Rates of Reaction

Some chemical reactions are fast (frying an egg), and some are slow (metal rusting). The rate at which a reaction occurs is a surprisingly complex area of chemistry, and one that examiners love to ask questions about at GCSE.

This set of notes will provide you with the perfect answers to all the questions that an examiner can think of (which isn't all that many looking at all the past papers!).

Collision Theory

A good place to start: Particles (atoms or molecules) collide all the time. When they collide they might react, but only if they have enough energy, and collide in the right orientation. This theory is called Collision Theory, and it is suggested that you learn this exactly as it appears here:

"In order for a reaction to occur particles must collide with energy greater than, or equal to, activation energy, and in the correct orientation."

Think  about it like this: If I'm driving my car at 50mph, and I hit something head on (not advisable), then the airbag will go off, because there is enough energy, and I collided in the correct orientation (position). What would the outcome be if I was driving at 5mph, and just clipped something? Well, fingers crossed, my airbag wouldn't go off and I would rebound off and carry on my journey. In this model the reaction is the airbag going off.

Here's another model

Factors Affecting Rates of Reaction (learn these exactly as they are written here)

Temperature:

Increasing the temperature of a reaction will give the particles more energy. This means that more collisions have energy greater than, or equal to, activation energy, and so more collisions will lead to reactions.

Concentration:

This only applies to liquids. Increasing the concentration means that there are more particles in a given space, so there are more frequent collisions. Increasing the frequency of collisions means that there is an increased chance of collisions leading to reactions.

Pressure:

This only applies to gases. Increasing the pressure means that there are more particles in a given space, so there are more frequent collision. Increasing the frequency of collisions means that there is an increased chance of collisions leading to reactions.

Surface Area:

This only applies to solids. Increasing the surface area increases the number of exposed particles that can react. This means that there will be more frequent collision so more collisions have enough energy to react.

Catalyst:

A catalyst works by offering an alternative reaction pathway, of lower activation energy. This means that more collisions lead to reactions. Catalysts are clever little things that are returned at the end of the reaction, chemically unchanged.

LEARN THESE DEFINITIONS BECAUSE EACH ONE IS WORTH UP TO 3 MARKS.

Meir , Stoke-on-Trent ST3 7DF, UK

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