CHEMISTRY COMMUNITY

Rachel Formaker 1E

Because the slow step is the rate-determining step, only changing the slow step will change the rate of the reaction.

Adding a catalyst to one of the fast steps will not affect the rate of reaction because the fast step does not determine the rate.

Rachel Formaker 1E

For a reaction profile, we mainly need to take into account the number of elementary steps, the activation energy of each step, and whether the reaction is exothermic or endothermic. For each elementary step, there is one energy barrier/"hill" in the reaction profile. The slow step will ha...

Rachel Formaker 1E

I think if you're told that it is an electrolytic cell, you should also be able to calculate standard cell potential by putting the reaction with the higher standard reduction potential at the anode rather than cathode. Basically, this would be the reverse of a Galvanic cell, so you could calculate ...

Rachel Formaker 1E

Just make sure that you are consistent with which numbers you use. Keep track of which set of numbers is k₁/T₁ and which is k₂/T₂

Rachel Formaker 1E

If you find experimentally certain points on the graphs we discussed in class, you can use the slope to find k. Since k is experimentally determined, this is the way to find k, not the other way around. I wouldn't say you necessarily need to memorize why certain orders correspond with certain graphs...

Rachel Formaker 1E

When there is product present, the product will also be reacting to form reactant. So this reverse reaction would occur in addition to reactant forming product, changing the overall rate at which reactant is consumed and product is formed. We therefore use initial rates to avoid having to calculate ...

Rachel Formaker 1E

Rates are always given as positive values. Since the equation for rate is rate= d[]/dt and the concentration of reactants is decreasing, simply using the above equation for the rate of consumption of reactants would yield a negative rate. To correct this and give a positive rate for rate of consumpt...

Rachel Formaker 1E

The order of a reaction tells us how the rate of reaction is affected by the concentration of the reactants. For a zero-order reaction, the rate of reaction is independent of the concentration of reactants, so changing the reactant concentration will have no effect on the reaction rate. For a first-...

Rachel Formaker 1E

I think Dr. Lavelle prefers that we use:
E^o_cell = E^o_cathode - E^o_anode
so that we don't have to switch the sign of either standard reduction potential

Rachel Formaker 1E

Since the reactions in Galvanic cells are spontaneous and E^o_cell = E^o_cathode - E^o_anode, the reaction with the more positive (or less negative) standard reduction potential should be at the cathode so that E^o_cell > 0.

Rachel Formaker 1E

Even though ΔG of a final reaction will be the sum of the ΔGs of its composite reactions, it is important to note that this is not true of standard potential, E o . Since E o is not a state function, you must manipulate the equations to use ΔG, which is a state function. If reaction 1 + reaction 1 =...

Rachel Formaker 1E

inert metal | reactant | product || reactant | product | inert metal The anode is typically on the left and the cathode is typically on the right. The double line represents the salt bridge, and will be replaced with a single line if there is a porous disk instead of a salt bridge. Lines are used to...

Rachel Formaker 1E

Because ΔG^o= -nFE^o = -RTlnK,

-nFE^o = -RTlnK

When standard cell potential is positive, K>1

When standard cell potential is negative, K<1

For positive E^o, larger the standard cell potential of a reaction, the larger the equilibrium constant.

Rachel Formaker 1E

Moles in the equation ΔG=-nFE is for moles of electrons, so even if the moles of a substance are not explicitly given, you should be able to figure out the moles of electrons through the redox half-reactions.

Rachel Formaker 1E

Heat is always required to melt a substance, so ∆H is positive.

Heat is released when a substance freezes, so ∆H is negative.

∆H_freezing = ∆H_fusion, and since you are given ∆H_fusion from Table 8.3, you just reverse the sign of ∆H_fusion to get ∆H_freezing = -6.01 kJ/mol

Rachel Formaker 1E

Since the problem says that the heat is being transferred from one reservoir to another, you can assume that the heat is leaving the first reservoir (-q) and entering the second reservoir (+q).

Rachel Formaker 1E

If the external pressure is higher than the internal pressure, there will be compression rather than expansion of the gas, so you're not comparing it to reversible expansion anymore. However, I would assume that reversible compression would be more work being done on the system than irreversible com...

Rachel Formaker 1E

ΔS of the system will increase because in free expansion, the volume of the system will still increase, so the particles can occupy more microstates, representing an increase in entropy.

