CHEM UNIT 3 AOS 2 - Collision theory

State collision theory

1. Particles must collide to have a chance to react

2. Particles must collide with sufficient energy to break the bonds in the reactants

3. Particles must collide in the correct position or orientation

Activation energy

The minimum amount of energy required to break the bonds of the reactants and start the reaction

What is reaction rate dependent on?

Activation energy → when the energy of a collision is greater than or equal to the activation energy, a reaction can happen

What are the 4 factors that affect rate of reaction? (SCaT Cat)

• Increase surface area (solids)

• Increase concentration (aqueous)

• Increase temperature (all)

• Add a catalyst (all)

Increasing concentration

• Increasing conc of a substance increases the number of reactant particles in a given volume

• This increases the frequency of collisions and therefore the number of successful collisions in a given time

Increasing pressure

• Increasing the pressure of a substance increases the number of reactant particles in a given volume

• This increases the frequency of collisions and therefore increases the number of successful collisions in a given time

Increasing surface area of a solid

• Only particles on the surface of a solid can participate in the reaction

• Increasing surface area increases the number of particles on the surface

• This increases the frequency of collisions and therefore the number of successful collisions in a given time

Increasing the temperature

• Increasing the temperature increases the average kinetic energy of the reactant particles and thus their average speed

• This increases the frequency of collisions and therefore increases the number of successful collisions in a given time

• However, more importantly, it also increases the proportion of particles that have sufficient energy to overcome the activation energy, increasing the number of successful collisions

What do catalysts do?

Provide an alternative reaction pathway which reduces the overall activation energy

Adding a catalyst

• Catalysts provide an alternative pathway by lowering the activation energy of the reaction

• This increases the proportion of particles that have sufficient energy to overcome the activation energy, increasing the number of successful collisions

What are the types of catalysts?

Homogenous and heterogenous

What is homogenous

Same physical state as the reactants and products of the reaction

What is heterogenous

Different physical state as the reactants and products of a reaction

What physical state would be most successful for catalysts and why?

Sponge like solid or powdered solids because they have a high surface energy such that the reactants adsorb to the surface which causes the bonds in the reactants to break

Why do chemist prefer heterogenous catalysts?

• Easier to separate from the products

• Easier to reuse

• Able to use at high temps

What is rate of reaction?

The change in concentration of a reactant or product per unit time

Lists methods of measuring rate

• Mass of solid formed

• Volume of gas formed

• Decrease in mass of container due to gas evolving

• pH

• Temperature

How would I measure how much gas was produced?

Using a gas syringe connected to a conical flask

What does it mean when no more gas is produced in the reaction?

The reaction has come to an end as one reactant has been exhausted (limiting reactant)

How do I read this graph?

• Start of the graph is when the reaction rate is the fastest as the concentration of the reactants is highest

• Rate decreases overtime as concentration of reactants decrease

• Graph plateaus as reactants are exhausted and the reaction comes to an end

How do I measure reaction rate from the production of a precipitate?

Recording the time taken for the solution to become cloudy

What does a reversible reaction mean?

A reaction can occur in both the forward and reverse directions

Why is it bad that some reactions dont reach completion?

• Consequences for industry

• Having large amounts of unreacted chemicals is expensive

What is yield?

The extent of the conversion of reactants into products

Define system

The chemical reaction

Define surroundings

Everything outside of the system that can exchange energy and/or matter with the system

Q > K

System shifts to the left to achieve equilibrium → more reactants are formed

Q < K

System shifts to the right to achieve equilibrium → more products are formed

Q = K

System is at equilibrium

What affects the value of K?

Temperature

What is Le Chateliers principle?

If an equilibrium system is subjected to a change, the system will adjust itself to partially oppose the effect of the change

What happens when pressure in increased?

• Le Chateliers principle states that the system will adjust itself to partially oppose the change

• To oppose the change, pressure must decrease

• The position of equilibrium will move in the direction of the fewest gas particles

What happens when pressure is decreased?

• Le Chateliers principle states that the system will adjust itself to partially oppose the change

• To oppose the change, pressure must increase

• The position of equilibrium will move in the direction of the most gas particles

What if there are equal numbers of reactant and product particles?

Change in pressure will not shift the position of equilibrium

What happens if I add an inert gas?

• Increases pressure

• No effect on position on equilibrium or K

What happens if I dilute my solution with water?

• All species will decrease in concentration 

• To increase concentration, the system will shift in the direction of the most particles

What happens if I add a catalyst?

No change in position of equilibrium or K

What happens when I increase the temperature of an exothermic reaction?

K decreases

What happens when I increase the temperature of an endothermic reaction?

K increases

Value of K (Less than 10^-4)

Negligible reaction occurs - high concentration of reactants and low concentration of products

Value of K (Between 10^-4 and 10^4)

Extent of reaction is significant - Significant concentration of reactants and products

Value of K (Greater than 10^4)

Almost a complete reaction - high concentration of products and low concentration of reactants