Noadswood Science, 2012

To be able to measure the rate of reaction

Wednesday, April 19, 2023

Rate

Different reactions can happen at different rates: - Reactions that happen slowly have a low rate of reaction Reactions that happen quickly have a high rate of reaction

For example, the chemical weathering of rocks is a very slow reaction – it has a low rate of reaction, whilst explosions are very fast reactions – they have a high rate of reaction

Rate

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

Measuring

How can you measure the rate of a reaction? Measure the rate at which a reactant is used up Measure the rate at which a product is formed

The method chosen depends on the reaction being studied – sometimes it is easier to measure the change in the amount of a reactant that has been used up; sometimes it is easier to measure the change in the amount of product that has been produced…

What can be measured?

Measuring

The measurement itself depends on the nature of the reactant or product…

Mass of a substance (solid, liquid or gas) is measured with a balance

The volume of a gas is usually measured with a gas syringe, or sometimes an upside down measuring cylinder or burette

Equation

The rate of reaction can be worked out simply, recording either the amount of reactant used / product formed and the time taken…

Rate of reaction = amount of reactant used or amount of product formed time taken

E.g. 24cm3 hydrogen gas is produced in 2 minutes – what is the rate of reaction?

Rate of reaction = 24 ÷ 2

Rate of reaction = 12cm3 hydrogen gas produced per minute

Experiment

Complete the rate of reaction worksheet…

CaCO3 + 2HCl CaCl2 H2O + CO2

Rates Of Reaction

What factors effect the rate of reaction?

Temperature

Concentration of a dissolved reactant

Pressure of a reacting gas

Surface area of reactants

Catalysts

Chemical reactions occur when particles of reactant collide with enough energy to react – anything that increases the chance of effective collision increases the rate of reaction (e.g. surface area, temperature, pressure, using a catalyst etc…)

Surface Area

Look at the sugar cube and granulated sugar dissolving in the water – which will dissolve first?

The granulated sugar dissolves much quicker than the sugar cube – this is due to the difference in surface area…

Surface Area

The reactions of solids can only take place at the surface of the solid – if the solid is broken into smaller pieces we get more area and a faster reaction…

Molecules collide with surface of

the substance

Extra surface for molecule

to collide with

Surface Area

If we grind up a solid to a powder we massively increase the surface area, massively increasing the rate of any reaction…

Slow

Fast

Temperature

How do the particles differ in cold water and boiling water?

In boiling water the particles collide more often and with more force – they are moving quicker and with more energy…

Temperature

If the temperature is increased: The reactant particles move more quickly More particles have the activation energy or greater The particles collide more often, and more of the

collisions result in a reaction The rate of reaction increases

Concentration

Concentration…

Pressure

Reactions involving gases are affected by the pressure of the gases present

Increasing the pressure squeezes the gas molecules closer together (making them more concentrated) – pressure speeds up reactions

Compress

Catalysts

For chemical reactions to occur: -

Existing bonds have to begin breaking so that new ones can be formed

The molecules have to collide in such a way that the reacting parts of the molecules are brought together

Catalysts can help with either or both of these processes

Catalysts

Catalysts increase the rate of reaction without being used up – they do this by lowering the activation energy needed

With a catalyst, more collisions result in a reaction, so the rate of reaction increases – different reactions need different catalysts

Catalysts are important in industry because they reduce costs, e.g. Biological soap powder uses biological catalysts (enzymes) Enzymes in pineapple help cooked ham to be more tender Manufacture of fertiliser via the Haber Process involves use of

an iron catalyst Catalytic converters in cars – the catalyst encourages

decomposition of nitrogen oxide (poisonous) back into nitrogen and oxygen