reaction rates course work. reactions occur when particles of reactant collide with enough energy to...

Reaction rates course work

Reactions occur when particles of reactant collide with enough energy to react.

Collision theory

Speeding up reactions

Increasing the chance of effective collision, increases the rate (speed) of reaction.

Surface Area

Concentration

Temperature

Catalyst

Factors include:

Surface area

Molecules collide with the surface of the solid

Extra surface for molecules to collide with.

Reactions of solids can only take place at the surface of the solid. If we break a solid into smaller pieces we get a larger surface area and a faster reaction.

Smaller particles larger surface area more collisions faster reaction

• If we grind up a solid to a powder we massively increase the surface area.

• We therefore massively increase the rate of any reaction

Very fast

Slow

Surface area

Particles in solution can only react with the solids exposed surface

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Concentration

Reactions in solution involve dissolved particles. The more crowded (concentrated) the solution, the faster the reaction.

Higher concentration more particles more collisions faster reaction

Sodium thiosulphate and rate

Temperature

• Particles need to have a minimum energy to react (activation energy).

• Heating the particles gives them energy.• At higher temperatures particles move faster. As a result

there are more collisions per second and so a faster reaction occurs.

Higher temperature faster particles more collisions faster reaction

Fair Testing

Only change one thing - keep everything else the same.

Variables will usually be:Concentration, volume, mass(which ever two you are not measuring in the reaction)

Catalysts

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Speed up a chemical reaction and remain unchanged as they do this. In biology these are called enzymes as they are biological catalysts

Reaction rates

Reactions can be followed by measuring changes in concentration, mass and volume of reactants and products.

We can then use these results to make graphs of the results

Calculating the Rate of Reaction

The rate of reaction can be calculated by measuring the :1. Time taken for the reaction

2. One other variable (something that changes) e.g.

Volume (cm3)

Mass (g)

Concentration (moles per litre = moll-1)

and

One other variable is measured by a reactant decreasing or a product being formed

Rate = Change (in variable)

Time taken

Example 1

Use the graph below to calculate the rate of reaction for the first 20 seconds.

Example 2

Use the graph below to calculate the rate of reaction between 5.2 and 10.8 seconds.

Rate and Time

For some reactions, a colour change will indicate the end of the reaction.

The only measurement carried out during this type of experiment is time. Therefore the rate equation is slightly changed from:

Rate = Change (in variable)

Time taken

No other variable measured

Rate = 1

Time taken

Units = per second (s-1)

To:

Concentration of sodium thiosulphate and the time taken for the cross to disappear

By doing a reciprocal graph it allows us to show the relationship between concentration and rate (speed at which reactants are converted to products due to successful collisions) .

Example 3

Calculate the reaction rates for the experiments below:

Reaction Temp (oC) Time (s)

1 40 35

2 60

Rate 1 = 1

35

Rate 2 = 1

12

= 0.029 s-1

= 0.083 s-1

12

Rate = 1

Time taken

Calculating Time from Rate GraphsExample 4

Use the graph below to calculate the time taken for the reaction

when a concentration of 0.6 moll-1 is used.

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1 1.2

Concentration (moll-1)

Ra

te (

s-1)

Rate = 1Time

We can rearrange the equation to:

Time = = 1

Rate1

12

Time = 0.08 s-1

Which of these would speed up the rate at which magnesium reacts with air?

A. Use a flame to heat the magnesium.

B. Use larger strips of magnesium.

C. Add water.

D. Coat the magnesium in oil.

Which of these would speed up the rate at which magnesium dissolves in acid?

A. Cool the acid.

B. Cut up the magnesium.

C. Add water.

D. Coat the magnesium in oil.

Why does breaking up solids increase the rate of reaction?

A. Makes more solid.

B. Creates more energy.

C. Increases surface area.

D. Increases the concentration.

Why does temperature increase the rate of reaction?

A. Acts as a catalyst.

B. Increases the concentration.

C. Increases number of molecules.

D. Makes collisions more frequent and harder.

Which of the following would not speed up a reaction?

A.Increasing surface area

B. Increasing concentration of the solution

C.Increasing the temperature

D. Diluting the solution