Measurement of Heat Energy

The amount of heat given off or absorbed in a reaction can be calculated by a) q=mc∆T(Table T) q= heat (in joules or calories) m= mass of substance (g) c= specific heat capacity of substance (J/g°C) ∆T= (Temperature final – Temperature Initial)

Measurement of Heat Energy q= mc∆T is used only when there is a change in temperature Specific heat capacity (c) 1.The amount of heat needed to raise 1.0g of a substance by 1.0°C 2.The value for specific heat of water is on Table B

Measurement of Heat Energy Example question: How many joules of heat are given off when 5.0g of water cool from 75°C to 25°C? q=x m= 5.0g c= 4.18 J/g°C ∆T = 75-25= 50°C q= mc∆T = (5.0)(4.18)(50) = 1045J

Measurement of Heat Energy Heat of Fusion 1.The amount of heat needed to melt 1.0g of a substance 2.q=mH F is used when calculating how much heat is absorbed when a substance melts 3.Remember: 1.heat absorbed during melting goes into raising the potential energy of the substance 2.Kinetic energy is constant (constant temperature), therefore you cannot use q=mc∆T 3.The value for heat of fusion of water is on Table B

Measurement of Heat Energy Example: How many joules does it take to melt 35g of ice at 0°C? q=xq=mH F = (35)(334) =11690J m=35g H F = 334J/g

Measurement of Heat Energy Heat of Vaporization 1.The amount of heat needed to vaporize (boil) 1.0g of a substance 2.q=mH v is used to calculate how much energy is absorbed when a substance boils 3. The heat of vaporization of water is 2260J/g (Table B)

Measurement of Heat Energy Example: How many joules are given off when 85g of steam condense to liquid water? In this example a phase change is occurring (no change in temperature) Since condensation is the opposite of vaporization, we can use q=mH v The only difference is that energy is being released not absorbed q= (85)(2260) = J