Rachel Formaker 1E

Although it does not specify 1 moleof N₂ gas in the book, 9.13 is listed in the solutions manual errors on Lavelle's website. In the corrected answer, Lavelle says to assume both ideal behavior and 1 mol of N₂ gas.

Rachel Formaker 1E

Isothermal means that the temperature does not change in the reaction. For all isothermal reactions, there is no change in internal energy, so ΔU=0. In this problem, we are told that ΔU=0 because the reaction is isothermal. Since no work is done in free expansion (w=0) and ΔU = q + w, when ΔU=0 and ...

Rachel Formaker 1E

Exothermic and endothermic refer to ΔH, or the amount of heat absorbed or released in a reaction. Spontaneous and non-spontaneous refer to ΔG, or the free energy change of the reaction. Because ΔG = ΔH - TΔS, ΔG depends on both ΔH and ΔS. So not all exothermic reactions (ΔH < 0) will be spontaneous ...

Rachel Formaker 1E

Since we cannot measure ΔG directly, we use equations with values that we can measure to calculate ΔG. We can measure equilibrium concentrations to calculate the equilibrium constant K, so we can calculate ΔG using K from measured concentrations: ΔG o = -RTlnK If the system is not at equilibrium, we...

Rachel Formaker 1E

At equilibrium, the free energy of the products is equal to the free energy of the reactants.
Since ΔG = G_products - G_reactants
When G_reactants = G_products, ΔG = 0

Therefore, at equilibrium, ΔG = 0

Rachel Formaker 1E

delta G o is for when the reactants and products are all in their standard states. This means that pressure is 1 atm and temperature is 25 o C (298 K). So if any of the given conditions violate these conditions, don't use delta G o . The general rule I use for homework and tests is that if the given...

Rachel Formaker 1E

There is heat transferred in this situation in order to keep temperature constant.
As a gas expands, it cools. Because the question tells us that temperature is constant, the gas must have absorbed heat from the surroundings to maintain its temperature and counteract the cooling from expansion.

Rachel Formaker 1E

When the pressurized liquid escapes as a gas, it undergoes a phase change where it changes from liquid to gas. Enthalpy of vaporization is always positive, meaning that heat is required to change the phase from liquid to gas. So heat is absorbed by the liquid as it changes into the gas phase while i...

Rachel Formaker 1E

Degeneracy is the number of possible states that a system can have. For example, if one particle can be in one of two sides of a flask (similar to the example in class) the degeneracy is W=2 because the system has two possible states (the particle on either side of the flask). An equation for degene...

Rachel Formaker 1E

At lower temperatures, enthalpy of a reaction (ΔH or q P ) has a bigger influence on the entropy of a reaction because of this equation: \Delta S = \frac{q}{T} When T is lower, ΔS is larger, meaning that q had a larger effect on entropy. For example, if q=10 and T=1, ΔS =10, but if q=10 and T=10, th...

Rachel Formaker 1E

In general, when a problem says to assume ideal behavior, it is referring to the ideal gas law:
PV=nRT

Rachel Formaker 1E

To modify the equation above, W=2 N for a 2-state system, where N is the number of particles. The example we used was where each particle could be in one of two bulbs of a flask This equation can be used for any number of states, just by changing the base. For a 3-state system: W=3 N The number of s...

Rachel Formaker 1E

Internal energy is a state function because it depends only on the initial and final states of the internal energy. Although work is not a state function, as long as the sum of work done and heat transferred is the same, the change in internal energy will be the same regardless of the pathway taken ...

Rachel Formaker 1E

The reason that no work is done in this picture is because the gas is expanding into a vacuum. There is no external pressure for in the vacuum, so since there is nothing for the gas to push against as it expands, it is not doing any work. The textbook calls this expansion against zero pressure "...

Rachel Formaker 1E

If the reaction is conducted at constant pressure , then heat transferred (q p ) will be equal to the enthalpy of the reaction (ΔH). Enthalpy is specifically the heat released or absorbed at constant pressure, so make sure that pressure is constant (reaction is not in a bomb calorimeter) before assu...

Rachel Formaker 1E

The reason the answer is c) and not b) is because the slopes of the heating curve for the solid and gas phases must be steeper than the slope of the heating curve for the liquid phase. This is because the heat capacity of the liquid phase is higher than the heat capacities of the solid and gas phase...

