CHEMISTRY COMMUNITY

Katie Lam 1B

The change in internal energy (delta U) for an isolated system is always 0 because no work is done on/by the system and no heat leaves/enters the system.

Katie Lam 1B

Catalysts are usually part of the original reactants. They are not formed in one of the reactions, like intermediates are.

Katie Lam 1B

The rate law for each individual step is just the rate constant times the reactant concentrations for the reactants (including intermediates) in that particular equation.

Katie Lam 1B

The units for the rate constant changes depending on the order of reaction, but the rate itself is always in mol/(L*s).

Katie Lam 1B

Both gases are distributed throughout the 0.75 L container, so you use that volume for both H2 and I2.

Katie Lam 1B

Rate is expressed as change in concentration over time, so the units are mol/(L*s).

Katie Lam 1B

Rate is always expressed in mol/(L*s), so k changes units depending on the order of the reaction.

Katie Lam 1B

The overall order of a reaction can be different from the order of the individual reactants. For example, a reaction may be first order with respect to X and first order with respect to Y. Even though the reactions are first order with respect to individual reactants, the overall order is second ord...

Katie Lam 1B

Galvanic cells have spontaneous reactions, while electrolytic cells have nonspontaneous reactions.

Katie Lam 1B

Carbon in graphite form can be used as well.

Katie Lam 1B

The placement of the arrow indicates whether oxidation or reduction occurs, so it is important. If it is Zn2+ + 2e- --> Zn, it is reduction since the zinc cation gains electrons. If it is Zn --> Zn2+ + 2e-, it is oxidation since zinc loses electrons.

Katie Lam 1B

Magnitude would refer to the distance from zero. For example, -5 and 5 are the same magnitude. However, -6 is a greater distance from 0 than +4, so the magnitude is greater.

Katie Lam 1B

It is assumed that the cations and anions involved in the half reactions are in aqueous solution, so water doesn't need to be explicitly written.

Katie Lam 1B

You just need to balance it so that the electrons that are transferred cancel out between the two half reactions.

Katie Lam 1B

If both delta H and delta S are negative, the reaction is only spontaneous at low temperatures since the T delta S part of the equation will be overall positive. In order for a reaction to be spontaneous, then, the negative delta H needs to be of larger magnitude than the positive T delta S so that ...

Katie Lam 1B

Since the hydrogen gas is still the same substance, the entropy will not change.

Katie Lam 1B

Either delta H and delta S are 0, indicating that no reaction occurred, or delta H = T delta S and they cancel out.

Katie Lam 1B

More complex molecules can occupy more microstates so the entropy values are higher.

Katie Lam 1B

Most biological processes are irreversible. In thermodynamics, though, systems are often approximated as reversible and the assumption that the pressure inside and outside the system is the same is made.

Katie Lam 1B

Because COF2 has more microstates, its degeneracy is higher. The different atoms in COF2 can take more positions than the identical atoms in BF3 so it is at a higher entropy state.

Katie Lam 1B

It is endothermic because it takes energy to break bonds and remove electrons from the atom. Forming cations is always endothermic.

Katie Lam 1B

Reversible reactions occur when a system is in thermodynamic equilibrium with its surroundings, while this is not true for irreversible reactions.

Katie Lam 1B

Irreversible expansion is irreversible because pressure is reduced much more quickly than in a reversible reaction, where the pressure changes very slowly and maximum work is done. In irreversible processes, the system is not in thermodynamic equilibrium the entire time while in reversible processes...

Katie Lam 1B

They are equivalent only under certain conditions, in this case at constant pressure.

Katie Lam 1B

However, when no expansion work is done, the change in internal energy is equal to the change in enthalpy.

Katie Lam 1B

As the volume increases and the system expands, energy is lost as work so there is a negative sign.

Katie Lam 1B

A constant-volume calorimeter was calibrated by carrying out a reaction known to release 3.50 kJ of heat in 0.200 L of solution in the calorimeter (q =3.50 kJ), resulting in a temperature rise of 7.32 'C. In a subsequent experiment, 100.0 mL of 0.200 m HBr(aq) and 100.0 mL of 0.200 m KOH(aq) were mi...

Katie Lam 1B

Since the question noted that there was an inner diameter, you can assume that it is referring to a cylinder.

Katie Lam 1B

The equilibrium constant K changes with temperature so Kw is 10^-14 only at 25 degrees Celsius.

