The Effect of Temperature on Reaction Rate

Dissolving ... a simple reaction

If you put some instant coffee granules or powder in water it will dissolve to make a brown solution of coffee.

If the water has come straight out of the cold tap it will probably take a while for the water to turn completely brown.

Water that comes out of a kettle will form a brown solution almost as soon as it mixes with the coffee powder ... this is because the water is much hotter.

Some important chemistry is happening here: if you want a reaction to go faster then give it some energy (heat).

What is going on ?

Have a look at the two rectangles ... they are meant to show the water before it goes into the coffee cup.

Cold Water

Hot Water

The green circles show what the molecules of water are doing (there are billions and billions of molecules in a cup of water - not just two!).

Water is a liquid that is completely made of molecules ... these molecules are always moving.
Sometimes the molecules move fast - sometimes slowly.


Now fill in these missing words:

Water is completely made out of that are always . When the water is hot the molecules move , but when it is cold the speed of the molecules is . You can make the molecules move faster by giving them some in the form of heat.

You have had tries to get this right

So how does temperature affect the Reaction Rate?

A chemical reaction between two molecules can only happen when those molecules bump into each other

If the molecules have a lot of energy then they move around fast

The faster the molecules move - the sooner they are likely to bump into each other

So, if you give the molecules energy you will speed up the number of times they bump into each other

This means that the reaction speeds up (the rate increases)

You can give molecules energy simply by heating them up


Now answer these questions:

Molecules are always doing what ?

If they have a lot of energy do the molecules move around fast or slow ?

What sort of energy will cause water molecules to move more ?

Will a 'hot' molecule move faster or slower than a 'cold' one ?

Can molecules react without bumping into each other ?

If the molecules have a lot of energy will they have to wait a long or a short time to bump into another one ?

When you make coffee, do the hot water molecules move fast or slow ?

If you timed it for 1 minute, would the cold water molecules bump more or less times than the hot ones into the coffee molecules ?

Would the coffee dissolve completely in the cold water ?

Would the cold water take a long or a short time to dissolve the coffee ?

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