7.1 Rates of reactions

Question 1

Collision theory

Answer 1

When reactants come together the kinetic energy they possess means their particles will collide and some of these collisions will result in chemical bonds being broken and some new bonds being formed.
Increasing the number of successful collisions means that a greater proportion of reactant particles collide to form product molecules.
Not all collisions result in a chemical reaction.
The minimum amount of energy needed is called the activation energy.
Unsuccessful collisions happen when the colliding species do not have enough energy to break the necessary bonds.
If they do not have sufficient energy, the collision will not result in a chemical reaction.
If they have sufficient energy, they will react, and the collision will be successful.

Question 2

Concentration

Answer 2

Increasing the concentration of a solution will increase the rate of reaction.
This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions per second.
The number of collisions is proportional to the number of particles present.

Question 3

Temperature

Answer 3

Increasing the temperature causes the rate of reaction to increase.
This is because the particles will have more kinetic energy than the required activation energy, therefore there will be more frequent and successful collisions per second, increasing the rate of reaction.

Question 4

Surface area

Answer 4

With an increase in the surface area of a solid reactant, the rate of reaction will increase.
This is because more surface area of the particles will be exposed to the other reactant, producing a higher number of collisions per second.
If you double the surface area you will double the number of collisions per second.

Question 5

Pressure

Answer 5

Increasing the pressure of a gas increases the rate of reaction.
Increasing the pressure means that there are the same number of reactant particles in a smaller volume.
This causes more collisions per second.
Leading to more frequent and successful collisions per second.
Therefore, the rate of reaction increases.

Question 6

Catalysts

Answer 6

Catalysts are substances which speed up the rate of a reaction without themselves being altered or consumed in the reaction.
They do this by lowering the activation energy required, hence providing an alternate reaction pathway requiring less energy.
Catalysis is a very important branch of chemistry in commercial terms as catalysts increase the rate of reaction (hence the production rate) and they reduce energy costs.

Question 7

Enzymes

Answer 7

Enzymes are biological catalysts made from protein.
Enzymes speed up chemical reactions in cells, allowing reactions to occur at much faster speeds than they would without enzymes at relatively low temperatures (such as human body temperature).

Question 8

Practical 6 (Measuring the Production of a Gas)

Answer 8

Procedure - Add hydrochloric acid into a conical flask.
Use a delivery tube to connect this flask to an inverted measuring cylinder.
Add marble chips into the conical flask and close the bung.
Measure the volume of gas produced in a fixed time using the measuring cylinder.
Repeat with different sizes of marble chips .

Question 9

Practical 6 (Observing a Colour Change)

Answer 9

Procedure - Measure 50 cm3 of sodium thiosulfate solution into a flask.
Measure 5 cm3 of dilute hydrochloric acid into a measuring cylinder.
Draw a cross on a piece of paper and put it underneath the flask
Add the acid into the flask and immediately start the stopwatch.
Look down at the cross from above and stop the stopwatch when the cross can no longer be seen.
Repeat using different concentrations of sodium thiosulfate solution (mix different volumes of sodium thiosulfate solution with water to dilute it).