11.5 – NOTES Heat of Combustion and Phase Changes

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11.5 – NOTES Heat of Combustion and Phase Changes

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

Example 1 - How much thermal energy would be produced by burning 12 Example 1 - How much thermal energy would be produced by burning 12.0 g of octane, C8H18? 573.6 kJ

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 25.0 mol of hexane, C6H14? 103,525kJ

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.

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

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

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

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 – 1567.5J Q Total – 6874.5J