Changes of State

Freezing, Melting, and Boiling

100

Heat of fusion of H2O is 6kJ/mol. The molar mass of H2O is 18g/mol. If you have 54g of ice, how much energy in terms of joules would you need to completely melt the ice.

18000 joules

200

Define molar heat of fusion.

amount of heat energy required to melt one mole of solid at its melting point

300

What is the molar heat of vaporization for water?

40.79 kJ/mol

400

2H2(g) + O2(g) -> 2H2O(l) If you're given 24 g of hydrogen gas with excess oxygen gas, how much energy (in kJ) is needed to convert the water that is produced into a gaseous phase? The molar heat of vaporization for water is 40 kJ/mol. The molar mass of H2 is 2 g/mol and the molar mass of O2 is 32 g/mol.

24 g H2 * 1 mol H2 / 2 g H2 = 12 mol H2 12 mol H2 * 2 mol H2O / 2 mol H2 = 12 mol H2O 12 mol H2O * 40 kJ / 1 mol H2O = 480 kJ 480 kJ

500

C7H12(s) + 10O2(g) -> 7CO2(g) + 6H2O(g) molar mass of heptyne: 98 g/mol molar mass of oxygen: 32 g/mol molar mass of carbon dioxide: 44 g/mol molar mass of dihydrogen monoxide: 18 g/mol molar heat of fusion for water: 6 kJ/mol molar heat of vaporization for water: 40 kJ/mol ARE YOU CONFUSED YET???? Given that 960 g of oxygen reacts with 196 g of heptyne, find how much energy is released when the water vapor that is formed condenses into liquid.

960 g O2 * 1 mol O2 / 32 g O2 = 30 mol O2 30 mol O2 * 1 mol C7H12 / 10 mol O2 = 3 mol C7H12 3 mol C7H12 * 98 g C7H12 / 1 mol C7H12 = 294 g C7H12 only have 196 g C7H12; C7H12 is limiting reactant 196 g C7H12 * 1 mol C7H12 / 98 g C7H12 = 2 mol C7H12 2 mol C7H12 * 6 mol H2O / 1 mol C7H12 = 12 mol H2O 12 mol H2O * 40 kJ / 1 mol H2O = 480 kJ 480 kJ

Equilibrium

100

Define equilibrium.

A dynamic condition in which two opposing changes occur at equal rates in a closed system

200

Explain Le Chatelier's Principle.

When a system at equilibrium is disturbed by application of a stress, it attains a new equilibrium position that minimizes the stress.

300

N2(g) + 3H2(g) -> 2NH3(g) Which way does the reaction shift if the concentration of N2 is increased?

Right

400

Given a sample of water at any point on curve AB on the H2O phase diagram, what effect would the following changes have on that sample? a. decreasing the volume at constant temperature b. adding heat energy at constant pressure

a. Decreasing volume = increased pressure = turns into liquid b. Adding heat energy = increased temperature = turns into gas

500

Decide whether the temperature of a liquid-vapor equilibrium system should be increased or decreased to make each of the following changes in the system. a. the rate of evaporation b. the rate of condensation c. the final concentration of vapor molecules d. the final number of liquid molecules

a. increased b. decreased c. increased d. decreased

Equilibrium Vapor Pressure

100

Which of the following characteristics of a vapor in equilibrium indicates the presence of strong intermolecular forces in the vapor's liquid?

It has low vapor pressure

200

What must the equilibrium vapor pressure of a liquid equal when it boils?

It must equal the atmospheric pressure.

300

How can the increase in equilibrium vapor pressure with increasing temperature be explained by the kinetic-molecular theory?

Increasing the temperature -> increases average kinetic energy -> increase # of molecules that can escape liquid phase -> increase concentration of molecules in vapor phase -> increases equilibrium vapor pressure

400

At room temperature, a liquid is in equilibrium with its vapor in a sealed container. What is the immediate result when the lid is removed?

The re-condensation rate lowers.

500

Put the liquids that contain these intermolecular forces into order of volatility. ionic bonds, dipole-dipole, LDF, hydrogen bonds

a. ionic bonds b. dipole-dipole bonds c. London dispersion forces d. hydrogen bonds

Phase Diagrams

100

What happens at point A?

Triple point; solids, liquids, and gases can coexist at that specific pressure and temperature

200

H2O exists as what state at point F?

Liquid

300

Which of the following points is the standard boiling point for water?

B

400

Which point is the critical point, and what is the critical point?

C; The point where liquids and gases become indistinguishable from each other

500

The phase diagram of H2O is different from the phase diagrams of most other substances. Explain the difference between the phase diagrams.

Water is unique in that when it is in solid form, it has less pressure than when it is in liquid form. Meanwhile, most substances have MORE pressure when it is in solid form than in liquid form. Therefore, the line separating the liquid and solid states in substances other than H2O slants toward the right instead of the left.

Sublimation and Deposition

100

Define sublimation?

Solid vaporizes into gas (skips the liquid phase)

200

What is it called when a substance goes from a gas phase to a solid phase directly?

Deposition

300

Dry ice does what at room temperature?

Sublimation

400

The formation of frost on a cold surface is an example of...

Deposition

500

Why do deposition and sublimation occur?

At sufficiently low temperature and pressure conditions, some liquids cannot exist. Under these conditions, a solid substance exists in equilibrium with its vapor instead of its liquid.