1. 11.5 – NOTES Heat of Combustion and Phase Changes

2. B.7 Using Heats of Combustion
Allows you to discover how much heat is given off in a reaction
Table 3.6 on page 248 gives heats of combustion for selected hydrocarbons
Be completely aware of units to cancel them out as you do the calculations

3. Example 1 - How much thermal energy would be produced by burning 12
Example 1 - How much thermal energy would be produced by burning g of octane, C8H18?
573.6 kJ

4. Example 2 How much energy in KJ is released by completely burning 25
Example 2 How much energy in KJ is released by completely burning mol of hexane, C6H14?
103,525kJ

5. Phase changes
When substances undergo a phase change, this is a physical change.
The temperature stays the same during a phase change, so you cannot use the equation Q = m * C * ΔT because it requires a change in temperature.

6. Heat of fusion is…
the energy needed to melt something (positive)
or the energy released when something freezes (negative)
333 J/g for H2O

7. Heat of vaporization is….
The energy needed to boil something (positive)
The energy released when something condenses (negative)
2259 J/g for H2O

8. Example 1 How many joules are required to boil 25 g of water?
Q = 56,475J

9. Example 2 How much energy is required to heat 15 g of water from - 10°C to 25°C?
Q1 – 312J Q2 - 4,995J Q3 – J
Q Total – J