Chemical Kinetics(MAH) (NXPowerLite) Opp

7/31/2019 Chemical Kinetics(MAH) (NXPowerLite) Opp

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ChemicalKinetics

ChemicalKinetics


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s 6.1 Rates of Reaction (2h)

Rate of reaction can be defined as the

decrease in the concentration of reactants per

unit time or the increase in the concentration of

product per unit time.

Consider the reaction

2H2(g) + 2NO(g) 2H2O(g) + N2 (g)

The rate of disappearance of H2 is the same as the rate

of disappearance of NO. It is also the same as the rateof appearance of H2 O and HALF the rate of

appearance of N2 from the stoichiometry of the

reaction in the balanced equation.


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INTRODUCTION TO REACTION RATES:

Work in your table groups to write down an example of A very, very fast chemical reaction

A chemical reaction that takes about 5 10 minutes to occur.

A chemical reaction that takes several years to happen

A chemical reaction that takes thousands or millions of years to occur.Cooking/digesting food are both examples of medium rate chemical

An example of a chemical reaction, perhaps related to food, that youwould want to slow down

speed up

Can you write down a link between how fast a chemical reaction isand the time it takes for that reaction to occur?


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Rate is..

For the disappearance of a reactant:

Change in some property/ time unit

The value is given a sign

For the appearance of a product:

The value is given a + sign


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At Grade 10 level you learnt that chemical reactions

occur when the particles in the reacting substances

collide with each other with sufficient energy to

produce new product chemicals. Why do gases, liquids and solutions tend to react

faster than solids at the same temperature?

Controlling reaction rates is an economic necessity.

We need to produce chemicals as cheaply as

possible, which usually means as quickly as possible.

Working in your table groups, try to list at least 5

different factors which affect how fast a chemicalreaction occurs.


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Collision Theory

As the reactants heat up, the particles move faster and so collide moreoften and with more energy. When they react, they must have enoughenergy to overcome the activation energy needed by the reaction. A roughrule of thumb which applies to many reactions (think about cooking!) isthat a temperature rise of 10K approximately doubles the rate of thereaction. Mathematically, this may take some explaining.

Reaction Rates and Industrial Processes:

Work in your table group and choose one of the following industrialprocesses. Produce an OHT / PowerPoint to help you tell the rest of theclass the key facts about your chosen process.

1. Haber Process

2. Contact Process

3. Manufacture of Margarine

4. Cat cracking

5. Biological washing powder 6. Enzyme technology


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Collision Theory states that for a reaction to take place

particles must collide with sufficient energy to overcome theActivation energy for that particular reaction


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METHODS USED IN KINETICS TO FOLLOW AREACTION

Method 1: Loss in mass as the reaction

progresses. e.g in a reaction which produces

CO2 Method 2: Volume of gas evolved as a reaction

progresses. e.g in a gas syringe or by

displacement of water in a graduated cylinder.

Method 3: Using a pH probe if there is a changein acidity as the reaction progresses.


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Method 2 Using a gas syringe


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More methods for following the course of a reaction

Method 4: Use a conductivity stick if there is a

change in the number of ions in solution.

Method 5: Use a colorimeter if there is achange in colour of one of the species.

There are several more methods, but I am sure

you now understand that we can use anysuitable method which measures changes as

the reaction progresses.


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What does a real reaction look like ?

Volumeof CO2

Time

What is happening to the rate of the

reaction with time?

How can we find the rate at any instant


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What is going on during the

reaction?

At the start: The reaction is fast becausehere we have the highest concentration of

reactants, therefore the greatest numberof successful collisions.

As reaction progresses: the rate declinesas there are fewer particles to collide.

At the end : the slope is zero (flat) no moreparticles to react


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When is the reaction HALF completed?

The rate at any instant is the gradient

(slope) at any instant. Clearly, if the rate changes all the time,

the time for the reaction is not the

time it takes for the complete reaction.

Lets say 48cm3 of CO2 wasgiven off on completion.

Half reaction when 24cm3,

Read off graph time to give24cm3


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Factors affecting rate

Concentration in terms of Collision Theory

Particle size of solids

clearly if the same of solid is used but theparticles are smaller (bigger surface area) willprovide more places for reaction

Temperature:

clearly particles will have a higher collisionenergy at higher temp. and more successfulcollisions.


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Fact 1: An increase in the concentration of a solution,

or the pressure of a gas, results in an increase in the

reaction rate

Explanation: If the concentration of a solution, or the pressure

of a gas, is increased, then there are more particles in a given

volume. Therefore there will be more collisions in a set amountof time and the probability of more successful collisions

becomes higher. As a result the reaction rate will increase.


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Fact 2: An increase in surface area of a solid results

in a increase in the rate of a reaction.

Explanation: If the surface area of a solid is increased, there

are more particles exposed to the other reactant. Therefore

there will be more successful collisions in a set amount of

time and the rate increases


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Temperature effect and collisions

The increased Kinetic Energy of collisions at

higher temperatures only accounts for a small

proportion of the increased rate. On average a 10oC rise doubles the rate of a

reaction.

The major reason for this is the proportion of

particles, at a higher temperature, which nowhave an energy greater than the Activation

energy.


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Fact 3: An increase in temperature results in an

increase in reaction rate.

Explanation: If the temperature is increased, the average

kinetic energy of the particles increases. There is therefore a

greater chance of more collisions having an energy greaterthan the activation energy and reaction rate increases. Also

(less importantly) because particles are moving faster, there

will be more collisions in a given time, therby increasing the

rate


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What the syllabus says.

6.2.5 State and explain qualitatively the Maxwell-Boltzmann energydistribution curve for a fixed amount of gas at different temperatures and its

consequences for changes in reaction rate.

Students should be able to explain why the area under the curve is constantand does not change with temperature.


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Catalyts (Enzymes)

Grade 10: A catalyst is a substance that

increases the rate of a reaction but is not

used up by the reaction. SL and HL -level: Catalysts affect the

rate of a reaction by providing an

alternative reaction pathway with alower activation energy


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Fact 4: Addition of a catalyst can lead to an increase

in reaction rate

Explanation: A catalyst acts by enabling a reaction to proceed via

a route of lower activation energy. There will therefore be more

collisions of sufficient energy to cause a reaction to occur, and ratewill increase


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SUMMARY


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Orders, rate equations and the

rate constant


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Mechanisms *Train Station Analogy

People can: Exit a train at 20 per second

Ascend the escalator at 10 per second

Pass the ticked barrier at 50 per second

People will only pass the ticket barrier at only 10 persecond because of the limiting escalator.

This is the rate determining step which is indicated bythe experimentally determined rate equation


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Equation Example

The reaction

NO2(g) + CO(g) NO(g) + CO2(g)occurs in two steps:

NO2 + NO2 NO+ NO3(slow step)NO3 + CO NO2 + CO2(fast step)

The slow step is the rate determining step andthe rate equation should be

Rate = k[NO2]2


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Orders and Reaction mechanism


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Rate = k [ A ]

Rate = k [(CH3)3CCl


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Going further; Mr Arrhenius


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Francis Sun

carrying out an

experiment to

determine the

activationenergy of a

reaction. Just

find the rate

over a range oftemperatures.

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Chemical Kinetics(MAH) (NXPowerLite) Opp