Liceo Scientifico “Giordano Bruno”Venezia-Mestre

Chemistry course Deep purpleChemical reactions and their different Speed Rates

Teacher

Donata Cesare

Reactions can occur:

very fast – like a firecracker

very slowly – like the time it takes for dead plants to make coal

at moderate speed – like food spoilage

By rate we mean the speed at which something changes

In chemistry, the speed rate of a reaction is expressed as the amount of reactant changing per unit time.

Example:

3 moles/year 5 grams/second

Factors affecting the speed rate Factors affecting the speed rate of a chemical reactionof a chemical reaction

• nature of the reactants

• concentration

• temperature

• catalyst

• surface area

The following events must occur before a reaction can proceed:

• the reactant particles must collide with each other

• the collisions must be strong enough to overcome the ‘energy barrier’, called the activation energy

• the reactants must form new bonds to create products

Factors affecting the speed rate of a Factors affecting the speed rate of a chemical reaction: chemical reaction: concentrationconcentration

……when washing up……when washing up

• If you have to wash grease off some dishes, which one would be more effective?

2 tablespoons in 1 cup of water 1 tablespoon in 1 cup of water

Why the one with more detergent?

• because it is more concentrated (there is more cleansing agent in the same amount of water);

• by increasing the concentration of reactants, the total number of particles will increase;

• more particles more collisions

Factors affecting the speed rate of a Factors affecting the speed rate of a chemical reaction: reaction: temperaturetemperature

The higher the temperature is, the faster the reaction starts

• molecules will move faster

• making more collisions

• making higher energy collisions

reaction at 300C reaction at 500C

The higher the temperature is, the higher the number as well as the energy of the collisions become; so temperature has a big effect on the speed rate of reaction.

higher temperature = faster reaction

Factors affecting the speed rate Factors affecting the speed rate of a of a chemical reaction: reaction: catalystcatalyst

A catalyst is a substance that increases the speed of a reaction, without itself undergoing any permanent chemical change.

A catalyst can be recovered at the end of a reaction and used again.

A catalyst reduces the activation energy of a reaction.

Factors affecting the speed rate Factors affecting the speed rate of a of a chemical reaction: reaction: catalystcatalyst

Catalysts lower the activation energy:when activation energy is low (blue line), the

number of colliding particles is greater.

enzyme: is a large molecule (usually a protein) that catalyzes (brings about) biochemical reactions;human body temperature = 37o C, much too low for digestion reactions to take place without any catalysts.

Take a piece of bread and keep chewing until it becomes sweet in your mouth because enzymes are breaking down the bread and changing it into simple sugars: these enzymes are biological catalyst.

Factors affecting the rate of a Factors affecting the rate of a chemical reaction: reaction: surface areasurface area

dissolving sugar into coffee..

grains are more effectivethan sugar lumps due to their surface area

Factors affecting the rate of a Factors affecting the rate of a chemical reaction: reaction: surface areasurface area

When solids are involved in chemical reactions only the surface particles are exposed. In this way they are the only ones that can collide with the particles of other reactants.

‘inner’ particles are protected and cannot collide with other particles until they become ‘exposed’

the surface particles are ‘exposed’ and can react

when you break up a lump into smaller pieces, the number of particles does not change but the number of surface particles increases

there is now a greater surface area with more exposed particles, so more collisions can occur hence the reaction is faster

Larger surface area = Larger surface area = faster reactionfaster reaction

……..which kind of wood will burn faster?

when on fire……

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