Phases and Phase Changes

Why do the intermolecular forces of a molecules in liquid phase become less significant above the boiling point?

Above the boiling point, molecules are evaporating and entering gas phase; molecules no longer in contact with one another

What is the driving force that causes fluids to flow?

Pressure difference

A cubical block has a mass of 24kg and a density of 3g/mL. Which of the following values most closely approximates the height of the block? A. 2 meters B. 5 meters C. 3 meters D. 8 meters

A. 2 meters

Phase diagrams are used to depict changes in the properties of a solution at different temperatures and pressures. Below is a phase diagram of a polar solution. What is the name of Point D? A. Triple point B. Critical point C. Freezing point D. Boiling point

A. Triple point

A hockey player cannot skate on dry ice due to its __________. A. positive slope of its solid-liquid equilibrium line B. negative slope of its solid-gas equilibrium line C. negative slope of its solid-liquid equilibrium line D. positive slope of its solid-gas equilibrium line

A. positive slope of its solid-liquid equilibrium line Because dry ice is frozen carbon dioxide, it does not have the same liquid-solid equilibrium line as water and traditional ice. Its liquid-solid equilibrium line is positive, which means that increased pressure will only cause the dry ice to remain solid. If it were negative, then increased pressure would cause melting as seen with water. Skating on solids is like skating on dirt or rocks. The other choices concerning solid and gas does not apply since no gases are involved.

Why does vapor pressure decrease as boiling point increases?

As boiling point increases, less vapor formed, which means less vapor pressure. Decrease in vapor pressure means increase in intermolecular forces, therefore increase in bp

At the microscopic and macroscopic level, what property is similar between molecule A in solid phase and molecule B in liquid phase? What property is different?

At microscopic level, both molecules have a definite volume. At macroscopic level, molecule A has definite shape while the molecule B does not have a definite shape

Which of the following is true regarding the density of solids? A. Solids usually have greater density than liquids because they have larger volumes B. A sample of a substance in its solid phase will have the same mass as the same sample in its liquid phase C. Solid water has a higher density than liquid water D. Density of a solid can be calculated by dividing the weight of the solid by the volume of the solid

B. A sample of a substance in its solid phase will have the same mass as the same sample in its liquid phase Changing a substance from its solid phase to its liquid phase only alters volume; it does not change the mass. Remember that the mass of a substance depends on the amount of molecules. Changing from solid to liquid will increase the spacing between the molecules (volume), but it will not change the amount of molecules (mass); The solid phase and the liquid phase of a sample of substance will have the same mass, but will not take up the less volume because they are tightly packed molecules, therefore, solid have greater densities than liquids

Consider the following factors: I. Temperature II. Pressure Which of these two factors will alter the density of a substance? A. Only I because it changes the volume of the substance B. Both I and II because both change the volume of the substance C. Only II because it changes the mass of the substance D. Both I and II because I changes the volume and II changes the mass of the substance

B. Both I and II because both change the volume of the substance Changing temperature will alter the distance between the molecules within the substance. Pressure also alters density by changing the volume of the substance.

The phase diagram for water (H2O) shows that it can go through all three phases at a pressure of 1atm. The phase diagram for carbon dioxide (CO2) shows that it can only be a solid or a gas at 1atm. Based on this information, which statement is true? A. CO2 will not sublimate at a pressure of 1atm B. CO2 has a higher triple point pressure than H2O C. CO2 does not have a liquid phase D. Raising the pressure when CO2 is a solid will result in CO2 liquid

B. CO2 has a higher triple point pressure than H2O

Phase diagrams are used to depict changes in the properties of a solution at different temperatures and pressures. Below is a phase diagram of a polar solution. What processes are occurring at Point C? A. None of these B. Condensation/vaporization C. Freezing/melting D. Deposition/sublimation

B. Condensation/vaporization

Phase diagrams are used to depict changes in the properties of a solution at different temperatures and pressures. Below is a phase diagram of a polar solution. What is the name of Point B? A. Triple point B. Critical point C. Freezing point D. Boiling point

B. Critical point

The following diagram shows the temperature of a substance as constant heat is supplied. Suppose the substance began in a solid state. The slope of the graph is zero at intervals B and D because __________. A. no energy is being supplied to the system B. the heat that is supplied is being used to overcome intermolecular forces C. the substance is undergoing fusion at these intervals D. no phase changes are taking place at these intervals

B. the heat that is supplied is being used to overcome intermolecular forces Instead of raising the temperature of the substance, the energy that is added during these phase changes is used to overcome intermolecular forces. When the slope is not zero, the phase is steady and the added heat energy is used to increase the molecular kinetic energy of the particles, resulting in a temperature increase.

