CHEMISTRY COMMUNITY

Emily Tieu 2K

I believe knowing internal energy can help understand the interactions and energy transfers in a system. One way to find the change in internal energy is through the equation delta U = q + w, so you'd have to know heat and work.

Emily Tieu 2K

A state function is not dependent on the path taken to obtain its current state. Because of this, state functions can be calculated through addition and subtraction (Final state - Initial state). Some examples of state functions are pressure, volume, temperature, heat capacity, etc. On the other han...

Emily Tieu 2K

I thought the final was fair, but personally, remote learning is difficult and is not as ideal as in-person classes. Some people learn better in different environments, but we can only make the best out of the situation. Wishing everyone a great spring break!

Emily Tieu 2K

Why are bond enthalpies always positive?

Emily Tieu 2K

Yup delta U = q when the volume is constant and we can prove that algebraically. When volume is constant, delta V = 0 since there is no change in volume and therefore w = 0. Since delta U = q + w and (w=0) we see that our statement delta U = q is true.

Emily Tieu 2K

Enthalpy is a state property, so its value is determined by its current state instead of the path taken to obtain that state. However, heat is dependent on the details of a path taken. If heat was at constant pressure, then the value would be the same as enthalpy.

Emily Tieu 2K

I'm not pre-med but I am still planning on applying for a STEM graduate degree. However, this quarter has specifically been tough and it looks like I'll barely pass the class with a C. I'm a freshman and am wondering if it's best to retake this class or continue on to 14C and get a better grade in t...

Emily Tieu 2K

Does anyone have a mnemonic device to help remember strong acids/bases?

Emily Tieu 2K

Kc specifically represents concentrations that are used to calculate the equilibrium constant K. You can calculate Kc by dividing the concentrations of products over reactants. On the other hand, K is a general term for when a reaction is at equilibrium and can be represented through concentrations ...

Emily Tieu 2K

Why do we put aqueous solutions closest to the salt bridge for shorthand notation?

Emily Tieu 2K

What determines the standard reduction potential for reactions? What does it mean when it is positive or negative?

Emily Tieu 2K

Hi, I'm having trouble solving for E. I know E naught is 0 and n should be 2 electrons. However, I'm not sure what Q is. They give us the reactant molarity (0.7M) and pressure in atm but how can we use pressure to find the product molarity?

Emily Tieu 2K

Hi, can someone help explain how to balance this reaction : Cl2O7(g)+H2O2(aq)⟶ClO−2(aq)+O2(g)

Emily Tieu 2K

Hi! Since this is my first time taking a final with Professor Lavelle, I was wondering if anyone has any tips on how to study for the final exam? Will the questions/ testing style be similar to our midterms? I heard that the 14A final was tough so any insight would be helpful!

Emily Tieu 2K

A state property is determined by its current state, meaning that only the final value is important and not the path taken to get to that value. Therefore, state properties can be added and subtracted to find their value (final state minus initial state). Examples of state properties are Entropy, Pr...

Emily Tieu 2K

What would Delta G and Delta G Naught be at equilibrium? What is the difference between the two?

Emily Tieu 2K

Yes you are correct. For a system of constant pressure, work = - P * deltaV. If deltaV were to change, then work will change, hence work is dependent on volume. If there is no change in volume then deltaV would equal 0, and by solving algebraically we would see that work would also equal 0.

Emily Tieu 2K

Would all exergonic reactions be considered a spontaneous reaction? If not, what is an example of an exergonic reaction that is not spontaneous?

Emily Tieu 2K

Hi, during lecture professor Lavelle kept mentioning different types of entropy. I'm confused on what types of entropy there are and the difference between them? I think the most important / common entropy he mentioned was positional (residual) entropy?

Emily Tieu 2K

Hi, it was mentioned in lecture that we cannot determine the outcome of whether a rxn is spontaneous/favorable solely on ∆U. We need to know both ∆U and ∆S (entropy) to know how the system will change.

Emily Tieu 2K

i'm not sure where you got that first equation for ∆U but i would assume that equation is some derivation/comes from the equations for q and w. i believe that ∆U does depend on q w and T just given that it represents internal energy. you can't change the internal energy without some type of heat or...

Emily Tieu 2K

Hi, the boiling point can be solved by setting delta G equal to 0! Then use the gibbs free energy equation to solve for the temperature of the boiling point of the compound (the specific temperature that causes phase change). Make sure to be aware of the units and convert ΔSvap into kJ and your fina...

Emily Tieu 2K

Hi, I believe you're correct about Prof Lavelle meaning net transfer of heat. I wrote in my notes that the isothermal reversible expansion process is slow (energy lost as work is replaced by heat) and that is why the system is in thermal equilibrium with its surroundings. In slow expansion, heat can...

Emily Tieu 2K

Hi, I'm sure there's plenty of reasons for studying isolated systems! While it is true that matter or heat cannot be transferred in an isolated system, it was mentioned in lecture that energy can be converted from one form to another. This idea is fundamental to the first law of thermodynamic where ...

Emily Tieu 2K

Hi, it's best to think of the second law of thermodynamics where temperature is not changing as heat enters the system. This means the system is an isothermal reversible expansion (the same system Prof Lavelle used as examples in lecture) and basically means the system expands slowly and energy lost...