Rachel Formaker 1E

To increase the internal energy of a closed system, you can heat the system and/or perform work on the system.
You can heat the system using an external heat source, and you can perform work on a system by compressing the system (like compressing a bike pump).

Rachel Formaker 1E

Adding matter to an open system increases the internal energy of that system, and removing matter from an open system decreases the internal energy of the system.

Rachel Formaker 1E

When a reaction produces heat (is exothermic), heat leaves the system. ΔH is negative because this represents heat leaving the system as it is produced.

Rachel Formaker 1E

The internal energy of an isolated system will not change over time.

In an adiabatic system, energy is not transferred as heat, but the internal energy can still change if energy is transferred to or from the system as work.
For adiabatic systems, ΔU=w because q=0.

Rachel Formaker 1E

Just to note, on the outline for the unit on Thermochemistry and the First Law of Thermodynamics, Dr. Lavelle says to omit section 8.18 concerning the Born-Haber Cycle.

Rachel Formaker 1E

Adding onto the previous response, if you are given the standard enthalpies of formation for the reactants and products, you use the equation:
Σ ΔH^o_rxn = Σ ΔH^o_f(products) - Σ ΔH^o_f(reactants)

Rachel Formaker 1E

Some examples of reactants or products not in their standard states is carbon as diamond (instead of graphite) or oxygen as ozone (O₃ rather than O₂).

So a possible example of a reaction with reactants not in standard states is the conversion of ozone into oxygen:
2O₃(g) --> 3O₂(g)

Rachel Formaker 1E

The length of the lines of the heating curves also represents the amount of heat needed to change the state of the substance. The longer the line, the more heat is required, indicating that the heat capacity is higher.

Rachel Formaker 1E

Dr. Lavelle also specified that Qv is found using a bomb calorimeter, which keeps volume constant, while Qp is found using a coffee cup calorimeter, which allows gases to escape to keep pressure constant.

Rachel Formaker 1E

Standard reaction enthalpy is specifically the reaction enthalpy for a reaction in which all the products and reactants are in their most stable forms and the pressure is 1 atm. Reaction enthalpy in general could be for a reaction in which ice is used instead of water, or ozone (O 3 ) instead of O 2...

Rachel Formaker 1E

Metal hydroxides are usually strong bases
Examples include NaOH, LiOH, KOH, etc.

Just as strong acids completely dissociate in water, strong bases also completely dissociate in water, so their conjugate acids are EXTREMELY weak, and you assume that none of the strong base remains in solution.

Rachel Formaker 1E

Isoelectronic means that the two ions have the same number of valence electrons.

F^- has valence 8 electrons
Because O^- and N^2- each have only 7 valence electrons, they are not isoelectronic with F^-

Rachel Formaker 1E

The lower the pK_b, the stronger the base.
Because pK_a + pK_b = 14 (so when pK_a increases, pK_b decreases), the larger the pK_a value for an acid, the stronger the conjugate base.

Rachel Formaker 1E

Increasing pressure by compression (decreasing volume) will cause the reaction to proceed toward the side with fewer moles of gas. Decreasing pressure by increasing volume will cause the reaction to proceed toward the side with more moles of gas. Dr. Lavelle specified in class that just saying that ...

Rachel Formaker 1E

Diaquadifluoromercury(III)

Rachel Formaker 1E

For an acid, when given the initial molarity and percent deprotonation, you multiply the initial molarity by the percent deprotonation (as a decimal, so over 100) to find the concentration of H 3 O + ions in solution. You then use this concentration to find pH in the normal way (pH=-log[H 3 O + ]) F...

Rachel Formaker 1E

If you are given initial concentrations and all of them are nonzero, calculate the reaction quotient Q. If Q<K, the reaction moves toward the products, so the ICE table would have +x on the products side and -x on the reactants side. If Q>K, the reaction moves toward the reactants, so the ICE table ...

Rachel Formaker 1E

The rule in general is for a given equation, you raise K to the power of whatever value you multiplied the chemical reaction by.

So if you multiply the equation by 3, the new value of K would be the initial K³

Rachel Formaker 1E

When a pi bond is delocalized, it is not restricted between two atoms. The electrons are spread out over several atoms in a molecule, giving the affected bonds characteristics (length and strength) between a double and single bond. For example, in benzene (C 6 H 6 ) there are 3 pi bonds delocalized ...