Why does the density of water decrease as the temperature of the water increases?

Because the volume of the water increases as temperature is increased. The ideal gas law states that temperature and volume are proportional (PV = nRT) and density equation states that density is inversely proportional to volume

The enthalpy of fusion for water is 80cal/g. The specific heat capacity for ice is 0.5 cal/(g∗ oC). The specific heat capacity for water is 1.0cal/(g∗ oC). How much heat is necessary to raise 2g of water from −20oC to 50oC? A. 95cal B. 140cal C. 280cal D. 120cal E. 220cal

C. 280cal The heat needed to raise the temperature from -20oC to 0oC. 2. Melting the ice (changing phases). 3. Raising the water temperature from 0oC to 50oC. Steps 1 and 3 are both solved by the equation q=mcΔT. Step 2 is solved using the enthalpy of fusion, and is multiplied by the number of grams being melted: q=(2g)(80calg). Combining the steps, we get the following expression. q=mcice(T2−T1)+mΔHf+mcwater(T3−T2) q=(2g)(0.5)(0+20)+(2g)(80)+(2g)(1.0)(50−0) q=280cal.

Raining is an example of what type of phase change? A. Deposition B. Evaporation C. Condensation D. Sublimation

C. Condensation Raining occurs when water vapors become liquid water in the sky.

Phase diagrams are used to depict changes in the properties of a solution at different temperatures and pressures. Below is a phase diagram of a polar solution. What processes are occurring at Point A? A. None of these B. Condensation/vaporization C. Freezing/melting D. Deposition/sublimation

C. Freezing/melting

hase diagrams are used to depict changes in the properties of a solution at different temperatures and pressures. Below is a phase diagram of a polar solution. At temperatures and pressures above those at Point B, the material is called a __________. A. Liquid B. Gas C. Supercritical fluid D. Plasma

C. Supercritical fluid

The molar heats of fusion and vaporization for water are given below: ΔHvap=40.7kJmol ΔHfus=6.01kJmol Which of the following explains why the heat of vaporization is much greater than the heat of fusion? A. Vaporization is an endothermic process while fusion is an exothermic process B. Liquids are in constant motion and hold more energy than solids C. The difference in kinetic energy of gas and liquid molecules is higher than the difference between solid and liquid molecules D. More heat is required to vaporize a given quantity of water

C. The difference in kinetic energy of gas and liquid molecules is higher than the difference between solid and liquid molecules

Which best describes the significance of a "critical point" on a phase diagram? A. The temperature and pressure at which a substance may exist in equilbrium between the solid, liquid, and gas phases B. The lowest temperature and pressure at which a substance may exist in a gaseous phase C. The highest temperature and pressure at which a substance may exist in distinct liquid and gas phases D. The lowest temperature and pressure at which a substance may exist in a liquid phase E. The highest temperature and pressure at which a substance may exist in distinct solid and liquid phases

C. The highest temperature and pressure at which a substance may exist in distinct liquid and gas phases

A molecule changed from a solid state into a gaseous state. Was the phase change an endothermic or exothermic process?

Endothermic Going from less energetic phase to more energetic phase is an endothermic process

A molecule changed from a gaseous state down into liquid state. Was the phase change an endothermic or exothermic process?

Exothermic Going from more energetic phase to less energetic phase is an exothermic process

Why do molecules evaporate most readily from the corners and next most readily from the surface?

In liquid phase, molecules are not packed in orderly fashion as they are in a solid. Corner molecules have the fewest neighbors so they have the fewest intermolecular forces so they evaporate more readily than interior molecules

Do molecules in liquid phase have both a definite shape and volume?

No, molecules in liquid phase ONLY has definite volume

What is the driving force behind water flow?

Osmotic pressure

When the water at 0 degree Celsius is reduced to 4 degrees Celsius, why does the density increase rather than decrease?

The density of water increases from a temperature T>4 till 4 degrees. At 4 degrees, the density of water is maximum. After 4 degrees, density of water decreases as water expands

Salt is added to water. Why is the bp for the salt water higher compared to bp for regular water?

