The Heating Curve Mr. Shields Regents Chemistry U07 L03

Phase Changes Let’s review what we’ve learned previously about First … What is a phase change? A change from one state of matter to another. What does endothermic and exothermic mean? Absorbs heat / Releases heat

Phase Change Overview What are the names of the phase changes we’ve studied? Gas Solid Liquid Energy Absorbed Energy Released Endothermic Exothermic a b c d e f ENDOTHERMIC Phase changes EXOTHERMIC Phase Chages a. Sublimation d. Deposition b. Fusion e. Condensation c. Vaporization f. Solidification

Phase Changes What actually happens to E as matter moves from Solid  Liquid  Gas: Energy is absorbed (endo) Temp increases KE increases molecules move further apart PE increases (Why?) Remember … PE is a function of the position of two bodies relative to one another So … PE inc as molecular separation increases

Heating Curve What happens when we keep adding energy to a solid ? Solid becomes a liquid then the liquid becomes a gas As this energy is added KE inc and so does Temp. but does temp Uniformly inc. over time? The answer is NO!

Heating Curve So what does happen if the temperature does not Uniformly increase? What happens is described by what is called the HEATING CURVE

Heating Curve If we heat a solid it’s temperature increases The energy needed to heat one gram of a substance 1°C is the specific heat value (c) for the substance. This can be calculated from the slope of the line when the temp. is increasing.

Steam Water Slope = Specific Heat Ice and Ice Ice Water and Steam Steam -20 20 40 60 80 100 120 220 760 800 Heating Curve for Water Steam Water Slope = Specific Heat Ice

Heating Curve The temp. will continue to rise until we finally reach the temperature at which the solid begins to melt It begins to go thru a phase change solid  Liquid This phase change is called melting

Solid/Liquid Phase Transition As the solid begins to melt something Unusual Happens. As we continue to add heat to the solid the temperature stops rising as the solid continues to melt. Solid Melting Solid But why does this happen?

Solid/Liquid Phase Transition Solids exist in a rigid, closely packed, highly structured pattern Liquids however have no such rigid structure. As we reach the solids m.p. there is just enough energy to begin overcoming the intermolecular forces between molecules holding them together in the solid state... Molecules begin to separate

Heat of Fusion Added additional heat energy goes into Separating more and more molecules As molecules move from solid to liquid the PE Increases but since the temperature doesn’t rise The KE remains constant. The energy necessary to melt 1 GRAM of a solid is called the … It is unique for every substance. HEAT OF FUSION (Hf)

Solid/Liquid Phase transition Remember… heat ALWAYS flows from hot to cold Until the last piece of solid melts the temperature the Solid/liquid mixture remains constant Once there’s no solid left all additional heat added begins to increase the temperature of the liquid. H C

Solid/Liquid Phase transition When heat is added what happens to KE, Temp. (T) & PE ? Only a single solid (S) phase present KE (T) increases PE is Constant Dual S&L Phase present KE (T) is Constant PE increase Only a single liquid (L) phase present PE is constant Solid S & L Liquid Heating curve

Liquid Phase transition B.P Once all the solid has turned to liquid, any additional heat added to the liquid increases the Temp. until the boiling Point (B.P.) is reached. Then . . . As in the transition from solid to liquid, 2 phases are now present & the Temp. of the boiling water remains constant Solid S & L Liquid

Heat of Vaporization Molecules in the liquid phase form a close but loosely organized structure Molecules in the gas Phase have no structure And are widely separated. To separate these molecules this much takes lots of energy. This energy is called the HEAT OF VAPORIZATION (Hv) Liquid S & L Solid Gas L & G

Heat of Vaporization For water, Hv = 2,260 Joules/g Note Hv > Hf As long as the liquid is boiling T and KE will be Constant PE will Increase as molecules Move further apart In the gas phase Liquid & gas Solid & liquid M.P B.P. The Heating Curve

Summary (b.p.) T const. / KE const. /PE inc ( 2 phases ) T inc. / KE inc / PE Const. ( 1 phase ) T const. / KE const. /PE inc ( 2 phases ) Hf = 334 J/g (m.p.) (b.p.) Hv = 2,260 J/g

Cooling Curve The opposite of a heating Curve is a COOLING CURVE. Brrrrr… Since the sample is cooling it must be releasing heat. As Temp decreases KE decrease and as a sample goes from gas To liquid to solid the PE must be decreasing.

Cooling Curve Phase I: T decreasing KE decreasing PE constant Phase II: T constant KE constant PE decreasing Phase III: T decreasing Phase IV: T constant Phase V: same as I and III Gas & Liquid Liquid solid Liquid & solid I II III IV V Gas Condensation freezing Note: This is the mirror image of a heating curve

STOP this is the end of the notes for p. 17-19

Specific Heat where ΔT = (Tf – Ti) The SPECIFIC HEAT of a substance is the amount of heat required to raise the temperature of 1 g of the material by 1°C. Each substance has it’s own unique specific heat… The lower the specific heat the better the conductor And …. q = m x C x ΔT where ΔT = (Tf – Ti)

Specific Heat One of the variables in Specific heat calculations Involves Temperature Change. What phases of the The heating curve Involve changes in T? Phase 1, 3, 5 The Heating Curve Phase V Phase III Phase IV Phase 1 Phase II

Specific Heat It’s in these phases that specific heat calculations are used to determine how much heat us needed to raise the temperature of the sample But how do we find the heat necessary to to fully melt or vaporize a sample of matter In regions where T does Not change (phase II & IV) The Heating Curve Phase 1 Phase II Phase III Phase IV Phase V