Chemistry(14 - 16)

Chemistry(14 - 16)

Rates of ReactionRates of Reaction

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Chemical ReactionsChemistry is effectively the study of the changes which occur when particles collide resulting in the making or breaking of chemical bonds.

All chemical reactions involve the making or breaking of bonds and new substances being formed, e.g...

The animation illustrates the combustion of methane...

CH4 + 2O2 2H2O + CO2

Chemical ReactionsChemistry is effectively the study of the changes which occur when particles collide resulting in the making or breaking of chemical bonds.

All chemical reactions involve the making or breaking of bonds and new substances being formed, e.g.

The animation illustrates the combustion of methane...

CH4 + 2O2 2H2O + CO2

Reactants and ProductsReactants and Products

This is an example of a combustion reaction.This is an example of a combustion reaction.

CHCH44 + 2O + 2O

22 2H 2H22O + COO + CO

22

The reactants are methane and oxygen.The reactants are methane and oxygen.

The products are water and carbon dioxide.The products are water and carbon dioxide.

In the chemical reaction shown in the equation above, methane In the chemical reaction shown in the equation above, methane ((CHCH

44) and oxygen () and oxygen (OO22) react to form water () react to form water (HH22OO) and carbon ) and carbon

dioxide (dioxide (COCO22).).

Fast and Slow ReactionsFast and Slow Reactions

Some chemical reactions happen very quickly.

Fast and Slow ReactionsFast and Slow Reactions

Some chemical reactions happen very slowly.

Collisions and Reactions

Collisions and ReactionsChemical reactions can only take place when the reactant

molecules collide with one another.

Collisions and Reactions

Collisions and Reactions

If the collision is energetic enough, then a reaction may occur.

Chemical reactions can only take place when the reactant molecules collide with one another.

The energy needed to bring about a chemical reaction is called the Activation energy.

Any factors which can make the collisions more violent, or make them happen more frequently will speed up a chemical reaction.

Collisions and ReactionsCollisions and Reactions

The quantity of reactants

The quantity of reactants

The concentration of fluid reactants

The concentration of fluid reactants

The surface area of solid reactantsThe surface area of solid reactants

The temperature of the reaction

The temperature of the reaction

LightLight The presence ofa catalyst

The presence ofa catalyst

Factors which Affect the Rate of Chemical Reactions

Factors which Affect the Rate of Chemical Reactions

Rate of ReactionRate of

Reaction

Concentration - Solutions

If more molecules of a reactant are dissolved in solution, then more collisions are likely to occur, and the reaction takes place quicker.

The concentration of solutions is measured in mol/dm3.

Solutions of the same concentration contain the same number of particles.

Concentration - Gases

If the gas pressure of a reactant is increased, then more collisions are likely to occur, and the reaction takes place quicker.

Gases at the same pressure and temperature contain the same number of particles.

Surface Area

In a reaction between a solid and a liquid, the reaction takes place at the surface of the solid particles. Liquid particles cannot all make contact with the solid to react at the same time.

If large particles of solidare broken into smallerparticles, there is a largerarea of the solid exposed tothe liquid - more collisionswill occur per second and thereaction is therefore faster.

Number of particles in contact with the surface. 3

Number of particles in contact with the surface. 8

CatalysisThe addition of a catalyst to a chemical reaction can affect the rate of reaction. A catalyst speeds up a chemical reaction, but is unchanged itself.

Only specific catalysts will work for certain reactions, e.g. Platinum is used as a catalyst in the catalytic converters found in modern car exhaust systems.

The platinum helps to convert harmful exhaust gases such as carbon monoxide and oxides of nitrogen, into harmless nitrogen, oxygen and carbon dioxide.

Temperature

If the particles are moving faster, then collisions will occur more often and be more violent, and the reactionis more likely to occur. Move the slide bar to see the effect of increasing temperature.

The Kinetic Theory explains why an increase in temperature makes particles move faster.

Temperature

If the particles are moving faster, then collisions will occur more often and be more violent, and the reactionis more likely to occur. Move the slide bar to see the effect of increasing temperature.

The Kinetic Theory explains why an increase in temperature makes particles move faster.

An increase in temperature will increase the rate of reaction - an increase of 10ºC will double the rate of many reactions.

Reaction at 10 ºC Reaction at 20 ºC

TemperatureTemperature

The low temperatures, in a refrigerator (4ºC), slow down the chemical reactions which cause food to decay. Therefore the foods ‘last longer’.

Temperature

Some chemical reactions such as photosynthesis are activated by light. Increased light intensity tends to speed up the reaction.

Light energy can make reactant particles more energetic, and therefore more likely to react upon collision.

Light

The chemicals impregnated on photographic filmreact only when they are exposed to light.

Light

Rate of Reaction

Industrial processes, and food preparation and cookery, involve chemical reactions whose timing needs to be carefully controlled. Chemists, therefore, study rates of reaction very carefully.

The time taken for a chemical reaction to occur is an important issue in many everyday situations.

Rate of ReactionThe rate of a chemical reaction is defined as the rate at which reactants are used up, or products are formed.

This can be represented by the calculation:

Rate of Reaction = Amount of reactant usedTime

Rate of Reaction = Amount of product producedTime

OR

Rate of Reaction

Rate of Reaction = Amount of product producedTime

In a reaction between 5g zinc and50cm3 sulfuric acid, 50cm3 of hydrogen gasis produced in 2 minutes.

What is the rate of reaction?

Rate of Reaction = 50cm3

2 minutes

Rate of Reaction = 25 cm3 per minute = 25 cm3/min

Measuring Rates

We need to be imaginative at times in the way we choose to follow the course of a reaction.

If the reaction being followed generates a gas as a product, the rate can be measured using the following apparatus:

Measuring RatesMeasuring Rates

Measuring RatesMeasuring RatesAnother method used to follow a reaction which generates a gas as a product, depends upon monitoring the loss of the gas.

In the reaction between sodium thiosulfate solution and dilute hydrochloric acid, a yellow precipitate of sulfur forms, and this can be used to measure the rate of reaction.

Measuring RatesMeasuring RatesIf we can ‘see’ a change occurring, this can be used to measure the rate of a reaction.

The reaction is started by adding a small measured volume of dilute hydrochloric acid to a larger measured volume of sodium thiosulfate solution.

The time taken for the cross to disappear is measured by a stopwatch.

Measuring Rates

If the reaction is carried out at different temperatures, the time taken for the cross to disappear changes.How would you control and measure the temperature of the reaction?

Tim

e ta

ken

for

cros

s to

dis

appe

ar /

s

Temperature / ºC

x

x

x

xx

The results can be plotted and a graph like this produced...

Measuring Rates

Measuring Rates

This apparatus was used to investigate the reaction of sulfuric acid with magnesium.

The results from this experiment are:

sulfuric acid + magnesium hydrogen + magnesium sulfateH2SO4 (aq) Mg(s) MgSO4(aq) H2(g) + +

Vol

ume

of h

ydro

gen

prod

uced

/cm

3

Time /minutes

Measuring Rates

How long does it take to produce 80cm3 of hydrogen?

20 minutes

SummaryThe rate of a chemical reaction can be measured by looking at the rate of loss of a reactant, or the rate of production of a product.

The rate of a chemical reaction can be affected by the...

Amount of reactants;

Surface area of solid particles;

Temperature in the reaction vessel;

Presence of light;

Presence of a catalyst.

End of ShowEnd of Show

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