● Kinetics refers to movement and is the study of how fast a reaction goes.

Topic 6. Kinetics

Examples of fast reactions:● Airbag inflation

● Egg in a hot pan

Examples of slow reactions:

● Esterification

● Rusting

6.1 Rates of reaction

Rate = 1/time = 1/s = s-1

H2 (g) + I2 (g) → 2HI (g)

● Because the graphs are curves (and not straight lines) the rate of reaction is not constant.

● The reaction rate at any point in time (for example 50s) is given by the tangent to the curve and is the gradient of the graph at that time.

1. Change in volume of gas produced

Measuring rates of reaction:

CaCO3 (s) + 2HCl (aq) → CaCl

2 + H

2O (l) + CO

2 (g)

2. Change in mass

CaCO3 (s) + 2HCl (aq) → CaCl

2 + H

2O (l) + CO

2 (g)

3. Change in transmission of light

H2 (g) + I2 (g) → 2HI (g)

4. Change in concentration (titration)

● At regular intervals a sample of the reaction mixture is taken and titrated against Hcl. The HCl reacts with the remaining NaOH.

● The smaller the volume of HCl solution required for neutralization, the further the reaction has progressed.

5. Change in concentration (conductivity)

● The small and fast moving hydroxide ions are consumed and replaced by slower moving bromide ions.

● The electrical conductivity decreases as the reaction proceeds.

C2H5Br (l) + OH- (aq) → C2H5OH (aq) + Br- (aq)

6. “Clock reactions”

Determine the reaction rate after 60 s and 120 s2 H

2O

2 (aq) → 2 H

2O (l) + O

2 (g)

Rate of reaction, (mol dm-3 s-1)

● How fast reactants are being converted to products during a chemical reaction.

6.2 Collision theory

● Particles in a substance have kinetic energy and move randomly.

● If a substance is heated, the kinetic energy of the particles increases and the temperature of the substance raises.

Collision theory:● For a reaction to take place between two particles, the

two particles must:

– Collide with each other

– Collide in the correct orientation (the reactive parts of the two particles come into contact with each other)

– Collide with sufficient kinetic energy

Activation energy, Ea

● The minimum amount of energy required to bring about a reaction.

= The minimum amount of kinetic energy that must be given to reactants before they will react.

= The minimum amount of energy necessary to break bonds.

Movement of particles in different states of matter:

● Solid

● Liquid/aqueous

● Gas

Maxwell-Boltzmann distribution

● The area under the graph represents the total number of particles and for a fixed mass of gas this area must be constant.

Geometry of collision

● The particles must collide in the correct orientation in order to react.

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/collis11.swf

How reactions happen

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/activa2.swf

https://www.youtube.com/watch?v=WeBE-_LRb2s

Factors affecting rate of reaction

Concentration

● Increasing the concentration of the reactants increases the number of collisions between the particles and therefore increases the rate of the reaction.

Zn + 2 HCl → ZnCl2 + H

2

Pressure

● If one or more reactants are gases an increase in pressure forces the particles together (=increases the “concentration”) and increases the collision rate.

Temperature

● When particles in gases, liquids or solutions are heated:

- they move faster and thus collide more- a larger portion of the colliding species will have enough energy to overcome the energy barrier

Particle size

● When one of the reactants is a solid, the reaction takes place on the surface of the solid.

● If the solid is broken down into smaller pieces, this surface area is increased.

CaCO3 (s) + 2HCl (aq) → CaCl

2 + H

2O (l) + CO

2 (g)

Light

● If the reactants are photosensitive, the rate of the reaction is greatly increased by exposure to sunlight or ultraviolet light.

Catalyst● A catalyst is a substance that is present in small amounts

and increases the rate of a chemical reaction but which is not consumed during the reaction.

● A catalyst provides a new pathway with a lower activation energy.

● Biological catalysts are known as enzymes and consist of proteins.

● A substance that decreases the rate of a chemical reation is called inhibitor.

Demo: Elephants toothpaste

● Pour ~50 mL of 30% hydrogen peroxide solution into the graduated cylinder.

● Squirt in a little dishwashing detergent and swirl it around.

● You can place 5-10 drops of food coloring along the wall of the cylinder to make the foam resemble striped toothpaste.

● Add ~10 mL of potassium iodide solution. Do not lean over the cylinder when you do this, as the reaction is very vigorous and you may get splashed or possibly burned by steam.

● ( An exothermoc reaction, the oxygen that is produced is captured as bubbles by the dishwashing detergent).

2 H2O

2(aq) → 2 H

2O(l) + O

2(g)