chemical kinetics

the rate of a chemical reaction

the increase in concentration of products or the decrease in concentration of reactants per unit time

how does the rate of reaction change during a reaction?

The rate of a reaction is not constant during a reaction, but is greatest at the start and decreases as the reaction proceeds.

6 techniques used for measuring the rate of reaction

1. change in volume of gas produced
2. change in mass
3. change in light transmission: spectrophotometry/colorimetry
4. change in concentration measured by titration
5. change in concentration measured using conductivity
6. clock reactions and non-continuous methods

change in volume of gas produced

* useful when one product is a gas
* using a gas syringe
* using water displacement from an inverted burette with a gas that has low solubility in water

\
change in light transmission: spectrophotometry/colorimetry

* one of the reactants or products is coloured and gives characteristic absorption in the visible region (wavelengths about 320–800 nm)
* an indicator can be added to generate a coloured compound that can then be followed in the reaction
* a colorimeter or spectrophotometer works by passing light of a selected wavelength through the solution and measures the intensity of the light transmitted by the reaction components
* as the concentration of the coloured compound increases, it absorbs proportionally more light, so less is transmitted.

change in mass

* keeping the mixture on a scale
* not useful when the evolved gas is hydrogen, as it will not produce too much of a change in mass

change in concentration measured by titration

* titrating against a known ‘standard’
* samples taken out from the mixture at regular time intervals

change in concentration measured using conductivity

* electrical conductivity of a solution depends on the total concentration of its ions and on their charges
* using a conductivity meter which involves immersing inert electrodes in the solution

clock reactions and non-continuous methods

something observable which can be used as an arbitrary ‘end point’ by which to stop the clock

collision theory - what factors are necessary for a collision to be successful

* energy of collision - activation energy is required to overcome the repulsion between the particles and often break bonds
* geometry of collision - proper orientation of particles that react together is a determining factor whether the reaction will take place
* not all collisions are successful!

factors affecting the rate of reaction

1. temperature
2. concentration
3. particle size
4. pressure
5. catalyst

temperature

* greater temperature = greater kinetic energy
* more particles will overcome the activation energy
* greater collision frequency
* increased rate of reaction

pressure

* for gases
* higher pressure = higher rate of reaction
* gas is compressed, which increases its concentration, so there is a greater chance that particles will collide

particle size

* decreasing the particle size increases the rate of reaction
* greater surface area
* greater chance for collisions

concentration

* increased concentration = more frequent successful collisions
* they are closer together so there is greater chance they will collide

catalyst

* a substance that increases the rate of a chemical reaction without itself undergoing permanent chemical change
* lowers activation energy, so more particles have enough energy to collide

The rate constant k is a constant for…

a particular reaction at a specified temperature

the order of a reaction with respect to a particular reactant

the power to which its concentration is raised in the rate equation

the overall order of reaction

the sum of the individual orders for all reactants.

units of rate for zero-order reaction

mol dm–3 s–1

units of rate for first-order reaction

s-1

units of rate for second-order reaction

mol–1 dm3 s–1

units of rate for third-order reaction

mol–2 dm6 s–1

concentration/time and rate/concentration graphs for reaction with rate=k (0-order)

\

concentration/time and rate/concentration graphs for reaction with rate=k\[A\] (1-order)

concentration/time and rate/concentration graphs for reaction with rate=k\[A\]^2 (2-order)

effect of doubling \[A\] for a zero-order, first- and second-order reaction

* 0: no change
* 1: rate doubles
* 2: rate x4

effect of increasing \[A\] x3 for a zero-order, first- and second-order reaction

* 0: no change
* 1: rate x3
* 2: rate x9

effect of increasing \[A\] x4 for a zero-order, first- and second-order reaction

* 0: no change
* 1: rate x4
* 2: rate x16

the rate-determining step of a reaction

the slowest step of the mechanism, determines the rate of the whole reaction, its transition state has the highest energy

The value of A, the Arrhenius constant, indicates…

the frequency of collisions and the probability that collisions have proper orientations.