Rates (Chem)

What does chemical kinetics study

How rates of reaction change under varying conditions

What molecular events occur during a reaction(reaction mechanism)

How is rate of reaction determined

The rate of product formed or used

Properties showing change in reaction

Change in volume(gases)

Change in colour intensity 

Change in acidity 

Change in electrical conductivity (total number of ions on either side of the reaction)

Change in pressure (gases)

What is rate measured as

A change in concentration per unit of time(mol/LS)

What are the 5 factors affecting rate of reaction

Temperature, Surface area, concentration, phases of reactants, effect of catalyst, nature of reactants

How does temperature affect rate of reaction

Increasing temp of reactants generally increases rate of rxn 

How does surface area affect rate of reaction

Increasing SA of reactants increases rate of rxn

How does concentration affect rate of reaction

Increasing concentration of a reactant increases rate of rxn

How does phases of reactants affect rate of reaction

Reactants in the solid phase are slower than those in liquid or solution or gases. This is a surface area effect as reactants in liquids or gases can be considered as finely divided particles done ✅ ✅ 👍 ✅ 👍 ✅ 👍

How do catalysts affect the rate of change

A catalyst allows reaction to proceed at a faster rate and a lower temperature without itself undergoing a permanent change

How does the nature of reactants affect the rate of the reaction

The atomic structure-such as more reactive because of a lower IE

Simple molecules react faster than complex ones

Aqueous ionic reactants have a faster rate than aqueous molecular reactants because bonds are broken by water in ionic and bonds still need to be broken in molecular

Collision theory

The rate of a chemical reaction is proportional to the number of effective collisions per second among reactant molecules

What effects the number of effective collisions per second

Concentration of reactants, orientation of reactant molecules, temperature of reactants

Collision theory- concentration of reactants

Lead to more collisions therefore increased total number of collisions

Decreasing the volume of a gaseous system increases the concentration

For gases at STP (1 mole) there are about 10^10 collisions per molecule per second

Collision theory- orientation of reactant molecules

nature of reactant

Favorable orientation- reactants have a short-lived intermediate and then produce products. This leads to an effective collision

Unfavorable orientation- reactants are in wrong orientation. No reaction, therefore not effective collision

Collision theory- temperature of reactants

Temperature is a measure of the average ke of molecules. Some molecules have greater speeds than others. This variation in speeds can be plotted on a graph showing the distribution of molecule speeds/ energies

Activation energy

This is this is the most important factor in determining the effectiveness of collisions among reactant molecules

Complexity of molecules, bond strength are unique to a reaction 

The minimum energy of a single particle needed for a reaction to occur when it collides with a stationary particle

Conclusion of collision theory

By increasing the temperature of reactant molecules, the ke distribution is shifted to the right resulting in many more molecules possessing the minimum ke required for effective collisions. Therefore, the rate of reaction is increased

What is the effect on the number of molecules possessing EA at some new lower temperature? And what is the effect on the value of EA at the new temperature

The value of EA does not change with a different temperature. The total fom with EA has decreased. Therefore the rate has decreased

Reactions one and two are at the same temperature, but ea1 is more than ea2 which is the faster reaction

R2 is faster because even though they have the same ke distribution, the EA for r2 is slower so more molecules have EA

Main point of collision theory

If a reaction has a large EA, the reaction rate will be slower because fewer particles will possess the activation energy I.e the minimum ke for an effective collision