CHEMISTRY COMMUNITY

Fiona Huang 3C

The integrated rate law is used when finding concentration at a some time during the course of the reaction. The equation is linear and arranged in y=mx+b format so that we can find concentration with respect to time.

Fiona Huang 3C

Catalysts lower the activation energy of a reverse reaction, as well as the forward reaction, because it orients the reactant molecules in a way where it can collide with enough energy to overcome the energy barrier.

Fiona Huang 3C

In an irreversible process, entropy is generated within the gas in the system, and whenever entropy is generated within the system, there is less opportunity to perform useful work. So, more work is done in a reversible process.

Fiona Huang 3C

The change in internal energy will be 0 in an isolated system. No change in volume means that work is 0.

Fiona Huang 3C

Free energy of activation is Gibbs free energy, which is the free energy difference between the transition state of a reaction and the ground state of the reactants.

Fiona Huang 3C

In isothermal, reversible reactions

Fiona Huang 3C

Do you guys think they'll give us the option to choose fully remote classes next year? If so, would anyone consider doing it? To live abroad, or save money, etc?

Fiona Huang 3C

I try to find out why I'm feeling burntout, because normally it's not just being physically tired. For me, it's typically because I'm not doing something I truly enjoy doing, and so doing any work related to it feels like a burden.

Fiona Huang 3C

The moles are found by balancing the oxi-red reactions to see how many moles of electrons are transferred

Fiona Huang 3C

Catalysts aren't considered reactants or products, so they shouldn't be shown on either side. Sometimes they are written above the arrow in the chemical equation.

Fiona Huang 3C

Adding on, studying chemical kinetics can allow us to know how to increase production of products, and control reaction conditions.

Fiona Huang 3C

A pH meter measures the acidity, or the hydrogen ion activity, in a solution. It determines the pH level, which is from 1 to 14.

Fiona Huang 3C

Adding on to what was said above, it is at a local energy maximum. Transition state has partial bonds and is highly unstable.

Fiona Huang 3C

Adding to what was said above, cathodes attract cations/positive charge.

Fiona Huang 3C

That should be the right equation! Did you include all the phases?

Fiona Huang 3C

Quick question: does it matter what species comes first in writing the cell diagrams? I know that the cathode will be on the right of the double line and the anode reaction will be on the left of the double lines, but does it matter where we put each species? For instance: Are these essentially the...

Fiona Huang 3C

Just kidding I switched the two reactions

Fiona Huang 3C

Also currently studying in Asia! There are workshops throughout the day, so there are definitely some that aren't in the middle of night for your time zone. Also lectures are almost always recorded, so my biggest issue isn't even the time zone, it's finding time to watch everything and not fall behi...

Fiona Huang 3C

Relating to this question and standard reduction potentials, conceptually, why does the half reaction with the larger standard reduction potential occur at the cathode? Larger standard reduction potential means that it has a higher tendency to get reduced (gain electrons), and electrons always flow...

Fiona Huang 3C

Hi, I can't seem to figure out what I'm doing wrong in this problem. Could anyone help? Thanks in advance :)

Fiona Huang 3C

Hey Stella, When approaching problems with a lot of numerical info like these, it helps to translate everything in the problem into a set of mathematical expressions: ΔS o vap @100 o C = 109J/Kmol C P liquid = 75.3J/Kmol C P vapor = 33.6J/Lmol ΔS o vap @85 o C = ? Based on those given values it's g...

Fiona Huang 3C

The process is irreversible and a free expansion. In a free expansion, the external pressure is 0 because the gas is expanding against no force. You also cannot calculate the change in volume because the gas is expanding freely. So, the work equation Pdv would equal 0.

Fiona Huang 3C

If the system compresses, there is a negative change in volume (piston being pushed in), so the work done by the system is positive because work is being done on the system.

Fiona Huang 3C

Why does degeneracy directly relate to entropy? Conceptually, how does this work?

Fiona Huang 3C

How can an exothermic reaction be nonspontaneous? What would be an example of this?

Fiona Huang 3C

Hi! d represents derivative (dy/dx), which is the rate of change. So for example, dS would be the same thing as ΔS, or the change in entropy.

Fiona Huang 3C

Knowing whether a system is endothermic or exothermic will only relate to heat but not the direction of work.

Fiona Huang 3C

Both equations are the same, except m is mass in grams and n is moles. If you are given specific heat capacity, it would be easier to use mass, and if you are given molar heat capacity, it would be better to use moles.

Fiona Huang 3C

Yes, when the system is expanding, work is negative because the system is doing work. Work is positive when the system is compressing because work is being done to the system.

Fiona Huang 3C

It depends on the units used, for instance if you are using the gas constant with units J/Kmol it would be better to use temperature in Kelvins to let the units cancel out.

Fiona Huang 3C

The entropy of the system can be negative, but the overall change in entropy for spontaneous process will always be positive. If the enthalpy has a large enough negative change, spontaneous reactions can have a negative change in entropy of the system, but the entropy of the surroundings will increa...

