1. TOPIC 6
Kinetics

2. KINETICS Describes how fast a reaction occurs
Rates of reactions can be measured multiple ways
Often by an increase in product or a decrease in reactant per unit time (mol dm-3s-1)

3. KINETIC THEORY
All particles are in continuous movement (all have kinetic energy)
Energy directly related to absolute temperature of substance
Increased temperature increases energy of particles

4. COLLISION THEORY
Reactions rearrange the matter that is involved (reactants)
For reactions to occur, particles involved must collide; collisions allow for the bonds to break and be made
Not all collisions are successful

5. Because collision must occur, the rate of a reaction is based upon…
The frequency of collisions
The number of particles with E > Ea
The appropriate collision geometry/orientation

7. Can be viewed as a barrier for reaction occurring
Minimum energy required for reaction to start is known as the activation energy (Ea)
Can be viewed as a barrier for reaction occurring

8. Rate of reaction depends on the proportion of particles that have values of kinetic energy greater than the activation energy

9. MAXWELL-BOLTZMANN DISTRIBUTION CURVES
Not all particles in a system possess the same kinetic energies (the average energy is the temperature of the system)
Distribution of the number of particles with certain kinetic energies is shown in a Maxwell-Boltzmann curve

10. MAXWELL-BOLTZMANN DISTRIBUTION CURVES
As temperature increases…
Average kinetic energy of particles increases
Curve broadens
Decreasing temperature has opposite effect
Area under the curve stays the same (the number of particles is constant and does not change)

11. If E < Ea, no reaction will occur
If E > Ea, the reaction will occur
Increasing the temperature increases the number of particles with the sufficient Ea

12. CATALYSTS
Catalysts are substances that increase the rate of reaction without itself undergoing permanent change
Works by lowering the activation energy (a greater fraction of the particles possess the activation energy, increasing the rate of reaction)
Catalysts are classified as either homogeneous (same state as reactants) or heterogeneous (different state at reactants). Examples: homo – CFCs; hetero – catalytic converter in cars

13. FACTORS AFFECTING RATE OF REACTION
1. Temperature–increases the number of particles with the necessary Ea; increases the frequency of collisions; an increase of 10°C roughly doubles the reaction rate

14. FACTORS AFFECTING RATE OF REACTION
2. Concentration –the more concentrated the reactants, the more collisions per second per unit volume; as the reaction proceeds, the [reactants] decreases, slowing the reaction

15. FACTORS AFFECTING RATE OF REACTION
3. Surface Area–in a solid substance, only the particles on the surface can react; increasing surface area or changing the state (liquid or solid) increases the number of particles that can collide

16. FACTORS AFFECTING RATE OF REACTION
4. Pressure–increasing the pressure on a reaction involving reacting gases increases the rate (the gas particles are compressed and the chance for a successful collision increases)
changing the pressure on a reaction which involves only solid or liquids has no effect
5. Catalysts – substances that increase the rate of reaction without itself undergoing permanent change

17. DETERMINING RATES
In the chemical reaction of calcium carbonate with hydrochloric acid, 0.25 g of carbon dioxide was generated in 60.0 s.
Deduce the balanced chemical equation for the reaction, including state symbols.
Calculate the average rate, in mol s-1.
CaCO3(s) + 2HCl(aq)  CaCl2(aq) + CO2(g) + H2O(l)
Determine molar mass of CO2 (44.01 g mol-1) and moles produced (5.7 x 10-3 mol). Avg rate = 9.5x10-5 mol s-1.

18. CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
Rate of reaction can be determined from the slope of the graph

19. RATES OF REACTION
The graph shows two different reactions showing the change in concentration of the reactant against time.
Reaction A is decreasing more quickly than reaction B; the curve is steeper.

20. RATES OF REACTION
The gradients are not constant and can only be given for a particular moment in time.
Drawing a tangent to the line at a specified time interval will give the rate of the reaction at that moment

21. METHODS TO MEASURE RATES
Change in volume of gas produced – trap gas (in an inverted burette/graduated cylinder or a syringe) and measure the volume over time.

22. METHODS TO MEASURE RATES
2. Change in mass –if a gas is produced, reaction vessel can be placed on a scale and the change in mass is equal to production of gas

23. Methods to Measure Rates
3. Change in concentration using transmission of light –if components are colored, can shine light through solution and measure change in transmittance (compared to a standard); colorimeters or spectrophometer

24. Methods to Measure Rates
4. Change in Concentration using titration –can not do continuously; must stop reaction and measure at specific times (quenching) 5. Change in conductivity –if ions are generated or used, can measure how the conductivity of a solution changes

25. Methods to Measure Rates
6. Non-continuous methods of detecting change during a reaction: “clock reactions”
Arbitrary point in reaction is chosen and the time to reach that point is measured