Chemstry Crunch #4.1 : Name: KEY Phase Changes Why? Most substances wll eventually go through a phase change when heated or cooled (sometmes they chemcally react nstead). Molecules of a substance are held together n ether the sold lqud, or gaseous phase by partcle attractons. Though we are very famlar wth seeng thngs lke snow meltng or water bolng, t s necessary to nvestgate what s occurrng at the partcle level n order to truly be able to explan how and why snow melts or water bols. Success Crtera: Construct a heatng or coolng curve when gven the temperature at whch phase changes occur. Interpret a heatng or coolng curve by beng able to dentfy: o the meltng/freezng pont temperature o the bolng/condensng pont temperature o the phase(s) present at a partcular temperature o the regons where Knetc Energy s constant OR changng o the changes that are exothermc or endothermc Prerequstes: Temperature =measure of average knetc energy of partcles n a substance Phases of matter: sold lqud, gas Partcles n a partcular phase have are arranged and attracted to each other n certan ways Endothermc Exothermc Vocabulary: Fuson (meltng) Soldfcaton Condensaton Bolng Vaporzaton Knetc energy
Modell: Representatons of Molecules n Three Phases A Sold fuson! meltng.. B Lqud bolng F... G condensaton 0 0 go C Gas Exercses: Label each arrow (D, E, F, G) wth the approprate phase change (fuson/meltng, soldfcaton/freezng, bolng, condensaton). Key Questons: 1. Whch arrows n Modell ndcate the addton of energy? _--=E and _F 2. Whch term, endothermc or exothermc, s used to descrbe the stuaton when energy s added nto a system from the surroundngs? endothermc 3. Whch arrows n Modell ndcate the release of energy? _-:D and_-:g - 4. What are the names of the phase changes that nvolve a release of energy to the surroundngs by the system? condensaton and freezng/soldfcaton *If you're nterested, ask your teacher about why meltng s called fuson.*
Model 2: A Plot of Temperature of a Substance as Heat s Added Over Tme u-------------e B meltng A Tme Increasng -t These dagrams show how a substance responds when heat s added to t at a steady rate, startng at a temperature below the meltng pont of the substance. A coolng curve can be made to show how a substance responds when heat s removed from t at a steady rate, startng at a temperature above the condensaton (bolnq) pont of the substance. --N()te that Freezng/Meltng «cur at the same temp... t just depends ()n f y()u are addllfj/rem()vng heat. *** Key Questons: 1. What s plotted on the x-axs and what s plotted on the y-axs of the graph? x-axs: tm=e...:.:.:;:nc:.:.r..:::e=as::.::.:.:;ng:::l--_ y-axs: te:.:.m:.!lpe=.:...r=at:.:::u:.:..re;;::;...1.,(o-=c)_ 2. Durng whch lne segments does temperature ncrease? AB, CD,.& EF 3. Durng whch lne segments s there no change n temperature? BC.& DE 4. If ths substance were water, at what temperature would segment BC occur? 0 C 5. If ths substance were water I at what temperature would segment DE occur? 100 C
r Informaton: MELTING, or "FUSION": We can represent phase changes wth partcle dagrams. The followng partcle dagram represents meltng. Meltng s also known as fuson, so chemsts refer to the amount of energy needed to make a substance melt as the "heat of fuson". meltng/fuson The added energy decreases the attractons between partcles, causng them to be less orderly. but stll attracted to one another. BOILING: We can represent phase changes wth partcle dagrams. The followng partcle dagram represents bolng. Bolng s a form of vaporzaton, so chemsts refer to the amount of energy needed to make a substance bol as the "heat of vaporzaton". o o o o o (j o -- In ths case, the added energy completely overcomes the attractons between partcles, causng them to become totally ds- orderly n ther conduct, wth no attractons to one another. 1. On a partcle level, why s energy needed n order to complete an endothermc phase change, such as meltng? Energy s needed to weaken attractons between partcles (need E to separate theml) 2. Energy s measured n Joules. Look on Reference Table B... how much energy s needed to melt one gram of sold water? 334 Joules per gram (334 J/g) so... 334 J 3. More energy s needed to make a substance bol than to make t melt. a. How can you tell 'hs from Model 2 (the graph)? It takes more tme to bol a substance than to melt t (segment DE s longer than Be). b. Why do you thnk ths s so, n terms of partcle attractons? (Hnt: Are lqud partcles attracted to each other? Are gas partcles attracted to each other?) Because gas partcles have absolutely NO attracton for one another (More energy s therefore needed to completely overcome attractons rather than just decrease them) 4. Based on your Reference Table B, how much energy s needed to bol one gram of lqud water? 2260 J
Exercse: Constructng a Coolng Curve A sample of a mysterous substance that s n the gaseous phase s cooled at a constant rate, startng from a temperature of 320 C. After 5 mnutes, t has cooled enough to reach ts bolng/condensng pont temperature of 275 C. The condensng process takes 20 mnutes. Then the temperature decreases for another 10 mnutes. The substance reaches ts freezng pont (meltng pont) at 120 C and remans at ths temperature for 10 mnutes. Then the sold s cooled for another 15 mnutes untl t reaches a temperature of 100 C. Usng ths nformaton, draw a coolng curve for the sample., '3 CO '2.():.) @k (00 ( Il "",......,/ v<. &\\ t'\.r- 'lc1 h,rlv\ -- w,(il ''''{> 1\ C A 7","''' I \ --.. Do '-... -0 Ilv t:: r"" ll-....!,,\ \ \ I 11'< A't \\1... \ ''1,(. U "'-... C l. 'L' \ I I """ 'I:.lIr \, - ' I"':\. o I I "- 1<1 1J"U,s 1 f\...., "'-J I'--... -.,;:oa - \\o k\." "r. \.:./...... ft;'-; :>tl -r<r) ' - I D o 10 J.c; '0 '19 50 (po ITtv'IQ. Cl"\(\') On the curve clearly label the followng tems n the approprate locatons (use arrows as needed to ndcate drecton or exact locaton on the curve. Some terms may be used more than once, as needed.): Sold Freezng Pont (fp) Meltng Pont (mp) Lqud Fuson Drecton of Gas SOldfcaton(fc(l0 Endothermc Changes Condensaton KE cl1aa'gng(o, ) Drecton of Bolng constant KE (\ \ Exothermc Changes