CHEMISTRY COMMUNITY

Jessica Castro 2H

The cathode is where the reduction reaction occurs (usually on the right side) whereas the anode is where the oxidation reaction occurs (usually on the left.

Jessica Castro 2H

1st order k units are s^-1
2nd order k units are L/mol/s
0 order k units are mol/L/s

Jessica Castro 2H

We would use this to determine the cell potential when the cell is not in standard conditions.

Jessica Castro 2H

Since the sample is brought to a pH of 6.1, the pH describes [H3O+] at equilibrium, NOT the initial concentration. Therefore, we can infer that the sample is at equilibrium and we can calculate [CO2] directly from K = [CO2]/[H3O+][HCO3-].

Jessica Castro 2H

Yes, a equilibrium, since both the forward and reverse reactions are occurring at the same rate, K = 1. Therefore, looking at the equation ΔG°= =RTlnK, plugging in K=1 would result in ΔG°= 0. Thinking at it conceptually, since both the forward and reverse reactions are occurring at the same rate, Gr...

Jessica Castro 2H

Cp is used when pressure is constant (isobaric) and Cv is used when volume is constant (isochoric).

Jessica Castro 2H

Since the question was asking for reducing power and the standard potentials for the reduction reactions were given, you did not need to flip the signs.

Jessica Castro 2H

A galvanic cell by definition is an electrochemical cell in which a spontaneous chemical reaction is used to generate an electric current. Since the chemical reaction in a galvanic cell is always spontaneous, delta G will always be negative for a galvanic cell.

Jessica Castro 2H

I don't think he expects us to memorize these formulas, however, it is good to know how to derive these equations from the ones given on the equation sheet: For ln(K2/K1) = -delta H/R(1/T2 - 1/T1), we can equate the two equations given for standard Gibbs free energy ΔG°= -RTlnK and ΔG°= ΔH°-TΔS° and...

Jessica Castro 2H

I'm sure either is fine, as long as you are consistent throughout the problem.

Jessica Castro 2H

When adding the Ecells of both reactions, we need to make sure that the electrons will cancel because we don't want electrons in the overall reaction. This is done by multiplying the equations by a least common multiple and by flipping one of the reactions so the electrons will cancel. By doing this...

Jessica Castro 2H

Since K is unitless, it is better to try to replace K with k/k' when possible to be consistent with units. It is also easier to understand the concentrations of the reactants when k is used instead of K.

Jessica Castro 2H

In a series of reactions, you will always start with the catalyst as a reactant. In contrast, an intermediate is first introduced as a product because an intermediate is formed then used.

Jessica Castro 2H

Since internal energy (U) depends on temperature, in an isothermal process where temperature does not change internal energy will not change either. Therefore, delta U = 0 for an isothermal process.

Jessica Castro 2H

For first order reactions, the equation for the plot is ln[A] = -kt + ln[A]0. If you solve for kt you would get kt = ln[A]0 - ln[A]. We know that for subtraction with ln, lnx -lny = ln(x/y). Therefore, kt = ln[A]0 - ln[A] = ln([A]0/[A]). There should be an ln next to the [A]0, but both equations sho...

Jessica Castro 2H

The amount of heat given is the heat generated by your body (which is the system). The question is asking for the entropy of the surroundings. Therefore, when plugging in the heat of the surroundings, remember q system = - q surroundings. The heat of a system is equal and opposite to the heat of its...

Jessica Castro 2H

Heat capacity (C) is simple the heat supplied / change in temp (C = q / delta T). Specific heat capacity (Cs) is the heat capacity at a given mass (Cs = C/m), therefore q = m x Cs x delta T. Molar heat capacity (Cm) is the heat capacity at a given amount of moles (Cm = C/n), therefore q = n x Cm x d...

Jessica Castro 2H

Remember that change in entropy is the heat at a constant temperature (delta S = q / t). Therefore the units will be the units of heat (J) over the units of temperature (Kelvin) --> J/K. It is in Kelvins because that is the absolute temperature.

Jessica Castro 2H

Thermodynamic properties refer to how the system exists, whether it can exist at any point in space regardless of how much or if it changes based on how much is in the system (the mass of the system). These are broken up into intensive and extensive properties. Intensive properties exist anywhere in...

Jessica Castro 2H

You can use the relationship between pressures and volumes (P1V1 = P2V2) and manipulate it to get (P1/P2 = V2/V1). Then just substitute P1/P2 into delta S = nRln(V2/V1).

Jessica Castro 2H

Internal energy is the amount of energy that exists in the system based on its chemical properties, basically the potential energy. Heat is a measure of energy transferred as temperature changes. Enthalpy is the change in heat of a reaction at constant pressure. Hope this helps.

