Rates of reaction

Simple collision theory

Particles must collide with enough energy (minimum activation energy - Ea) and in the correct orientation for a reaction to occur.

Increasing concentration (solutions) for rate of reaction

• More particles in the same volume.

• Particles are closer together.

• Collision frequency increases.

• More successful collisions per second.

• Reaction rate increases.

Increasing pressure (gases) for rate of reactionGas particles are forced into a smaller volume.

• Particles are closer together.

• Collision frequency increases.

• More successful collisions per second.

• Reaction rate increases.

Catalysts

A catalyst increases the rate of a reaction by increasing the number of successful collisions per second.
It is not used up in the overall reaction.
It can be recovered unchanged at the end.
It takes part in intermediate steps but is regenerated later.

A catalyst provides an alternative reaction pathway with a lower activation energy (Ea).
Activation energy is the minimum energy particles need for a successful collision.
Because the activation energy is lower:
A greater proportion of particles have enough energy to react.
More collisions are successful.
The reaction rate increases.

Enthalpy diagrams

Types of catalysts

A homogeneous catalyst is in the same physical state (phase) as the reactants.

A heterogeneous catalyst is in a different physical state (phase) from the reactants

Economic importance of catalysts

Catalysts reduce costs because they:

• Increase reaction rates, so more product is made per second.

• Allow reactions to run at lower temperatures and pressures.

• This reduces energy costs, especially electricity or fuel needed for heating.Economic importance

Sustainability benefits catalysts

Many industrial reactions normally need high temperatures to proceed quickly. Catalysts allow the same reactions to happen at lower temperatures.

This leads to:
Less fuel burned (often fossil fuels)
Lower energy demand
Reduced energy waste

Because less fossil fuel is burned:
Less carbon dioxide is released
Smaller greenhouse gas footprint
Helps reduce climate change impact

Investigation reaction rate, mass change

Method

• Reactants are placed on a balance.

• The reaction is started (often in a flask on the balance).

• The mass is recorded at regular time intervals.

Why mass changes

• A gas may be produced and escapes from the flask (e.g. CO₂).

• This causes the mass to decrease over time.

What is done with results

• Plot mass vs time

• Calculate rate from the gradient of the graph

Investigation reaction rate, gas volume

Method

• Reaction carried out in a sealed flask connected to a:

  • gas syringe or

  • upturned measuring cylinder in water

• Gas produced is collected and measured at set time intervals.

What is recorded

• Volume of gas (cm³) vs time

Analysis

• Plot volume vs time

• Steeper curve = faster reaction

• Rate = gradient of graph

Investigation reaction rate, dissapearing cross

Method

• A reaction produces a cloudy precipitate (solid forms).

• A cross is placed under a flask.

• Time is measured until the cross can no longer be seen.

What is recorded

• Time taken for cloudiness to obscure the cross

Rate calculation

• Reaction rate is often taken as:

Rate∝1/time

Boltzman distribution curve

• The curve shows number of particles vs energy

• Particles have a range of energies (Boltzmann distribution)

• Most have moderate energy; few have very high or low energy

• Only particles with energy ≥ activation energy (Ea) react

• Rate depends on proportion of particles above Ea

Boltzmann distributions and reaction rate, temperature

• At higher temperature, particles have more kinetic energy overall.

• The Boltzmann distribution becomes:

  • wider and flatter

  • shifted to the right

• A larger proportion of particles have energy ≥ Ea

• So the area under the curve beyond Ea increases

• More succsesful collisions per second reaction rate increases

Boltzmann distributions and reaction rate, catalyst

• A catalyst does not change the Boltzmann distribution

• Ea line moves left on the graph

• A larger proportion of particles now exceed Ea

• More succsessful collisions with particles exceeding minimum activitation energy thus reaction rate increases

Boltzmann distribution diagram catalyst (left) and temperature (right)