Fiona Huang 3C

The equation is derived from the first law of thermodynamics (ΔU = Q - W).
The derivation is long, but basically the equation means that the specific heat capacity of gas at constant pressure (Cp) is greater than the gas' specific heat capacity at constant volume (Cv) by R (8.314 J K-1 mol-1).

Fiona Huang 3C

If heat of combustion is always negative, when are we supposed to use the absolute value of the heat of combustion in calculations?

Fiona Huang 3C

How do we determine which systems do work on the surroundings? (Sapling week 3/4 question 13)

Fiona Huang 3C

A closed system just means that matter cannot be transferred in or out, but energy still can. For example, if a beaker is sealed, it is a closed system, but work and heat can still be transferred, so it is exchanging energy with its surroundings.

Fiona Huang 3C

The pKa tells you how much an acid will dissociate. If the value is low, more acid will dissociate, which means the acid will be stronger. If the pKa value is large, less acid will dissociate, so the acid will be weaker, and the conjugate base will be stronger.

Fiona Huang 3C

Yes, we could assume that temperature would be at standard state, which is 25 degrees C, or 298 K.

Fiona Huang 3C

SamayaJoshi1A wrote:Can someone clarify why we have these numbers set as the STP?

0 degrees Celsius and 1 atm is the freezing point of pure water at sea level atmospheric pressure. Gases are dependent on temperature and pressure, so we use these values as a reference condition.

Fiona Huang 3C

10^(-pKa) would equal the Ka value. This is important to convert back and forth between between pKa and Ka.

Fiona Huang 3C

Yes, we would assume that H2O is a liquid unless otherwise stated, and because water is the solvent, we can ignore it in the equilibrium equation.

Fiona Huang 3C

In the textbook, K is referring to Kp, so they have to specify whenever they are using Kc, which is the ratio of equilibrium concentrations of products over the equilibrium concentrations of reactants.

Fiona Huang 3C

Hi! If the pH is higher than the pKa for acids, it would want to be more acidic, so it would produce more protons (H3O+), and therefore favor the products. And if the pH is lower than the pKa, it would produce more HA and favor the reactants. For bases, I think that it would be the opposite. If pH i...

Fiona Huang 3C

Bformal is the same as the initial concentration of B. The formal concentration of B is the sum of the concentrations of B and BH+. On sapling, they calculated the concentration of B at equilibrium, not the initial concentration of B, which is why they had to add B (x) and BH+ (B-x) to get to the in...

Fiona Huang 3C

Hi, I think Bformal is the same as the initial concentration of B. The equation is still [BH+]/[Binitial]. On sapling, they calculated the concentration of B at equilibrium in the solution, not the initial concentration of B, which is why they had to add B (x) and BH+ (B-x) to get to the initial, or...

Fiona Huang 3C

Thank you guys! So is it the opposite for bases? When pH>pka, the neutral species would be predominant?

Fiona Huang 3C

Why is it that when the pH value is above the pKa, the A− will become more predominant in solution?
Do these types of questions have anything to do with the Henderson-Hasselbach Equation?

Fiona Huang 3C

The quadratic formula doesn’t directly have anything to do with the chemical equilibrium itself, it’s just a mathematical formula to figure out the value of x. The formula helps us solve a quadratic equation in the form of ax^2 + bx + c = 0. Because we set up an ICE table to find the value of x, whi...

Fiona Huang 3C

If you have an acid dissociating or when H3O+ is formed, you would use Ka. When a base is dissociating, you would use Kb. Also, Ka x Kb = Kw = 10-14.

Fiona Huang 3C

They don’t refer to the same value. The reaction quotient measures the amount of product/reactant at any given time in a reaction, while the Kc value is the concentration at equilibrium. Knowing the reaction quotient and comparing it to the equilibrium constant helps figure out what direction the re...

Fiona Huang 3C

Where does the value for the gas constant come from? I understand that the value equals 8.3144598 J/mol·K, however how was this value calculated? Avogadro's Law says that at STP (which is 1.00 atm and 0 degreesC), 1.00 mole of a gas takes up 22.4 L. If you rearrange the ideal gas law PV=nRT, you ge...

Fiona Huang 3C

The ice box is just a helpful way of keeping track of changing concentrations or pressures or each specifies in an equilibrium reaction. The I stands for initial concentration/pressure, the C stands for the change in concentration/pressure, and the E is what the value is when the system is at equili...

Fiona Huang 3C

In addition, could someone give me an example of a heterogenous equilibrium? I am confused on how we could have one or more product/reactant in "different phases" or what that really means in the context of a reaction. A common reaction that is heterogenous is ice and water at equilibrium...

Fiona Huang 3C

Thank you for the response, but as a follow up question, why are solids and liquids not affected? I am not clear on that. The molar concentration of a pure substance, such as a solid or a liquid, does not change. So, they aren’t taken into account because their concentrations don’t change throughou...

Fiona Huang 3C

K is only affected by temperature changes.

Fiona Huang 3C

A catalyst helps the reaction proceed faster by lowering the activation energy or changing the reaction mechanism. Equilibrium will be reached faster, but the concentration of reactants and products stays the same, so K won’t change.

Fiona Huang 3C

First you balance your equation, then you raise the concentration or partial pressure of the product to that coefficient.

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