5. Phases

Question 1

When a substance undergoes a physical change, what kind of bonds are broken / formed?

Answer 1

physical changes (melting, freezing, etc) break or form intermolecular bonds

NO intramolecular bonds are made / broken.

Question 2

What is the fundamental reason a solid melts when temperature rises?

Answer 2

the intermolecular forces try and hold together the solid. The internal kinetic energy of the molecules tries to jump the molecules around.
When kinetic energy is stronger then the IMFs, the solid will melt.

this can be applied to any phase change.

Question 3

What is it called when a solid becomes a gas? When a gas becomes a solid?

Answer 3

S –> G = sublimation

G –> S = deposition

Question 4

What is it called when a solid becomes a liquid? When a liquid becomes a solid?

Answer 4

S –> L = melting (fusion)

L –> S = freezing (crystallization)

Question 5

t or f, phase changes that make molecules more ordered must release energy.

Answer 5

true –> condensation, deposition, and crystallization all release energy to become more ordered.

Question 6

What is heat of transition? What is the equation?

Answer 6

heat of transition is the amount of energy needed for a phase transition to occur.

q = n x ΔH(phase change)

e.g. ΔHfusion or ΔHcondensation

Question 7

What is a calorie?

Answer 7

The amount of heat required to increase 1 gram of water by 1 degrees Celsius.

1 Cal = 4 J

Question 8

What occurs if energy is added to a substance but not enough energy for a phase change?

Answer 8

Energy causes internal kinetic motion = change in temperature.

e.g.

energy is added to ice (temperature rises) until enough energy is supplied for the ice to melt (T is constant)

Question 9

t or f, when a substance undergoes a phase change, temperature is constant.

Answer 9

true

Question 10

What is specific heat (c)? What is heat capacity (C)?

Answer 10

specific heat, c = intrinsic property of how resistant a substance is to changing in temperature

heat capacity, C = m x c
m = mass
heat capacity accounts for the mass of a material

Question 11

What is the equation for calorimetry (when a substance has energy change but not enough for a phase change)?

Answer 11

q = mcΔT

Note: C = mc

q = CΔT

since its ΔT, units do not matter

Question 12

t or f, the specific heat of ice and water are the same.

Answer 12

false, c changes with each phase.

Question 13

What does a large specific heat indicate?

Answer 13

q = mcΔT

ΔT = q / mc

ΔT and c are inversely proporotional. A large c indicates the substance does not like to change temperature.

ALWAYS refer to equations when asked questions about relationships

Question 14

How do IMFs affect c?

Answer 14

stronger IMFs will resist kinetic energy (resist changes in T). Therefore, strong IMFs have larger specific heats.

Question 15

Phase diagrams explain how pressure and temperature effect the phase of a substance. What are the boundary lines?

Answer 15

boundary lines represent temperature and pressure values in which two phases are in equilibrium with each other. For example, at 1 atm and 0 degrees Celsius, ice cubes in water would be plotted right on a phase diagram boundary line between solid and liquid.

Question 16

Phase diagrams explain how pressure and temperature effect the phase of a substance. What is the triple point, and the critical point?

Answer 16

triple point = the one T and P where all three phases are in equilibrium with each other (all 3 boundary lines come together)
critical point = the point where the liquid-gas boundary line stops. After this point, a substance is a super-critical fluid displaying properties of both gases and liquids

Question 17

Typically, Low temperature high pressure favors?

Answer 17

low T, high P = solids

high T, low P = gases

Question 18

Explain how the phase diagram of water is different than most other substances (such as CO2).

Answer 18

Water is denser than ice. Therefore, at certain temperatures, increasing pressure favors liquid water (more compact) over ice. On a phase diagram, this is seen by the solid-liquid boundary line which is slanted to the left. thus, at 0 degrees Celsius, increasing pressure favors water.

On a CO2 phase diagram, this boundary slants to the right.

this is why we can skate on ice.

Question 19

t or f, the heat of transition for vaporization is always greater than for fusion.

Answer 19

true

Question 20

for q = mcΔT, what are the units of m?

Answer 20

This just depends on the units of c. if c uses grams then m = grams, if c uses moles then m = moles.