Rachel Formaker 1E

Another difference is that all Bronsted acids are Lewis acids, but not all Lewis acids are Bronsted acids. This is because when a molecule is a proton donor, that proton is an electron pair acceptor, making the molecule both a Bronsted acid(proton donor) and a Lewis acid (electron pair acceptor). So...

Rachel Formaker 1E

Since the resonance structures are the same in terms of atom placement and differ only in electron placement, the shapes for different resonance structures will be the same. Shape depends on the number of regions of electron density, so whether there is a single, double, or triple bond has no effect...

Rachel Formaker 1E

The coefficient when describing a hybridized orbital describes the energy level of the orbital being hybridized. So in general, when there are 4 regions of electron density, there will be sp 3 hybridization. Using the coefficient 4 in 4sp 3 means that the element with the hybridized orbitals is in P...

Rachel Formaker 1E

To find the charge of a complex, you add the charges of the atoms/ions that make up the complex. In tetracyanonickelate(II): 4 x CN - = 4- 1 x Ni 2+ = 2+ Charge of complex = 2- You know that the nickel in this complex has a 2+ charge from the Roman numerals in the name. Since Table 17.4 is on the ou...

Rachel Formaker 1E

The solutions manual also states that because the triple bonded carbons are strained in the ring formation, they may form a diradical in which the triple bond is replaced by a double bond with a single radical electron on each carbon. This would also result in a very reactive molecule because of the...

Rachel Formaker 1E

We need to know the bond angles for the common VSEPR shapes (linear, trigonal planar, tetrahedral, trigonat bipyramidal, etc). However, we do not need to memorize the exact values for experimentally determined bond angles, like those resulting from lone pairs "pushing" the bonded atoms clo...

Rachel Formaker 1E

The I 3 - molecule has the VSEPR notation AX 2 E 3 . Because it has 5 regions of electron density, begin by looking at the trigonal bipyramidal shape. The bond angles in trigonal bipyramidal are 120 o between the equatorial atoms, and only 90 o for the axial atoms. Because the lone pairs have more r...

Rachel Formaker 1E

If the performance of the supercomputer is only based on the speed of the electrons, the number of electrons is unrelated to performance. If you increase the number of electrons, all of the electrons will still have the same speed, there will just be more of them. This is similar to the results of t...

Rachel Formaker 1E

Generally, with chemical formulas, atoms written next to each other are bonded to each other. So in HOCO, writing the formula this way tells you that the two oxygen atoms are bonded to the carbon, and the hydrogen is bonded to an oxygen. This can be especially helpful in hinting at the common groups...

Rachel Formaker 1E

When drawing Lewis structures, the bonds can be placed at any angle around the central atom. We are not responsible for drawing VSEPR models, but it will help to understand the 3D shape, and you must know the bond angles for the given shapes.

Rachel Formaker 1E

Both types of equilibrium constant, whether in regard to molarity or partial pressure, refer to the composition of the equilibrium. Either constant will tell you if the equilibrium sits to the left or to the right.

Rachel Formaker 1E

In general, since lone pairs have the greatest repulsion, put them wherever they will be farthest from each other or from other atoms. This will minimize the repulsion in the molecule and result in the most stable structure.

Rachel Formaker 1E

As a general rule, yes, an odd number of valence electrons in a molecule means the molecule will be a radical.

Rachel Formaker 1E

In addition, hydrogen is a very small atom, so its nuclear charge has a strong pull on the electrons in a bond. There is also very little to no shielding because hydrogen only has one electron, so the effective nuclear charge felt by the electron is very high.

Rachel Formaker 1E

The elements that typically have less than an octet are hydrogen, helium, lithium, and beryllium. Boron also may form molecules where it only has 6 electrons. For expanded octets, any element in period 3 or later with a d-orbital can have an expanded octet. This includes sulfur, phosphorus and chlor...

Rachel Formaker 1E

In the equatorial plane, there are 3 regions of electron density, which can be separated by approximately 120 o bond angles. This is favorable compared to axial atoms above and below the equatorial plane, which have only approximately 90 o bond angles with equatorial atoms. Since lone pairs have mor...

Rachel Formaker 1E

To clarify, when there is a bracket with a charge around a Lewis structure with multiple atoms, that means that the molecule shown is a polyatomic ion. The bonds between the atoms within the ion itself are covalent, but the ion will form ionic bonds with other ions. When there are bonds shown as lin...

