Topic 16.3: Activation energy

16.3.1 Describe qualitatively the relationship between the rate constant (k) and temperature (T).

We know that the rate of reaction depends on two things: the rate constant, k and the concentrations of reactants, raised to a power. Since increasing temperature has no change of concentration, thus it effects the value of k. So k is a general measure of the rate of reaction at a particular temperature

16.3.2 Determine activation energy (Ea) values from the Arrhenius equation by a graphical method

This is the Arrhenius equation that links all the factors together.

k is the rate constant
A is the Arrhenius constant
R is universal gas constant
T is temperature (K)
Ea is activation energy

By rearranging it, we could plot it on the graph using linear law.

Thus the graph shall look like this:

The x-axis is 1/Temp
The y-axis is ln k
The y-intercept will be ln A
The gradient is Ea/R

Thus finding the gradient will give us Ea/R. Then times the result by the gas constant 8.31 J K^-1 mol^-1 to receive the final answer of Ea.

Ea is in kJ mol^-1

Topic 16.2: Reaction mechanism

16.2.1 Explain that reactions can occur by more than one step and that the slowest step determines the rate of reaction (rate-determining step)

Most reactions that occur at a measurable rate occur as a series of simple steps, each involving a small number of particles, as described in the collision theory. This sequence steps is known as the reaction mechanism. The elementary steps usually cannot be observed directly, thus a theory.

Intermediates are used to split the reaction. The slower step in the sequence is the rate-determining step. This step determines the overall rate of reaction.

The term molecularity is used to an elementary step to indicate the number of reactant particles. Unimolecular reaction is single particle reactant reaction. Bimolecular reaction involves two reactant particles. Anything higher is unrealistic due to collision theory.

16.2.2 Describe the relationship between reaction mechanism, order of reaction and rate-determining step

In the reaction mechanism, the order of reaction is slightly different. Any reaction that happens after the rate-determining step is not included in the overall order of reaction. However, any reaction before the rate-determining step is included.

If there are two of the same molecules used as reactants. In the order of reaction, the concentration has twice the effect hence making it second-order.

Topic 16.1: Rate expression

16.1.1 Distinguish between the terms rate constant, overall order of reaction and order of reaction with respect to a particular reactant

Rate is measured as the concentration, mass or volume (or anything else that can be directly related) of reactants or products changes with time. Rate constant is a constant for a particular reaction at a specific temperature.

Order of reaction with respect to a particular reactant is the power to which its concentration is raised in the rate equation. The overall order for the reaction is the sum of the individual orders for all reactants

16.1.2 Deduce the rate expression for a reaction from experimental data

Rate = k [A] [B]

Find the concentration of B that is constant. Find the change in [A] compared to the initial rate of reaction. Thus determine the order of reaction. Zero-order has no effect. First order is direct proportional. Second order is proportional squared. Repeat to find B as well.

Then conclude the rate expression

Rate = k [A]^n [B]^m


16.1.3 Solve problems involving the rate expression

After finding the rate expression. Input all the date into the equation to find the rate constant.

Be sure to get the state symbols correct.

• Zero order

k = mol dm^-3 s^-1

• First order

k = s^-1

• Second order

k = mol^-1 dm^3 s^-1

• Third order

k = mol ^-2 dm^6 s^-1


16.1.4 Sketch, identify and analyse graphical representations for zero-, first- and second-order reaction

Simply state the graphical representations. Such as, e.g., in zero-order reactions, the concentration has no effect to the rate.

Zero

First

Second

Topic 16: Kinetic

Topic 16 of the IB HL Chemistry syllabus is the Kinetic. IBO recommends to spend 6 hours on this topic.

This topic has 3 sub-chapters: "Rate expression", "Reaction mechanism" and "Activation energy". Each are separated with numerical values in order of mentioned.

These are advanced HL syllabus statements, it is recommended to bring a Casio Graphical Calculator instead of Texas. Casio Calculators have the periodic table installed already.

Topic 6.2: Collision theory

6.2.1 Describe the kinetic theory in terms of the movement of particles whose average energy is proportional to temperature in kelvins.

The essence of kinetic-molecular theory is that particles in a substance move randomly as a result of the kinetic energy that they possess. However, because of the random nature of these movements and collisions, not all particles in a substance at any one time have the exact same value of kinetic energy. Thus, it is the average kinetic energy. Kinetic energy is directly proportional to temperature.


6.2.2 Describe the term activation energy, Ea.

The minimum energy required for a chemical reaction to take place.

6.2.3 Describe the collision theory.

Reactions take place as a result of particles colliding and then undergoing a reaction. The particles must have sufficient energy and correct orientation.

6.2.4 Predict and explain, using the collision theory, the qualitative effects of particle size, temperature, concentration and pressure on the rate of a reaction.

• Particle size

The smaller the particles, the faster the reaction. Reaction happens on surface and large area = smaller SA. 

• Temperature

Increase temperature, increase rate of reaction. More sufficient energy particles and more collisions

• Concentration

Increase concentration, increase rate of reaction. More collisions at closer proximity 

• Pressure

Increase pressure, increase rate of reaction. More collisions at closer proximity.

6.2.5 Sketch and explain qualitatively the Maxwell-Boltzmann energy distribution curve for a fixed amount of gas at different temperatures and its consequences for changes in reaction rate.

The area under the curve must stay constant. The average temperature which is proportional to KE shifts accordingly. The peak also shifts accordingly.

6.2.6 Describe the effect of a catalyst on a chemical reaction

• Catalyst

Presence of catalyst, increase rate of reaction. Catalyst finds an alternative pathway thus lowering the activation energy.

6.2.7 Sketch and explain Maxwell-Boltzmann curves for reactions with or without catalysts.

Because catalyst can find alternative pathways. Thus, the Ea shifts to the left allowing more particles to react at a lower KE.

Topic 6.1: Rates of reaction

6.1.1 Define the term rate of reaction

The rate of a reaction is the increase in concentration of products (or the decrease in concentration of reactants) per unit time. It is measured in mol dm^-3 s^-1

6.1.2 Describe suitable experimental procedures for measuring rate of reaction

There are several suitable experimental procedures

• Change in volume of gas produced

Gas syringe or inverted beakers to collect gas produced

• Change in mass

If the reaction is giving off a gas, the corresponding decrease in mass can be measured by standing the reaction mixture directly on a balance.

• Change in transmission of light: colorimetry/spectrophotometry

As the concentration of coloured compound increase, it absorbs porportionally more light, so less is transmitted. A photocell generates an electric current according to the intensity of the light transmitted.

• Change in concentration measured using titration

Through the method of quenching, we could obtain freeze frames from the experiment. Thus showing all the points.

• Change in concentration measured using conductivity

Conductivity can be measured directly using a conductivity meter.

• "Clock reaction"

Simply measures the time taken to reach its final point. Thus the rate given is an average.


6.1.3 Analyse data from rate experiments

There will be a steady decreasing rate of temperature.

Assuming that the decrease of temperature is uniform, we could deduce the original temperature without heating process.

Thus, find the temperature change and calculate the for enthalpy change.

Topic 6: Kinetics

Topic 6 of the IB HL Chemistry syllabus is the Kinetics. IBO recommends to spend 5 hours on this topic.

This topic has 2 sub-chapters: "Rate of reactions" and "Collision theory". Each are separated with numerical values in order of mentioned.

These are SL syllabus statements, it is recommended to bring a Casio Graphical Calculator instead of Texas. Casio Calculators have the periodic table installed already.