CHEMISTRY COMMUNITY

Chem_Mod

For weak acids and bases the ICE tables are exactly the same as "regular" equilibrium problems. However, for strong acids and bases there are no ICE tables as the species dissociates 100%.

Chem_Mod

Regarding UA schedule for week 3 due to holiday. On Monday Week 3 (holiday), you may attend Ashley Van Belle, Step-up, 7-9pm in Covel 210 (switched with Lyndon for week 3 only) On Tuesday Week 3, you may attend Lyndon Bui, Workshop, 6-8pm in Covel 210 Normal Young Hall hours are in effect Sunday and...

Chem_Mod

Hi all,

Attached please find this week discussion notes.

Have a great long weekend!

Best,
Joyce

Chem_Mod

Here is the worksheet for this Sunday's session covering acid and base calculations:

Chem_Mod

Ka and Kb are used for acid and base respectively, usually dealing with the dissociation process in water. Kc is the equalibrium concentration and is used widely used in different reactions.

Chem_Mod

It would be discussed later in thermodynamics, but basically, if the enthalpy is negative, then the product is more stable than the reactant, vice versa

Chem_Mod

Kp=Kc*RT^(delta n)

Delta n is the difference of the sum of the stoichiometric coefficients between the products and the reactants (gaseous only)

Chem_Mod

A hint would be write out the expression for the K for both cases, you would see the K for b is basically the square root of a

Chem_Mod

If you want to create a new post, you would click on a sub topic located on the board index under 14B. You can then click "New Topic" in the upper left hand corner (under the subtopic name). You have to be reading a specific sub-topic to be able to create a "New Topic". If new po...

Chem_Mod

Weak bases tend to have Kb values that are less than 10^-3. Strong bases tend to have Kb values that are more than 10^-3.

Chem_Mod

We normally apply the 5% rule to acid/base types of reactions. Essentially though, the idea is to divide the change in dissociation (usually "x") by the original concentration the dissociation came from (whatever quantity you subtract x from).

Chem_Mod

About six.

Chem_Mod

There is more details in the organic section, you can take a look at the nomenclature principles.

Chem_Mod

This might be from solving the quadratic equation. Apply the equation and solve for the answer.

Chem_Mod

pH =-log[H3O+], if you plug in the 1.5, you should get about 0.176

Chem_Mod

We assume that x is much smaller compared to the number that x is being subtracted from (with in 5%). It simplifies the calculation from solving a quadratic equation.

Chem_Mod

Yes, you you can still ask question on chemistry community

Chem_Mod

Yes

Homework is due every week. The homework handed in the day of the test is the same material on the test.

As I have mentioned in class the homework is the best preparation.

Chem_Mod

You will be given the formula.

See the link "Constants and Equations" on my class website.

Chem_Mod

This depends on the question, usually gas reactions with given pressures will be a good sign.

Chem_Mod

This is a short cut for changing pressure by reducing V, reaction going toward the direction with less moles of gases will help reduce the effect of increasing the pressure. The more standard approach will be looking at the effect of concentration change

Chem_Mod

A lower pKa value indicates a stronger acid. That is, the lower value indicates the acid more fully dissociates in water.

Chem_Mod

Remember if it is neutral, the concentration of H3O+ and OH- are the same and their product is equal to the Kw

Chem_Mod

It take energy to break the chemical bond in the diatomic gas which will need energy input

Chem_Mod

Slides for week 2

Chem_Mod

For Chem 14B simplifying the cubic equation when K is small is part of the syllabus. Solving cubic equations is not part of the syllabus. Therefore examples with large K and solving cubic equations will not occur in Chem 14B. See my detailed Outlines. Below is an example simplifying a cubic equation...

Chem_Mod

Week 1 homework should definitely be equilibrium.

Week 2 homework can be either equilibrium, or acids and bases, or some of both as it is all ‘recently covered material’.

Week 3 homework should definitely be acids and bases.

