Chemistry II
In a particular location the boiling point of ethyl alcohol was found to be 73∘C, while it typically boils around 78∘C. What might be the approximate atmospheric pressure at this location? Select the correct answer below: 28 kPa 47 kPa 80 kPa 105 kPa
80 kPa This is a bit below the normal boiling point for this substance, so we must be a bit below normal atmospheric pressure which is approximately 101 kPa. The answer closest to this value is 80 kPa.
What mass of CO2 will sublimate given 500 kJ of energy at room temperature? (ΔHsub=26.1kJmole)
843 g 500 kJ(1 mole26.1 kJ)(44.0 g1 mole)=843 g
How much heat is required to evaporate 2.5 kg of water (H2O) at room temperature? (For water at room temperature use, ΔHvap=44.01kJmol) Your answer should include two significant figures.
First, calculate the number of moles of H2O using a formula mass of 18.015gmol from the periodic table: 2.5kg×1000.g1.0kg×1.0mol18.015g=138.77mol Next, calculate the total change in enthalpy. ΔH=138.77mol×(44.01kJmol)=6.1073×103kJ The answer should have two significant figures, so round to 6.1×103kJ.
What volume of water (H2O), in milliliters, can vaporize at room temperature given 150. kJ of energy? (For water, ΔHvap=44.01kJmol). Assume the density of water is exactly 1.00gmL. Your answer should have three significant figures.
First, find the number of moles: 150. kJ44.01kJmol=3.40832mol Next, convert the amount of the compound from moles to grams using the formula mass from the table: 3.40832mol×18.015gmol=61.4009g Next, convert the amount of the compound from grams to milliliters: 61.4009g×mL1.00g=61.4009mL Finally, round to the correct number of significant figures (three): 61.4mL
When a solid melts at its transition temperature (normal melting point): Select the correct answer below: The solid melts slowly in one direction (the reverse process does not occur). The solid melts rapidly in one direction (the reverse process does not occur). The solid melts slowly and reversibly. The solid melts instantaneously and reversibly.
The solid melts slowly and reversibly. When a crystalline solid is heated, the average energy of its atoms is increased, eventually overcoming the forces holding the solid particles in their fixed positions in the lattice. At this temperature, the solid begins to transition slowly to a liquid state through reversible melting (the solid and liquid phases are in equilibrium).
Which substance will sublimate when in the solid phase at room temperature and pressure? Select the correct answer below: carbon dioxide water mercury helium
carbon dioxide Solid carbon dioxide, or dry ice, will sublimate at room temperature and pressure.
An increase in elevation will cause the boiling point of a liquid to: Select the correct answer below: increase decrease stay the same depends on the liquid
decrease As external pressure decreases at higher elevation, the equilibrium vapor pressure required for a substance to boil will decrease, so the boiling point itself will decrease.
Vapor pressure will increase with: Select the correct answer below: increasing molecular mass decreasing molecular mass decreasing temperature none of the above
decreasing molecular mass Heavier molecules will be more difficult to vaporize, while lighter molecules will be easier to vaporize.
In comparing diethyl ether and water, which will have the greatest vapor pressure? Remember, diethyl ether is more volatile than water. Select the correct answer below: water at 25∘C water at 75∘C diethyl ether at 25∘C diethyl ether at 75∘C
diethyl ether at 75∘C
When a liquid is in equilibrium with its vapor, the rate of vaporization and the rate of condensation are in: Select the correct answer below: static equilibrium stationary equilibrium dynamic equilibrium none of the above
dynamic equilibrium When molecules transition from the gaseous state to the liquid state, the phase change process is known as condensation. When molecules transition from the liquid state to the gaseous state, the phase change process is known as vaporization. When these two rates are equal to each other, neither the amount of liquid nor the amount of vapor changes, and the two are in equilibrium with one another. The molecules are continuing to exchange with one another, so the equilibrium is dynamic because the rates of each are equal.
The boiling point of a liquid is defined as the temperature at which the equilibrium vapor pressure is: Select the correct answer below: 0 atm 1 atm 100 atm equal to the pressure exerted on the liquid by its surroundings
equal to the pressure exerted on the liquid by its surroundings Although much chemistry is done at atmospheric pressure, or 1 atm, which means boiling points typically involve a vapor pressure of 1 atm, the correct answer is the more rigorous definition listed.
The relative strength of intermolecular forces as follows: hydrogen bonding>dipole-dipole>London dispersion Which of the following intermolecular force, when present in a substance, will typically result in the lowest vapor pressure? Select the correct answer below: dispersion forces dipole-dipole hydrogen bonding There is no relationship between type of intermolecular force vapor pressure, so any of the above is equally likely.
hydrogen bonding Compounds with stronger intermolecular forces will have lower vapor pressures (hydrogen bonding > dipole dipole > London dispersion). The type of molecule affects the vapor pressure. If the intermolecular forces between molecules are relatively strong, the vapor pressure will be low because fewer particles will be able to escape the liquid. If the intermolecular forces between molecules are relatively weak, the vapor pressure will be high because more particles will be able to escape the liquid.
The magnitude of the enthalpy of vaporization increases with: Select the correct answer below: increasing strength of intermolecular forces decreasing strength of intermolecular forces decreasing molecular mass none of the above
increasing strength of intermolecular forces As the liquid particles interact more strongly, the enthalpy of vaporization will increase.
Enthalpy of vaporization: Select the correct answer below: is the same at all temperatures for a particular substance is the same at all temperatures for every substance is different at different temperatures for a particular substance none of the above
is different at different temperatures for a particular substance Enthalpy of vaporization will typically decrease as temperature increases.
A substance with a higher boiling point will exhibit a: Select the correct answer below: higher vapor pressure lower vapor pressure higher density lower density
lower vapor pressure The harder a liquid is to boil, the lower the vapor pressure will be at a given temperature.
The ______ of a substance is the temperature at which its liquid and solid phases are in equilibrium. (select all that apply) Select all that apply: boiling point condensing point melting point freezing point
melting point freezing point The point at which liquid and solid phases are in equilbrium is known as the freezing point if transitioning from liquid to solid, and the melting point if transitioning from solid to liquid.
While a pure substance is melting, additional heat will cause the temperature of a substance to: Select the correct answer below: increase decrease stay the same depends on the substance
stay the same While a substance is melting, its temperature will remain constant until all of it has reached the liquid phase.
The enthalpy of sublimation for a particular substance will be equal to: Select the correct answer below: the enthalpy of fusion plus the enthalpy of vaporization the enthalpy of fusion minus the enthalpy of vaporization the enthalpy of vaporization minus the enthalpy of fusion none of the above
the enthalpy of fusion plus the enthalpy of vaporization If the enthalpy of fusion involves the transition from the solid phase to the liquid phase, and the enthalpy of vaporization involves the transition from the liquid phase to the gas phase, then the enthalpy of sublimation, involving the transition from the solid phase to the gas phase, should be the sum of the other two enthalpies, according to Hess's law.
Which of these values will be the greatest for a given substance? Select the correct answer below: the enthalpy of fusion the enthalpy of vaporization the enthalpy of condensation the enthalpy of sublimation
the enthalpy of sublimation Sublimation involves the transition from the solid phase directly to the gas phase, which will require much more energy than any transition between adjacent phases.
How are the energy absorbed when melting and the energy released when freezing related for a particular substance? Select the correct answer below: they are equal they are inverse in magnitude they are opposite in sign depends on the substance
they are opposite in sign Since these values represent opposite processes, they will have the same magnitude but opposite signs.They will represent the same amount of energy, just absorbed in one process and released in the other.
