Equations and Calculations

Balancing equations'll cause you a few palpitations - as soon as you make one bit right, the rest goes pear-shaped.

  • Created by: chunks-42
  • Created on: 05-05-15 17:25

Equal Numbers of each Atom on Both Sides

1. Balanced equations have the same number of each atom on both side. They're.. well... you know.. balanced.

2. You can add more atoms by adding whole compounds. You can do this by putting a number in front of a compound or changing one that's already there. You can't mess with formulas - ever.

E.g. Balance the equation C2H6 + O2 --> CO2 + H2O

First work out how many of each atom you have on each side.

The right side needs 2 C's, so try 2 CO2. It also needs 6 H's, so try 3H2O.

The left side needs 7 O's, so try 3 1/2 O2. This balances the equation. Phew...

C2H6 + 3.5 O2--> 2 CO2 + 3 H2O

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Ionic Equations

In ionic equations, only the reacting particles are included. You don't have to worry about the rest of the stuff.

E.g. Balance the ionic equation Cr2O7 2- + H+ + e- ---> Cr3+ + H2O

Again, first work out how many of each atom you have on each side.

The right side needs 2 Cr's, so try 2Cr3+. It also needs 7 O's, so try 7 H2O.

The left side needs 14 H's, so try 14 H+. No the charges just need balancing.

The left side needs five additional electrons. So the balanced ionic equation is:

Cr2O7 2- + 14 H+ + 6 e- ---> 2 Cr3+ + 7 H2O

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Calculating the mass using balancd equations

E.g. Calculate the mass of iron oxide produced if 28g of iron is burnt in air:

2Fe + 1.5 O2 ---> Fe2O3

The molar mass of Fe = 56 g mol-1, so the number of moles in 28g of Fe = mass/mr= 28/56 = 0.5M.

From the equation: 2 moles of Fe produces 1 mole of Fe2O3, so 0.5 moles of Fe produces 0.25 moles of Fe2O3.

M of Fe2O3 = (2 x 56) + (3 x 16) = 160 g mol-1

Mass of Fe2O3 = no. of moles x M = 0.25 x 160 = 40g. And that's your answer.

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Calculating gas volumes using balanced equations

It's pretty handy to be able to work out how much a gas reaction will produce, so that you can use large enough apparatus. Or else there might be a rather large bang.

e.g. How much gas is produced when 15 g of sodium is reacted with excess water at r.t.p.?

2Na + 2H2O --> 2NaOH + H2

M of Na = 23 g mol-1, so number of moles in 15 g of Na = 15/23 = 0.65 moles.

From the equation, 2 moles Na produces mole H2, so you know 0.65 moles Na produces 0.65/2=0.326 moles of H2.

So the volume of H2 = 0.326 x 24 = 7.8 dm3

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State symbols

State symbols are put after each compound in an equation. They tell you what state of matter things are in.

s= solid, l= liquid, g= gas, aq= aqueous (solution in water)

To show you what I mean, here's an example -

CaCO3(s)(solid) + 2HCl(aq)(aqueous) ---> CaCl2(aq)(aqueous) + H2O(l)(liquid) + CO2(g)(gas)

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