CHEMISTRY COMMUNITY

Emma Popescu 1L

The standard E cell potential will be 0. However, current is driven by the difference in concentration of the ions, which move from a higher concentration to a lower concentration.

Emma Popescu 1L

The more positive the electrode, the more oxidizing the reactant. By looking at the electrochemical series, you can determine which reactants would be better oxidizing or reducing agents.

Emma Popescu 1L

An inert electrode (Pt) should be used if the reactants cannot act as an electrode. For example, if you have ions or gases then Pt is needed. However, if you have a solid metal, then it can be used as the electrode.

Emma Popescu 1L

Cell potentials are intensive properties. This means that they are independent of the quantity of material present. Thus, multiplying half-reactions when balancing a redox equation does not affect the value of the potential for the reaction.

Emma Popescu 1L

This learning objective relates to the homework questions 7A.1 and 7A.3. You need to look at the overall stoichiometric coefficients of the equation.

Emma Popescu 1L

In order to find the number of electrons transferred you need to find the two half reactions. Then you need to multiply the half reactions in order to have the same number of electrons transferred in each reaction, which allow the electrons to cancel in the overall reaction. This would give the over...

Emma Popescu 1L

The half reaction with the highest standard potential is the cathode. E cell needs to be positive in order for the reaction to be spontaneous.

Emma Popescu 1L

If the reactant or product is an ion, it is usually aqueous, but if it has no charge then it is in its standard state (solid or gas).

Emma Popescu 1L

There is also a typo in this question and in the solutions manual. You should find the range in pH from 0 to 14.

Emma Popescu 1L

The work done by the cell is is equal to the change in gibbs free energy. Based on the equation deltaG=-nFE, E needs to be positive so that energy is released (deltaG is negative) and thus the cell does work.

Emma Popescu 1L

For the oxidation, 3I- ->I3- +2e-. Since the charges need to balanced on both sides, 2e- need to be added (the charge of the left side is -3 and the charge of the right side is -1). Since there are 2 moles of e- transferred, n=2

Emma Popescu 1L

To find n (which are the moles of electrons in a balanced redox reaction) just look at the redox reaction and determine how many electrons are involved. It might be easier to see how many e- are involved by finding the half redox reactions.

Emma Popescu 1L

Based on the rule of logs:
0.06V=-0.0129V*In[H+]^2
0.06V=-0.0129V*2*In[H+] since log(x^2)=2log(x)
0.06V=-0.0258V*In[H+]
0.06V=-0.0258V*2.303log[H+]
pH=0.06V/(2.303)*(0.025693V)=1

Emma Popescu 1L

This is a concentration cell so Enot=0V. The Nernst equation can be used to find the cell potential.

Emma Popescu 1L

Using the appendix you find that F2+ 2e- -> 2F- gives a value of Enot of +2.87V. Then by using the equation Enot (cathode)- Enot (anode), you can find Enot cell. Then using the equation InK=nFEnot/RT you can find K. To find Ka, just find the square root of K.

Emma Popescu 1L

The reducing agent is the reactant being oxidized and the oxidizing agent is the reactant being reduced. For most of these reactions, you need to look at the change in oxidation state of the element.

Emma Popescu 1L

In a basic solution you add OH- to balance H and in an acidic solution you add H+

Emma Popescu 1L

Br2 is both the oxidizing and reducing agent since it is the only reactant. You have the balance the reduction half reaction and the oxidation half reaction and then combine both reactions.

Emma Popescu 1L

You have to first balance the Os and then balance the Hs giving 6OH- + 3H2O +Br- ---> BrO3- + 6H2O. Then you need to balance the charge by adding electrons in order to ensure the charge is equal on both sides of the equations. This gives the half equation 6OH- + 3H2O +Br- ---> BrO3- + 6H2O +6e-

Emma Popescu 1L

All free uncombined elements have an oxidation number of zero. The oxidation numbers of O2 and O3 are both 0.

Emma Popescu 1L

The internal energy is dependent on temperature and an isothermal reaction takes place at constant temperature. Thus, in any isothermal process involving only ideal gases, delta U=0. This is because the energy gained as heat (q) is equal to the amount of work done by the system (q = -w).

