Rates of Reaction and Collision Theory

Objectives

:

• To explain factors that affect the rate of chemical reactions.

• To explain the collision theory and what happens to cause a chemical reaction.

• To carry out practicals and calculate the rate of reactions.

Chemical Reactions A reaction occurs when particles collide with each other.

We can tell that a reaction has occurred by looking for the following:

• a change in mass • a colour change • energy released (exothermic reaction) • bubbles or a gas being released • a new compound produced

Rates of reaction

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

Rates of Reaction

Chemical reactions occur when different atoms or molecules collide: For the reaction to happen the particles must have a certain amount of energy – this is called the ACTIVATION ENERGY.

The rate (speed) of a reaction happens depends on four things: 1) The temperature of the reactants, 2) Their concentration 3) Their surface area 4) Whether or not a catalyst is used

Activation energy

( Chemical reactions involve the formation of bonds between atoms but often before new bonds can be Energy is given out as This means that there has to be enough energy

activation energy

energy ) to start breaking the old bonds before a reaction can occur.

Reactants Old bonds start to break New bonds form

Changing the rate of a reaction..

4 factors that affect the rate of reaction are:

• Surface area • Concentration • Temperature • Catalyst Chemical reactions can be speeded up by increasing the temperature , surface area or the concentration of the the reactants.

Catalysts

also speed up chemical reactions. However at the end of a reaction the catalyst has not chemically changed, it just speeds up the reaction.

Surface area

The reactions of solids can clearly only take place at the surface of the solid. If we break a solid into smaller pieces we get more area and a faster reaction.

Molecules collide with the surface of the solid Extra surface for molecules to collide with.

Surface area

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

We therefore massively increase the rate of any reaction Slow Very fast

Concentration

Reactions in solution involve dissolved particles that must collide before reaction is possible.

The more crowded (concentrated) the solution, the faster the reaction.

Collisions infrequent Collisions frequent

Temperature

At higher temperatures molecules move faster. As a result there are more collisions per second and so a faster reaction occurs. Slow molecules are also less likely to lead to a reaction than fast ones. More collisions per second Fewer collisions per second

Catalysts

A catalyst is a substance that speeds up a reaction without getting used up in the process.

For example: In the present of Nickel catalyst, vegetable oil and hydrogen bond together to make margarine !!

• Catalysts are used over and over again, they may need cleaning but are never used up.

• Different reactions need different catalysts.

Measuring the rate of reaction?

There are several ways to measure the rate of reaction, these include: • collect the amount of gas released over a

period of time.

• record the colour change or precipitate

produced over a period of time.

• record a change in mass over a period of time So, you have to measure the amount of reactant used up or the amount of product produced per unit of time.

Measuring rate of reaction

Two common ways: 1) Measure how fast the products are formed 2) Measure how fast the reactants are used up

Rate of reaction graph

Slower rate of reaction here due to reactants being used up Fast rate of reaction here Slower reaction Time

Rates and Graphs

These show the increasing amount of product or the decreasing amount of reactant.

Shallow gradient Slow reaction Steep gradient Fast reaction Time

Acid and metal

Reactive metals (e.g. Magnesium) react with acid to make hydrogen gas. magnesium + hydrochloric acid  magnesium chloride + hydrogen Mg(s) + 2HCl(aq)  MgCl 2 (aq) + H 2 (g) As the gas given off leaves the flask the total mass of the flask and its contents decreases slightly.

Readings of the mass (g) can be taken. Usually at 1 minute intervals.

HCl Mg

Acid and metal

1. Measure the agreed volume of acid / water into the conical flask.

2. Have a loose plug of cotton wool to prevent “spitting” of droplets of liquid.

3. Have a piece of magnesium of known mass ready.

4. Add the magnesium, place the cotton wool in the neck and start taking mass readings immediately.

0 60 120

Cotton wool

Time (s) Reading (cm 3 ) 0

HCl Mg

Acid and marble

Marble chips are calcium carbonate.

They react with acid to make a gas (carbon dioxide) calcium carbonate + hydrochloric acid  calcium chloride + carbon dioxide CaCO 3 (s) + 2HCl(aq)  CaCl 2 (aq) + H 2 O(l) + CO 2 (g) The gas given off can be collected in a syringe and readings taken every 30 seconds or so.

Glass tube Gas syringe Hydrochloric acid Marble chips

Acid and marble

1. Measure the agreed mass of marble chips.

2. Set up the syringe, flask and connector 3. Measure the acid /water.

4. Add the marble chips and quickly insert the bung and start stop clock.

5. Take syringe readings at 30 second intervals.

Time (s) 0 Reading (cm 3 ) 0