unit 1 study guide (part 1) - cracking...
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General Chemistry II, Unit I: Study Guide (part I)
CDS Chapter 14: Physical Properties of Gases • Observation 1: Pressure-‐Volume Measurements on Gases
o The “spring” of air is measured as pressure, defined as the force applied over an area 𝑃 = !!.
o Units of pressure: § Atmosphere (atm): 1 atmosphere is the pressure exerted by gases in the atmosphere at sea
level § Torr (mm Hg): 1 mm Hg = !
!"#atm
§ Pascal (Pa): 1 atm = 760 torr = 101325 Pa = 101325 !!!
o Using an experiment that traps small quantities of air inside a syringe, one can measure the pressure of a gas while varying its volume.
§ Experimental data shows that as volume increases, pressure decreases. § However, this relationship is not linear:
§ Plotting the inverse of the pressure (1/P) versus volume yields a linear relationship. The
straight line formed by 1/P vs. V also connects with the origin (0,0). § The linear equation relating pressure and volume is !
!= k×V where k is a proportionality
constant. o The data proving the inverse relationship between pressure and volume is consistent across
multiple gases when the amount of gas is held constant. § However, when the amount of gas is varied, the proportionality constant k, which
represents the slope of the linear graph, changes:
§ § The new line, n2, still passes through the origin, so the original equation !
!= k×V is
retained. However, it is clear that k now depends on the number of moles of gas. o Boyle’s Law: The product of pressure and volume is a constant for a given amount of gas at a
fixed temperature. • Observation 2: Volume-‐Temperature Measurements on Gases
2 o The first three paragraphs can basically be summarized as “temperature is subjective until it’s
actually important.” o Placing mercury in hot and cold water yields different changes in the volume of the liquid.
Therefore, the volume of mercury is a measure of how hot something is. § In addition, two different objects with the same subjective “hotness” give the same volume
of mercury – temperature is not dependent on the identity of the object being measured. o Fahrenheit and Celsius are arbitrary ways of measuring temperature; they do not reveal what
physical property is actually being measured. o New experiment: trap a small sample of air (at room temperature and pressure) in a syringe and
monitor the temperature (using a mercury thermometer) as the volume is varied. In this experiment, the pressure is held constant by a piston that moves against atmospheric pressure.
§ § There is a simple linear relationship between the volume of a sample of a gas and its
temperature. This can be expressed by a correspondingly simple linear equation, V = αt + β, where t is the temperature in Celsius, α is the slope of the line and β is the y-‐intercept.
• From the CDS experiment, α = 0.335 mL/°C and β = 91.7 mL. Therefore, the quantity α/β must be a temperature because the volume units (mL) cancel out, leaving only °C.
• Therefore, the linear equation can be rewritten as V = α t + !!. This gives an x-‐
intercept of -‐273.15 °C. • Repeating this experiment many times continues to yield a ratio of -‐β/α = -‐273.15
°C. • While we do not know the meaning of this temperature, we can assume it is
important and give it the title “absolute zero,” since any temperature lower than it would produce an impossible negative gas volume.
• For simplicity, a new absolute temperature scale, Kelvin, can be defined with the same unit size as Celsius. Since 273.15 °C = 0 K, the above linear relationship between volume and temperature can be rewritten as V = αT.
o Charles’ Law: The volume is proportional to the absolute temperature (in Kelvin). § The constant α depends on the pressure and quantity (number of moles) of gas.
• The Ideal Gas Law Law Related properties Type of
relationship Constants Equation
Boyle’s Law Pressure and volume
Inverse Quantity, temperature P×V = k! N,T
Charles’ Law Volume and temperature
Linear Quantity, pressure V = k! N, P T
Avogadro’s Law
Volume and quantity
Linear Pressure, temperature V = k! P,T N
Ideal Gas Law
Pressure, volume, temperature and number of moles
R = 0.086057 L atmK mol
PV = nRT
3 o Boyle’s, Charles’ and Avogadro’s laws can be combined into an Ideal Gas Law that simultaneously
describes all the relationships between the four properties of gases. § All of the gas laws are just special cases of the Ideal Gas Law. For Boyle’s Law, when n and T
are held constant, nRT in the Ideal Gas Law is held constant, so the product PV is also a constant, establishing an inverse relationship between pressure and volume. For Charles’ Law, when n and P are held constant, (nR/P) in V = (nR/P)T is constant, establishing a linear relationship between volume and temperature. For Avogadro’s Law, when P and T are held constant, (RT/P) in V = (RT/P)n is constant, establishing a linear relationship between volume and number of moles.