Chem_Mod

See Announcements on my class website.

Chem_Mod

Strong acids and bases are defined as those which dissociate 100% in solution. Identifying an acid as strong or weak is memorization. There is a list of strong acids and bases in the textbook.

Chem_Mod

Water being added does not shift the reaction in any particular way because it is a liquid solvent that is in excess. Partial pressure and moles/concentration are all analogous, when thinking about Le Chatelier's Principle. Therefore, if the partial pressure of CO2 (a reactant) is decreased then the...

Chem_Mod

As a UA who has taken the course and has been a UA for multiple quarters, I found the notes I took during lecture along with the assigned homework problems to be the most helpful. Both of these resources can adequately prepare you for the midterm and the final. If you have questions/need extra help,...

Chem_Mod

Equilibrium constants change from temperature. Other changes, like changing concentration, will shift a reaction out of equilibrium, but the reaction will eventually return to equilibrium with the same K. Only temperature permanently alters a K. Changing pressure does not affect K, but changing conc...

Chem_Mod

We can consider heat to be a product of an exothermic reaction. Therefore raising the temperature (adding heat) adds more "product," so the reaction shifts towards reactants.

Chem_Mod

K is the standard we use to determine if a reaction is at equilibrium or determine what the concentrations should be at equilibrium. If we change the concentrations, we change Q and may cause the reaction to temporarily not be at equilibrium, but we do not change K.

Chem_Mod

There are some reactions with ∆H = 0 called thermoneutral reactions, and temperature will not alter the equilibrium concentrations. However, they are rarely encountered.

Chem_Mod

The activities of pure liquids and solids are technically included in the equilibrium constant, but their activities do not change during the reaction, so the value of 1 is accurately used for their activities (or concentrations).

Chem_Mod

If a reactant or product is written as X(aq), it is dissolved in water, and water is present, even if not directly involved in the reaction. If it is listed as X(s), do not assume there is water unless it is explicitly written as a reactant or product or another reactant or product is aqueous. The w...

Chem_Mod

Will you write out the question for those who don't have the 7th edition?

Chem_Mod

Will you write out the question for those who don't have the 7th edition?

Chem_Mod

Key for worksheet 1. Please note corrections: 1) #3 and #8, some reactants/products are in the solid phase. 2) 8c: I2 favors neither direction because it is in the solid phase, so it does not affect equilibrium, not because it is an inert gas. Inert gases are the last column of the periodic table (n...

Chem_Mod

There are several difference units that can be used to describe the pressure of a gas (bar, atm, mmHg, torr etc...). While any of them can be used to describe the pressure of a gas, when doing calculations using the ideal gas law (PV=nRT), you need to make sure to use the correct gas constant (R) wh...

Chem_Mod

There was a note on the door letting students know where to go as that room was locked and key could not be found. First day hiccup which is now fixed.

Chem_Mod

An equilibrium is said to either favor the reactants or products. However, adding a reactant or product (or changing something such as the pressure/volume when gasses are involved) SHIFT the equilibrium. The professor didn't want the word "shift" to be used when talking about which side of...

Chem_Mod

There are very very few reactions (if any at all) in which a pure solid and liquid react to yield another pure solid or liquid. If a pure solid and liquid did react, the resulting product would be dissolved to some extent in the pure liquid and thus have a concentration which would appear in the equ...

Chem_Mod

The relationship between pressure and volume is described by Boyle's Law which is PV=a constant. This shows us that pressure and volume have an inverse relationship. This means increasing the volume leads to a decrease in pressure while decreasing the volume leads to an increase in pressure. This is...

Chem_Mod

The equilibrium constant of a reaction is essentially a ratio of the rate constant of the forward reaction to the rate constant of the reverse reaction (we have not discussed this in class). These rate constants are temperature dependent. This is why the K is dependent on temperature.