Jessica Castro 2H

Looking at the equation for the change in entropy (delta S = q / T), we can see that temperature (T) is inversely related with the change in entropy (delta S). Therefore, at lower temperatures, there will be a greater change in entropy. Looking at the concept of entropy conceptually, entropy changes...

Jessica Castro 2H

For part b, it is asking what percentage of the total heat was used to specifically heat the water. Therefore, we need to divide the heat of water by the total heat used (Q of water / Q total) and multiply by 100 to find the percentage. I think we use the info from the table to find the heat used fo...

Jessica Castro 2H

The standard units for Rydberg's constant are J/K/mol. But if you need to use pressure in your equation, the conversion is 1 L atm = 101.325 J

Jessica Castro 2H

I'm not sure if I'm doing this problem right. I first found Cpm (Cvm for b) then found delta T then added that to the intial T to find the temperature. Then for change in enthalpy, since it equals the heat in constant pressure, I just used the given heat. However, the answer it doesn't match the ans...

Jessica Castro 2H

"Molecular processes" is referring to the transfer of energy as heat between the system and the surroundings. Therefore, at thermal equilibrium, energy is being transferred between the system and its surroundings at the same rate.

Jessica Castro 2H

The first step would be to solve for Ccal. Since q = -Ccal (delta T), solving for Ccal = -q(deltaT). Since q = -3.5kJ, we would end up with a positive Ccal. Then use Ccal to find qcal. Then recall that q reaction + q cal = 0, therefore q reaction = - q cal.

Jessica Castro 2H

For this problem, we are looking for the total heat (Q total)for the whole process. To find Q total, we need to find the heat for both copper and and water. Use the respective values for copper and water to find Q copper and Q water. Then add Q copper and Q water to find the Q total.
Hope this helps!

Jessica Castro 2H

The K expression only applies to substances that change in concentration, which affect the reactant at equilibrium. Since solids and liquids have a constant concentration during the reaction, they do not affect the amount of reactant at equilibrium. Therefore, we can ignore solids and liquids when f...

Jessica Castro 2H

It doesn't say on Lavelle's outline that we are required to know the energy profile diagrams. However, I think it's best to have an understanding of what the graph would look like based on the direction of the reaction and vice versa and be able to relate the two.

Jessica Castro 2H

If we compare K and Q when the temperature changes, then we can also use Q to determine whether the reaction is endothermic or exothermic based on the direction of the reaction. For example, if the reaction shifts right (favors product), then the reaction requires heat, therefore the reaction is end...

Jessica Castro 2H

In class, I remember Lavelle saying either way is fine. However, on m midterm, I put d ahead of s and my TA drew an arrow to put it after p. So just in case, put it in spd order.

Jessica Castro 2H

For a molecule whose shape is seesaw, it's arrangement of electrons is trigonal bypyramidal. In a trigonal bipyramidal shape, we have angles 90, 120, and 180 degrees. When we make one of the bonds on the equatorial in a trigonal bypyramidal shape a lone pair, all the angles are less than the origina...

Jessica Castro 2H

Dipole-dipole interactions are electrostatic interactions between any partially negative atom and any partially positive atom. However, a hydrogen bond is specifically between a partially positive hydrogen atom and a partially negative atom (N, O, or F). Since N, O, and F, are much more electronegat...

Jessica Castro 2H

Someone had the same question, so here's what I told them.

F is an exception to this rule, in that F is the only halogen that does affect the pH of a solution. Since KF is a very weak acid, the conjugate base of KF is a basic solution, therefore producing a pH greater than 7.

Jessica Castro 2H

F is an exception to this rule, in that F is the only halogen that does affect the pH of a solution. Since KF is a very weak acid, the conjugate base of KF is a basic solution, therefore producing a pH greater than 7.

Jessica Castro 2H

It depends whether or not a hydrogen can form a hydrogen bond. For hydrogens, they must be partially positive to form a hydrogen bond (with N, O, or F). For example, the hydrogens in methane (CH4) have no partial charge. Therefore, they can't form hydrogen bonds. Since oxygen is a very electronegati...

Jessica Castro 2H

DNA is held by hydrogen bonds. Therefore, for something to bind to it, it must also bind to a polar molecule (bonded by hydrogen bonds). Since cisplatin has its two chlorines on the same side, the dipole doesn't cancel, creating partial negative charges on the chlorines. This allows cisplatin to bin...

Jessica Castro 2H

Polarity is also determined on whether or not the dipoles cancel. Tip: try to know the trend of electronegativity to determine if a dipole exists or not.

Example: XeF4 has two lone pairs, but the two lone pairs cancel. Therefore XeF4 is nonpolar.

Hope this helps!

Jessica Castro 2H

My logic for this problem was to see which molecule is more likely to disassociate and accept protons. Since BrO^- has an extra electron on O, we can infer that O will want to form a bond with H+. If we write out the equation, we can see that OH- will form and Br- will be left alone, making BrO- the...

