3.1.5 Kinetics

What is the definition of rate?

The change in concentration of a substance in unit time

How do you find rate from a concentration vs time graph?

Calculate the gradient

• initial rate is the gradient at t=0

Define activation energy, Ea

Minimum energy required for a reaction to occur

Define catalyst

A substance that increases the rate of a reaction without being used up

How does a catalyst work?

Provides an alternative route of lower activation energy

Why does rate of reaction decrease over time?

Concentration of reactant molecules decreases so there is a lower collision frequency. Fewer successful collisions in a given time so rate of reaction is slower.

Explain how increasing temperature affects rate

Rate increases because molecules have more kinetic energy so more molecules have E ≥ Ea and the collision E increases. The collision frequency also increases so there are more successful collisions in a given time

Why does a small increase in temperature lead to a large increase in the rate of reaction?

At a higher temperature, many more molecules have E ≥ Ea so there are many more successful collisions per unit time

Explain how increasing concentration/pressure affects rate

Rate increases because there are more molecules in a given volume/ same number of molecules in a smaller space. The collision frequency increases so there are more successful collisions in a given time

Explain how increasing surface area affects rate

Rate increases because more of the solid is exposed, giving more surface contact with the reagent. The collision frequency increases so there are more successful collisions in a given time

Explain how adding a catalyst affects rate

Rate increases because the catalyst provides an alternative route of lower activation energy. More particles have E ≥ Ea so there are more successful collisions in a given time

Describe the Maxwell-Boltzmann distributions

• axes: y- number of molecules, x- energy

• curve begins at origin because 0 molecules have 0 energy

• peak of curve is most probable energy

• area under curve is total number of molecules

• curve does not touch x axis because there is no maximum energy

How does an increase in temperature affect the Maxwell-Boltzmann distribution?

• curve shifts right and becomes flatter

• value of Emp will increase

• number of molecules with Emp will decrease

• area under the curve remains the same

• Ea remains the same

• number of molecules with E ≥ Ea increases

How does increasing the concentration/pressure affect the Maxwell-Boltzmann distribution?

• curve is larger but with the same peak

• Emp stays the same

• number of molecules with Emp increases

• area under curve increases

• Ea stays the same

• number of molecules with E ≥ Ea increases

How does adding a catalyst affect the Maxwell-Boltzmann distribution?

• shape of curve stays the same

• value of Emp stays the same

• number of molecules with Emp stays the same

• area under curve stays the same

• Ea decreases

• number of molecules with E ≥ Ea increases

When can successful reactions occur?

When collisions take place between particles with E ≥ Ea of correct orientation

Why do most collisions not lead to reaction?

Most collisions are not successful because when most particles collide they have E < Ea (only a small number of molecules have E ≥ Ea)

Why do irreversible reactions still go to completion although only a small fraction of collisions lead to a reaction?

Molecules gain energy due to collisions so there will always be some molecules with E ≥ Ea

• collisions also cause some molecules to lose energy so they slow down

What can you measure to calculate the rate of a reaction?

The rate at which a reactant is used up or the rate at which a product is made eg volume of gas or concentration of acid

How can you investigate how the rate of a reaction changes with temperature?

Investigate the effect of temperature on the rate of reaction of sodium thiosulfate and hydrochloric acid by an initial rate method

• the reaction produces a precipitate of sulfur

• measure the time taken for a fixed amount of sulfur to be produced by timing how long it takes for a cross, marked under the bottom of the reaction vessel, to disappear as it is obscured by the sulfur precipitate

• dilute hydrochloric acid will be added to sodium thiosulfate solution at different temperatures in a series of experiments

Variables: temperature, concentration, volume, how dark the cross is

Which variables should be controlled?

Temperature, concentration, volume, how dark the cross is

• the amount of S needed to make the cross disappear should be the same each time so Δc will be constant

What are hazards of the experiment and how can they be reduced?

• SO₂ is produced which is a toxic gas so a fume cupboard should be used

• use low concentrations so the amount of sulfur dioxide formed in each experiment is small (the sulfur dioxide is soluble so the amount released into the air is small)

What are errors of the experiment?

• Human reaction time- greatest effect on shortest time/ highest concentration of thiosulfate

• Apparatus error- greatest percentage error in using the measuring cyclinder

How can the experiment be improved?

• lower concentrations- will mean it takes longer for the ppt to form (longer time= smaller timing error)

• water bath to control temperature variable which could affect the rate of reaction

• spectroscopy to monitor colour change so human error is eliminated