Water has a stronger attraction to the ionic impurities (i.e. Na+) than it does to water molecules. Interaction with ionic impurities allows that oxygen atom to assume a full positive charge vs. the partial positive charge it has when interacting with other water molecules. ionic-dipole bond stronger than hydrogen bond

Why is a water phase diagram different from the typical phase diagrams? Why is it slanted to the left?

Water is denser than ice; an isothermal increase in pressure compressed the solid (ice) into liquid (water)

Do molecules in solid phase have both a definite shape and volume?

Yes, molecules in liquid phase has definite volume and shape

What is the relative energetics for the three phase changes?

deltaHsublimation > deltaHvaporization > deltaHfusion

What happens when water stops flowing?

osmotic pressure = hydrostatic pressure

The following diagram shows the temperature of a substance as constant heat is supplied. Suppose the substance began in a solid state. What points represent the freezing point of the substance? Possible Answers: A. A & B B. B & C C. C & D D. D & E E. E & F

B. B & C

A scientist prepares an experiment to demonstrate the second law of thermodynamics for a chemistry class. In order to conduct the experiment, the scientist brings the class outside in January and gathers a cup of water and a portable stove. The temperature outside is -10 degrees Celsius. The scientist asks the students to consider the following when answering his questions: Gibbs Free Energy Formula: ΔG = ΔH - TΔS Liquid-Solid Water Phase Change Reaction: H2O(l) ⇌ H2O(s) + X The scientist prepares two scenarios. Scenario 1: The scientist buries the cup of water outside in the snow, returns to the classroom with his class for one hour, and the class then checks on the cup. They find that the water has frozen in the cup. Scenario 2: The scientist then places the frozen cup of water on the stove and starts the gas. The class finds that the water melts quickly. After the water melts, the scientist asks the students to consider two hypothetical scenarios as a thought experiment. Scenario 3: Once the liquid water at the end of scenario 2 melts completely, the scientist turns off the gas and monitors what happens to the water. Despite being in the cold air, the water never freezes. Scenario 4: The scientist takes the frozen water from the end of scenario 1, puts it on the active stove, and the water remains frozen. Which of the following are possible quantities for the "X" present on the right side of the Liquid-Solid Water Phase Change Reaction? I. Entropy II. Heat III. Water Vapor A. II only B. I and III only C. I, II, and III D. I only E. I and II

A. II only As water freezes, it releases heat into the universe to balance the local loss of entropy encountered by ordering atoms into a rigid solid structure. Entropy, in contrast, is not a commodity that can be released by a reaction in the same way as heat. Also, water vapor would not be released on only one side of the equation. Water vapor would exist with in equilibrium with both solid and liquid, but would not appear as an additional product with the solid phase in this reaction.

A scientist prepares an experiment to demonstrate the second law of thermodynamics for a chemistry class. In order to conduct the experiment, the scientist brings the class outside in January and gathers a cup of water and a portable stove. The temperature outside is -10 degrees Celsius. The scientist asks the students to consider the following when answering his questions: Gibbs Free Energy Formula: ΔG = ΔH - TΔS Liquid-Solid Water Phase Change Reaction: H2O(l) ⇌ H2O(s) + X The scientist prepares two scenarios. Scenario 1: The scientist buries the cup of water outside in the snow, returns to the classroom with his class for one hour, and the class then checks on the cup. They find that the water has frozen in the cup. Scenario 2: The scientist then places the frozen cup of water on the stove and starts the gas. The class finds that the water melts quickly. After the water melts, the scientist asks the students to consider two hypothetical scenarios as a thought experiment. Scenario 3: Once the liquid water at the end of scenario 2 melts completely, the scientist turns off the gas and monitors what happens to the water. Despite being in the cold air, the water never freezes. Scenario 4: The scientist takes the frozen water from the end of scenario 1, puts it on the active stove, and the water remains frozen. Which of the following best describes the Liquid-Solid Water Phase Change Reaction above? A. Physical reaction B. Both an aqueous redox and physical reaction C. Aqueous redox reaction D. Both a chemical and physical reaction E. Chemical reaction

A. Physical reaction There is no change in the organization of chemical bonds, no change in the bonding partners, no change oxidation numbers

Phase diagrams are used to depict changes in the properties of a solution at different temperatures and pressures. Below is a phase diagram of a polar solution. What are the phases in sections 1, 2, and 3, respectively? A. Solid, liquid, gas B. Gas, solid, liquid C. Liquid, solid, gas D. Solid, gas, liquid

A. Solid, liquid, gas

Which of the following describes the concept of sweating? A. Sweating results in a decreased body temperature because evaporation is endothermic B. Sweating results in increased body temperature because evaporation is endothermic C. Sweating results in increased body temperature because evaporation is exothermic D. Sweating results in decreased body temperature because evaporation is exothermic

A. Sweating results in a decreased body temperature because evaporation is endothermic Sweating reduces skin temperature because the sweat that is secreted to the skin evaporates, which is an endothermic process. Thus, heat is absorbed from the body and skin to drive the evaporation of the sweat on the skin.

