Rate of Reactions

instantaneous rate

The reaction rate at a point in time during the reaction

adsoprtion

ability of a solid substance to hold gases and liquids on their surfaces.

Rate of Reactions

The change in concentration per unit time of any one reactant or product

catalyst

a substance that alters the rate of a chemical reaction without being used up itself.

Autocatalysis

When a reaction is catalysed by one of the products.

Intermediate formation theory

Catalysts work by forming an intermediate compound
With no catalyst the reaction is slow.
Add catalyst and it quickly forms an intermediate with one of the reactants.
The intermediate now reacts quickly with the other reactant to form the product and release the catalyst.

Surface Adsorption Theory

For heterogenous catalysis only. (catalyst = solid, reactant = gas mostly)

Finely divided platinum and nickel have the ability to do this

Steps: .Adsorption – A reactant adsorbs onto surface of catalyst, by forming temporary bonds with it. This increases the concentration of the reactant at that area.

Reaction at surface-

Other reactant approaches and reacts with attached reactant at the surface of the catalyst. Collisions happen, bonds are broken and product formed.

Desorption – Product leaves the surface of the catalyst and the process repeats.

The greater the surface area of the catalyst the better its performance.

Example of homogenous catalyst

Hydrogen peroxide being catalyised by potassium iodide
Both are liquids

Example of heterogeneous catalyst

Manganese and hydrogen peroxide

Example of autogenous catalyst

Between permanganate and iron ions the reaction is catalysed by the product Mn+2 ions

Oxidation of Potassium Sodium Tartate With Hydrogen Peroxide Using a Cobalt Chloride Catalyst

-Pink colour at start, Due to colbat ions
-Fizzing, A reaction taking place releasing CO2
-Colour changes to green, New substance is formed
-Colour goes back to pink, catalyst is released- green coloured substance is intermediate

Conditions need for a dust explosion

Presence of combustible, finely divided particles for example, flour or coal dust.
Enclosed space,
Spark
Oxygen

Factors effecting rate of reaction

Concentration
Temperature
Particle size
Nature of reactants
Presence of catalyst

Collision Theory

For a reaction to occur the particles must collide.
The colliding particles must have a minimum amount of energy called the Activation Energy

Activation Energy

is the minimum energy a colliding particle must have for a reaction to occur

Effective Collisions

collisions that cause product to form.

Effect of concentration on rate of reaction

Increasing concentration increases the rate by increasing the number of collisions
and hence the number of effective collisions increases.

Why does the reaction slow down as it progresses?

Because the reactants are being used up
Concentration is decreasing.

Effect on temperature on rate of reaction

Increasing temperature increases the rate:
-Particles have more energy so there are more collisions per second, so more effective collisions.
-More of the colliding particles have the activation energy required.
Second reason b is more significant as it causes a much larger increase in rate.

effect of particle size on rate of reaction

Smaller particles have a greater surface area.
The greater the surface area the greater the number of collisions.
A higher proportion of collisions are effective.
So the rate of reaction increases.

Effect of surface area on rate of reaction

Increased surface area means that there is an increased area for the reactant particles to collide with.
The smaller the pieces, the larger the surface area. This means more collisions and a greater chance of reaction.

Nature of Reactants Affecting Reaction Rate

•Nature of reactants means whether ionic or covalent.

•Ionic reactions are fast.

Ionic are free and have no bonds to break

Whereas covalent bonds most be broken first and are therfore slower

Homogeneous catalysis

when the catalyst and reactant are in the same phase.
Eg. Both solutions are liquids

Heterogenous catalysis

When the catalyst and reactant are in different phases.
Eg. Liquid and solid

Properties of catalysts

Catalysts are specific - they usually catalyse only one reaction or type of reaction.
Negative catalyst or Inhibitors slows down the rate of reaction rather than speed it up
A catalyst poison destroys the action of a catalyst.
In reversible reactions, the catalyst alters the rate of both the forward reaction and the backward reaction to the same extent

Everyday application of catalysts

Exhausts fumes in engines- A catalytic converter is used to convert these harmful gases into less harmful ones.
Catalyst are: Platinum, Palladium and Rhodium
Heterogeneous Catalysis

Enviromental Benefits

Reduces pollution by:
Less gases that cause acid rain
Less smog due to soot.
Less carbon monoxide which is toxic.

How catalysts work

by lowering the activation energy necessary for a reaction to occur.
it provides an alternative pathway for the reaction, one with a lower activation energy.

exothermic reaction

Give out heat eg. Combustion of Fuels

Endothermic Reaction

Takes in heat e.g cooling packs