Jessica Castro 2H

Drawing the Lewis structure is your best guide. Once you draw it out, use your knowledge about polarizability and electronegativity to determine the distribution of charge and the type of intermolecular force.

Jessica Castro 2H

An easy way to do a VSEPR model is to start with the Lewis structures. Don't forget about formal charge. Once you have your designated lone pairs about the central atom, treat them as an area of high electron concentration, then space out all areas of high electron concentration away from each other...

Jessica Castro 2H

In respect to the 2 carbon atoms the shape will be tetrahedral.

Jessica Castro 2H

By calculating the formal charge of each atom, we can tell that this is an ionic bond. Remember in ionic bonds, the valence electrons are TRANSFERRED, while in covalent bonds, the atoms are shared. That is why in this ionic bond, ammonium (the cation) and chloride (the anion) are not connected.

Jessica Castro 2H

Electronegativity measures an atom's affinity or attractiveness to electrons. The more electronegative an atom is, the greater the pull of electrons is towards the atom's nucleus. I like to think of a tug of war. Let the middle of the rope represent the valence electrons in the bond. Electronegativi...

Jessica Castro 2H

Electronegativity measures an atom's affinity or attractiveness to electrons. The more electronegative an atom is, the greater the pull of electrons is towards the atom's nucleus. I like to think of a tug of war. Let the middle of the rope represent the valence electrons in the bond. Electronegativi...

Jessica Castro 2H

Yes, when oxygen is present, it is the atom that typically forms double bonds because looking for two more valence electrons. The number of bonds an atom can form is based on the number of valence electrons it needs to have a full octet.

Jessica Castro 2H

All this question is asking is which atom (of ^+2, which will make it an ion-cation) has the given configuration? However, remember: each orbital contains a pair of (2)electrons, so if the ion contains a charge greater than 2 (i.e. +3 charge), it will likely be referring to the atom in the next orbi...

Jessica Castro 2H

Most elements try to follow the Octet Rule in order to obtain a more stable state. However, as Dr. Lavelle mentioned during lecture, Hydrogen, Helium, Lithium, and Beryllium are exceptions to the Octet Rule because they don't have enough electrons in which obtaining 8 valence electrons would make th...

Jessica Castro 2H

The number of dots around the nuclear symbol of an element corresponds with its number of valence electrons. Tip: Remember that for all elements (except for hydrogen and helium), the outer shell would fill up to 8 valence electrons. However, always start with one on each side (the first 4), then pai...

Jessica Castro 2H

Coulomb's Law represents the effect of inter nuclear distance on the electrostatic potential energy of the charges between non H- atoms. This equation shows that the electrostatic potential energy and the inter nuclear distance between the charges are inversely proportional.

Jessica Castro 2H

I believe you would use the equation for the energy level of an H- atom, but use it for the initial and final n. In this case, you would be solving for n(initial). Set this difference equal to the equation for the energy of a photon, since it takes energy to change from one energy level to another.

Jessica Castro 2H

Just a note: The name for the quantum number and what the quantum number describes is not the same thing. (n) can be called the shell or the principle quantum number. It describes the energy level and size of the atom. (l) can be called the subshell or the angular momentum quantum number. It describ...

Jessica Castro 2H

De Broglie's equation can only be used for particles with rest mass. Since photons are massless, this equation does not apply for light.

Jessica Castro 2H

Convert the weight of the baseball to kg and the velocity to m/s, then use the equation for wavelength we learned in class.

Jessica Castro 2H

The number of shells and orbitals depend on the number of electrons of the atom, thus the energy level of the atom. Shells (represented by circles around the nucleus of an atom), represent the energy levels of an atom. The energy levels are labeled by n, with a number in front to represent which ene...

Jessica Castro 2H

Since most other units that contain a measurement of length (i.e. velocity is m/s, etc.), it is best to convert measures of length into meters (m). On another note, most units are calculated using the SI units. Unless the given values and the other needed units are not in SI units, it is best to con...

Jessica Castro 2H

From the book, we know that the mass of a molecule equals the number of atoms multiplied by the number of atoms (m=nM). If we solve for gallium's and astatine's mass using their respective molar masses and Avogadro's number, we compare the two to see how to make them equal (multiply by 3). By gettin...

Jessica Castro 2H

I talked to one of the TAs and she said that they will be grading the homework based on picking problems that are relevant to the material that was taught that week and trying your best to complete them correctly and understand the concepts of each problem. Also, I suggest doing more than 7 problems...

Jessica Castro 2H

The coefficients in a chemical equation represent the ratio of moles required among molecule for the reaction to occur. They do not represent the actual number of moles. Also, remember that moles are a representation of a number (i.e. a dozen = 12, whereas 1 mole = 6.022 x 10^23).

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