Rate of reactions - CHEMISTRY

SURFACE AREA INCREASE 🥳

If the surface area of a solid reactant increases, the amount of particles exposed to collisions increases, leading to more frequent and successful collisions. This increases the rate of reaction.

CONCENTRATION/PRESSURE 💅

As concentration/pressure increases, there are more reactant particles per unit volume, therefore the frequency of collisions increases, also increasing frequency of successful collisions occurring in a given time, thus increasing reaction rate.

TEMPERATURE (FREQUENCY OF COLLISIONS)🥵

Exemplar response: When temperature increases, the average kinetic energy of the particles increases, increasing the frequency of collisions that occur in a given time, and hence the frequency of successful collisions. This increases the rate of reaction.

USE OF A CATALYST 🔮

Provides an alternative reaction pathway with a lower activation energy, increasing the proportion of collisions that have sufficient energy to overcome the activation energy, increasing the frequency of successful collisions, and hence increasing rate of reaction.

A catalyst doesn’t affect the enthalpy change because…

It does not change the chemical energy of the reactants or products, only changing the activation energy.

TEMPERATURE (PROPORTION OF SUCCESSFUL COLLISIONS) ♨

When temperature increases, the average kinetic energy of reactant particles increases, meaning particles collide with more force, increasing the proportion of collisions that have sufficient energy to overcome the activation energy, hence increasing the frequency of successful collisions, and hence the rate of reaction (MORE IMPACTFUL THAN PREVIOUS REASON) 

 

Collisions are successful if:

• Must collide

• Particles collide in correct orientation to allow bond-breaking to occur

• Collide with sufficient kinetic energy to overcome the activation energy barrier

Open system

Allows both matter and energy to be transferred to or from its surroundings.

Closed system

Allows the transfer of energy to or from the surroundings, but not matter.

Isolated system

Neither matter nor energy is transferred to or from the surroundings.

Product concentration rate

How quickly concentration of product in a reaction is increasing/decreasing

 

• Initially, when there is a high concentration of reactants available to become products…

concentration of products increases quickly. But later, this rate decreases and eventually reaches 0, as reactant concentration decreases.

How do catalysts work?

• Provide an alternative reaction pathway

• Forms temporary bonds with reactants, weakening existing bonds within reactants, and hence lowering activation energy.

• After products are formed, molecules leave surface of catalyst, and catalyst can be used again whenever.

• Catalysts also hold reactants on the surface in a certain way, improving orientation so more collisions can be successful

Homogenous catalysts

• Same state as reactants, and therefore cannot separate by themselves

Heterogeneous catalysts

• Different state to reactants, and therefore can separate by themselves.

• Favoured by industry for separability and ability to use at high temperatures.

• Solid catalysts often powdered to increase SA and further speed reactions.

Common catalysts

• Manganese dioxide

How does orientation impact rate of reaction?

• If most collisions are incorrectly oriented, reaction rate will be lower.

• If most collisions are correctly oriented, reaction rate will be higher.

What is a catalyst?

A catalyst is a substance that increases the rate of a chemical without being consumed, by providing an alternative reaction pathway with a lower activation energy.

STIRRING A MIXTURE DOES…

INCREASE RATE OF REACTION AS IT LEADS TO MORE FREQUENT AND SUCCESSFUL COLLISIONS