CHEMISTRY COMMUNITY

alexandra

The unique rate will be the same for ONE unique reaction. Every different reaction will have a different unique rate.

alexandra

Delta H equals q at constant pressure

alexandra

It's just given. The textbook gives a bunch of components that go into solving for it but we don't need to know that for class

alexandra

Divide both sides by -0.0257V x 2.303 to get pH= 0.06V/ (2.303)(0.025693V) = 1.0. This is because -log[H+] equals pH, and dividing by this amount will leave us with -log[H+] = 1. Hope this helps!

alexandra

So entropy approaches zero as the temperature approaches 0 kelvin. I had thought the answer was zero but apparently theres some residual entropy that arises from the fact that the molecule is not a perfect crystal and there is some movement within the molecule. Hope this helps!

alexandra

You would do the exact same derivation process that we did in class for the lower orders.

alexandra

the catalyst will increase the rate of both the forward and reverse reaction

alexandra

There is no shift, the reaction will just reach equilibrium faster.

alexandra

This is just the same as saying that the q of system = - q of surroundings. the system is the reaction and the surroundings is the calorimeter.

alexandra

it stands for potential of hydrogen!

alexandra

the catalyst is present at the begining and end. the intermediate is produced in the reaction and then consumed

alexandra

You're gonna use the equation k = ae^-ea/rt. write this equation twice (one for cat, one for uncat). Plug in all of the data given. then, divide the two equations (with cat on top). this will give you the answer.

alexandra

Use the pre equilibrium approach to find the rate law if slow step is not first step because you can’t have an intermediate in the rate law

alexandra

Write rate law of slow step and set it equal to 1/r times d[R]/dt for rate of product formation Write K = [P]/[R] for step before slow steps and rearrange it so its [intermediate] = K [R] Plug K[R] in for intermediate in d[R]/dt = rk [][] Substitute k/k prime in for K Replace the whole mess of k’s f...

alexandra

the catalyst is present at the begining and end

alexandra

You look at the reduction potentials in the back of the book. High reduction potential means the species really "wants" electrons, so it has a lot of oxidizing power (and vice versa).

alexandra

Delta G = delta g knot minus RTlnK

alexandra

You should reverse a reduction reaction when an oxidation is occuring. Reduction reactions are presented in the table in the book, so when the half reaction is actually an oxidation you should reverse the reaction to find the oxidation reaction.

alexandra

Yes, intermediates can be in the rate law of an intermediate step.

alexandra

Could someone walk me through the rest of the steps to find the answer to this problem? Thank you.

alexandra

What kinetics and electrochem equations (that are not on the equation sheet provided) should we know for the final?

alexandra

Why does the equilibrium constant K equal k/k prime instead of k prime/ k? I would've thought it would be the latter because K is [products]/[reactants].

alexandra

Would you mind explaining the answer to #13 part c?

alexandra

Do we need to memorize the electrochemical series? Or should we just use reduction potential to decide which species will reduce the other one? Is this even going to be on the final?

alexandra

Do we need to know naming of molecules? Will it be on the final?

alexandra

If you integrate a function, it means that you are looking at the area under the curve on the graph. You can see this visually by literally looking at the area under the graphed line. If you have the graphed line, take a pencil and shade everything between that line and the x axis (constrained on th...

alexandra

A) The molecule ending in Br has the higher activation energy because it takes more energy to be the predominant species (reflected in the fact that it is the predominant species at high temperatures). High temperatures means there is more energy available to overcome the high energy barrier of a hi...

alexandra

Do we need to memorize the electrochemical series?

alexandra

What is on the y axis when a graph of a first order reaction is a straight line?

alexandra

It’s because the anode is generally changing the solid into an aqueous product, producing the product. The cathode is taking the reactant from aqueous solution and changing it into solid.

alexandra

Why are only reactants included in rate laws?

alexandra

What are the units of K when the reaction is zero order?

alexandra

E is always positive for galvanic cells because E being positive means the reaction is spontaneous.

alexandra

A reaction at an order of one will have an increasing rate proportional to the increasing concentration. A reaction at an order of 2 will have an increasing rate that is the squared value of the increasing concentration. (And so forth). A reaction at an order of 0 will not have it’s rate change when...