Rachel Formaker 1E

The atom in the center in the molecules here is always the one with the lowest ionization energy, so these do follow the guideline laid out in class. The guideline for ionization energies is that ionization energy decreases as you go down a group and increases right to left across a period. In the s...

Rachel Formaker 1E

Lewis acids and bases are defined in terms of electrons, while regular (Bronsted) acids and bases are defined in terms of protons. A Lewis acid accepts an electron pair and a Lewis base donates an electron pair. A Bronsted acid donates a proton and a Bronsted base accepts a proton. If something is a...

Rachel Formaker 1E

Nitrogen has a half-filled 2p subshell, so that there is one electron in each orbital. This creates an unusually stable atom because of half-shell stability. Because nitrogen is relatively stable on its own, it has a relatively low electron affinity. It is similar to the reason that chromium has an ...

Rachel Formaker 1E

Another way to think about this is that a low ionization energy corresponds to likelihood to form a cation (because it is easier to remove the electron) and a high electron affinity corresponds to likelihood to form an anion (because more energy is released when an electron is added). This is helpfu...

Rachel Formaker 1E

The ionization energies of the Group 15 elements is higher than that of the Group 16 elements because in Group 15, each p-orbital has one electron, which is relatively stable because it is a half-filled subshell. Group 16 elements have one set of paired electrons in the p-orbital, and the repulsion ...

Rachel Formaker 1E

Nodal planes are planes on which there is no chance that an electron will be found. The s-orbitals have no nodal planes, but the p- and d- orbitals do. They are mainly of interest when we are trying to figure out the location of an electron in a given orbital. The electron will never be at the nucle...

Rachel Formaker 1E

The magnetic quantum number of an electron tells us precisely which orbital within a subshell that the electron occupies. The spin magnetic quantum number has values of either +1/2 (spin up) or -1/2(spin down). This tells us if the electron spins counterclockwise (spin up) or clockwise (spin down). ...

Rachel Formaker 1E

To clarify, the 2p electron subshell has 3 orbitals, 2p x , 2p y , and 2p z , each of which can hold 2 electrons. According to Hund's Rule, if electrons in the same subshell can occupy different orbitals, then they will do so. So when an atom has 2 electrons in the 2p subshell, they will occupy the ...

Rachel Formaker 1E

To elaborate on the last answer: When you calculate ΔE, you do final-initial. In this case, that means E n (final) - E n (initial) When the electron is moving from a higher energy state to a lower energy state, ΔE will be negative. When the electron is moving from a lower energy stat...

Rachel Formaker 1E

I think your confusion is mainly mixing up speed, energy, and frequency. The speed of light is constant (3.00 x 10 8 m/s), and is equal to the frequency times wavelength of light. The frequency(v) of light is different for different types of light, and the energy(E) of light is related to the freque...

Rachel Formaker 1E

Because each element has a different number of protons, each element's nucleus has a different positive charge. This variation in positive charge for each element causes the electrons in each element to be bound to the atom by a different amount of energy. The slight differences in configuration cau...

Rachel Formaker 1E

Constants usually have the units that they do because of the units of the quantities that they relate.
So in the equation E=hv, Planck's constant relates frequency(in seconds^-1) and energy (Joules). In order for this equation to make sense, Planck's constant must have the units of Joule-seconds.

Rachel Formaker 1E

For the purposes of chemical equations, aqueous solutions and liquids are different, and it is important to make this distinction when writing out the equation.
An aqueous solution is indicated by the symbol (aq), while a liquid is indicated by the symbol (l).

Rachel Formaker 1E

As far as I know, the common prefixes need to be memorized.
The ones we went over in lecture are

giga: 10⁹
mega: 10⁶
kilo: 10³
deci: 10^-1
centi: 10^-2
milli: 10^-3
micro: 10^-6
nano: 10^-9
pico: 10^-12

Rachel Formaker 1E

Discussing the formula unit of an ionic compound is equivalent to discussing a molecule of a covalently bonded compounds. It is technically incorrect to call a unit of NaCl a molecule, although that may be what you have heard in past chemistry classes. Call it a formula unit instead. Fundamentals Se...

Rachel Formaker 1E

When we are given the name of the compound in a problem and we are supposed to find the chemical formula for an equation with no other information, the compound is usually an ionic compound and we are supposed to be able to figure out the formula using the oxidation numbers of its elements. For exam...

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