CHEMISTRY COMMUNITY

Kayla Vo 1B

Knowing the graphs can be helpful since the slope (ln[A] vs. T graph) can tell you the value of k for each order reaction. In addition, the graph can help you identify what order the reaction is.

Kayla Vo 1B

For week 10 we only have to turn in 7 problems since it would be the last discussion. You don't have to turn anything in finals week.

Kayla Vo 1B

To solve this problem try using the van't Hoff equation and solve for K2 (make sure the temperature is in Kelvin). Using that value, you can solve for the concentration of H+ and then the pH.

Kayla Vo 1B

If you need to improve your problem solving skills, I think the step up and workshop sessions are great for that. If you are unable to make it to some of the sessions, you can also find some of the worksheets with practice problems and solutions on chemistry community. Also, some TAs post their powe...

Kayla Vo 1B

Also you can use the equation to calculate the equilibrium constant at a different temperature (T2).

Kayla Vo 1B

The topics that were discussed in lecture were the applications of electron transfer reactions, ion-selective electrodes, electrolysis, and the introduction to kinetics.

Kayla Vo 1B

Spontaneous reactions release free energy, thus the sign of ΔG must be negative. Looking at the Gibbs free energy equation, when ΔH is negative and ΔS is positive, the sign of ΔG will always be negative and therefore the reaction will be spontaneous at all temperatures.

Kayla Vo 1B

According to the course website, Test 2 covers Gibbs free energy and electrochemistry (mostly the stuff in Chapter 14.1-14.11). Nernst equation, which was taught in lecture this Monday, will not be on the test.

Kayla Vo 1B

The test will be on Gibbs free energy and electrochemistry but not including the Nernst equation, which Professor Lavelle taught in lecture this Monday.

Kayla Vo 1B

Test 2 will be on Gibbs free energy and electrochemistry, which Professor Lavelle will cover in lecture on Wednesday and Friday.

Kayla Vo 1B

According to my TA, midterms are being passed out after Wednesday's lecture (2/20) and your TA will be handing them out

Kayla Vo 1B

Midterms will be passed out after Wednesday's lecture and your TA will be passing them outside the lecture room

Kayla Vo 1B

Most conversions that we will need to know on the midterm are provided on the equation sheet. The equation sheet can be found on the course website so you can see which ones are or are not provided.

Kayla Vo 1B

If the problem gives you temperature and the system is not isothermal, deltaS=nCvln(t2/t1) is used to determine the change in entropy. To calculate the change in entropy of an ideal gas in a reversible isothermal expansion, use deltaS=nRln(V2/V1).

Kayla Vo 1B

I believe we will be going over past exam problems during lecture on Monday to prepare for the midterm.

Kayla Vo 1B

Degeneracy is the number of ways of achieving a specific energy state. In other words, there are different positions and these configurations determine the energy state. W=(number of possible positions of an atom)^(number of particles). Gases have a higher degeneracy because they occupy larger volum...

Kayla Vo 1B

NA is the number of particles per mole. For example, if the problem gives us mols to work with, we know that the degeneracy W=(number of possible positions of an atom)^(number of moles x Avogadro's number). However, if you are given particles to work with, W=(number of possible positions of an atom)...

Kayla Vo 1B

I believe by definition that the standard enthalpy of formation (Hf) represents the standard reaction's enthalpy per mole for the formation of a substance from its elements in their most stable form. Hf therefore is represented in units of kJ/mol. In addition, bond dissociation energies (aka bond en...

Kayla Vo 1B

To add on, the standard state basically is the physical state (solid, liquid, or gas) that a substance would be in under standard conditions (1.00 atm and 25 degrees Celsius). So, the standard state for carbon is solid and for water is liquid.

Kayla Vo 1B

The standard enthalpy of formation for an element in its standard state is zero. Standard state is like the physical state ( can be either solid, liquid, or gas) that a substance would be in under standard conditions (1.00 atm and 25 °C). Therefore, in standard conditions, the standard state for car...

Kayla Vo 1B

Also, a change in enthalpy means the change in heat of a system. Enthalpy is expressed usually in units such as J or kJ/mol. Heat capacity is calculated by dividing the total energy by the change in temperature. Heat capacity is expressed in J/K or J/degree Celsius.