alexandra

E knot is under standard conditions, and one of the standard conditions is that everything is at a concentration of one mol

alexandra

Galvanic has positive e cell and is spontaneous, electrolytic is nonspontaneous so it requires energy input.

alexandra

Is the standard hydrogen electrode in the anode or cathode?

alexandra

What do the commas mean in cell diagrams?

alexandra

Can someone explain 6k.5 part a in the textbook? How is O3 to O2 a reduction if the oxidation numbers don't change? Both have a charge of 0.

alexandra

Can someone explain how to solve 6k.d 3 and why you solve it that way? Thanks

alexandra

Are there any oxidation numbers that we have to know from the periodic table? Or can we just know h and o?

alexandra

How can we tell if h has an oxidation number of negative one instead of positive one?

alexandra

It’s not that the nernst equation is different at 25 degrees Celsius. We simply want to simplify the equation so we plug in a temperature. That way we get rid of a variable and can just use a constant for that given temperature. we could also plug in temperature to simplify the ln version.

alexandra

H is plus one and O is negative 2

alexandra

To find entropy change at non transition temp:
Heat the substance to Boiling point (ΔS = ClnT2/T1)
Allow it to phase change ( ΔSvap = ΔHvap/TB)
Cool the vapor back down to temperature (ΔS = C lnT2/T1)

alexandra

For q of calorimeters we do not include moles. if calculating q of anything else (including the substance inside the calorimeter) we use q = nCT (or we can use grams).

alexandra

How do I know when to use j/mol as opposed to j when finding delta G or work?

alexandra

Could someone explain how to find the enthalpy of the reaction in 4.15 of the book? The answer solution does something weird that doesn't follow our formula from class.

alexandra

When do we use delta U = 3/2 n R delta T as opposed to delta U(m) = 3/2 R delta T ? What are the uses and difference of these two equations?

alexandra

When do we use the equation delta U = 3/2 n R delta T?

alexandra

which form of the vant hoff equation do we need for the midterm?

alexandra

Why does delta U equal zero during the isothermal expansion of an ideal gas? Or in a closed system?

alexandra

A living plant can exchange both energy and matter with its environment. Examples: it can take in solar energy and also take in gasses through its stomata

alexandra

Here are the steps you should take for this problem. First find delta H, then use delta H to find delta S. You find Delta H by finding the total number of joules added after the molecules get to their boiling point. You do this by multiplying 575 J/s by 60 s/min by 5.08 minutes (60 s/min was simply ...

alexandra

When to use irreversible:
Changing T
Constant Pexternal
expands/ compresses all at once

When to use reversible:
Isothermal (constant T)
Changing P
Reversible
Max work

alexandra

Molecules can have higher residual entropy as molecular complexity increases. This is because there are more possible bond vibrations that can absorb added energy. Also, if the molecule is symmetrical it will have a lower residual entropy compared to a similar molecule. The book BRIEFLY mentioned th...

alexandra

Hi! Here are the steps: First you're going to find delta H. Then, you'll find delta S. You find Delta H by finding the total number of joules added after the molecules get to their boiling point. This is done by multiplying 575 J/s by 60 s/min by 5.08 minutes. 60 s/min was simply your conversion fac...

alexandra

The definition of heat capacity is that C = q/(delta T). This is because heat capacity is measuring the change in temperature (delta T) associated with a transfer of heat (q). When we rearrange this equation we get q = C (delta T). Then, we scale up the equation by multiplying by mols because heat c...

alexandra

It seems you haven't attached the problem! Maybe try submitting your question again.

alexandra

First you're going to find delta H, then find delta S. You find Delta H by finding the total number of joules added after the molecules get to their boiling point. You do this by multiplying 575 J/s by 60 s/min by 5.08 minutes (60 s/min was simply my conversion factor). This gives us 175260 Joules =...

alexandra

Hi! So calorimeters are special because they allow us to accurately measure the temperature change of a system in order to make deeper analyses of the system. The conditions of a system are adiabatic and closed, which is also known as isolated. So yes, it is essentially an isolated system that allow...

