Heat: Calculations and Diagrams

Key Terms Heat Capacity: The amount of heat needed to increase the temperature of an object by 1°C Heat is a form of energy, temperature is a measurement of energy Heat Capacity depends on mass and chemical composition Metals tend to have low heat capacities, nonmetals have higher heat capacities

How is Specific Heat Related? Specific Heat: amount of heat needed to raise 1 g by 1°C Measured in J/(g·°C) or cal/(g·°C) 1 Calorie = 1 kilocalorie = 1000 calories 1 Joule (J) = 0.2930 cal 4.184 J = 1 cal

How is Specific Heat/Heat Capacity Calculated? Use this equation: q = mCpΔT Where… q = heat energy (Joule or cal) m = mass (g) Cp = specific heat of substance [J/(g·°C) or cal/(g·°C) ] ΔT = change in temperature (°C) ΔT = Tf - Ti

Guided Sample Problem # 1 The temperature of a 95.4 g piece of copper increases from 25.0°C to 48.0°C when the copper absorbs 849 J of heat. What is the specific heat of copper? q = mCpΔT q = heat energy (Joule or cal) = 849 J m = mass (g) = 95.4 g Cp = specific heat of substance = unknown ΔT = change in temperature (°C) = 23.0°C ΔT = Tf – Ti = 48.0 – 25.0 = 23.0

q = mCpΔT 849 J = (95.4 g)(x)(23.0°C) 849 J = 2194.2 g·°C (x) C = C = 0.387 J/ g·°C

Guided Practice # 2 When 435 J of heat is added to 3.4 g of olive oil at 21°C, the temperature increases to 85°C. What is the specific heat of olive oil?

Practice #3 How much heat (in Joules) is required to raise the temperature of 250.0 g of mercury 52°C? Substance Specific Heat J/ g·°C cal/ g·°C Water 4.18 1.00 Aluminum 0.90 0.21 Mercury 0.14 0.033 Silver 0.24 0.057 Grain alcohol 2.40 0.58

Heat of Fusion and Heat of Vaporization Heat of Fusion = amount of heat needed to melt/freeze a substance Uses the equation: q = mHf Heat of Vaporization = amount of heat needed to boil/condense a substance Uses the equation: q = mHv Both use the unit: J/g

Guided Practice #4 What is the amount of heat needed to melt 64 g of ice at 0 degrees Celsius?

Guided Practice #5 What is the heat of vaporization of 70 g of chlorine gas if 5,600 J of heat is needed to boil a sample of the gas?

Converting Temperatures Kelvin scale = most precise temperature scale. Based on Absolute Zero (theoretical temperature) Relates to Celsius with the equation: K = C + 273

Guided Practice #6 What is the temperature in Kelvin of a sample with 120 degrees celsius?

How in the world do I decide which equation to use? If the problem talks about changing temperatures (or gives you 2 different temperatures), and uses the word heat: Use q = mCpΔT If the question uses the words melt/freeze, boil/condense (or variations of those words) AND does not give you more than 1 temperature: Use q = mHf or q = mHv