speed of reactions.pdf

7/29/2019 Speed of Reactions.pdf

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Speed of ReactionsSpeed of Reactions

A chemical reaction can be represented by aChemical Equation.

The starting chemicals are called Reactants

and the new ones made the Products. The reactants go on the left-hand side of

the arrow and the products on the right-hand side.

ie. Reactants Products

Mg +HCl Mg(Cl)2 + H2eg.


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Factors affecting the speed of

reactions(Rate of reactions)

There are four factors that affect the Rate ofReactions.

They are:Concentration

Particle Size

TemperatureCatalyst.


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Let us consider each in turn

For a reaction to occur, reacting molecules mustcollideand stick to together.

For a successful collision they must be movingfast enough ie. They must have sufficient Kinetic

or movement energy. The minimum energy they

must possess is called the Activation Energy.

Concentration: The Higher the concentration

the greater the rate of reaction. This is becausehigher concentrations have greater numbers of

reacting molecules and therefore a greater chance

of successful collision


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Factors affecting Reaction Rates contdFactors affecting Reaction Rates contd.

Particle Size: The smaller the particle size thequicker the rate of reaction. This is a result of

smaller particles allowing a greater surface areaof reacting molecules to be in contact andtherefore a greater chance of a successful

collision. Temperature: The higher the temperature

the greater the rate of reaction. When we heat a

substance we give the particles more energy. As aresult they move faster thus increasing the chanceof a successful collision(it can be said that more

particles now have more than the ActivationEnergy).


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Factors affecting Reaction Rates(contd)Factors affecting Reaction Rates(contd)

Catalysts: A catalyst speeds up the rate of reaction.

It does so by providing a surface on which reactingmolecules can meet and collide.

A catalyst will remain unchanged before and after a reaction.

Catalysts in industrial reactions often have to be changed asthey become Poisoned.

Catalysts are said to be specific ie. A catalyst that works for

one reaction does not work for any other. Enzymes:These are Catalysts in living things(Biological

Catalysts). Enzymes only work when the reaction is at the

correct temperature and pH.


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Measuring The Speed of Reactions

Marble chips are added toHydrochloric Acid and the

volume of gas given off wasnoted every 10secs.

Marble chips are added to

Acid and the mass of the flasknoted ever 30secs as the CO2gas is given off.


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How do we measure the Rates of ReactionHow do we measure the Rates of Reaction

Time(seconds)

Volume

of

gas(cm

3

)

This line represents

Mg metal + 2M HClacid added together

This line represents the final volume of

gas produced.This will be the same if

the mass of Mg and no. of moles of

Acid remain constant

------- This line represents a smallerParticle Size

------- This line represents a higherTemperature------- This line represents a greaterConcentration

The steeper theslope the

quicker thereaction

The following is obtained by adding ametal to an acid and measuring thevolume of H2 gas given off every 10secs


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Measuring Reactions rates. We can measure the rate of reaction by followingthe change in mass throughout the course of the

reaction.

Let us consider chalk and excess acid added

together and the mass of the flask taken every30secs

Time (s)

Mass (g)

CaCO3 + HCl CaCl2 + CO2 + H2O

The Mass of the flask

gets lighter as the CO2is released

The steeper the slope the

faster the reaction.The

line levels off when all

the Chalk has been used

up

When the line levelsoff the reaction has

finished

speed of reactions.pdf

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