Emma Popescu 1L

In an isothermal process, the temperature is constant. The energy lost in the expansion of the gas is gained by incoming heat so the internal energy change is zero, since internal energy is dependent on temperature.

Emma Popescu 1L

There are more possible arrangements of atoms for larger, more complex molecules, increasing the number of possible microstates. So the value of W will be larger, thus causing a larger value of S.

Emma Popescu 1L

Also when the reaction is reversible delta S total=0 so delta S system= - delta S surroundings

Emma Popescu 1L

My TA said he will most likely curve the midterm because most people did very bad.

Emma Popescu 1L

Molar entropy is the entropy content of one mole of substance (under a standard state if it is standard molar entropy). The entropy of a substance depends on the amount of substance (extensive property) so it can be described in terms of moles.

Emma Popescu 1L

A spontaneous reaction releases energy (it is favorable), and so the sign of ΔG (which is the energy available to do work) must be negative.

Emma Popescu 1L

He hasn't taught this yet. We will learn it after the midterm.

Emma Popescu 1L

An open system allows matter and energy to be exchanged with surroundings. In this case, moles of gas (being matter) and heat (energy) can be transferred to the environment.

Emma Popescu 1L

For this question, why do we use the specific heat capacity of liquid water instead of ice?

Emma Popescu 1L

Another example of a closed system would be a pot with a lid because energy can be exchanged with the surroundings.

Emma Popescu 1L

The energy of a closed system can be changed by heating/cooling or compression/expansion. So Change in U=q + w. But if the change in volume is 0 (constant volume) then there is no expansion work.

Emma Popescu 1L

I found that the pOH was 12.4

Emma Popescu 1L

You use PV=nRT and then find the number of moles (n). They forgot to give us the molar mass of Xenon on the test but you have to multiply moles by molar mass and it will give you the mass.

Emma Popescu 1L

Based on Boyle's law Boyle's volume of a gas have an inverse relationship as long as temperature is held constant. As seen from PV=nRT, if volume increases, then pressure decreases and vice versa.

Emma Popescu 1L

Polyprotic acids can donate more than one proton. Protons are donated with the acidity constant (K) decreasing significantly with each proton lost Ka1>>ka2>>ka3. However, for most polyprotic acids (except Sulfuric acid), only the first deprotonation (Ka1) should be taken into account since all subse...

Emma Popescu 1L

H2SO4 is the only strong polyprotic acid and it is the most common for which deprotonation can be complete. The second deprotonation adds to H3O+ concentration so the overall pH of the solution is slightly lower due to first deprotonation alone. All the other polyprotic acids should be treated as we...

Emma Popescu 1L

It is a base so you need to find the pOH from the pH. Using the pOH, you can then find the concentration of OH- and from that calculate Kb.

Emma Popescu 1L

You have to look at the ratio of change for each reactant/product. Since the change is double for C (10) and A (-10) compared to B (5), that means that the reaction is 2A -> B + 2C.

Emma Popescu 1L

I also got 8.62. You have to convert pKa to pKb

Emma Popescu 1L

If the volume decreases and more moles of gas are on the left, the reaction shifts to the right. If the volume decreases and there are more moles of gas on the right, then the reaction shifts to the left. The reaction responds to minimize the effect of increasing pressure (by decreasing volume).

Emma Popescu 1L

Le Chatelier's principle states that chemical reactions adjust so as to minimize the effect of changes, which applies to temperature, pressure and concentration.

Emma Popescu 1L

The bond of X2 is broken which requires energy so it is endothermic.

Emma Popescu 1L

Adding an inert gas at constant volume will change the total pressure but not the partial pressures of the other gases in the container. The number of moles per unit volume remains the same so the equilibrium does not change

Emma Popescu 1L

Increasing the pressure by adding an inert gas at constant volume has no effect because only the partial pressures of the reactants and products are important. Adding an inert gas at constant volume will change the total pressure but not the partial pressures of the other gases in the container. The...

Emma Popescu 1L

Based on le Châtelier’s Principle, when we make changes to a reaction at equilibrium, the equilibrium will respond by trying to undo the change. For example, if the concentration of products increases, then the reaction forming reactants is favored (reverse reaction). Additionally, if the forward re...

Emma Popescu 1L

The change in solvent concentration is insignificant because it is in excess.

