Rates of Reaction

Common factors that determine the rate of reaction

Temp

Catalyst

Concentration

SA of solids

Pressure of gases

Ways to measure rates of reaction

Change in mas

Collecting a volume of gas

Changes in colour

Changes in appearance like the formation of a precipitate

Rate of reaction mathematical formula

Graph showing how much product is left v time

Experiment to meausre how SA affects rate of reaction, and diagram

Use the reaction between HCl

large marble chips

Medium chips

Lots of small marble chips/powder

Measure the mass of CO2 against time every 30 seconds

How does SA affect the rate of reaction

Larger SA= faster rate of reaction

Control variables when experimenting the effect of SA in the rate of a reaction

Temp

Concentration of HCl

Volume of HCl

Mass of CaCO3

Effect of Acid Concentration on Rate of Reaction Mg + HCl

Method: Add a strip of Mg to 40cm3 of HCl and time how long it takes for Mg to disappear. Repeat for different concentrations.

Variables:

Independent: Acid concentration

Dependent: Time for Mg to react completely

Controls: Volume of acid, mass & surface area of Mg, temperature, mixing amount

Reactant against time graph

What is the gradient equal to on this graph

the rate of reaction at that time

Effect of Temperature on Rate of Reaction (Disappearing Cross)

Method: Warm 40cm3 of thiosulfate solution to required temp, add 5cm3 of HCl, place over a black cross, and time how long it takes for the sulfur precipitate S to obscure the cross.

Variables:

Independent: Temperature

Dependent: Reaction time (used to calculate rate: 100/time)

Controls: Volumes and concentrations of both solutions, same cross (X)

Conclusion: As temperature increases, the rate of reaction increases.

What is a successful collision

When a collision has enough energy to lead to a chemical reaction

What is activation energy

The minimum energy needed for a collision to be successful

The result of a larger SA of solids for rate of reaction (collision theory)

For the same mass, smaller particles have more surface area.

There is a higher frequency of collisions.

Therefore, so more frequent successful collisions so higher rate of reaction.

The result of increasing concentration of solutions on rate of reaction (collision theory)

Higher concentration means there are more particles per unit volume.

There is a higher frequency of collisions.

Therefore, so more frequent successful collisions so higher rate of reaction.

The result of temperature of solutions on rate of reaction (collision theory)

Higher temperature increases the kinetic energy of the particles (they move faster).

There is a higher frequency of collisions.

The fraction of collisions that are successful increases (as collisions happen with more energy)

Therefore, so more frequent successful collisions so higher rate of reaction.

Why does a catalyst work

As it provides an alternative pathway with lower activation energy so that a greater proportion of collisions are successful

Catalytic Decomposition of Hydrogen Peroxide

Aim: Investigate how different catalysts, catalyst mass, and surface area affect the decomposition rate of hydrogen peroxide H2O2

Equation: 2H2O2 (aq) --> 2H2O (l) + O2 (g)

Method: Add catalyst to 50cm3 of 1% H2O2 in a conical flask, seal with a bung, swirl continuously, and measure the volume of O2 gas collected in a gas syringe at regular time intervals.

Variables Tested:

Type of Catalyst: Manganese(IV) oxide vs. Copper(II) oxide

Mass of Catalyst: 0.5 vs. 0.25

Surface Area: Powder vs. Granules

Key Control: Same volume 50cm3 and concentration 1% H2O2

What does the rate of reaction depend on

The number of collisions per second (frequency)

The energy of the collisions (fixed propn will be successful)

What is a catalyst

A substance that will increase the rate of a chemical reaction without being used up. They are unchanged at the end of the reaction

Two types of catalyst

Heterogeneous Catalyst- the catalyst is in a different physical state from the reactants

Homogeneous catalyst- the catalyst is the same state as the reactants (enzymes)