Katie Lam 1B

You use the molarity of H30+ and OH- to find the pH and pOH by taking the -log of each.
-log[H30+]=pH
-log[OH-]=pOH

Katie Lam 1B

Usually only the reactants have concentrations since none of the products have been formed yet (Q=0) so the reaction proceeds to the products. If the initial product concentrations are also given, it's not a bad idea to check Q to see which direction the reaction proceeds.

Katie Lam 1B

Yes, because a Bronsted acid is a proton donor so water gains a proton.

Katie Lam 1B

In Table 11.2, it lists values of K and Kc for various chemical equilibrium reactions. For top half of the table, the values of K and Kc are the same for each reaction (ex: K=4.0x10^31 and Kc=4.0x10^31), but for the second half of the table, K and Kc are different for each reaction (ex: K=3.0x10^-11...

Katie Lam 1B

They only gave the overall pressure of the flask so in order to determine the partial pressures of X2 and X, you need to figure out the percentage of the total pressure for X2 and X. This is done by counting the number of molecules of X2 and X. Out of the 17 molecules in the flask, 12 were X and 5 w...

Katie Lam 1B

The oxidation number refers to the charge of the ion. You determine the oxidation number by comparing the overall charge of the compound to the charges of anions within the compound in order to determine the charge on the transition metal cation.

Katie Lam 1B

With the help of Table 17.4, determine the coordination
number of the metal ion in each of the following complexes:
[PtCl2(en)2]2+

How do we know that en (ethylenediamine) is bidentate?

Katie Lam 1B

It is named ferrate because it is an anion within a coordination compound.

Katie Lam 1B

It could be trigonal bipyramidal with 3 lone pairs and a 180 degree bond angle.

Katie Lam 1B

Delocalization occurs when there is resonance. For example, if there are two resonance structures--one with a single bond between 2 atoms and one with a double bond between two atoms, there won't be exactly 2 or 4 electrons bonded in that area. The electrons aren't specifically associated with these...

Katie Lam 1B

It has to be drawn this way since there is a tetrahedral structure to the molecule. Each phosphorus atom is connected to all 3 of the other phosphorus atoms, but this is hard to convey using a 2D Lewis structure.

Katie Lam 1B

It would make sense for the unpaired electron to be on the atom with lower electronegativity since the atom with higher electronegativity has a stronger pull on the electrons and is more likely to fulfill its octet.

Katie Lam 1B

Sigma bonds are the first covalent bonds that atoms can make. Since it is a covalent bond, the electrons are shared. If two atoms have a single covalent bond, it's a sigma bond. When they have double bonds, there is a sigma and a pi bond, and both types of bonds share electrons between the atoms.

Katie Lam 1B

The second ionization energy is also higher than the first because there is less shielding.

Katie Lam 1B

Hypochlorous acid, HClO, is found in white blood cells, where it helps to destroy bacteria. Write two Lewis structures with different atom arrangements for HClO and select the most likely structure by identifying the structure with formal charges closest to zero. Consider only structures with single...

Katie Lam 1B

The Roman numerals refer to the charge on that atom. In this case, chromium has a +3 charge so you know how to write the formula for a neutral molecule.

Katie Lam 1B

Yes, one bond can only involve two atoms. However, one atom can have more than two bonds.

Katie Lam 1B

I went to Dr. Lavelle's office hours today and learned that De Broglie's equation cannot be applied to light or electromagnetic radiation. It can only be applied to particles with rest mass, like electrons, so you can't use the speed of light in De Broglie's equation.

Katie Lam 1B

You're allowed to use the Rydberg equation if you know how it works. Like Tess said, he showed us the other method because it is more easily explainable.

Katie Lam 1B

In 1.0 s, a 60 W bulb emits 11 J of energy in the form of infrared radiation (heat) of wavelength 1850 nm. What is the energy per photon of light emitted? How many photons of infrared radiation does the lamp generate in 1.0 s? I understand how to find the energy per photon (1.074 x 10-19 J), but how...

Katie Lam 1B

Each photon excites an electron to a higher level, so 1 million photons excites 1 million electrons (there is more than one hydrogen atom in the sample).

Katie Lam 1B

The number of moles of Cl- ions is different than the moles of CuCl2, so you use a stoichiometric ratio (2 mol Cl-/1 mol CuCl2) to get the correct moles of Cl- ions.

Katie Lam 1B

When a problem states that nitrogen is formed, can we always assume that the product is gaseous nitrogen (N2)?

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