o Deriving the Ideal Gas Law: Mathematical step Explanation PV = k! N,T Start with Boyle’s Law.
k! N,T = k!" N ×T In accordance with Charles’ Law, the volume must increase with the temperature in Boyle’s Law if the pressure is held constant. Therefore, the constant kB must be proportional to T.
PV = k!" N ×T Substitute the new constant kB into Boyle’s Law.
k!" N = k×N
In accordance with Avogadro’s Law, the volume must increase with the number of particles when pressure is held constant. Therefore, the constant kB2 must be proportional to N. Since there are no more relationships to establish, the new variable k can be left without a subscript.
PV = kNT Substitute the new variable k in.
n =NN! The number of moles, n, can be found by dividing the number of
particles (N) by Avogadro’s number (NA). PV = kN!nT Substitute the mole ratio above in.
PV = nRT The constants k and NA (Avogadro’s number) are combined into the constant R.
• Observation 3: Partial Pressures o From Boyle’s Law: The total pressure of a mixture of gas depends only on the number of moles of
gas, regardless of the identities and amounts of the gases in the mixtures. o From the Ideal Gas Law: The pressure exerted by a mole of molecules does not depend on what
those molecules are. o The process of mixing two gases together:
§ Inject 0.78 moles of nitrogen gas at 298 K into a container of fixed volume 25.0 L. The pressure of this gas is 0.763 atm (from Ideal Gas Law).
§ Inject 0.22 moles of oxygen gas at 298 K into a second identical container of fixed volume 25.0 L. The pressure of this gas is 0.215 atm.
§ Inject 0.22 moles of oxygen gas into the first container. The pressure of this mixture of nitrogen and oxygen gas is 0.978 atm, which is the sum of the pressures of the gases in separate 25 L containers.
o The partial pressure of each gas is the pressure of each gas as if it were the only gas present. § The partial pressure of each component can be calculated using the Ideal Gas Law:
P!" =!!"!"!
or P!! =!!"!"!
o Dalton’s Law of Partial Pressures: The total pressure of a mixture of gases is the sum of the
partial pressures of the component gases in the mixture.
McMurry & Fay 9.1–9.5, 9.8 Mole fraction (9.5) X =
Moles of componentTotal moles in mixture
Can be used to find the partial pressure of a component gas:
4 P! = X! × P!"!#$
The Behavior of Real Gases (9.8) • The behavior of a real gas is different from that of an ideal gas. • Deviations from the Ideal Gas Law occur because IGL assumes that the
volume of the gas particles themselves is negligible. o At high pressure, the volume of a real gas is larger than
predicted by the IGL. • Kinetic Molecular Theory also assumes that there are no attractive
forces between gas particles. This is not true at high pressures. o At high pressure, the particles are much closer together and the
attractive forces become significant. • Deviations from Ideal Gas behavior can be rectified using the van der
Waals equation.
Van der Waals equation (9.8) This equation uses two correction factors (a and b) to compensate for deviations from the Ideal Gas Law.
𝑃 +𝑎𝑛!
𝑉!𝑉 − 𝑛𝑏 = 𝑛𝑅𝑇
𝑃 =𝑛𝑅𝑇𝑉 − 𝑛𝑏
−𝑎𝑛!
𝑉!
CDS Chapter 15: The Kinetic Molecular Theory • Introduction
o What happens when a substance changes from solid to liquid or liquid to gas? Why do some substances do this so readily?
o Kinetic Molecular Theory provides a way to relate macroscopic and molecular properties and takes into account the essential fact that atoms and molecules are constantly moving.
• Foundation o Physics concepts:
§ 𝑃 = !!
