TOPIC 2

Speed of Reactions

Speed of ReactionsSpeed of Reactions

A chemical reaction can be represented by a A chemical reaction can be represented by a Chemical Equation.Chemical Equation.

The starting chemicals are called The starting chemicals are called ReactantsReactants and the new ones made the and the new ones made the Products.Products.

The The reactantsreactants go on the go on the left-hand sideleft-hand side of of the arrow and the the arrow and the productsproducts on the on the right-right-hand side.hand side.

ieieReactants Products

Mg +HCl Mg(Cl)2 + H2

Factors affecting the speed of reactions (Rate of reactions)

There are four factors that affect the There are four factors that affect the Rate of Rate of Reactions.Reactions.

They are:They are:ConcentrationConcentration

Particle SizeParticle SizeTemperatureTemperature

CatalystCatalyst..

Let us consider each in turn

For a reaction to occur, reacting molecules must For a reaction to occur, reacting molecules must collidecollide and stick together. and stick together.

For a successful collision they must be moving For a successful collision they must be moving fast enough ie. They must have sufficient fast enough ie. They must have sufficient Kinetic Kinetic or movement energyor movement energy. The minimum energy they . The minimum energy they must possess is called the must possess is called the Activation Energy.Activation Energy.

ConcentrationConcentration: The : The Higher the concentration Higher the concentration the greater the rate of reactionthe greater the rate of reaction. This is because . This is because higher concentrations have greater numbers of higher concentrations have greater numbers of reacting molecules and therefore a greater chance reacting molecules and therefore a greater chance of successful collision of successful collision

Factors affecting Reaction Rates continued

Particle SizeParticle Size: The : The smaller the particle size the smaller the particle size the quicker the rate of reaction.quicker the rate of reaction. This is a result of This is a result of smaller particles allowing a smaller particles allowing a greater surface areagreater surface area of of reacting molecules to be in contact and therefore a reacting molecules to be in contact and therefore a greater chance of a successful collision.greater chance of a successful collision.

TemperatureTemperature: : The higher the temperature the The higher the temperature the greater the rate of reactiongreater the rate of reaction. When we heat a . When we heat a substance we give the particles substance we give the particles more energymore energy. As a . As a result they move faster thus increasing the chance of a result they move faster thus increasing the chance of a successful collision(it can be said that more particles successful collision(it can be said that more particles now have more than the now have more than the Activation EnergyActivation Energy).).

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

CatalystsCatalysts: : A catalyst speeds up the rate of reactionA catalyst speeds up the rate of reaction . . It does so by providing a It does so by providing a surfacesurface on which reacting on which reacting molecules can meet and collide. molecules can meet and collide.

A catalyst will remain unchanged before and after a reaction.A catalyst will remain unchanged before and after a reaction. Catalysts in industrial reactions often have to be changed as Catalysts in industrial reactions often have to be changed as

they become they become PoisonedPoisoned.. Catalysts are said to be Catalysts are said to be specificspecific ie. A catalyst that works for ie. A catalyst that works for

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

Catalysts).Catalysts). Enzymes only work when the reaction is at the Enzymes only work when the reaction is at the correct temperature and pH. correct temperature and pH.

Measuring The Speed of Reactions

Marble chips are added to Hydrochloric Acid and the volume of gas given off was noted every 10secs.

Marble chips are added to Acid and the mass of the flask noted ever 30secs as the CO2 gas is given off.

Time(seconds)

Volume of

gas(cm3)This line represents Mg metal + 2M HCl acid 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 smaller Particle Size

------- This line represents a higher Temperature

------- This line represents a greater Concentration

The steeper the slope the quicker the reaction

The following is obtained by adding a metal to an acid and measuring the volume of H2 gas given off every 10secs

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

Measuring Reactions rates We can measure the rate of reaction by following the We can measure the rate of reaction by following the

change in masschange in mass throughout the course of the reaction. throughout the course of the reaction. Let us consider chalk and excess acid added together and Let us consider chalk and excess acid added together and

the mass of the flask taken every 30 secsthe mass of the flask taken every 30 secs

Time (s)

Mass (g)

CaCO3 + 2HCl CaCl2 + CO2 + H2O

The Mass of the flask gets lighter as the CO2 is released

The steeper the slope the faster the reaction.The line levels off when all the chalk has been used up

When the line levels off the reaction has finished