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Topic 16. Chemical kinetics HL

16.1 Rate expression and reaction mechanism

For the reaction xA + yB → zC​ 

the rate expression is

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r: rate of reaction (mol/dm³s)

k: rate constant (the unit depends on the overall order of reaction)

[A]ˣ: concentration of reactant A to the power of the coefficient x (mol/dm³)

The overall order of the reaction is x + y (the sum of the coefficients of the reactants).

The rate equation only looks at the reactants.

The rate constant of a reaction increases exponentially with an increase in temperature. 

Calculating r (rate of reaction)

The rate of reaction can be calculated by finding the slope/gradient of the tangent of a given point.

The orange line on the graph is the tangent. Steps:

1. Using a ruler, draw the tangent of the given point as best as you can.

2. Find the slope of that tangent using two points (preferably the x and y-intercepts).

* the IB paper 2 mark scheme will provide a range for the answer, so as long as your tangent is fairly accurate it will be alright.

Order of reaction graphs

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From Ms. Fu's powerpoint

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From Ms. Fu's powerpoint

This graph represents the relationship between time and the concentration of REACTANTS. It can also be the concentration of products, but the graph would look different (it would increase, not decrease).

Thinking of the second graph as a differentiated version of the first can help you remember both. 

Pay really close attention to the axes!

This graph represents the relationship between the concentration of REACTANTS and the rate of reaction. Unlike the previous graph, this one only works for reactants, not products.

Determining the unit and value of k (rate constant)

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Pearson textbook pg. 290

Using this equation:

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You don't have to memorize the units since you can easily solve for them, but it will save time.

Example problem:

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Pearson textbook pg. 290

For 16.1, just practice as many of these rate constant determining questions as you can.

Determining the order of reactions through initial rates

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Pearson textbook pg. 294

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Pearson textbook pg. 295

Reaction mechanisms

The reaction mechanism determines elementary steps.

The overall reaction cannot occur faster than the rate-determining step, which is the slowest step of the reaction.

The order of reaction for the rate-determining step = The order of reaction for the entire reaction

Reactants in a fast step before the slow step (rate-determining step) are included in the rate expression.

 

Catalysts and species that are of zero-order still take part in the reaction.

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From the IBO

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From Ms. Fu's powerpoint

Unimolecular steps have only one species as a reactant: A → products

Bimolecular steps have two species that collide to form a transition state: A + B → products

16.2 Activation energy

The rate constant "k" is dependent on temperature (the rate constant increases exponentially as temp increases).

We can use the Arrhenius equation to find the activation energy of a reaction.

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We can take the natural log of both sides to get a different form of the same equation:

Think of it like the y = mx + b equation

  • we can view lnk as the y variable

  • we can view - Ea/R as the slope

  • we can view 1/T as the x variable

  • we can view lnA as the y-intercept

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The figure to the right is called the Arrhenius Plot

We can solve for the activation energy by finding the slope of the graph.​

A: energy requirements of collisions; takes into account        the orientation of the molecules

Ea: activation energy (kJ/mol or J/mol)

R: gas constant (8.31 J/K*mol)

T: temperature (K)

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Pearson textbook pg. 302

Calculating "k" from two equations

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Pearson textbook pg. 303

Remember to convert the temperature from C to K

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