6.1- Collision Theory and Rate of Reaction

Defining Rate of Reaction

As a chemical reaction proceeds the reactants get used up and the products are formed. At any specific time in the course of the reaction the amount (mass, moles, concentration etc) of the reactants compared to the amount at the beginning of the reaction gives us a measure of how far the reaction has gone.

A car's speed is measured in distance travelled/ time taken. The rate of a chemical reaction is measured as change in reactant (or product) amount / time taken

We usually use concentrations of reactants – but we could just as easily use concentrations of products. If we can measure how either changes with time then this provides us with the best definition of reaction rate.

• What units would you expect Rate of Reaction to have?

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The stoichiometry reactions give no indication of rates of reaction.

But it does tell us the relative rate of consumption of reactants and formation of products.

If we look at the Contact Process: 2SO_2(g) + O_2(g) ⇌ 2SO_3(g)

It’s clear that for every 2 mol of Sulphur Dioxide used up only 1 mol of Oxygen is consumed.

So, if the rate of removal of Sulphur Dioxide is 2 moles per minute, the rate of removal of Oxygen must be 1 mole per minute. And the rate of removal of Sulphur Dioxide is 2 moles per minute, then the rate of formation of Sulphur Trioxide is also 2 moles per minute.

3. If Hydrogen is consumed at 2.8 moldm^-3hr^-1 what is the rate of formation of Ammonia?

Exercise

If concentration drops from 6.2 x 10^-3 moldm^-3 to 2.8 x 10^-3 moldm^-3 in

20 sec then the average rate over this part of the reaction is:

Average rate = __________________________? ___________units

You should be able to:

a) Sketch these graphs

b) Explain their shapes

c) Use these graphs to predict the shape of another graph under different conditions

2. Which experiment shows the reaction with the highest temperature? Use Collision Theory to explain why you have chosen this reaction.

3. Two experiments were carried out using an excess of Acid and Magnesium powder. The top line shows the graph when 2g of Mg was used. What can you deduce about the reaction represented by the lower line?

4. If the graph below shows the reaction between excess acid and 5g of Carbonate powder, on the same axes sketch the graph for the reaction between excess acid and 5g of Carbonate chips. Label this line (A)

Sketch another line showing the graph for 2.5g of Carbonate powder at a much higher temperature. Label this line (B)

The Average rate of the reaction measures its change over the whole reaction.

In this case the Concentration drops from 1700 to 0 over 500 seconds

Average Rate of Reaction = _____________________

But the rate changes over the course of the reaction so we may need to know the rate at a particular time – the Instantaneous Rate.

Rates change as the reaction continues. So to compare the effect of changing conditions we tend to use the rate at 0 seconds – the Initial Rate.

This involves drawing a tangent to the curve at 0 – not pleasant!!

You then need to calculate the gradient:

On the graph below, draw a tangent and use it to calculate the initial rate of the reaction.

How does it compare to the rate at 110 sec? Why is there a difference?

• If the reaction creates or consumes an acid or a base we could monitor changes in ____________________.

The most accurate piece of equipment for this would be a __________________________________________________

• If the reaction creates or consumes ions we could monitor changes in ____________________.

The most accurate piece of equipment for this would be a __________________________________________________

• If the reaction creates gas we could collect the gas in a____________________ or a __________________.

If the reaction creates gas we could allow the gas to escape and measure changes in ___________________.

• If the reaction creates or consumes coloured compounds we could monitor changes in absorption of _______________________

The most accurate piece of equipment for this is called a __________________________________________________

We could collect Carbon Dioxide “Over Water” or in the gas-syringe. Why is the gas-syringe much more accurate?

Students usually study this in Year 7-9 but we met the ideas this year when talking about Ideal Gases

In an Ideal Gas it is assumed that:

a) Gas particles move

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b) The volume occupied by the gas particles is

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c) Collisions between particles are _______________________________

d) The average energy of the particles is proportional to the _________________________________________________________

1. Why is it reasonable to treat the volume of gas particles to be zero even though this cannot possibly be true?

2. Few gases are entirely Ideal – mostly because their collisions are not Elastic. Why is this generally true?

For reactions to occur the reactant particles must come into contact. Since they are moving they must c_______________.