§ 𝐹 = 𝑚𝑎 • Observation 1: The Limitations of the Ideal Gas Law
o Despite the variety of unique molecular characteristics of different gases, the Ideal Gas Law predicts exactly the same pressure for every type of gas. In short, this is too good to be true.
§ Experiment to find when 𝑃 ≠ !"#!: keep T constant and vary the particle density (n/V).
§ The Ideal Gas Law predicts that a plot of P versus nRT/V will yield a straight line. The actual result:
§ Even after increasing the density of the particles tenfold, the gases still stay close to the
Ideal Gas Law. However, at high densities the pressures start to deviate from each other.
5 § Plotting the ratio of PV/nRT versus n/V gives an even clearer view of these deviations. If the
Ideal Gas Law does not work, PV/nRT will not always be equal to 1:
§ This clearly shows that the Ideal Gas Law is not accurate at high densities.
• The deviation from IGL is different for each gas. • The deviation increases further for even higher densities:
§ Two types of deviation:
• Negative deviation – as the density increases, the value of PV/nRT drops below 1; the pressure of the gas increases less than the pressure predicted by IGL (this is true of most gases).
• Positive deviation – as the density increases, PV/nRT rises above 1; the pressure of the gas becomes greater than the pressure predicted by IGL.
• Observation 2: Densities of Gases and Liquids o The density of a gas is very low compared to that of a liquid – the volume of 1g of water vapor takes
up 1700 times more space than the volume of 1g of liquid water. § Note: this volume is only valid at P = 1atm. § With a lower pressure, the volume of a gas increases; with a higher pressure, its volume
decreases. § Since the same is not true of liquids (pressure does not change volume), the differences in
the volumes of liquids and gases are not due to changes in the size of their molecules. o The only conclusion is that the molecules in the gas must be much farther apart in the gas than
in the liquid. § At low gas densities, the molecules are so far apart that their individual characteristics are
unimportant. • Observation 3: Dalton’s Law of Partial Pressures
o Recall from the previous study that the pressure of a mixture of gases in a fixed-‐volume container is the sum of the individual pressures of its constituent gases.
o Assume we have oxygen and nitrogen mixed together in a container. The Law of Partial Pressures suggests that the oxygen molecules move in the same way that they would if the nitrogen molecules weren’t there.
o Conclusion: if the molecules are so far apart from one another (see previous observation), then they never affect each other. Therefore, they never exert forces on each other.
• Postulates of the Kinetic Molecular Theory
#
6 o A gas consists of individual particles in constant and random motion. o The distance (on average) between particles is very much larger than the sizes of individual
particles. o Because of the large distances between particles, the individual particles (on average) do not exert
any forces on each other, so that they neither attract nor repel one another. o The pressure of the gas is due entirely to the force of the collisions of gas particles with the walls of
the container. • The Ideal Gas Law and the Kinetic Molecular Theory
Mathematical step Explanation
1 P =FA Start with the equation for pressure, which is equal to the force exerted
over an area. 2 F!"#$%&'( = ma Remember that force is equal to mass times acceleration.
3 a ∝ 2v The acceleration is proportional to two times the velocity v, since the particle hits the wall and changes direction without losing any of its energy.
4 F!"#$%&'( = 2mv Substitute the acceleration into the force equation.
5 𝑓 ∝NV
The total force generated by all of the small impacts is determined by how many of these impacts there are. IF the particles hit the wall more often, the force will be higher. The density of particles in the container is a factor in how many particles hit the wall. Therefore, the frequency of the collisions of the particles with the walls of the container is proportional to N/V, where N is the number of particles and V is the volume of the container.
6 𝑓 ∝ A The surface area of the interior of the container is also a factor in determining the pressure.
7 𝑓 ∝ v The speed of the particles is also a factor in determining the pressure.
8 𝑓 =NV
Av This is the combined expression for the frequency of collisions with the container wall.
9 F!"!#$ = 2mvNV
Av Multiplying the expression for frequency by the force of each collision yields the total force.
10 P =2mv NAvVA
→ P =kNmv!
V
Since pressure is force per area, A is divided out of the expression. The 2 is removed because it is a proportionality constant – it is replaced with k. This equation agrees with the Ideal Gas Law’s pressure, quantity and volume relationship – however, it’s missing the temperature.