Chem_Mod

Welcome back Chem 14B students! For those of you who aren't familiar with me, my name is Karen Leung, one of your UAs this quarter. I will be hosting workshops every Sunday 7-9pm this quarter, and I use worksheets I have made to facilitate my workshops. They will be posted on this SAME post here eve...

Chem_Mod

As I announced in class: Covel sessions started Wednesday Week 1.

I will follow up on this.

Chem_Mod

This is more of a physics topic but if your interested this is the general idea: To calculate the weight of the liquid we use the equation: weight (force)=mg (mass x gravity constant). The mass of the liquid can be determined using both the volume of the liquid and its density (m=Vd : where V=volume...

Chem_Mod

Changing the pressure/volume of a reaction does not change the equilibrium constant but changing the temperature ALWAYS changes the equilibrium constant. This is becasue an equilibrium is in essence, a ratio of the rate constant of the forward reaction over the rate constant of the reverse reaction....

Chem_Mod

In reality, no gas is ideal. An ideal gas is a theoretical gas which we give certain properties which allows us to simplify calculations. We can use these calculations as well as correction factors to make calculations about real gases. The ideal gas law is used to make calculations about ideal gass...

Chem_Mod

The chemical equation needs to be balanced becasue in the equilibrium equation, each species is raised to the power of its stoichiometric coefficient (which is obtained from the balanced chemical equation).

Chem_Mod

Boyle's Gas Law (which we did not yet cover) says that pressure and volume are inversely related. Therefore, increasing the volume results in a decrease of pressure while a decrease of volume results in an increase of pressure. when an inert gas is added to the system in equilibrium at constant volu...

Chem_Mod

Pure liquids and solids are not included in the equilibrium equation. This is becasue they dont really have a concentration. On another note, please be aware that this doesn't mean water isnt included in the equilibrium equation. If a reaction involves water as a gas then it IS included in the equil...

Chem_Mod

Equilibrium constants are never memorized as they are dependent on temperature (which means one would have to memorize them for every temperature). You will always be given the appropriate information needed to answer a question and should not be worried about memorizing any equilibrium constants.

Chem_Mod

Just to clarify things, increasing the pressure (which can be done by decreasing the volume) or decreasing the pressure (which can be done by increasing the volume) does not result in a change in the equilibrium constant. It DOES however change the CONCENTRATION of the gasses as concentration is def...

Chem_Mod

A quadratic equation by definition is one which can be expressed as Ax^2+Bx+C=0. The solution to these equations are values of x which when plugged into the equation make the equation shown true, and thus equal to zero.

Chem_Mod

A quadratic equation by definition is one which can be expressed as Ax^2+Bx+C=0. The solution to these equations are values of x which when plugged into the equation make the equation shown true, and thus equal to zero.

Chem_Mod

Hi,

Attached please find notes and worksheet answer for week 1.

Best,
Joyce

Chem_Mod

If a reaction is multiplied by a number X, the equilibrium constant K' of the new reaction will be K^(X)

Chem_Mod

The change of concentration for solid and liquid are so small so that they can be neglected.

Chem_Mod

The concentration of both reactants and products do not change.

Chem_Mod

If you look at all the components in the equilibrium constant K, you can find that for a and c, both of them only contain gases, for b there is no gas. However, you can also write a and c in terms of concentration.

Chem_Mod

For 1, you can use concentration in the calculation
For 2, if water is, for example, in the gas phase in the reaction

Chem_Mod

Much better to learn by discussing them.

Chem_Mod

You need to set up a I.C.E table, only one pressure was given but you can find the percent of dissociation in the picture which points to the change during the reaction.

Chem_Mod

Yes. Because, partial pressure in a gas reaction is essentially a way to represent concentration and you can use the ideal gas law to convert.

Chem_Mod

My advice will be to write out the expression for K in each step reaction and for the overall reaction. You will see why it is multiplication rather than addition.

Chem_Mod

In addition to paste the question, please specify what is your concern on this?

