HL CHEM 6: Kinetics BY HEIMAN KWOK 12N03S 08.03.14

6.1 the rate of reaction

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

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

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)

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 INSERT IMAGE HERE P.205

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 INSERT IMAGE HERE P.205

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 INSERT IMAGE HERE P.206

4. Titration – concentration Quenching – titrating while taking ‘freeze frame’ shots of the conc. at a particular interval of time

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

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

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 INSERT PHOTO FROM p.207

Volume of gas given off (1) Downward displacement of water using a delivery tube and basin of water

Volume of gas given off (2) Bung and syringe

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

6.2 kinetic theory + collision theory

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 Insert bottom left image from P/208

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

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)

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. INSERT LOWER GRAPH OF P.212

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. INSERT TOP DIAGRAM OF P.213

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)

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 INSERT GRAPH BOTTOM p.213 + P.214

16.1 rate of expression