C5 Energy Changes

Chemical changes and batteries

There are different types of batteries

Modern alkaline cells produce a larger voltage

The first mass produced were called Primary Cells

Dry cells are made out of Zinc and carbon

These are non-rechargeable

A battery is made up of 2 or more cells joined together to create a higher voltage

The more reactive metal will always displace the less reactive

If a ion does change in a reaction it is a spectator ion

The bigger the difference in reactivity between the metals the higher the voltage

This means you can leave them out and write a ionic equation

Bond energy calculation

Making and breaking bonds

If the energy needed to break the bond is more it is endothermic

If the energy released is more it is exothermic

Bond energy

The balance between the energy need to make a bond and the energy released when a bond is broken decides whether it is endothermic or exothermic

The energy needed to break a bond is called the bond energy

This is measured in KJ/mol

How to work it out

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Example :Ammonium Nitrate (N2 + 3H2 = 2NH3)

First work out the first side of the equation

  • N=N bond = 945
  • H-H bond = 436 (x3)
  • Total = 2253

Then Work out the other side of the equation

  • H-N bond = 391 (x6)
  • Total = 2346

Finally work out the total and figure out whether it is exothermic or endothermic

  • Overall energy change = 2253 - 2346
  • = -93 KJ
  • =Endothermic because the final total is negative (going out to the surroundings

Using energy transfers

Warming up

Self-heating cans work by pressing a button on the base

These are exothermic reactions

It cost a million to develop

The development took years

Some early versions didn't heat up enough

This brakes a seal and water and calcium oxide mix

Then its heated up again

Some hand warmers can only be used once

An example of this is oxidation of iron

It turns into hydrated Iron(III) Oxide

These hand warmers can last for hours

Sodium Chloride is used as a catalyst

Some hand warmers are reusable

These last for about 30 minutes

The salt is often Sodium Ethanoate