Rate Of Reaction & Catalysts

Noadswood Science, 2012

Tuesday, April 18, 2023

To understand how catalysts affects the rate of reaction

Rate Of Reaction

A variety of factors affect the rate of reaction, including: -

Temperature

Concentration of a dissolved reactant

Pressure of a reacting gas

Surface area of reactants

Catalysts

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

Other catalysts, especially enzymes, absorb molecules in a way that not only stretches bonds but also brings the reacting parts of reactants right next to each other...

Catalyst Example

In the presence of a nickel catalyst vegetable oil and hydrogen react to form margarine – nickel adsorbs hydrogen gas onto its surface in such a way that the bond holding the hydrogen molecule together becomes stretched

This partial breaking of the bond lowers the activation energy making hydrogen more reactive... H H

H HH HH HHH HH

Ni NiNi Ni NiNi

Hydrogen bonds stretched

Catalysts

Catalysts are used in the manufacture or application of a huge number of products: - Biological soap powder uses biological catalysts (enzymes) Enzymes in pineapple help cooked ham to be more tender Plastics are made using catalysts Manufacture of fertiliser via the Haber Process involves use

of an iron catalyst Synthetic materials like polyester are made using a catalyst Catalytic converters in cars – the catalyst encourages

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

Experiment

Plan and carry out an experiment investigating how catalysts affects the rate of reaction (record your results)…

Get the 3x catalysts: liver; celery; and manganese dioxide

Measure 25cm3 hydrogen peroxide into a conical flask with a delivery tube leading to a measuring cylinder submerged in water

Add the catalyst and measure the oxygen produced every 10 seconds for 2 minutes

Repeat using different catalysts

Graph your results and write a conclusion

Experiment

CatalystOxygen produced (ml) every 10 seconds

10 20 30 Etc…

Liver

Celery

Manganese dioxide

Experiment

Why did we not use boiled liver?

The enzymes in the liver (catalysts) would denature (change shape) if the liver was boiled, and would no longer work

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

Catalyst Disadvantages

Catalysts can be very expensive to buy, and often need to be removed from the product and cleaned

Different reactions also use different catalysts, so if more than one product is made then different catalysts will need to be bought

Catalysts can also be ‘poisoned’ by impurities (stopping them working) meaning the reaction mixtures must be kept clean – all of these cost money!

Summary

The rate of a reaction increases if: -

The temperature is increased

The concentration of a dissolved reactant is increased

The pressure of a reacting gas is increased

Solid reactants are broken into smaller pieces

A catalyst is used

Summary

For a chemical reaction to occur, the reactant particles must collide – collisions with too little energy do not produce a reaction

Summary

Changing concentration or pressure – if the concentration of a dissolved reactant is increased, or the pressure of a reacting gas is increased: There are more reactant particles in the same volume There is a greater chance of the particles colliding The rate of reaction increases

Changing particle size – if a solid reactant is broken into small pieces or ground into a powder: - Its surface area is increased More particles are exposed to the other reactant There is a greater chance of the particles colliding The rate of reaction increases

Summary

Changing the 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

Using a catalyst – 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