AQA Chemistry 3: Bond Energy Calculations & Water

So far bond energy calculations, hard water and solubility covered. All of C3 should be done eventually

*Credit to 'AQA science GCSE chemistry nelson thornes' textbook

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Introduction to Bond Energy Calculations 1 (energy

Bond energy: energy needed to break the bond between two atoms, measured in kJ/mol, we can use bond energies to work out the energy change ('delta' H)

Here are a few bond energies (kJ/mol) for some chemical bonds:

  • C-C: 347, C-O: 358, O=O: 498, C-N: 286

How to calculate Energy Change in a chemical reaction:

1) How much energy is needed to break bonds in the reactants, 2) How much energy is released when new bonds are formed in the products

3) When we want to calculate step 2, we use the same values of bond energies but put a negative sign in front of the number as the energy is being released, and the bond energy values are for when the bond is being broken (e.g bond energy for forming a C-C is -347 kJ/mol)

  • Bond Breaking (in the reactants) is an endothermic process
  • Bond Forming (in the products) is an exothermic process
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Bond Energy Calculations 2 - Worked Example (energ

Here is a worked example to explain the energy change calculation in further detail:

Here is the equation for making ammonia in the Haber process: N2 + 3H2 --> 2NH3


  • The equation tells us that we need one mole of nitrogen and 3 moles of hydrogen
  • Therefore, we need 3 lots of the H-H bond (3x436) and 1 lots of the Nitrogen triple bond (945). 
  • In total that gives us +2253 kJ of energy needed to break the bonds in the reactants


  • These atoms form 2 moles of ammonia, which means we need 6 lots of the N-H bond (because 2 x 3 = 6) (6 x -391)
  • In total the energy needed to form the bonds is -2346 kJ

Therefore, the overall energy change in the reaction is +2253-2346 kJ = -93kJ

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Water and Solubility

The Water Cycle:

Evaporation from lakes, rivers and oceans --> Water vapour condenses to the tiny water droplets that clouds are made from --> These droplets fall as rain, replenishing the water that evaporated


  • Most ionic substances are soluble in water, but many covalent substances cannot because there is little attraction between their molecules and water molecules

Solute: The substance you're dissolving

Solvent: The liquid in which a solute is dissolved to form a solution.

Solubility: The amount of a solute which we can dissolve in a certain amount of solvent, measured in grams of solute per 100g of solvent at a particular temperature

Saturated Solution: One in which as much solute as possible has been dissolved. As a hot saturated solution cools down some of the solute will come out of the solution - it will crystallise out

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Removing Hardness

* Hard Water is caused by dissolved substances such as calcium and magnesium salts which produce scum and scale

To soften hard water, we need to remove these ions.

Method 1: Washing Soda

  • Washing Soda = Sodium Carbonate
  • Precipitates out calcium and magnesium ions as insoluble carbonates
  • Ions are no longer in solution, so ions don't react with soap
  • Ca 2+ + CO3 2-  --> CaCO

Ca: Hardness, CO3 2-: Sodium Carbonate

Method 2: Ion-Exchange Column

  • Columns contain sodium ions which are exchanged for calcium and magnesium ions in hard water
  • The column is washed with a salt solution (sodium chloride) to keep the sodium ions topped up
  • Dishwasher contains it's own water-softening system
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