But they must collide in the correct o_______________ and with sufficient e___________ to cause bonds to __________ or the reaction will not succeed.

Collisions usually involve 2 particles. Three-particle collisions are rare.

A collision that leads to some kind of chemical change is called an 'effective collision' or a “Successful collision”.

Any factor that results in a greater number of collisions per second AND/OR more effective collisions increases the rate of reaction. The number of collisions per second is often called the Collision F__________.

v Which 3 factors account for higher temperatures increasing rate?

v How does increasing pressure/concentration increase rate?

These should be familiar from earlier in the course.

What is happening at the tip of the profile?

This is called a Transition State – it’s the intermediate species formed by the collision of the two particles.

Catalysts – the GCSE definition will still do. A catalyst _______________

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What is a Homogeneous Catalyst?

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How do they work?

Often the catalyst reacts with one reactant to form one of the products and another substance. This substance may then react with the other reactant, forming the other product.

Eg. Reaction: W + X à Y + Z Slow reaction, high E_act

Catalyst added.

Step 1 W + Cat à Y + A Fast reaction, low E_act

Step 2 A + X à Cat + Z Fast reaction, low E_act

Added together W + Cat + A + X à Y + A + Cat + Z

Cancel W + Cat + A + X à Y + A + Cat + Z

Overall W + X à Y + Z

The only process you need to know about in detail is Ozone Depletion.

British Students often study this at GCSE but you will now need to be able to produce the relevant equations.

• Why do we need a functioning Ozone layer?

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Ozone (O₃) is naturally broken up by UV light to form Oxygen and an Oxygen radical (O.)

• Write an equation for this process

• What is meant by a radical? Why are they so reactive?

Without catalysis the reverse process occurs, giving out heat

• Write an equation for this process

• How do these two processes protect us from the UV? What happens to all the energy in the UV waves?

Radicals may be formed in jet engines but the biggest cause has been Chlorine radicals (Cl.).

• Write an equation for the formation of a Chlorine radical and another radical from Freon (CF₂Cl₂)

• Write an equation for the reaction of a Chlorine radical with Ozone making Oxygen a new radical

• Write an equation for the reaction of the new radical with Ozone making Oxygen another Chlorine radical

• What is the overall reaction?

Most Heterogenous catalysts are metals which gas reactants stick to (adsorb), weakening their internal bonding and lowering the Activation Energy.

It’s important that the product does not adsorb well or it will coat the catalyst and stop it working.

We could summarise as:

A__________________________ --> Reaction --> D__________________________________

Catalytic Converters in car exhausts contain precious metals like ____________(Pt), ________________(Rh), and ___________________________ (Pd).

These help unreacted fuel, Nitrogen Monoxide and Carbon Monoxide react before they escape the exhaust pipe.

• Write a chemical equation for the formation of Nitrogen and Carbon Dioxide from Nitrogen Monoxide and Carbon Monoxide

• Write a chemical equation for the formation of Water vapour and Carbon Dioxide from Octane and Oxygen

Some Heterogeneous Catalysts “react with” the reactants like Homogeneous Catalysts. A good example is the Constact process where Sulphur Trioxide is formed from Sulphur Dioxide and Oxygen with the presence of Vanadium Pentoxide Catalyst.

Try to work out the equation for step 2

Nb This process is actually Heterogeneous

Overall SO₂ + ½ O₂ --> SO₃

Step 1 SO₂+V₂O₅→SO₃+V₂O₄

Step 2

• Suggest a reason why particles end up at different temperatures.

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Know how to sketch a Maxwell-Boltzmann distribution and how to label the axes

• Why must the distribution start from the Origin?

• Why must the curve never touch the x-axis again?

• What does the area under the curve represent?

• Add a line representing the Activation Energy and label it E_Act (uncatalysed)

• Add another line representing the Activation Energy when a catalyst has been added and label it E_Act (catalysed)

• Use the graph to explain how catalysts increase the rate of reaction

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• Add a line showing the distribution at a higher temperature

• Add a line representing the Activation Energy and label it E_Act

• Use the graph to explain how higher temperatures increase the rate of reaction

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