Emma Popescu 1L

The molar concentration of a pure substance does not change in a reaction so solids and liquids are not included in K expression.

Emma Popescu 1L

Q is the reaction quotient while K is the equilibrium constant. Q is used to determine which direction a reaction will shift to reach equilibrium. If K > Q, the reaction will proceed forward but if K < Q, the reaction will proceed in the reverse direction.

Emma Popescu 1L

Q is the reaction quotient. It can be used to determine which direction a reaction will shift to reach equilibrium based on the value of K, the equilibrium constant. If Q = K then the system is already at equilibrium.

Emma Popescu 1L

We will get it back the second week of winter quarter

Emma Popescu 1L

I don't think so. However, we should know how to compare which molecule would have a higher dissociation energy.

Emma Popescu 1L

He also said we should do acid rain problems from the textbook

Emma Popescu 1L

I was also really confused by this problem. However, this link helped: https://chemed.chem.purdue.edu/genchem/ ... metals.php

Emma Popescu 1L

Dr. Lavelle said in lecture that we should work on acid rain problems from the textbook. However, none of the homework problems refer to acid rain. I was wondering what pages in the textbook refer to acid rain and what problems is he referring to?

Emma Popescu 1L

What pages can I find information about organic vs. inorganic acids in the textbook?

Emma Popescu 1L

By drawing out the lewis structures, you can see the electron arrangement around the atom. If the atom can donate an electron pair (for example has lone pairs) then it is a lewis base. If the atom is likely to gain electrons, then it is a lewis acid.

Emma Popescu 1L

Does it say this in the textbook or did Dr. Lavelle mention it in lecture? If it is in the textbook, what page?

Emma Popescu 1L

Ammonia is a weak base. It reacts with water to produce ammonium ions and hydroxide ions.

Emma Popescu 1L

If you have the pH of a solution, you can calculate its pOH or vice versa. For example, if the pH is 2 then the pOH will be 14-2=12

Emma Popescu 1L

Intramolecular forces are the forces that hold atoms together within a molecule (covalent, ionic bonds). Intermolecular forces are forces that exist between molecules (dipole-dipole, hydrogen bonding, London dispersion interactions).

Emma Popescu 1L

The molecule with the highest molar mass (and so size and number of electrons) has the highest polarisability so the electrons in the electron cloud can easily be distorted so it has the strongest london dispersion interactions.

Emma Popescu 1L

I understand that CH2Cl2 is polar because a tetrahedral shape isn't completely symmetrical. However if a shape is symmetrical like a square planar would it be considered nonpolar? For example if it was a square planar with two Cl and two H atoms around a central atom, would it be considered non pol...

Emma Popescu 1L

A non polar molecule can have polar bonds. If the VSEPR shape is symmetrical, the dipoles can cancel out making the molecule non-polar.

Emma Popescu 1L

Victoria Otuya 4F wrote:Does a non-polar molecule mean it has a Dipole-Dipole intermolecular force?

A polar molecule has dipole-dipole interactions. A non-polar molecule has London dispersion interactions (induced-dipole induced-dipole)

Emma Popescu 1L

Bond length is determined by the strength of the bond so the stronger the bond, the shorter the length. Single bond is the longest, then double bond and triple bond is the shortest. However, the size of the atom can determine the length of the single bond.

Emma Popescu 1L

You should be able to assume the which molecule would have a higher or lower melting point based on their shape and strength of interactions/bonds.

Emma Popescu 1L

If one atom is more electronegative than the other, there will be a dipole moment. Although the bonds are polar, the molecule will not if the shape is symmetrical causing the dipoles to cancel out.

Emma Popescu 1L

I think we just need to know the number of regions of electrons dedicates the shape and Lone pair>triple>double>single bonds in terms of repulsion.

Emma Popescu 1L

Hydrogen bonding is a special type of Dipole-Dipole as it is considered stronger than a regular dipole-dipole force.

Emma Popescu 1L

You do not need need to memorise any values for electronegativity. The only thing you need to know is the trend across a period and down the group in order to be able to assume which element is more/less electronegative than another.

Emma Popescu 1L

505106414 wrote:Is it okay to break the octet rule in order to minimize formal charge?