Kayla Vo 1B

pH is a logarithmic scale (from 0 to 14) that determines how acidic or basic an aqueous solution is based on the concentration of hydronium ions.
pH= -log [H3O+}

Kayla Vo 1B

Percent ionization is equal to : concentration of weak acid ionized (H+ concentration) / initial concentration of the weak acid x 100%. For strong acids, ionization is around 100% but for weak acids, the percent ionization changes depending on the concentration. The more diluted an acid is, the grea...

Kayla Vo 1B

Essentially 100 degrees Celsius steam burns skin more than 100 degrees Celsius water because the enthalpy of steam is greater. More heat energy is needed to be supplied in order to vaporize water compared to boiling water. Therefore, when steam comes into contact with skin, the amount of heat energy...

Kayla Vo 1B

You can convert between atm and bars when dealing with partial pressures depending on the question/problem. 1atm is approximately equivalent to 1.01325 bars

Kayla Vo 1B

Solvents and solids are not included when calculating the equilibrium constant K, so they dont change the value of the equilibrium constant. If a compound is stated to be aqueous, you can assume there is water. However, you can not assume there is water if there is only a solid compound stated and n...

Kayla Vo 1B

To clarify using the formula above, delta n is calculated via (c+d) - (a+b), where a and b are the coefficients of the gaseous reactants, and c and d are the coefficients of the gaseous products.

Kayla Vo 1B

That equation from the book is used when you want to write Kp in terms of Kc. For this problem you can just use the formula given in lecture to find the equilibrium constant.

Kayla Vo 1B

In other words for part A, you are looking for the equilibrium constant of the reverse reaction. Since we know that K=41 for the forward reaction, K for the reverse reaction would be the inverse of that of the forward reaction. Thus, the equilibrium constant of the reverse reaction would be K^{-1} a...

Kayla Vo 1B

As said above, K' usually is written to refer to the equilibrium constant of the reverse reaction whereas K represents the equilibrium constant of the forward reaction. You don't have to take any derivatives, it is just a way of notation.

Kayla Vo 1B

For naming, if a ligand is polydentate or already has a prefix, then you would use bis, tris, tetrakis, etc.

Kayla Vo 1B

Going down a group has more impact on the electronegativity of an atom than going across a period. As you go down a group, there will be another valence shell added. This means that the outermost electrons feel less pull from the positive nuclear charge. Because of this, chlorine is less electronega...

Kayla Vo 1B

A Bronsted acid is a proton (H+) donor and a Bronsted base is a proton acceptor. A Lewis acid is an electron acceptor while a Lewis base is an electron donor. Because the Bronsted definitions restricts acids to be H+ donors, ions like Al 3+ (which are Lewis acids since they can accept electrons) are...

Kayla Vo 1B

The bond angles depend on the existence of lone pairs because the lone pairs repel each other and other atoms in molecules. Bond angles decrease as the number of lone pairs increase. For example, in a tetrahedral shape, the bond angle would be 109.5 given that there are 4 atoms. If one of the atoms ...

Kayla Vo 1B

In general on the periodic table, polarizability decreases across a period (due to increasing effective nuclear charge) and increases down on a group (due to atomic size increasing which allows for electron clouds to distort more easily).

Kayla Vo 1B

To determine if a molecule is polar, find the electronegativities of each atom in a molecule and denote if the molecule is slightly negative or positive. From there draw the dipole moments going from the slightly positive atom to the slightly negative atom. If the dipole moments cancel out, then the...

Kayla Vo 1B

BrO3- has a total of 26 valence electrons. In the Lewis structure, Br is placed in the center since it is lower in electronegativity. With all single bonds connecting the atoms, the formal charge of the O atoms are each -1 while the Br is 2. However, it is best to reduce the formal charges as much a...

Kayla Vo 1B

Another way to determine if a molecule is polar is to look at the dipole moments between the atoms. If the dipole moments cancel each other out, the molecule would be nonpolar. On the other hand, if the dipole moments produce a net dipole moment, the molecule is polar.

Kayla Vo 1B

I'm sure that you just have to only know the periodic trend of electronegativity (increases across a period and decreases down a group)

Kayla Vo 1B

Yes chlorine can have more than an octet because it can hold electrons in its d-orbital. Usually elements in period 3 or higher (e.g P, S, Cl, etc.) can have an expanded octet.