Emily Tieu 2K

You are completely off track. I'm kidding. You only forgot to add one step, multiplying the enthalpy of fusion by the amount of ice that you have. Make sure to convert the grams of ice to moles (49.4/18.02) which should give you 2.72 moles of ice. So your new values should be the same except 2.72 b...

Emily Tieu 2K

Why don't we need to specify Cp or Cv for liquids and solids?

Emily Tieu 2K

Hi, if the temperature scales are different then why is it that the change in 1 C is the same as the change in 1 K?

Emily Tieu 2K

What is the difference between a reversible reaction and an irreversible reaction? What are key information to know about them when solving thermochemistry problems?

Emily Tieu 2K

"An ice cube with a mass of 49.4 g at 0.0 C is added to a glass containing 359 g of water at 45.0 C . Determine the final temperature of the system at equilibrium. The specific heat capacity of water, Cs, is 4.184 J/g⋅C, and the standard enthalpy of fusion, ΔH∘fus, of water is 6.01×103 J/mol. A...

Emily Tieu 2K

This question was asked during lecture but can someone clarify the answer to why vapor would be hotter when it touches our skin compared to boiling water? Is steam the same as water vapor?

Emily Tieu 2K

When water vapor condenses, it transitions from a gas phase to a liquid phase. This phase transition releases heat (i believe from the formation of hydrogen bonds, which releases energy) and therefore is considered an exothermic reaction. An exothermic reaction is a reaction that releases energy in ...

Emily Tieu 2K

If the phase change goes from solid to liquid, liquid to vapor, or solid to vapor, you would add the bond enthalpies because the phase change is considered endothermic (requiring heat) because of the intermolecular interactions present and because bonds require energy to break apart. Also, by taking...

Emily Tieu 2K

I might have missed it from Lavelle's lectures but are there other properties that I should know of? Professor Lavelle only mentioned state properties and non-state properties. State property values would be determined by their current state and would not depend on the path taken to obtain that sta...

Emily Tieu 2K

From Professor Lavelle's lectures, I remember he said if a reaction goes from high to low enthalpy, it would be considered an exergonic reaction and would favor reactants. However, I also have a question about heating reactions... I believe a reaction going from high to low enthalpy would release he...

Emily Tieu 2K

If you have the Ka value, you can solve for the concentration of H3O+ if that is a product of your reaction. Once you have the concentration of H3O+, we know pH = -log [H3O+] and can solve for pH. Hope this helped!

Emily Tieu 2K

Professor Lavelle mentioned a few factors that don't affect pH (such as group 1&2 on the periodic table, halogens, etc...) but what factors do alter pH?

Emily Tieu 2K

In his lectures Professor Lavelle stated Ka or Kb values smaller than 10^-4 are small enough to consider a negligible X. However, to validate the approximation it's best to check if it's less than 5% of the initial. If it's not (I think this was the case for one of the sapling problems) then we must...

Emily Tieu 2K

For the sapling homework I actually made X negligible for a few of the problems. If the given Ka or Kb value was significantly small (less than 10^-4) then I assumed it was negligible. When Ka or Kb is small, you can multiply the molarity concentration given to you with the equilibrium constant and ...

Emily Tieu 2K

A general tip is to remember the seven strong acid and bases. To add on to what others have said, if you're looking specifically at salts, split the ionic compound. For instance NaF put into water would turn into Na+ and F-. Then find the conjugate acid/base of each ion to figure out if it's weak or...

Emily Tieu 2K

If it is not one of the 7 strong acids, the acid is generally considered weak. I think it's best to memorize the list, including the strong bases.

Emily Tieu 2K

Kw is also related to pH since pH + pOH = 14 and pK values since pKa + pKb = 14.

Emily Tieu 2K

ICE Boxes are typically used to find the equilibrium constant calculated through concentrations (hence Kc). As everyone else said, you can use mol values but make sure to calculate your answers to molarity with the correct volume before calculating Kc. I personally think finding the molar concentrat...

Emily Tieu 2K

Just out of curiosity... will the time it takes for the reverse rxn to reach equilibrium be the same as the time it takes for the original rxn?

Emily Tieu 2K

For ICE Boxes, how do we know when the change in molar concentration is positive or negative when given a chemical reaction?

Emily Tieu 2K

How do we know which x-value to choose from when both are positive values after computing the quadratic formula?

Emily Tieu 2K

I believe an ideal gas is a type of thermodynamic system where gas particles occupy no volume, there are no intermolecular interactions, and all collisions are elastic. These types of gases can be used for the ideal gas law. As for inert gas, they are gases that tend to not react and are typically n...

Emily Tieu 2K

As mentioned, n/V is a simple way for us to find the concentrations for gases by manipulating the ideal gas law. In PV = nRT, we know the variable n represents moles and the variable V represents volume. Just make sure V is in Liters since the units for concentration are typically mol/L.

Emily Tieu 2K

I believe it's because solids have a fixed volume and do not dissolve when put into a solvent. Therefore, the solid concentration remains unchanged and does not affect equilibrium.

Emily Tieu 2K

The most common R values are 8.3145 J/(K*mol) and 0.08206 (L*atm)/(K*mol). Choosing which one to use depends on the given units of the other variables in the equation. For example, if the pressure (P) is given in units of bar pressure, you can convert bar pressure to atm using dimensional analysis a...

Emily Tieu 2K

^ This is also my first time taking Professor Lavelle. To clarify, is it 50 times by week 10 or 5 times every week?

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