Chemistry Lab - Reaction Rates

Purpose: To determine the effects of temperature, concentration, catalysts, surface area,

and the nature of the reactants on the rates of specific chemical reactions.

Materials: “see lab sheets”Procedure: “see lab sheets”

Prelab:

Part 1:MV- temperature of the waterRV - time for the complete reaction

Part 2MV- RV-

Part 3MV-RV-

Part 4MV-RV -

Part 5 The effect of different acids:MV-RV-

The effect of different metals:MV-RV-

definitions:

collision theory - the theory that states for molecules to react with each other, they must collide with sufficient energy to break existing bonds, rearrange their atoms, and form new bonds.

activation energy -

catalyst -

Data :

Part 1

room temp water ice water hot water

Part 2

Part 3 reaction occured in test tubes C,D and E no reaction in A and B

Part 4

Part 5

Analysis Questions :1,2,3,4,7,8,9,10,11,13

Conclusion :

Factors which effect reaction rates:

The effect of Temperature

• When gases or liquids are heated the particles gain kinetic energy and move faster (see diagrams below).

• The increased speed increases the chance (frequency) of collision between reactant molecules and the rate increases.

• However this is NOT the main reason for the increased reaction speed!

• Most molecular collisions do not result in chemical change.• Before any change takes place on collision, the colliding molecules

must have a minimum kinetic energy called the Activation Energy shown on the energy level diagrams below (sometimes called reaction profile/progress diagrams).

• Going up and to the top 'hump' represents bond breaking on reacting particle collision.

• The purple arrow up represents this minimum energy needed to break bonds to initiate the reaction, that is the activation energy.

• Going down the other side represents the new bonds formed in the reaction products. The red arrow down represents the energy released -

exothermic reaction.• It does not matter whether the reaction is an exothermic or an

endothermic in terms of energy change, its the activation energy which is important in terms of temperature and reaction speed.

• Now heated molecules have a greater average kinetic energy, and so at higher temperatures, a greater proportion of them have the required activation energy to react.

• This means there is an increased chance of 'fruitful' higher energy collision that greatly increases the speed of the reaction.

The effect of Concentration

In general, increasing the concentration of reactant A or B will increase the chance or frequency of a successful collision between them and increase the

speed of product formation (slower to faster, illustrated below).

The effect of a Catalyst

• The word catalyst means an added substance, in contact with the reactants, that changes the rate of a reaction without itself being chemically changed in the end.

• Catalysts increase the rate of a reaction by helping break chemical bonds in reactant molecules.

• This effectively means the Activation Energy is reduced (see diagram below).

• Therefore at the same temperature, more reactant molecules have enough kinetic energy to react compared to the uncatalysed situation. The catalyst does NOT increase the energy of the reactant molecules!

• Although a true catalyst does take part in the reaction and may change temporarily chemically, it does not get used up and can be reused/regenerated with more reactants.

• It is chemically the same at the end of the reaction but it may change a little physically if its a solid e.g. ....

Different reactions need different catalysts and they are extremely important in industry: examples ..

• nickel catalyses the hydrogenation of unsaturated fats to margarine• iron catalyses the combination of unreactive nitrogen and hydrogen

to form ammonia

• enzymes in yeast convert sugar into alcohol• zeolites catalyse the cracking of big hydrocarbon molecules into

smaller ones• most polymer making reactions require a catalyst surface or

additive in contact with or mixed with the monomer molecules.

The effect of Surface Area - solid particle size

• If a solid reactant or a solid catalyst is broken down into smaller pieces the rate of reaction increases.

• The speed increase happens because smaller pieces of the same mass of solid have a greater surface area compared to larger pieces of the solid.

• Therefore, there is more chance that a reactant particle will hit the solid surface and react.

• The diagrams below illustrate the acid–marble chip reaction (slower

to faster, but they could also represent a solid catalyst mixed with a solution of reactants.