alexandra

work is negative when the system is doing work. Work is positive when work is being done on the system

alexandra

How do you switch from L*atm to Joules?

alexandra

Why does Delta H =q at constant presssure?

alexandra

When does delta U equal zero? The textbook says that delta U equals zero for the isothermal compression or expansion of an ideal gas. Can someone please explain? What cases could we not use delta U equals zero? What are the implications?

alexandra

If a reaction is irreversible, it means that the acid or base is very strong and thus will completely ionize. Therefore, the concentration of the acid or base will be equal to the concentration of the conjugate acid or base. We can easily find the concentration of the ion or conjugate acid/base beca...

alexandra

If the specific heat capacity is given in Celcius, then the T should be in Celcius. Same for kelvin. Either works but you have to make sure they're coordinating.

alexandra

Some different sources (achieve vs the textbook) have solved for percent ionization differently. Is percent ionization concentration of ion over initial concentration of acid or base? Or is the denominator the equilibrium concentration of the acid or base?

alexandra

Hi! What changes K? Is it only Temperature? Or can pressure change K too? If we change the pressure of the system by compressing the system, does the reaction shift or does the equilibrium concentration change?

alexandra

What are the important equations to know for this chapter?

alexandra

Question 5.33 in the book discusses that X2 <--> X is endothermic. How do we know this?

alexandra

Question 5.33 in the book discusses that X2 <--> X is endothermic. How do we know this?

alexandra

Less than 5 percent means that it's okay to approximate (ignore the x in the denominator). This is just because this value corresponds with 10^-3 and is similarly negligable.

alexandra

He said that 10^-3 is the cutoff but for numbers that are very close we should operate our best judgement. Generally I'd rather be safe so I don't approximate. It's a bit more work but it should get you to the right answer.

alexandra

A strong base will add OH- to the solution, so it will increase the pH. For instance, KOH is a strong base that will fully dissociate into K and OH, which will increase the concentration of OH and therefore decrease the pOH (pOH = -log [OH]). A smaller pOH means a higher pH.

alexandra

Bases will accept H+ or donate OH- and acids will donate H+. If you write a chemical reaction, look at what happens to the H+ or OH- concentration in water.

alexandra

Hi! So if it's a base, it's going to increase the OH- in the solution (so it'll either donate OH- or accept H+). If it's an acid, it'll donate H+ and increase the H+ concentration. You can tell if it's weak or strong in a few ways. First, weak bases/acids have low K values as they are not fully depr...

alexandra

We would use pH = -log [h+] or convert to pOH and then find [ OH-]. This concentration is the concentration at equilibrium. Put this value over the initial concentration of the acid or base. Granted, you would need to be given the concentration of the acid or base initially.

alexandra

Which factors that we discussed will affect K? I know that K remains unchanged after some changes, and sometimes K can change. Which factors will change K and which will not?

alexandra

Pressure does not affect liquids and solids so strongly. Liquids are pure substances and thus their concentrations remain the same despite change in pressure.

alexandra

There were some equations in class where we ignored the x on the bottom. When are we allowed to do this?

alexandra

For an endothermic reaction, increasing temperature will cause K to increase, and decreasing temperature will cause K to decrease. The same relationship is true for exothermic. Heating a reaction and cooling a system both have an affect on K for the reaction.

alexandra

Hi! A solid can not have a concentration as it is a pure substance!

alexandra

The conceptual reason why we have to balance the chemical reaction first is because the chemical reaction is showing how the molecules "switch off" in the reaction. For example, given the formula 2R --> 1P, balancing the formula is important because it shows that one mol of P is going to b...

alexandra

Additionally, you can calculate both Ksp and Kc using the same general formula, with the key exception that Ksp uses partial pressure and Kc uses concentration.

alexandra

Thank you so much for sharing these tips!

alexandra

We do not include solids or liquids in these calculations. This is because solids are pure substances and thus their concentration can not change. Liquids are not included because if a solute is in a liquid solvent, the solute is in such a negligible amount that the concentration is not changing sig...

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