3.2.2 Reaction Rates

Define rate of chemical reaction

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

Give 5 factors that affect the rate of reaction

• temperature

• pressure

• concentration

• surface area

• use of a catalyst

What does simple collision theory state is neccessary for a reaction?

• molecules must collide with each other

• they must have energy greater than or equal to activation energy

• molecules must collide in the correct orientation

What does ‘correct orientation’ refer to?

organic mechanisms

What happens if molecules collide with less energy than reaction energy?

no reaction takes place

What happens if molecules collide in the incorrect orientation?

no chemical reactions take place

What is a collision that doesnt result in a reaction called?

unsuccessful collision

How does concentration effect reaction rate?

increasing concentration increases rate

Why does concentration increase reaction rate?

more molecules in a given volume and thus more frequent successful collisions

How does increasing pressure effect reaction rate?

increasing pressure increases rate of reaction

Why does increasing pressure increase rate of reaction?

more molecules in a given volume, more frequent successful collisions

How does increasing temperature affect rate of reaction?

it increases rate of reaction

Why does increasing temperature increase reaction rate?

the average energy of the molecules increases as kinetic energy increases, a greater proportion of molecules have eenergy greater than or equal to activation energy → more frequent successful collision

Define activation energy

the minimum energy required for a reaction to take place by the breaking of bonds in the reactants

What is a catalyst

a substance that increases the rate of reaction without being consumed by the overall reaction. It allows the reaction to proceed via a different route with lower activation energy.

Give 2 types of catalyst

homogeneous and heterogeneous

What is a heterogeneous catalyst?

a catalyst that exists in a sifferent physical state from the reactants

What is a homogeneous catalyst?

a catalyst that exists in the same physical state from the reactants

How does a catalyst increase rate of reaction?

• lowers activation energy

• greater proportion of the molecules have energy greater than or equal to activation energy

• more frequent successful collisions occur

What are the advantages of using a catalyst?

• lower temperature can be used

  • reduced energy demand

  • less CO2 emissions

  • less cost

  • increased sustainability

• alternate pathway

  • higher atom econcomy

  • less waste

What conditions do enzymes operate at?

• 30-40*C

• room pressure (1 atm)

What type of catalyst are enzymes?

homogenous

Give 3 examples of heterogeneous catalysts?

• (iron) in manufacture of ammonia

• ziegler-natta

• Rh/Pd/Pt as catalytic converter

Describe the distribution of energies of the molecules of a reaction mixture

• few molecules have very high activation energy

• the majority of molecules have medium to ow activation energy

What is a boltzmann distribution?

shows the distribution of energies of molecules at a particular temperature

Draw a boltzmann distribution

What is the modal energy?

the energy that the greatest number of molecules occupy

What is the total area under a curve equal to?

the number of molecules

What does the shaded area under a boltzmann curve?

the number of molecules with energy greater than or equal to activation energy

Describe the shape of a boltzmann curve?

unsymetrical

How does a boltmann curve change when tempterature increases?

• lower peak

• total area is the same

• shifts to the right

Draw a boltzmann distribution with lower and higher temperature

How do you show a catalyst on a botlzmann distribution curve?

Ea shifts to the left

Give 5 ways of measuring rate of reaction

• change of mass

• titration

• colorimeter

• volume of gas released

• change in pH or electrical conductivity

What is continuous monitoring?

measuring concentration/mass/volume at regular time intervals

If a gas is a product, how is rate measured?

• change of mass

• volume of gas evolved

How is change in mass measured?

placing the beaker of the reaction on a balance and measuring mass at regular time intervals 

How is volume of gas lost measured?

• gas syringe

• upside down measuring cylinder

If a coloured solution is used/produced how can rate be measured?

colourimeter

If a reaction involved acid or base how can rate be measured?

• pH meter at regular time intervals

• samples taken at regular time intervals and titrated

Which way to measure change in pH is more accurate?

samples taken, via titration

What does a concentration time graph involve?

change in concentration of a reactant or product with time

How can you calculate rate of reaction at t using a conc-time graph?

drawing a tangent, and change in y over change in x

How can you calculate initial rate of reaction using a conc-time graph?

tangent at 0, change in y over change in x