1. Topic 6.1 – Rates of Reaction

2.  Studies the rate (speed) at which a chemical process occurs.  Kinetics also sheds light on the reaction mechanism (exactly how the reaction occurs).

3.  A chemical reaction is a process in which new substance or substances is/are formed  During the course of a chemical reaction the starting materials chemically change to become the products.  Reactants → Products

4.  Reaction rates describe how fast reactants are used up and products are formed.  The reaction rate is determined by monitoring the change in concentration of either reactants or products per unit time. Rate = change in concentration / time  Since the concentration unit that is used is molarity (mol/L), the units for reaction rate are:

5.  Example:  If the concentration of reactants changes from 2.0 moles per liter to 1.6 moles per liter in 20 seconds  then the change in concentration = 1.6 – 2.0 = - 0.4  the time taken = 20 seconds  therefore the rate of reaction = - 0.4 /20 = - 0.02 moles per liter per second  a negative value indicates that the reactants are disappearing with time.

6.  Example:  For the reaction A→B  Average rate of change in A = ∆[A]/∆t Average rate in A = 0.30 – 1.00/40– 0= - 1.75 x 10 -2 mol L -1 s -1  Average rate of change in B = ∆[B]/∆t Average rate in A = 0.70 – 0.0/40– 0= 1.75 x 10 -2 mol L -1 s -1

7.  Example: Note that the average rate decreases as the reaction proceeds. This is because as the reaction goes forward, the concentration of reactants decreases.

8.  The average rate tells us the rate between two intervals, but does not tell us the rate at a specific time, called the instantaneous rate.  To find the rate at a specific time:  1. make a graph of concentration vs. time  2. draw a tangent to the curve at the desired time  3. from the slope of the tangent line, the instantaneous rate can be obtained.

9.  For example, for the reaction A → B, the instantaneous rate at 3 seconds can be obtained as follows:

10. Since all reactions slow down over time, when we compare rates of reactions under different conditions, it is best to compare the initial rate of each reaction. The initial rate is obtained by drawing a tangent to the curve at time = 0.

11.  So far we have only discussed the simple reaction of A →B, in which the stoichiometric ratio is 1:1.  What if the ratio is not 1:1? For example:  In general, for the reaction:  When we speak of the rate of a reaction without specifying a particular reactant or product, we will mean it in this sense. 2 HI (g)  H 2 (g) + I 2 (g) a A + b Bc C + d D

12. or Rate = − 1212  [HI]  t =  [I 2 ]  t Rate = −  [HI]  t =  [I 2 ]  t

15.  If we are to measure the rate of the reaction then we must be able to measure something that tells us the concentration of either the reactants or products.  In most cases the concentration is not measured directly, but by means of a signal that is related to the changing concentration.  Common techniques include measuring:  Change in volume of gas produced  Change in mass  Colorimetry/spectrophotometry  Change in concentration using titration  Change in concentration using conductivity  Non-continuous methods for detecting change during a reaction

16.  If one of the products is a gas, the volume of the gas can be collected. Measuring the change in volume at regular intervals enables a graph to be plotted of volume vs. time.  For example: Mg(s) + 2HCl (aq)  MgCl 2 (aq) + H 2 (g)  A gas syringe is the apparatus best suited for this purpose.

17.  Many reactions involve a change in mass and it may be convenient to measure this change directly.  If the reaction is giving off a gas, the corresponding decrease in mass can be measured by placing the reaction mixture directly on a balance. This method allows for continuous readings, so a graph can be plotted of mass vs. time.  For example: CaCO 3 (s) + 2HCl(aq)  CaCl 2 (aq) + CO 2 (g) + H 2 O(l)

18.  A colorimeter or spectrophotometer passes light of a selected wavelength through the solution being studied and measures the light absorbed by the reaction components.  As the concentration of the colored compound increases, it absorbs proportionally more light, so less light is transmitted and this is recorded on a meter connected to a computer.  For example, 2HI(g)  H 2 (g) + I 2 (g)  Iodine is the only colored component here. As its concentration increases during the reaction, there will be an increase in absorbance of light of the appropriate wavelength.  This method allows for continuous readings to be taken and a graph of absorbance against time can be plotted.

20.  In some reactions it may be possible to measure the concentration of one of the reactants or products by titrating it against a known standard.  Samples are taken at regular intervals and then analyzed by titration.  It is like taking a snap shot of the reaction at a specific time during the reaction. This process is repeated at several intervals of time to see how the concentration changes.  A graph of concentration vs. time can then be plotted.

21.  The total electrical conductivity of a solution depends on the total concentration of its ions and on their charges.  If this changes when reactants are converted to products, it can be used to follow the progress of the reaction.  Conductivity can be measured directly using a conductivity meter, and readings can then be converted to concentrations of ions present.  For example: BrO 3 - (aq) + 5 Br - (aq) + 6H + (aq)  3 Br 2 (aq) + 3 H 2 O (l)  The decrease in the concentration of ions (12 ions on the reactants side and 0 on the product side) will give a decrease in electrical conductivity of the solution as the reaction proceeds.

22.  Sometimes it is convenient to measure the time it takes for a reaction to reach a chosen fixed point, that is, something observable.  For example, the following can be measured:  The time it takes for a certain size of magnesium ribbon to react completely (no longer visible) with acid:  Mg(s) + 2HCl(aq)  MgCl 2 (aq) + H 2 (g)  The time it takes for a solution of sodium thiosulfate with dilute acid to become opaque by the precipitation of sulfur, so that a cross viewed through paper is no longer visible