If a system is said to be adiabatic, what does that mean? A. System is perfectly insulated B. Pressure of system does not change C. Temperature of system does not change D. Heat is being lost from the system

A. System is perfectly insulated If system is perfectly insulated, that means the heat (q) is constant

Imagine a graph that shows the amount of heat being added to water. Heat (q) is on the x-axis, and the temperature of the water is on the y-axis. As more heat is added to the water, the temperature will increase, however, there are two parts of the graph that appear to plateau, and it takes a set amount of heat in order to continue raising the water's temperature. What is the explanation for these plateaus on the graph? A.Water expands when heated, and the heat must warm a larger area. This results in a temporary plateau in temperature increase. B. Heat is dedicated to breaking the hydrogen bonds between water molecules at these plateaus. C. Water gives off heat very quickly, and a great amount of heat must be accumulated before the water temperature will continue to rise. D. It eventually requires a greater amount of heat to raise the temperature of water every Kelvin unit.

B. Heat is dedicated to breaking the hydrogen bonds between water molecules at these plateaus.

What conditions best favor the gaseous state? A. High temperature and high pressure B. High temperature and low pressure C. Low temperature and low pressure D. Low temperature and high pressure

B. High temperature and low pressure

What is the phase change at the contact point between an ice hockey player's skate and the ice? A. Evaporation B. Melting C. Condensation D. Deposition

B. Melting Melting occurs because the skates apply pressure to the ice. Due to the negative slope of solid-liquid equilibrium line for water, increased pressure results in a change from solid to liquid

A scientist prepares an experiment to demonstrate the second law of thermodynamics for a chemistry class. In order to conduct the experiment, the scientist brings the class outside in January and gathers a cup of water and a portable stove. The temperature outside is -10 degrees Celsius. The scientist asks the students to consider the following when answering his questions: Gibbs Free Energy Formula: ΔG = ΔH - TΔS Liquid-Solid Water Phase Change Reaction: H2O(l) ⇌ H2O(s) + X The scientist prepares two scenarios. Scenario 1: The scientist buries the cup of water outside in the snow, returns to the classroom with his class for one hour, and the class then checks on the cup. They find that the water has frozen in the cup. Scenario 2: The scientist then places the frozen cup of water on the stove and starts the gas. The class finds that the water melts quickly. After the water melts, the scientist asks the students to consider two hypothetical scenarios as a thought experiment. Scenario 3: Once the liquid water at the end of scenario 2 melts completely, the scientist turns off the gas and monitors what happens to the water. Despite being in the cold air, the water never freezes. Scenario 4: The scientist takes the frozen water from the end of scenario 1, puts it on the active stove, and the water remains frozen. In scenario 1, the Gibbs Free Energy and Keq of the Liquid-Solid Water Phase Change Reaction, as the reaction begins, is best characterized as __________. A. negative and decreasing, respectively B. negative and increasing, respectively C. zero and decreasing, respectively D. undefined and decreasing, respectively E. positive and increasing, respectively

B. negative and increasing, respectively As scenario 1 begins, the reaction is spontaneous as written, and so the Gibbs Free Energy is negative. Additionally, the Van't Hoff equation proves that Keq increases with decreasing temperature in exothermic reactions. As this reaction is exothermic and placed in low-temperature conditions, the relative abundance of the products will become the prevailing state. Keq, therefore, increases.

