kinetics I

State collision theory

Reactions can only occur when collisions that take place between particles have sufficient energy and collide in the right direction

What is formula for rate

Amount of reactant used/product made / time

Define activation energy

Is the minimum amount of energy particles need to collide in order to start a reaction

What does the maxwell Boltzmann distribution show

Shows the spread of energies that molecules of gas or liquid have at a particular temperature

What does the area under the Boltzmann graph show

The total number of particles present

How does the Boltzmann graph change when the temperature decreases

The peak gets higher and the graph shifts to the left so more narrow

How does the Boltzmann graph change when the temperature increases

The peak gets lower, shifts to the right and gets wider

Define the rate of reaction

The change in concentration of per unit time

Where is the rate of reaction on a concentration - time graph

The gradient

How does higher pressures and concentration affect rate of reaction

There are more particles per unit volume and so the particles collide with a greater frequency so higher frequency of successful collisions

What happens to the shape of the distribution graph when concentration increases

It doesn’t change the peak will only move lower with lower concentration

What happens to the activation energy when the concentration increases

Molecules have more energy so greater activation energy but doesn’t mean a greater proportion

What is the affect of increasing surface area on the rate of reaction

Causes successful collisions to occur more frequently between the particles and so increases the rate of reaction

Define catalyst

Increases the rate of reaction without getting used up in the process

How does adding adding a catalyst affect the reaction profile for a reaction

The activation energy has a lower peak

How does adding a catalyst affect the rate of reaction

It lowers the activation energy so at the same temperature a greater proportion of particles have energy equal to or greater than the activation energy, so more successful collisions more frequently- faster rate

How is a catalysed reaction shown on the distribution graph

It’s the same but the activation energy is further to the left

How do heterogenous catalyst work

• adsorption - reactant molecules bond to the surface of the solid catalyst

• The catalyst weakens the bonds in the reactant and break to form radicals which react with each other to form new substances

• Reacts via an alternative pathway with Lower activation energy

• Desorption - products leave the catalysts surface

How do homogenous catalyst work

Forms intermediate species by reactants combining with catalyst which react to form product, catalyst is reformed again

How can u increase rate using heterogenous catalyst

Increase the surface area, increases the rate as more particles can react with the catalyst at the same time

What are the uses of a catalyst

• lower temp needed for the reaction to occur so less energy co2 and less money

• Speeds up reaction

• Can change the properties of a product

What are the three ways to measure the rate of reaction

• how long it takes for a precipitate to form

• Amount of mass lost - in reactions were gas is formed

• Volume of gas produced

How to measure rate by how long it takes for precipitate to form

• place a cross on paper and add the beaker, time how long it takes for the cross to dissapear

• It’s hard to know when so make sure the same observer is used to avoid errors

How to measure rate by measuring amount of mass lost

• place reaction on mass balance and measure the mass loss at a given time interval

• Do in a fume cupboard if toxic gas produced

How to measure rate from volume of gas produced

• measure amount of gas produced using gas syringe and measure this over a specific time

How do you calculate the initial rate of reaction

At 0 mins, draw a tangent and find the gradient

How does the reaction profile for a homogenous catalyst look like

What are the three reasons why a heterogenous catalyst increases the rate of reaction

• it increases the temperature of the reactants

• Provides an alternative pathway with lower activation energy

• Increases enthalpy change of the reaction

What are the environmental benefits of catalysts

• the reactions happen at lower temp so less fuel needed and fewer emissions from fuels

• Higher atom economy so fewer are materials needed and less waste products produced