study the speed of reaction by measuring the volume of gas evolved.

Procedure$. #he apparatus is set up as shown in igure +. #he

layer of oxide on the magnesium ribbon is removedusing a piece of sandpaper. #his ensures thatmagnesium reacts with the dilute hydrochloric acid.

#he magnesium ribbon is then put in a small testtube.

+. #he conical flas" is sha"en to mix the magnesiumribbon and acid. #he stopwatch is started at the

same time.



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#he theory that we use to explain how different variableschange the rate of reaction is called the collision theory.

or a reaction to ta"e place, the particles of the

substances that are reacting have tocollide. If theycollide, with enough energythen they will react. #heminimumamount of "inetic (movement) energy that twoparticles need if they are going to react when they collideis called the acti#ation energy.

#here are therefore two main ways of increasing the rateof a reaction:

$) increase the num(er of collisions

+) increase the amount of mo#ement )*inetic+energyso that more collisions lead to a reaction



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, thought e!periment on collision theory

1 group of students are ta"en into the school hall andblindfolded. #hey are as"ed to move around the hall. If two

students crash into each other and (oth students fallover, then they stay lying on the ground % they have3reacted3.

%ow can we increase the rate at which students falldownhere are two obvious choices:

+ &ut more studentsinto the hall. #his will lead to morecollisions between students.

$+ 1s" the students in the hall to run more -uic*ly. #hiswill also lead to more collisions and to a greater chance ofa collision leading to the students falling over.

4e will use this 3thought experiment3 to try and explainrates of reaction.

.actors affecting the speed of reaction5ou will be expected to remember the factors that affectthe speed of reactions, and to plot or interpret graphsfrom rate experiments.

actors affecting the speed of reactions are:

6oncentration of solution

&ressure

&article sie of solids

#emperature

6atalyst

+ 'oncentration of solution



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7oo"ing bac" to the 8students in the 9all thoughtexperiment, we are putting more students into the 9all.

$+ Pressure

7oo"ing bac" to the 8students in the 9all thought

experiment, we are putting the students into a smaller9all.

/+ Particle si0e of solids

1+ TemperatureIf the temperature is increased, the rate increasesbecause not all particles react when they collide. #heymust have a certain minimum energy to react when theycollide.1t a higher temperature, the particles have more energy

so they react more often when they collide.



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2+ 'atalystIf a catalyst is added, the rate increases because catalystsallow the reaction to go by a different route. #he minimum

energy needed for a reaction when particles collide is less,so more reactions ta"e place.

The graph above summarises the differences in the rate ofreaction at different temperatures, concentrations andsize of pieces. The steeper the line, the greater the rate ofreaction. Reactions are usually fastest at the beginningwhen the concentration of reactants is greatest. When theline becomes horizontal the reaction has stopped.

Please proceed to do exercise from:

WORKSHEET 2

'atalysts

4hat is a catalyst;00000000000000000000000000000000000000000000000000000000000

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4hat are the characteristics of a catalyst;0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

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4hat are the factors affecting the speed of catalysedreactions;

#he speed of catalysed reactions is affected by changes inconditions, ust li"e reactions without catalysts. #he speedof a catalysed reaction is increased by:

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9ow does a catalyst wor";6atalysts provide an alternative route for the reaction. Inthe catalysed route, 000000 energy is needed to brea"bonds and so the activation energy is 00000000. 1s a result,bond brea"ing ta"es place more easily and more oftenwhen particles collide. #his is why catalysed reactions arefaster.

#he effect of catalysts on activation energies can beshown by energy profile diagrams such as shown in igure.



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Figure 3 the energy profile diagrams showing the effect of a catalyst on activation energy.

1 simple analogy of how catalysts wor" is a group ofcyclists riding up and over a hill. #he hill route is the

normal one, re*uiring a lot of energy. #he short cutthrough the tunnel is li"e the catalysed route. It re*uiresless energy and the cyclists, li"e the reaction, go faster.(igure -)

Figure showing how catalyst wor! by ta!ing an alternate pathway.

3hat are the common catalysts for someindustrial processes&

In many industrial processes, catalysts are used to speedup various reactions. Below are some examples:a) Iron%b)


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Figure " showing how an en#yme wor!s.

3hat are the properties of en0ymes&44444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444

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3hat are the uses of en0ymes in industry&444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444

Please proceed to do exercise from:

WORKSHEET 3WORKSHEET 4


15558933 52 speed of reaction

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