Chem_Mod

Samantha Hoegl Roy 2C wrote:
Chem_Mod wrote:Hi,
Could you show where you get this?

page 412 in the 7th edition, topic 5H.3

I see, you just need to know how to convert Kc and Kp if necessary, don't bother too much about remembering the long equation.

Chem_Mod

Kp is more convenient if the reaction contains gases. Depend on each question, one might be better than others, but you can write any of them if there is no instruction in the question.

Chem_Mod

Hi,
Could you show where you get this?

Chem_Mod

Slides for week 1

Chem_Mod

Fixed.

As I announced in class students can attend any office hours:
https://lavelle.chem.ucla.edu/wp-conten ... OH_ALL.pdf

Chem_Mod

You can find the discussion slides for each week below

Chem_Mod

No, equilibrium constant is signified by the letter K while reaction quotient is signified by the letter Q. K implies the reaction is at equilibrium meaning that it has reacted fully while Q is for reactions that are not at equilibrium.There is only one K, however you can solve for Q at any point wh...

Chem_Mod

We never include solids or liquids in equilibrium expressions.

Chem_Mod

Activity is essentially how a certain concentration of a solution is acting relative to its actual concentration. It comes from the fact that very concentrated solutions will either express a property more highly or less highly due to non-ideal interactions. It is a little bit more conceptual than y...

Chem_Mod

We do not go into detail on chemical activity, but it is a unitless quantity that we have replaced (by approximation) with concentration. This means for equilibrium constants, do not use units.

Chem_Mod

Take a look at the detailed answers in the homework problems on this topic to see the individual answers that differ from acid to acid.

Chem_Mod

Correct

Chem_Mod

The non-metals (C, N, S, etc.,) form oxides that produce acidic oxides such as:
H₂CO₃
HNO₃
H₂SO₄

Chem_Mod

Because the other 5 positions already have bonds.

Chem_Mod

Sometime after finals week.

Chem_Mod

Thank-You!

Many great comments!

Encourages me to keep going and assisting young minds!

Chem_Mod

It was a super active quarter!
Great you found Chemistry Community helpful!
:-)

Chem_Mod

I am not aware of a worksheet 4.
Email whoever gave you one.

Chem_Mod

Only bonds can have ionic or covalent character.

Chem_Mod

Several students left water bottles, etc., in exam rooms.
Go to your exam room to collect them.

Chem_Mod

You change the salt because water now becomes a very saturating ligand (considering the metal is now surrounded by water) so it replaces chlorine. Metal cations usually act as acids because their positive charge will lead them to form ionic compounds with hydroxide ions in solution, therefore increa...

Chem_Mod

Can you please post the full question so that everyone can have it for reference?

Chem_Mod

Let's do an example. For part a, the counterion that is a weak base is the phenolate ion (C6H5O-). This would pick up proton to become phenol in solution, so the equation would be the phenolate ion plus hydronium, yields phenol.

Chem_Mod

Can you please post the question so that everyone can easily reference it?

Chem_Mod

Trifluoroacetic acid is stronger because the negative charge that arises from deprotonation is better stabilized by the inductive effect from the three fluorines. Formic acid is the stronger acid in the second pair because hydrogen is less electron donating than a methyl group, so the negative charg...

Chem_Mod

In each case, the central metal atom can lower the pH by complexing with hydroxide ions, which increases the effective [H+], thereby lowering pH.

Chem_Mod

Can you please post the full text of the question so everyone can have it in front of them?

Chem_Mod

Sometimes nitrogens are still willing to give up protons if they have a way of stabilizing the resulting negative charge. Thymine's structure lends it the ability to balance the charge by resonance, whether the charge is positive or negative, so it is find either accepting or giving a proton.

Chem_Mod

Can you post the question please for reference?

Chem_Mod

Can you explain more about what you mean by the "quantum theory of bonding?"

Chem_Mod

Please post the question too, but yes you can convert to m.

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