Physics Made Easy

Reacting Quantities and Equations

Writing chemical equations: this may be second nature to you, but if not, here’s the steps to remember-

  1. If it helps, write out the equation in words first.
  2. Write out the equation using chemical symbols.
  3. Balance the equation.
  4. Remember state symbols!

Ionic equations: when writing ionic equations, include only substances which actually take part in the reaction. Ignore spectator ions which remain the same on both sides of the equation, e.g. instead of writing out

2Na+ + 2Br + Cl2 -> 2Na+ + 2Cl + Br2

just write

2Br + Cl2 -> 2Cl + Br2

Concentration: use square brackets to represent concentrations e.g. [H+] means concentration of hydrogen ions. At a concentration of 1 mol dm-3 (1 M) one mole of solute would be dissolved in one litre of solvent.

Calculating no. of moles:

No. moles = mass/molar mass

In solution:

No. moles = volume in cm3/1000 x concentration

Reaction yields:

Percentage yield = actual yield/theoretical yield x 100%

where actual yield is the mass of product formed during the reaction and theoretical yield is the maximum mass of product that could be formed as calculated using the chemical equations and mass of reactants used.

Molar gas volume: 1 mole of gas occupies 22.4 dm3 at stp (0° C and 1 atm) and 24 dm3 at rtp (25° C and 1 atm). 1 dm3 is equal to 1000 cm3 or 1 litre.

Temperature:Three main temperature scales are used: the Fahrenheit scale, where 0oF is the melting point of a particular compound (can’t remember which) and 100oF is body temperature; the Celsius/Centigrade scale where 0oC and 100oC are the melting and boiling points of water respectively and the Kelvin scale, where 0 K is Absolute Zero (the theoretical lowest possible temperature where particles would have no energy at all). I will be working mainly with Centigrade and Kelvin- to convert a temperature from Centigrade to Kelvin, add 273 (273.15 to be precise) e.g. 25oC = 298 K.

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