Reaction Rates

What factors affect the rate of reaction?

• Concentration or pressure

• Temperature

• Catalyst

• Surface area of solid reactants

What does the rate of reaction measure?

How fast a reactant is being used up, or how fast a product is formed, can be defines as a change in concentration of reactant or product in a given time

What methods can you use to follow the rate of reaction?

• Monitoring the removal of reactant (decrease in concentration)

• Monitoring the formation of product (increase in concentration)

• If a reaction produces a gas:

  • Monitoring volume of gas produced, using a gas syringe

  • Monitoring loss of reactants, using a balance

What factors make an effective collision?

• Particles must collide with the correct orientation

• Particles must have sufficient energy to overcome the activation energy barrier of the reaction

How do you calculate the initial rate of reaction?

• Plot a graph of volume (or concentration) against time

• Draw a tangent at t=0

• Calculate the gradient

• Gradient = Rate of reaction

Activation energy (E_a)

The minimum amount of energy required to start a reaction by the breaking of bonds

Catalyst

A substance that increases the rate of a chemical reaction without being used up in the process; a catalyst provides an alternate reaction pathway for the reaction with a lower activation energy

Homogeneous catalysis

A reaction in which the catalyst and reactants have the same physical state (most frequently aqueous or gaseous)

Heterogeneous catalysis

A reaction in which the catalyst has a different physical state from the reactants

Adsorption

A process that occurs when a gas, liquid or solute is held to the surface of a solid (happens in heterogeneous catalysis)

Desorption

The release of an adsorbed substance from a surface (happens in heterogeneous catalysis)

Draw the enthalpy diagram for an exothermic reaction

Draw the enthalpy diagram for an endothermic reaction

Is the change in enthalpy for an exothermic reaction positive or negative?

• Negative

  • Chemical system loses energy

  • Surroundings gain energy

Is the change in enthalpy for an endothermic reaction positive or negative?

• Positive

  • Chemical system gains energy

  • Surroundings lose energy

What is the economic importance of catalysts?

They lower activation energy, meaning that less electricity or fossil fuels are needed (to provide as much energy), cutting costs and increasing profit.

Boltzmann distribution

The spread of molecular energies in gasses

What does the area under the curve of the Boltzmann distribution represent?

Total number of molecules

Why does the Boltzmann distribution start at the origin?

No molecules have zero energy

Why does the Boltzmann distribution never cross the x-axis?

Theoretically, molecules can have infinite energy

What is the effect of temperature on the Boltzmann distribution?

At a higher temperature, the peak is lower and shifted right, this means that a greater proportion of molecules have the required activation energy. The means that a greater proportion of collisions result in a reaction. Also the frequency of collisions increases, as molecules are moving faster.

What is the effect of a catalyst on the Boltzmann distribution?

Higher proportion of molecules have the required activation energy with a catalyst. When molecules collide, they are more likely to react.

Catalytic converter

• A device used to reduce the emissions of an internal combustion engine

• Contains a ceramic honeycomb mesh coated in platinum and rhodium

• Found in petrol engines

• Heterogeneous catalyst

  • Reactant molecules adsorbed on the surface

  • They then leave ( after reaction) via desorption