You can break the octet rule only if the element can hold more than 8 electrons (starting 3p)

Emma Popescu 1L

Dipole moments occur because of a separation of charge (caused by a difference in electronegativity). The more electronegative element has the greater share of the electrons than less electronegative element causing it to have a net negative charge. The larger the difference in electronegativity, th...

Emma Popescu 1L

Branched molecules will have a lower boiling point than straight chain molecules because the intermolecular forces are reduced.

Emma Popescu 1L

Dissociation energy is based on bond strength. The shorter the bond, the stronger it is resulting in a higher dissociation energy. For example, triple bonds have the highest dissociating energy and single bonds have the lowest.

Emma Popescu 1L

There are no exceptions: single bond (longest) > double bond > triple bond (shortest) except for resonance structures in which the electrons are delocalised across all bonds.

Emma Popescu 1L

You don't need to show the calculation for formal charge as most people directly use the shortcut, but showing work can give you some marks incase you make a calculation error.

Emma Popescu 1L

I don't think we need to know the material from Topic 1C. He did not talk about most of the information from that section.

Emma Popescu 1L

I am confused as to why we use 5 as the uncertainty in velocity. Aren't we suppose to use 10 as it is +-5m/s?

Emma Popescu 1L

A wavefunction is a mathematical function with values that vary with position. It can give the probability of finding the particle in a region proporitional to the value of the wavefunction squared in that region. Schrodinger's equation is used to calculate the wavefunction for any particle.

Emma Popescu 1L

No, Dr. Lavelle said to omit Topic 1C from the textbook.

Emma Popescu 1L

The Schrodinger equation is used to calculate both the wavefunction and the corresponding energy. There is a lot of information about it in Topic 1C in the textbook but we do not need to read that section.

Emma Popescu 1L

Yes, the work function should always be in Joules so the units can later cancel out

Emma Popescu 1L

As you go down a group, electron affinity decreases as there is more shielding which decreases the electrostatic attraction between the nucleus and electrons, resulting in less energy released when e- is added to a gas phase atom. However, as you move across the period, electron affinity increases a...

Emma Popescu 1L

This confused me as well and I am not sure about the textbook. However, in a test the electron configuration should definitely be written from lowest to highest energy level.

Emma Popescu 1L

The 4s is filled first because it is at a lower energy than 3d. However, Dr. Lavelle wants us to put them from lowest to highest energy level (3d and then 4s) because 4s is ionized first.

Emma Popescu 1L

Emma Popescu 1L

Cu and Cr are only one electron away from being in a more stable state. A completely full or half full d sub-level is more stable than a partially filled d sub-level.

Emma Popescu 1L

We won't need to calculate the value of atomic radius. If needed, the values should be given to us.

Emma Popescu 1L

3d has lower energy when 4s is filled. You should list them in order of energy level

Emma Popescu 1L

Light does have momentum however the equation m*v cannot be used in this case to calculate momentum

Emma Popescu 1L

They remove points on tests if the answer does not include the correct number of significant figures (more or less).

Emma Popescu 1L

DesireBrown3K wrote:Would we be able to go to office hours and review our test with a TA for any problems we got incorrect?

Yes, our TA said to come to office hours if we have questions about the test

Emma Popescu 1L

The net ionic equation only lists the species participating in the reaction. We should learn it later in the course so don't worry too much about it for now.

Emma Popescu 1L

The test will provide the chemical formula or equation so we don't need to know polyatomic ions for this test

Emma Popescu 1L

My TA said we will go over writing formulas later in the course and for now they will provide the formulas on the test.

Emma Popescu 1L

They include basic equations and unit conversions

Emma Popescu 1L

My TA said that they are not very strict regarding significant figures on tests

Emma Popescu 1L

Maya Pakulski 3D wrote:
Jaklin Astvatsatryan 3G wrote:
Lauren Sanchez 3D wrote:Just to make sure, the homework problems are now due during our actual discussions right?

Yes. Our TA's will be collecting them the day of your discussion.

Our answers on chemistry community will always be due Sunday night though right?

yes, the 5 posts on chemistry community are due Sunday

Emma Popescu 1L

Our TA said we shouldn't worry about that until chemistry 14B as they will usually give as the formula

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