11 P ∝ KE =12mv!
Notice that the highlighted part of the expression in the last step is very similar to the expression for the kinetic energy of a particle (KE = !
!mv!).
Therefore, the pressure is proportional to the kinetic energy of the particles.
12 P ∝nVT From the Ideal Gas Law: pressure is proportional to the particle density
(n/V) times the temperature (T).
13 P ∝NV×12mv!
From step 10: the pressure is proportional to the particle density (N/V) times the kinetic energy of the particle. Note the similarities with step 11.
14 T ∝ KE!"# Therefore, the temperature is proportional to the average kinetic energy of the gas particles (not all the particles have the same speed).
15 KE!"# =32RT This is the mathematical expression for the average kinetic energy. R is the
same constant that appears in the Ideal Gas Law.
16 P =3NRT2V
→nRTV
Substituting the average kinetic energy expression into step 10 and dropping the fraction 3/2 (which is, like the number 2 in step 10, just a proportionality constant) yields the same Ideal Gas Law that was found in CDS Chapter 14.
7 • Analysis of the Ideal Gas Law
Observation Kinetic-‐Molecular Theory explanation Boyle’s Law – the pressure of a gas is inversely proportional to the volume of the gas (when n and T are fixed).
Decreasing the volume for a fixed number of molecules increases the frequency with which the particles hit the walls of the container, producing a greater force and higher pressure.
Pressure increases with the number of particles.
More particles will create more collisions with the walls, producing a greater force and a higher pressure.
Pressure increases with temperature. Increasing the temperature increases the speed of the particles, since temperature is proportional to kinetic energy. This increases the frequency of collisions and the force of each collision (two of the three factors that determine pressure). Therefore, the increase in pressure is proportional to v2.
Deviations from the Ideal Gas Law at high particle density
The particles are very close together at high density. Therefore, we cannot assume that the particles do not interact with each other. As the particles interact, they exert forces on each other and change their speeds. Negative deviations are caused by reduced particle speeds. When the speeds of the particles are reduced, fewer collisions with the walls of the container occur and each collision has a smaller force, both of which reduce the pressure. Attractive forces between the particles cause the slower speeds. Large negative deviations occur in gases where the molecules have strong intermolecular attractions. Positive deviations occur when the opposite happens: repulsions between the molecules speed up the molecules, creating more collisions with greater force. As density increases, first intermolecular force the molecules experience is attraction. Therefore, the attraction of particles is important even when the density is relatively low. It is only when the density gets very high that repulsions begin.
McMurry & Fay 9.6–9.7 Relationship between temperature and the kinetic energy of molecular motion (9.6)
𝐸! =32𝑅𝑇𝑁!
=12𝑚𝑢!
This can be rearranged to solve for the average speed u of a gas particle at a given temperature:
𝑢! =3𝑅𝑇𝑚𝑁!
𝑢 =3𝑅𝑇𝑚𝑁!
=3𝑅𝑇𝑀
where M is the molecular mass.
Diffusion (9.7) The mixing of gas molecules by random motion under conditions where molecular collisions occur.
Effusion (9.7) The escape of a gas through a pinhole into a vacuum without molecular collisions.
Graham’s Law (9.7) The rate of effusion of a gas is inversely proportional to the square roots of its mass:
8
Rate of effusion ∝1m
When comparing two gases at the same temperature and pressure, an equation can be formed that shows that the ratio of the effusion rates of the two gases is inversely proportional to the ratio of the square roots of their masses:
Rate!Rate!
=𝑚!
𝑚!=
𝑚!
𝑚!
Because temperature is a measure of average kinetic energy and doesn’t depend on the identity of the gas being measured, different gases at the same temperature have the same average kinetic energy. This can be tied to Graham’s Law by the following:
12𝑚𝑢!
!"# !=
12𝑚𝑢!
!"# !
𝑚𝑢! !"# ! = 𝑚𝑢! !"# ! 𝑢!"# !
!
𝑢!"# !! =
𝑚!
𝑚!
𝑢!"# !𝑢!"# !
=𝑚!
𝑚!
Therefore, the rate of effusion of a gas is proportional to the average speed of the gas molecules.