MCAT Study Guide Chemistry Ch. 7 – Physical Changes 2017-08-15T06:45:06+00:00

I.          7.1:  PHYSICAL CHANGES

No intramolecular bonds are made or broken; physical changes affect only intermolecular forces

A.     PHASE TRANSITIONS

1.     Temperature tells us about amount of internal KE

2.     Average KE affects state or phase (solid, liquid, gas)

3.     ↑KE = ↑ entropy

4.     Phase transitions are the result of breaking/forming intermolecular bonds

5.     Terminology:

a)     Fusion – melting (solid → liquid)

b)     Crystallization – freezing (liquid → solid)

c)     Sublimation – solid to gas

d)     Deposition – gas to solid

 

II.          7.2:  HEATS OF PHASE CHANGES

A.     ΔH → heat of transition; the amount of heat needed or released to change phases

B.     q = n・∆Hphase change → q = heat needed, n = moles of substance

C.    Calorie = amount of heat needed to raise 1 g H2O by 1° C (1 Cal = 1000 cal)

1.     This is water’s specific heat

D.    1 cal = 4.2 J

 

III.          7.3:  CALORIMETRY

A.     When a substance absorbs or releases heat, either its temperature increases OR it will undergo a phase change but not both at the same time

B.     Specific Heat

Amount of heat required to raise certain amount of material 1° C

1.     C = mc  →  C is heat capacity, c is specific heat

2.     q = mc∆T  →  q = heat added or released, c = specific heat

3.     Specific heat tells us how resistant a substance is to changing its temperature

4.     Specific heat varies with each substance and with the phases of the substance

C.    Coffee cup calorimeter

1.     Open system

2.     Reaction occurs in water, which absorbs/releases heat

3.     Good for solution-based chemistry

4.     Enthalpy of reaction can be determined

5.     qcal=mwaterCs,waterΔT=−qp,system

6.     Operated under constant pressure

D.    Bomb calorimeter

1.     Isolated system

2.     Good for combustion reactions

3.     Operated under constant volume, which means heat flow (qv) is equal to the internal energy change, ∆U

4.     qcal = mwater Cs,water ∆T + Chardware ∆T

 

IV.          7.4:  PHASE TRANSITION DIAGRAM

 

MCAT Study Guide Chemistry - Kim Matsumoto


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