TOPIC 6CHEMICAL KINETICS

6.1Collision Theory and Rates of Reaction

ESSENTIAL IDEAThe greater the probability that molecules

will collide with sufficient energy and proper orientation, the higher the rate of reaction.

NATURE OF SCIENCE (2.7)The principle of Occam’s razor is used as a guide to

developing a theory – although we cannot directly see reactions taking place at the molecular level, we can theorize based on current atomic models. Collision

theory is a good example of this principle.

UNDERSTANDING/KEY IDEA 6.1.B

The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time.

● The rate of a chemical reaction is the increase in concentration of products or the decrease in concentration of reactants per unit time.

rate of a reaction = Δ[P]/ Δt or

rate of a reaction = Δ[R]/ Δt ● Units for rate = mol dm-3s-1

Reaction Rate Calculation

APPLICATION/SKILLS

Be able to analyze graphs and numerical data from rate experiments.

GUIDANCEBe able to calculate reaction rates from tangents of graphs of concentration, volume or mass vs time.

GUIDANCEBe able to interpret graphs of changes in concentration, volume or mass against time.

ANALYSE DATA

• You may be given a set of data such as concentration or changes in volume or mass vs time.

• You should be able to graph the data and find the rate at any point using the tangent.

• Be careful with units.

2NO2(g) → 2NO(g) + O2(g)Reaction Rates:

2. Can measure appearance of products

1. Can measure disappearance of reactants

3. Are proportional stoichiometrically

• To find the tangent at a certain point, take the change in concentration (change in “y”) divided by the change in time (change in “x”).

• Rates of reaction are always expressed as positive numbers even if the slope is negative.

2NO2(g) → 2NO(g) + O2(g)Reaction Rates:

4. Are equal to the slope tangent to that point

Δ[NO2]

Δt

5. Change as the reaction proceeds, if the rate is dependent upon concentration

1. When during the reaction is the rate the fastest? Slowest?

2. Determine the rate at t= 0s

3. Determine the rate at t= 600s

1. Fastest: start (higher [reactants] = more collisions)Slowest: End- lower [products]

2. Rise: 0.04 mol dm-3

Run: 205 s= 2 x 10-4 mol dm-3 s-1

3. Rise: 0.024 mol dm-3(ish)Run: 400s

= 6 x 10-5 mol dm-3 s-1

Which chemical reaction has a faster rate?

UNDERSTANDING/KEY IDEA 6.1.C

Concentration changes in a reaction can be followed indirectly by monitoring changes in mass, volume and color.

• In all of the following techniques, the goal is to measure the change in concentration vs time.

• Reaction rate is dependent upon temperature so it is crucial to control the temperature.

• This is best done by a thermostatically controlled water bath.

Techniques for measuring rate of reaction

• Concentrations are not usually measured directly. There is usually some sort of signal which relates to the change in concentration of either the reactants or the products.

• The raw data collected from these “signals” usually have units other than the units for concentration which is measured in mol/dm3.

• There are six common techniques which will be discussed.

1. CHANGE IN VOLUME OF GAS PRODUCED

• If one of your products is a gas, then you can collect the gas and measure the change in volume at regular time intervals.

• The graph is change in volume vs time.• Collection can be done with a gas syringe or

by water displacement in a burette.● Water displacement is limited if the gas is

soluble in water.● Warm water should be used since most

gases are less soluble in warm water.

2. CHANGE IN MASS

• If a reaction involves a change in mass (gas being produced or combustion), you can measure this directly.

• The reaction vessel can be placed directly upon a balance and the mass can be recorded against time by being hooked to a data collection set up.

• The graph would be mass against time.

3. CHANGE IN TRANSMISSION OF LIGHT – COLORIMETRY OR SPECTROPHOTOMETRY

• If one of the reactants or products is colored, you can measure the change in the color on a colorimeter.

• Continuous readings can be made of the light transmission.

• The graph is absorbance vs time.

Crystal Violet and NaOH Demo

4. CHANGE IN CONCENTRATION MEASURED USING TITRATION

• This is not a continuous method because as you are titrating one sample, the rest is still reacting.

• Samples must be withdrawn at regular time intervals and then analyzed by titration.

• In order to stop the reaction at each time interval, a substance is introduced to stop the reaction at that moment. This is called quenching.

5. CHANGE IN CONCENTRATION MEASURED USING CONDUCTIVITY• Conductivity is a measure of the number of

ions in a solution. • If a reaction is using up or producing

ions, you can measure this by a conductivity meter.

6. NON-CONTINUOUS METHODS OF DETECTING CHANGE DURING A REACTION – CLOCK REACTIONS

• There are times when you cannot record the continuous change in the rate of a reaction.

• It may be more convenient to measure the time is takes for a reaction to reach a certain fixed point.

• This means something you have chosen to use as an indicator to “stop the clock”.

• The limitation is this method gives only an average rate over the time interval.

UNDERSTANDING/KEY IDEA 6.1.A

Species react as a result of collisions of sufficient energy and proper orientation.

COLLISION THEORY• In order to react, particles must collide with

kinetic energy greater than the activation energy and have the correct collision geometry.

● Restating: To have a successful reaction you need 2 factors:

● Sufficient energy (greater than Ea)● Correct geometry

Activation Energy (Ea)• Activation Energy (Ea)- the min. amount of

energy a species needs in order to undergo a specific reaction● Necessary to overcome repulsion between

molecules or breaking some bonds in reactants before they can react

● Reactants achieve a transition state from which products can form • Particles with enough KE will overcome the

Ea to react• Too little KE = no reaction

Endothermic Reactions

POTENTIAL ENERGY DIAGRAM

PE

Transition State

Exothermic Reactions

POTENTIAL ENERGY DIAGRAM

PE

Transition State

APPLICATION/SKILLS

Be able to describe the kinetic theory in terms of the movement of particles whose average kinetic energy is proportional to temperature in Kelvin.

KINETIC THEORY OF MATTER

• All matter consists of particles which are in constant motion.

• The KE for gases is greater than that of liquids which is greater than that of solids.

• Temperature (measured in Kelvin) is proportional to the average KE of the particles in a substance.

• The higher the temperature, the higher the kinetic energy.

More about kinetics

• Kinetics refers to movement.• Movement in chemistry refers to the

progress of a reaction.• Therefore, kinetics is the study of how

fast the reaction goes.• The reaction mechanism is a sequence

of bond breaking and bond making which suggests “how” the reaction happens.

APPLICATION/SKILLSBe able to construct Maxwell – Boltzmann energy distribution curves to account for the probability of successful collisions and factors affecting these, including the effect of a catalyst.

MAXWELL-BOLTZMANN DISTRIBUTION CURVE

• The Maxwell-Boltzmann curve shows that particles in a gas at a particular temperature show a range of values of kinetic energy.

• The area under the curve represents the total particles in a sample.