Chemistry - Rates of reactions

What is the rate of reaction

The speed at which the reactants become products

What is an example of a fast reaction

fireworks

What is an example of a medium reaction?

magnesium+ an acid

What is an example of a slow reaction?

rust

How can you measure the rate of reaction?

Measure how fast reactants are being used up
Measure how fast products are being formed

What is the rate of reaction equation?

Quantity of reactants used
Time taken
OR
Quantity of products produced
Time taken

If you used ‘quantity of reactants used up’, what would the graph look like

| (downwards curve)

___

If you used ‘quantity of products produced’, what would the graph look like?

---

/ 

/
| (upwards curve)

What factors can affect the rate of reaction?

Temperature
Surface area
concentration/pressure
catalyst

What is collision theory

For particles to react, they have to collide with each other with sufficient energy

What is ‘sufficient energy’

The activation energy (if there is less energy than this, nothing will happen)

What impacts whether a reaction will take place?

Amount of energy
Frequency of collisions

How does temperature increase the rate of reaction?

Particles gain more energy
Move faster
Collide more frequently
Collide with more energy
More likely to collide with activation energy

Higher rate of successful collisions

What is concentration/pressure

Number of particles per unit of measure

How does concentration/pressure increase the rate of reaction

More particles per unit
Collisions more frequent

Higher rate of successful collisions

How does surface area increase the rate of reaction?

Higher area
More space for particles to collide
Collisions are more frequent

Higher rate of successful collisions

What is a catalyst?

Substances used to speed up a reaction, that aren’t used up in the reaction themselves (don’t include them in the equation)

How does the use of a catalyst increase the rate of a reaction

Provides alternative reaction pathway
Lowers activation energy
Collisions with this new lower activation energy are higher

Higher rate of successful collisions

 / \

__/ \ _/\ __ _____
No catalyst Catalyst

What are some examples of catalysts

Cobalt
Nickel
Celery (actually from greek)

Why does the rate of reaction fall over time

As the reactants react, there are less particles to collide, which means there are less successful collisions

How do you find the rate of reaction at a given point on a curve

Draw a tangent to the curve at that point with a sharp pencil
Measure the gradient of the tangent (rise over run)

ADD disappearing cross method

1. Measure out 10cm³ of sodium thiosulphate solution into the 50cm³ measuring cylinder
2. Dilute this by adding 40cm³ of water to the same 50cm³ measuring cylinder. Add this to the conical flask.
3. Put the conical flask on the laminated card with the black cross
4. Using the 10cm³ measuring cylinder measure out 10cm³ of dilute hydrochloric acid.
Add the hydrochloric acid to the sodium thiosulphate, start the stopwatch and give the flask a swirl.
5. Stop the clock when you can no longer see the cross
6. Record the time taken
7. Repeat using different concentrations
8. Repeat the whole experiment and calculate a mean for each concentration

What is turbidity

A measure of how clear water is.

Why might results vary for the disappearing cross experiment

Eyesight, subjectivity