1. HL CHEM 6: Kinetics
BY HEIMAN KWOK 12N03S

2. 6.1 the rate of reaction

3. Rate of Reaction
The increase in concentration of products per unit time OR the decrease in concentration of reactants per unit time (always positive!)
𝑚𝑜𝑙 𝑑 𝑚 −3 𝑠 −1
Change in conc.
over time
∆ 𝑃 ∆𝑡 𝑜𝑟 −∆ 𝑅 ∆𝑡

4. Graphing rates of reaction
The gradient of a tangent to a curve will determine the rate of reaction at instantaneous time.
The initial rate is highest (steepest) and gradually the rate of reaction decreases (collision theory)
TIP: larger the triangle, more accurate the gradient
NOTE: draw – scale chosen, clear data points and units; sketch – showing trend, axis clearly labelled

6. NEEDED? Measuring Rates of Reaction
How quickly one of the reactants is being used up / time - (Time taken for the reaction to finish)
How much of a product (usually gas as its easier to collect) is produced / time
REMEMBER: the higher the rate, the faster the reaction takes place (shorter time needed)

7. 1. Volume of Gas produced
Gas syringe – moves outwards as the gas collects and calibrated to record the volume directly.
OR displacement of water in an inverted burette or measuring cylinder – only if gas has low solubility in water
NOTE: most gases are less soluble in warm water than in cold water – use warm water to minimise error
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8. 2. Mass Change of Reactants
Standing the reaction mixture directly on a balance to measure ^
YET – unlikely to work well if gas produced is hydrogen cuz it is too light to give sig. change in mass
Allows continuous readings if electric balance connected to a computer
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9. 3. Change in transmission of light: Colorimetry
If reactant or produce is coloured – giving a characteristic absorption in the visible region – wavelengths from 320 – 800mm
Sometimes an indicator can be chosen to generate a coloured compound
Can by calibrated so a x absorbance of light = y concentration
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10. 4. Titration – concentration
Quenching – titrating while taking ‘freeze frame’ shots of the conc. at a particular interval of time

11. 5. Conductivity - concentration
Directly measured using a conductivity meter which involves immersing inert electrodes in the solution
Sharp decrease or increase in ions (which carry charge) will allow the calculated of the rate of reaction
Can by calibrated so a x conductivity = y concentration

12. 6. pH change
Not often used as the pH scale is limited in its range and the change is often from acid/ basic to neutral

13. 7. Non-continuous methods of detecting change – ‘clock reactions’
Using something arbitrary as the end point to stop the clock
Only gives the average rate of reaction
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15. Volume of gas given off (1)
Downward displacement of water using a delivery tube and basin of water

16. Volume of gas given off (2)
Bung and syringe

17. Use of electronic balance
Mass of gas given off
Use of electronic balance

18. 6.2 kinetic theory + collision theory

19. Kinetic Theory All particles are continually moving
As temp increases = particles gain energy
Temp in Kelvin (K)/ Absolute temp. is proportional to the AVERAGE kinetic energy of the particles
HENCE
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20. Collision Theory
Not all collisions are successful/ cause reaction; to be a fruitful collision the have to:
1 ) The particles must collide with greater kinetic energy than the activation energy ( 𝑬 𝒂 ) - The minimum energy required for a chemical reaction to take place (J) to achieve the transition state
2 ) The particles must have the correct geometry/ orientation so that reactive sites of the particles come into contact

21. Factors that affect the rate of reaction
frequency of collisions; kinetic energy/speed of reactant particles; collision geometry/orientation;
NOTE: rate of reaction is only ‘how fast’ the reaction goes NOT ‘how far’ which is the yield of a reaction (ch7)

22. Temperature – factors affect the rate of reaction
Higher temperatures = 1) increase in no. of collision frequency and 2) more importantly increase in the no. of collisions between particles with sufficient KE for a fruitful reaction
NOTE: many reactions DOUBLE their reaction rate for every 10 °C rise in temp.
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23. Concentration/ Pressure – factors affect the rate of reaction
Increasing the conc. increases the rate of reaction as the frequency of collisions between reactant particles increases and hence successful collisions increase in suit.
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24. Particle Size/ Surface Area – factors affect the rate of reaction
Decreasing the particle size increase the rate of reaction
Very important in heterogeneous reactions where the reactants are in different phases such as solid reacting with a solution
Smaller particle size = increased surface area = more contact = higher probability of collisions between the reactants (more vigorous reactions)

25. Catalyst – factors affect the rate of reaction
Catalyst is a substance that increases the rate of reaction without itself undergoing chemical reaction
Most ^ provide an alternate rout for the reaction which has a lower activation energy – hence more particles have sufficient KE for a successful collision resulting in more reaction
Doesn’t change yield yet without ^ many reactions would be too slow – hence have a huge impact of feasiblity and efficiency
Every biological reaction is controlled by a catalyst – known as an enzyme
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26. 16.1 rate of expression