Presentation on theme: "Just to review before we start…"— Presentation transcript:
1Just to review before we start… What is the melting point of this substance?50˚CThe boiling point?100˚C
2Topic: Calculating Energy Changes at Phase Changes (Hv and Hf)
3It takes energy to heat stuff up! for pure substance in single phase - can calculate how much E needed using:Q = mCTQ = energy in Joulesm = mass in gramsC = specific heat capacityT = change in temperature = Tf - Tion other hand, when something cools down, energy is released!
4Temperature Time IV I II III V Q = mCliquidT Q = mCgasT Q = mCsolidTTemperatureIVIIIIIIVC = specific heat capacity(amount heat required to raise temp of 1g of pure substance by 1C)C is a physical constantunique for every pure substanceCAN YOU FIND THE SPECIFIC HEAT OF WATER ON YOUR REFERENCE TABLE?Time
5Why can’t I use Q = mCT for II and IV?? TemperatureIVIIIIIIVBecause T = 0, temp isn’t changing!!!!Time
6So, how do we calculate the amount of energy required during a phase change? HF = Heat of Fusion (Q = mHF)HV = Heat of Vaporization (Q=mHV)We use one of these two constants instead of specific heat and delta TQ = mCT
7Hf = Heat of Fusion is amount energy required to change 1 gram of pure substance from solid to liquid at its MP (meaning you aren’t changing the temperature)Is a physical constantCheck out Reference Table B, what is the heat of fusion for water?The Equation Q = mHf
8How much heat is absorbed when 10 grams of ice melts at 0oC? Heat absorbed =mass of substance x heat of fusion of substanceQ = mHf = (10 g)(334 J/g) = 3340 JWhere does this energy go?Particles must overcome forces of attraction to move farther apart during phase change (s → l)
9HV = Heat of Vaporization is the amount energy required to convert 1 gramof pure substance from liquid to gas at its BP (meaning you aren’t changing the temperature)Is a physical constantCheck out Reference Table B, what is the heat of vaporization for water?The Equation Q = mHv
10How much energy does it take to vaporize 10 g of water? Q = mHvQ = (10 g)(2260 J/g) = J
11H2O changing from liquid to gas requires 22,600J/g It takes a lot more energy to go from liquid to gas than from solid to liquid. Why?H2O changing from liquid to gas requires 22,600J/gH2O changing from solid to liquid requires 3,340J/g* greater energy required to change from liquid to gas because particles are spreading farther apart!
12Temperature Time IV I II III V Q = mHV Q = mHF Q = mCsolidT Q = mCLIQUIDTQ = mCgasTQ = mHVQ = mHFTemperatureIVIIIIIIVTime