ENERGY CHANGES

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CH202: Describe exothermic and endothermic reactions in terms of heat and temperature

Exothermic:

  • In an exothermic reaction, heat is given out to the surroundings.
  • This means that:
    • The temperature increases.
    • Heat energy is given out to the surroundings (so the surroundings get hotter).
    • The products have less energy than the reactants. (See CH203)

Endothermic:

  • In an endothermic reaction, heat is taken in from the surroundings.
  • This means that:
    • The temperature decreases.
    • Heat energy is taken in from the surroundings (so the surroundings get colder).
    • The products have more energy than the reactants. (See CH203)
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CH203: Draw reaction profiles for exothermic and endothermic reactions

EXOTHERMIC

Clue: More people have exited the shop (products have less energy) so it is EXOthermic.

ENDOTHERMIC

Clue: More people have entered the shop (products have more energy) so it is ENDOthermic.

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CH204: Describe bond breaking and bond forming

There are two key things to remember that occur in every chemical reaction:

  • Bond breaking is endothermic – heat energy is taken in to break the bonds.
  • Bond forming is exothermic – heat energy is given out to the surroundings when new bonds form.
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CH205: Explain what makes a reaction exothermic overall

For a reaction to be exothermic overall, more energy must be given out to the surroundings (when bonds form) than taken in (when bonds are broken).

For a reaction to be endothermic overall, more heat energy must be taken in from the surroundings (when bonds are broken) than given out (when new bonds are formed).

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CH206: Calculating Energy Changes (H)

In a chemical reaction, you need a certain amount of energy to break bonds (endothermic) and a certain amount of energy is given out to form new bonds (exothermic). You can, therefore, use a reference table (such as the one on the right) to work out if a reaction is exothermic or endothermic.

Example: Calculate the energy change in the reaction between hydrogen and oxygen to form water:

2H2 + O2 → 2H2O

Step 1: Work out the energy needed to break the bonds in H2 and O2:

2H2 + O2 →

We have 2 hydrogen molecules and one oxygen molecule.

From the table above, you can see that:

  • 436kJmol-1of energy is needed to break the bonds in each hydrogen molecule. We have 2 molecules of hydrogen, so 2 x 436 = 872kJmol-1
  • We have one oxygen molecule, so 498kJmol-1 is needed.

Therefore, the total energy to break the bonds is 872 + 498 = 1370kJmol-1 is needed to break all bonds.

Step 2: Work out the energy needed to form the bonds in H2O:

→ 2H2O

Each water molecule has 2 hydrogens attached to one oxygen. Therefore, each H2O has two O=H bonds. 

  • 464kJmol-1of energy is needed to break the O- H bond.
  • Each water has 2xO-H bonds, so each water = 928kJmol-1.

We have 2 water molecules, so 928×2 = 1856kJmol-1 needed to break all bonds in 2H2O.

Step 3: Work out the energy change, which is bond breaking (reactants) – bond forming (products):

  • 1370kJmol-1 – 1856kJmol-1 = -486kJmol-1
  • Make sure to put the sign (+ or -) – this is worth one mark!
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