Kayla Vo 1B

In addition it is also good to know that a sigma bond can exist without a pi bond, but a pi bond cannot exist without a sigma bond.

Kayla Vo 1B

Another way to put it is that the molecule has 2 lone electron pairs and a Steric number of 5 (number of atoms bonded to a central atom of a molecule + the number of lone pairs attached to the central atom). Therefore, in terms of the VSEPR model it would be T-shaped.

Kayla Vo 1B

I believe bond lengths usually are given in the problem and when they are given, they are usually in the units pm (picometer) or A (angstroms).

Kayla Vo 1B

Oxygen is more electronegative than nitrogen because oxygen has a greater nuclear charge (more protons). Electronegativity in a sense measures how tightly an atoms holds its electrons, where atoms with high electronegativity are more likely to gain/share electrons rather than lose them. Nitrogen is ...

Kayla Vo 1B

Yes, usually when a diatomic molecule (e.g H2) is formed a nonpolar covalent bond (electrons are equally shared) is made. Usually this type of bond results when the atoms have similar or the same electron affinity.

Kayla Vo 1B

λ=h/p (Broglie wavelength) is used when you need to find the wavelength of a particle (e.g. an electron) with a certain speed. On the other hand, E=hc/λ is used to find the wavelength of a photon of a certain energy

Kayla Vo 1B

I believe that since it was not covered in lecture, diagonal relationships of elements will not be asked on the exam. However, it wouldn't be bad to have a general idea of the concept and knowing that elements have similar properties.

Kayla Vo 1B

In NO3- the total number of valence electrons is 24 (N has 5 valence electrons, 3 O have 18 electrons, plus 1 more electron due to the ion's charge). When doing Lewis dot structures, the atom with the lowest ionization energy is placed in the center (in this case N) and the total number of valence e...

Kayla Vo 1B

The principal quantum number, n, determines the size of the orbital (or how far the electrons extend from the nucleus) and its energy. The magnetic quantum number, ml, divides the subshells into individual orbitals. The number denotes the orientation of the orbitals (for example px, py, pz) and is c...

Kayla Vo 1B

Most constants and equations are provided during the exam so you don't have to memorize them all.

Kayla Vo 1B

I assume that for part d, a higher frequency doesn't always imply that the emitted electrons have higher kinetic energies because the frequency has to be within a certain threshold (thus the wavelength has to also be within a certain threshold) in order for electrons to even be emitted.

Kayla Vo 1B

Another way to think of it is that a greater amount of energy is required to free an electron from the inner shell rather than an electron from the outer shell. Because of this, electrons in the outer shell have a higher (potential) energy compared to those in the inner shell.

Kayla Vo 1B

In the wave model of electromagnetic radiation, intensity is proportional to the square of the amplitude. However, in the particle model, intensity is proportional to the number of photons present at an instant.

Kayla Vo 1B

Check out outline 2 on the class website and you'll see the list of concepts that should be known for the exam!

Kayla Vo 1B

During lecture it was said that black body radiation will not be discussed much during the course. However it would be good to be familiar of what it is regardless

Kayla Vo 1B

It's also good to keep in mind that in the wave model of electromagnetic radiation, intensity is proportional to the square of the amplitude. However, in the particle model, intensity is proportional to the number of photons present at an instant.

Kayla Vo 1B

Yes this equation can be used to solve problems like dilutions. Just be sure that your units always are converted to liters and mols/L (M) so that they cancel out during calculations.

Kayla Vo 1B

As said in the previous posts, the products of a complete combustion are always CO2 and H2O. However it's also good to keep in mind that during an incomplete combustion (not enough oxygen is provided for the reactant to completely burn) other products, such as C and CO (carbon monoxide), can also be...

Kayla Vo 1B

Correct. 2NaCl would mean that there are 2 formula units of NaCl. You usually first calculate the number of moles (divide the mass of the sample by the molar mass) and then multiply it by Avogadro's number (6.022 x 10^23) to calculate the number of formula units. This procedure is the same as if you...

Kayla Vo 1B

The number 705.0090 would have 7 significant figures based on the rules listed above.

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