The molar heats of fusion and vaporization for water are given below: ΔHvap=40.7kJmol ΔHfus=6.01kJmol 70kg of water solidify and 15kg of water vaporize within a closed system. What is the change in energy of the surroundings? Possible Answers: A. The system experiences no overall change in energy B. 30000J of energy is absorbed from the surroundings C. 20000kJ of energy is released to the surroundings D. 10000kJ of energy is absorbed from the surroundings

D. 10000kJ of energy is absorbed from the surroundings First, calculate the energy change when 70kg of water solidifies. Convert the mass to moles and multiply by the heat released during freezing. This value will be equal in magnitude, but opposite in sign to the heat of fusion. 70kg∗(1000g/1kg)∗(1mol/18g)∗(−6.01kJ/1mol) = −20,000kJ From this calculation we find that 20,000kJ of heat is released into the surroundings (a negative sign denotes an exothermic process). Next, find the energy change associated with the vaporization of 15kg of water, using the given heat of vaporization: 15kg∗(1000g/1kg)∗(1mol/18g)∗(40.7kJ/1mol) =30,000kJ We find that 30,000kJ of energy is absorbed when this quantity of water is vaporized. Adding the two together we find a total of 10,000kJ. −20,000kJ+30,000kJ=10,000kJ Since this is a positive number, that means that the energy is absorbed from the surroundings (endothermic).

The molar heats of fusion and vaporization for water are given below: ΔHvap=40.7kJmol ΔHfus=6.01kJmol A certain amount of snow is formed in the atmosphere, and 5.2∗106kJ of energy is released. How much water was solidified? 1lb=454g A. 2.3∗106 lb B. 5.0∗103 lb C. 1.6∗107 lb D. 3.4∗104 lb

D. 3.4∗104 lb Q=n(−ΔHfus)

Phase diagrams are used to depict changes in the properties of a solution at different temperatures and pressures. Below is a phase diagram of a polar solution. What processes are occurring at Point E? A. None of these B. Condensation/vaporization C. Freezing/melting D. Deposition/sublimation

D. Deposition/sublimation

ΔHvap=40.7kJmol ΔHfus=6.01kJmol Given the following information, which substance would be expected to have an intermolecular force strength similar to that of water? Substance ΔHfus ΔHvap Acetone ΔHfus = 5.72 kJ/mol ΔHvap = 29.1 kJ/mol Ethanol ΔHfus = 4.60 kJ/mol ΔHvap = 43.5 kJ/mol Hydrogen ΔHfus = 0.12 kJ/mol ΔHvap = 0.90 kJ/mol Ammonia ΔHfus = 5.65 kJ/mol ΔHvap =23.4 kJ/mol A. Acetone and ammonia B. Ethanol and ammonia C. Hydrogen D. Ethanol

D. Ethanol Strong intermolecular forces result in more resistance to changes that result in greater distance between molecules (greater entropy), as the forces cause the molecules to "stick" to one another. When a liquid is vaporized, the strength of the intermolecular force is overcome; similar heats of vaporization indicate similar intermolecular force

100J of energy is added to a 1g sample, which undergoes a phase change from liquid to gas as a result. The temperature remains constant throughout the process. What does the 100J quantity represent? A. Specific heat capacity B. Heat of vaporization C. Enthalpy of vaporization D. Specific heat of vaporization

D. Specific heat of vaporization specific heat of vaporization = heat of vaporization is given for a single gram of substance, with the units of Joules per gram Enthalpy of vaporization heat of vaporization are the same quantity. Specific heat capacity = energy needed to raise the temperature of one gram of a sample by one degree Celsius.

What will happen to the density of iron if you increase the temperature to its melting point? A. The density will increase because the mass will increase B. The density will decrease because the mass will decrease C. The density will increase because the volume will decrease D. The density will decrease because the volume will increase

D. The density will decrease because the volume will increase Look at density equation

he molar heats of fusion and vaporization for water are given below: ΔHvap=40.7kJmol ΔHfus=6.01kJmol Which of the following is not true? A. Vaporization is an endothermic process B. Solidification is an exothermic process C. It takes 12.02kJ to melt two moles of water D. The heat of condensation is 40.7kJ/mol

D. The heat of condensation is 40.7kJ/mol

Phase diagrams are used to depict changes in the properties of a solution at different temperatures and pressures. Below is a phase diagram of a polar solution. What molecule has a phase diagram similar to the one presented? A. Bromine B. Mercury C. Hydrogen D. Water

D. Water Unique to only a few molecules, the solid phase in our diagram is less dense than the liquid phase, meaning that the solid/liquid phase line has a negative slope (this can be seen as segment AD in the above image). Water is one of the select few compounds with this characteristic.

he following diagram shows the temperature of a substance as constant heat is supplied. Suppose the substance began in a solid state. If 100g of the substance begins at interval E and ends at interval C, the substance __________ and energy is __________. A. solidifies . . . released B. vaporizes . . . released C. condenses . . . absorbed D. condenses . . . released

D. condenses . . . released