Chemical Kinetics IJSO Training (Phase 3) Dr. Kendrew K. W. Mak Department of Chemistry The Chinese University of Hong Kong Textbook: John Green, Sadru Damji, Chemistry for the International Baccalaureate Diploma Programme, 2nd Ed.

The “How Fast?” Question The reaction of methane（甲烷）with oxygen CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g) At room temperature – the reaction is extremely slow. At high temperature – very rapid combustion（燃燒）or even explosion（爆炸）! Fast reaction: The neutralization（中和作用）between a strong acid and a strong base Slow reaction: The rusting（鏽蝕）of iron Rate of reaction can be defined as the decrease in the concentration　（濃度）of reactants per unit time or the increase in the concentration of products per unit time.

The Rate of a Chemical Reaction A measure of the rate at which products are formed, which is equal to the rate at which the reactants（反應物）are consumed. Expressed as the change in concentration divided by the change in time. The numerical value will vary according to the number of moles（摩爾）of the substances involved in the stoichiometric equation（化學計量方程式）:

The Rate of a Chemical Reaction In most cases the rate of reaction decreases with time because the concentration of the reactants decreases with time and the reaction rate usually depends on the reactant concentration. It is a common practice to compare reactions by their initial rates（初速） (the gradient at t=0)

The Rate of a Chemical Reaction

The Rate of a Chemical Reaction

The Rate of a Chemical Reaction Any property that differs between the reactants and the products can be used to measure the rate of the reaction. e.g. Absorption（吸收）of colored light electrical conductivity（導電性） volume/mass/pressure of a gas concentration of a reactant/product determined by titration（滴定法）

Some Techniques for Measuring Rates Titration（滴定法） Collection of an evolved gas / increase in gas pressure Measurement of the mass of the reaction mixture Light absorption Electrical conductivity（導電性）

Some Techniques for Measuring Rates - Titration Removing small samples from the mixture at certain time intervals（時距）. Determine the concentration of either one of the reactants or products by titration. Plot the concentration data against time. Example: Draw small samples from the mixture. Quench（冷卻）the reaction with a solid base (e.g. CaCO3) Titrate the samples with sodium thiosulfate（硫代硫酸鈉）. End point: yellow (I2) to colorless (I-) Add starch（澱粉）near the end point（終點）to help clearer visualization.

Some Techniques for Measuring Rates – Collection of an evolved gas / increase in gas pressure Collect the gas with a gas syringe（氣筒）or a graduated vessel（附有刻度的容器）over water. Record the gas volume at fixed time intervals, or Carry out the reaction in a fixed-volume container and monitor the change in gas pressure. Example: Monitor the reaction by measuring the volume of H2 produced

Some Techniques for Measuring Rates – Measuring the mass of the reaction mixture The total mass of a reaction decreases if one of the products is a gas. Example: Monitor the reaction by measuring the decrease in total mass due to evolution（釋出）of CO2. Better result if: the gas has a high molar mass（摩爾質量） the gas has a very limited solubility（溶解度）in water.

Some Techniques for Measuring Rates – Light absorption Reactions that form precipitates（沈澱物）. Measure the time taken for the mixture to turn opaque（不透明）. Example: If the reaction involves a colored reactant or product, monitor the reaction by measuring the color intensity of the mixture using a colorimeter（比色計）or a spectrophotometer（光譜儀）. Absorbance（吸光度） concentration

The Collision Theory（碰撞理論） Collision are vital for chemical change, either to provide the energy required for a particle to change (e.g. for a bond to break), and/or to bring the reactants into contact. Not all collisions are successful. Sufficient energy is required to: 1. to overcome the repulsion between the electron clouds（電子雲）of the particles. 2. to break some of the bonds in the particles to allow the reaction to take place.

The Collision Theory In order to react, the two particles（粒子）involved must: Collide with each other The collision must be energetic enough to overcome the activation energy（活化能）of the reaction. The collision must occur with the correct geometrical alignment（幾何排列）, i.e. it must bring the reactive parts of the molecule into contact in the correct way. The minimum amount of energy required for reaction is known as the activation energy (Ea).

The Activation Energy (Ea) Reaction of very low activation energy (occurs almost instantaneously（即時地）) Reaction of H+ and OH- to give H2O Reaction of very high activation energy (negligible at room temperature) Reaction of sugar with oxygen

Heterogeneous reaction Factors Affecting the Reaction Rate Homogeneous reaction （同相反應） Reactants and products are all in the same phase (e.g. gas or solution) The properties of reactants and products – in particular, molecular structure and bonding The concentrations of the reactants and sometimes the products The temperature at which the reaction occurs The presence of a catalyst（催化劑）and, if one is present, its concentration Reactions take place at the surface or interface（界面）between two different phases (e.g. solid and solution) Heterogeneous reaction （異相反應） The four factors listed above The area of the surface (or interface) at which the reaction occurs.

Effect of Concentration on Reaction Rate The rates of most reactions change with the reactants’ concentrations. The rate at which particles collide is increased if the concentration of a reactant is increased. For gases, increasing the pressure is equivalent to increasing the concentration. For heterogeneous reactions （異相反應）, increasing the surface area in contact will also increase the collision rate, therefore powdered calcium carbonate（碳酸鈣） reacts much faster with hydrochloric acid（氫氯酸） than lumps of calcium carbonate.

Temperature and Reaction Rate A very common way to speed up a reaction is to increase the temperature. Reaction rates increase with temperature. At a higher temperature, a higher portion of reactant molecules have enough energy to overcome the activation-energy barrier.

Catalyst（催化劑）and Reaction Rate A catalyst is a substance that increases the rate of a chemical reaction without undergoing any overall change. Catalysts achieve this by providing an alternative reaction mechanism（反應機理）with a lower activation energy by which the reaction can take place, so that a greater proportion of collisions will have the required energy.

Catalyst and Reaction Rate In the presence of a catalyst, a greater proportion of collisions will have the required energy.

Rate Expression and Order of Reaction Rate expression – showing relation between the concentration of the reactants and the reaction rate （速率表示式） e.g. A reaction involves reactants A and B Rate of reaction = k : rate constant（速率常數） 　 does not vary with concentration but varies with temperature m, n : order of reaction with respect to reactants A and B m + n : overall order of the reaction

Determining the Rate Law from Initial Rates Experiment Initial Concentration Initial Rate mol L-1 s-1 [CH3COOCH3] [OH-] 1 0.100 M 0.050 M 0.00048 ↓no change ↓ X 2 2 0.00096 3 0.200 M 0.00192 Reaction rate = k[CH3COOCH3][OH-] First order in CH3COOCH3, first order in OH- Reaction rate = 0.00048 molL-1s-1 = k(0.100 molL-1)(0.050 molL-1) k = 0.00048 molL-1s-1 (0.100 molL-1) (0.050 molL-1) = 0.096 L mol-1s-1

Rate Expression and Order of Reaction - Example A reaction involving the reactants A, B, and C Experiment [A] [B] [C] Initial rate 1 0.600 0.150 0.450 1.15 x 10-3 2 1.200 3 0.900 2.30 x 10-3 4 20.70 x 10-3 Rate = k[A]a[B]b[C]c a, b, c = ? Give the rate equation for the reaction. Calculate the rate constant（速率常數）(k). ans: a=0, b=2, c=1 k=0.114