310 likes | 321 Views
Energy Changes & Phase Changes. Heating & Cooling Curves. It takes energy to heat stuff up!. For a pure substance in a single phase , we can calculate how much using Q = mC T. Q = energy in Joules m = mass in grams C = specific heat capacity T = change in temperature = T f - T i
E N D
Energy Changes & Phase Changes Heating & Cooling Curves
It takes energy to heat stuff up! • For a pure substance in a single phase, we can calculate how much using Q = mCT. • Q = energy in Joules • m = mass in grams • C = specific heat capacity • T = change in temperature = Tf - Ti • On the other hand, when something cools down, energy is released!
Q = mCT • C = specific heat capacity = amount of heat required to raise the temperature of 1 gram of a pure substance by 1C. • C is a physical constant. It is unique for every pure substance. • Values of C are tabulated. • CH2O = 4.2 J/g Memory Jogger
Heat Flow: hot to cold Environment System – 1 phase
But what about phase changes? • Sometimes more than one phase of a substance is present. • For example, when melting ice, both liquid water and ice are present. • Furthermore, the temperature is constant, so T = 0, even though the beaker of ice water is absorbing heat from a hot plate.
All chemical & physical changes are accompanied by energy changes. • Phase changes are physical changes. • Sometimes energy is absorbed, sometimes energy is released. • The energy change for a given phase change can be measured or calculated.
Potential Energy • Energy of relative position. • Molecules are always attracted to one another. • You have to put energy into the system to pull molecules apart from one another. • So the farther apart they are, the higher their potential energy.
Melting Ice • Ice water on hot plate: ice is melting. • The ice is absorbing heat from the hot plate and using it for the phase change. • The temperature of ice-water mix is constant -- the heat energy from the hot plate is going into the phase change or potential energy of the system. • The heat energy is not going to the kinetic energy. Remember, temperature is …
Identify a phase change as endothermic or exothermic. Endothermic Exothermic Gas Condensation Freezing Deposition Fusion Vaporization Sublimation P.E. Liquid Solid
Heating & Cooling Curves • 1 way to investigate energy changes. • Measure temperature as a function of time at a constant heating or cooling rate.
I II III IV V Liquid & Gas Gas Solid & Liquid Liquid Solid Boil pt. K.E. K.E. K.E.↔ K.E.↔ P.E.↔ K.E. P.E. Temperature P.E. Melt pt. P.E.↔ P.E.↔ Time
Melting & Boiling Points • Plateaus = Phase changes = Potential energy changes. • Notice that as long as 2 phases are present, the temperature is constant. • Melting point, Boiling point. Tiger
What happens to the temperature as heat is added at the boiling point? • Nothing, until only 1 phase is present!
Heating Curve • Tiger Graphic
To analyze a heating/cooling curve: • Does the curve go uphill or downhill? • Label the phases present in each region. • Describe what happens to the K.E. in each region. • Describe what happens to the P.E. in each region. • Locate the melting point and boiling point.
What is the melting point of this substance? The boiling point?
Experimental Approach The heating rate is 150 J/min. If the substance takes 4 minutes to melt, how much heat energy was used to melt it?
3 equations for Q • Q = mCT • Q = mHf • Q = mHv • Have to figure out which one to use for a given problem. • Depends which section of heating curve. • Look for hints in the problem.
Q = mCgT Q = mClT Q = mHv Q = mCsT Q = mHf Temperature Time
Temperature changed Temperature increased Temperature decreased Initial temperature Start temperature Final temperature Ending temperature From ____ to ____ Water Q = mCT: pure substance, single phase
Q = mHf : liquid and solid present • Ice • Freezing • Melting • At 0C (for H2O) • At constant temperature
Heat of Fusion • Amount of energy required to change 1 gram of a pure substance from a solid to a liquid at its melting point. • Heat of Fusion = Hf = physical constant. • Hf for water = 333.6 Joules per gram (Table B)
How much heat is absorbed when 10 grams of ice melts at 0oC? • Heat absorbed = mass of substance X heat of fusion of substance • Q = mHf = (10 g)(333.6 J/g) = 3336 J • Where does that energy go? • Particles must overcome forces of attraction to move farther apart.
Q = mHv : liquid and gas present • Steam • Boiling • Condensation or Condensing • At 100C (for H2O) • At constant temperature
Heat of Vaporization • Amount of energy required to convert 1 gram of a pure substance from a liquid to a gas at its boiling point. • Heat of vaporization = Hv = physical constant • Hv for water = 2259 J/g
How much energy does it take to vaporize 10 g of water? • Q = mHv • Q = (10 g)(2259 J/g) = 22590 J • It takes a lot more energy to go from liquid to gas than from solid to liquid. Why?
Heats of fusion & vaporization • Determined in calorimetry experiments.