h. cordy-mckenna (honywood school 2008) understanding rates of reactions use the green button to...

H. Cordy-McKenna (Honywood School 2008)

Understanding Rates of Reactions

Use the green button to advance slides that contain animations

http://teachable.net/res.asp?r=1015


What takes part in a reaction?

• Reactants – These are the chemicals that react together (on the left side of the chemical equation)

• Products – These are the chemicals that get made in the reaction (on the right side of the equation)

• Sometimes there may only be one reactant or one product. Can you think of an example?


Copper + Oxygen Copper oxide

Copper oxide + Sulphuric acid Copper sulphate + Water

Reactants Product(s)


Particles and Collisions

• Particles need to hit each other (collide) to react

• Anything that makes particles collide more often will speed up a reaction

• Faster collisions have more energy so they are more likely to react together


Measuring the Rate of a Reaction

• You can measure how fast a reaction produces a gas


Measuring the Rate of a Reaction• You can measure how fast a reaction loses mass (when a

gas is given off)• Notice how both the reactions below lose the same mass

by the end


Measuring the Rate of a Reaction

• You can measure how long it takes for the reaction to take place.


Low Temperature

• Particles move slower and have less kinetic energy

• They collide less frequently

• They collide with less energy

• The reaction is slower


Higher Temperature

• Particles move faster and have more kinetic energy

• They collide more frequently

• They have more energy when they collide

• The reaction is faster


Concentration

• If a solution is more concentrated then there are more particles per volume (e.g. cm3) than before (they are more crowded)

• Which of these is the most concentrated?


Low Concentration

• Particles are not as crowded so they collide less frequently

• The reaction is slower


High Concentration

• Particles are more crowded so they collide more frequently



Surface Area

• Let’s say you have just killed George Bush and want to dissolve him in a vat of acid to get rid of the evidence. What could we do to his body to speed up the reaction before the police arrive?


George’s Head

• On the next slide we will see arrows that show where the acid particles could collide with George’s head.

• The more chances for collisions then the faster his head will dissolve in the acid!


LOADS more surface area for the acid particles to collide with


Surface Area

• The greater the surface area the more collisions there will be

• The reaction happens faster


Catalysts

• These speed up reactions without getting changed or used up

• How?

• They provide a surface where the reactants meet and react

• They also lower the activation energy needed for the reaction to happen


Catalyst Demo

• Click here for a great video of “Elephant’s Toothpaste”

• The catalyst used causes hydrogen peroxide to break down and release oxygen

• Fairy Liquid traps the oxygen in bubbles with great results!

• The catalyst is NOT CHANGED or used up at the end, and could be re-used


Rate of Reaction Graphs

• In exams they often show you a graph of a reaction

• It might show you how quickly a gas is made

• It might show you how quickly mass is lost

• The steeper the line the faster the rate of the reaction

• A reaction stops when no further change takes place (e.g. no more gas is made)


A

B C

When is the reaction fastest? (Press a letter)


A

B C

When is the reaction slowing down? (Press a letter)


A

B C

When has the reaction stopped? (Press a letter)


Use the pen to draw two more curves to show what would happen if this reaction happened at: (a) A higher temperature (b) A lower temperature

Click here for the pen (and also to select “Arrow” when you have finished drawing


Higher temperature

Lower temperature

Why does each reaction have the same end point in terms of gas produced? (Assume you are using the same concentrations and masses of reactants)

Click the green button to move on…


Summary

• Reactions need collisions between particles• Rate of reaction is increased when particles

collide more frequently• Higher temperature, pressure and

concentration increase the rate of collisions• More surface area = More area for particles

to collide with• Catalysts speed up reactions without getting

changed or used up


Pressure in gases

• Increasing the pressure in a gas forces the particles close together

• They collide more often


h. cordy-mckenna (honywood school 2008) understanding rates of reactions use the green button to...

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