CHEMISTRY COMMUNITY

chloewinnett1L

Why does a positive standard cell potential correspond to K>1?

chloewinnett1L

The solutions manual says that the proper redox reaction for the formation of water from hydronium and hydroxide ions involves the two following reactions: O2(g)+4H+(aq)+4e->2H2O(l) (cathode) and O2(g)+2H2O(l)+4e->4OH- (anode). Using these two reactions gives you an overall standard cell potential o...

chloewinnett1L

The solutions manual says that the proper redox reaction for the formation of water from hydronium and hydroxide ions involves the two following reactions: O_{2}(g)+4H^{+}(aq)+4e\rightarrow 2H_{2}O(l) (cathode) and O_{2}(g)+2H_{2}O(l)+4e\rightarrow 4OH^{-} (an...

chloewinnett1L

How can you tell if a metal (or a solid in general) is conductive? Is there a way to check on the periodic table, or are there some general rules of thumb?

chloewinnett1L

When do you include hydronium ions in your galvanic cell diagram? And, because I've heard different things from different UAs, can we combine these ions with the reduced anions rather than writing them as two separate aqueous solutions? For example, if your cathode contained H+ and NO3-, both aqueou...

chloewinnett1L

****This is an important question because I've heard two different things from two different UAs and you probably have too!**** In an entropy problem in which both temperature and volume are changing, you isolate the two entropy changes. When you calculate the entropy change for temperature, do you ...

chloewinnett1L

What exactly is S_tot? Is this a combination of the change in entropy of surroundings minus the change in entropy of the system? Furthermore, how can you use S_tot to determine spontaneity?

chloewinnett1L

Still have this question. Is it possible to use R instead of C when calculating the entropy of reversible isobaric compression due to change in temperature, as the solution manual says?

chloewinnett1L

What is k prime and how does it differ from k? I've seen k' used in multiple contexts- for example, used as the rate constant for the rate law of a reverse reaction and used as the rate constant for the rate law of an isolated concentration in a termolecular reaction.

chloewinnett1L

How do we determine which step in a multistep reaction is the slow step? Or will that always be given?

chloewinnett1L

Does anyone have a good way to remember what the units of k are for zero/1st/2nd order reactions?

chloewinnett1L

Is the "unique rate" of a reaction the same thing as k? If not, what is it and how do we find it if it isn't given?

chloewinnett1L

What does the graph of a second order reaction tell us, and how does it relate to the differential rate law and integrated rate law for a second order?

chloewinnett1L

I know Prof. Lavelle mentioned that intermediates are not products, but what exactly are they? How do we account for them when consolidating the steps of a multistep reaction?

chloewinnett1L

H20 shouldn't be included in the cell diagram! Also, if you're ever in doubt about what to put in your cell diagram, just make sure you have included each component of the half reactions. So you should have the solid, any ions involved in reduction/oxidation, and the conductor (if it is separate fro...

chloewinnett1L

The order refers to the exponent of concentrations of different species that must interact in order to form a product. For example, a zero order reaction, such as the evaporation of water or the decomposition of a single solid, does not depend on concentration and thus has an exponent of zero. If a ...

chloewinnett1L

Another example could be the evaporation of water. The rate at which liquid water becomes water vapor does not depend on a "concentration," but rather on a multitude of other things (amount of water/temperature/pressure/etc)

chloewinnett1L

This problem involves balancing the redox reaction Cl2(g)->HClO(aq)+Cl2(g). The balanced reduction half reaction reports aqueous Cl- ions as products, even though they are not included in the skeletal equation at all. Could someone please clarify why that is? Do the HClO molecules dissociate in wate...

chloewinnett1L

If, for example, Cu2+(aq) has greater oxidizing ability than H+(aq), does that mean that it has an increased ability to act as an oxidizing agent and reduce the h2 species or that it has the greater ability to BE oxidized by another reducing agent? The inverted language is getting more and more diff...

chloewinnett1L

In part D) of this problem, you are asked to write a cell diagram for the skeletal equation Au+(aq)->Au(s)+Au3+(aq). In the solutions manual, the Au+(aq) isn't included in the anode and cathode, only in the cathode. Why? I understand that it is both the oxidizing and the reducing agent, so why isn't...

chloewinnett1L

Why does a positive free energy correspond to instability? I would assume that a negative free energy and thus spontaneous decomposition/formation would correspond to instability.

chloewinnett1L

Under what conditions could a porous disk be used instead of a salt bridge? And why would this need to be porous only for anions?

chloewinnett1L

Why do we always want to know voltage as a positive value rather than a negative value?

chloewinnett1L

In general, what are the rules for charges? I know that pure elements always have a charge of zero, and oxygen is always 2-, unless if it is paired with hydrogen? Or something along those lines?

chloewinnett1L

Look at the units- specific heat capacity is typically in units of J/g*C, and molar heat capacity is typically in units of J/Kmol. This means that in order to find the actual amount of heat energy in Joules, you need to multiply SHC by temperature and mass, and you need to multiply MHC by temperatur...

chloewinnett1L

Delta U is zero when q and w are both zero. So, if there is no work done by or on the system, and there is no transfer of heat energy into or out of the system, the change in internal energy will be zero.

chloewinnett1L

W=-Pext(deltaV) is used when there is a constant external pressure outside of a system, and the system irreversibly expands into its surroundings, doing a certain amount of work. W=-nRTlnv2/v1 is used for a much more nuanced system, in which the external pressure is not constant but is instead subje...

chloewinnett1L

You end up with essentially the same answer either way, but at this point it's a matter of pride and I need validation that my way makes sense

chloewinnett1L

When calculating the heat required to melt the ice and then heat the resulting liquid water to its final temperature, why do we multiply the mass of ice and the specific heat capacity of liquid water to calculate the energy required to heat the water? Since liquid water and ice have different densit...

chloewinnett1L

Even though the problem asks you to find the final temperature and the change in enthalpy at both constant pressure and constant volume, is there even such thing as an enthalpy change when pressure is not constant? In that case, would it just be a change in q? If so, why is the problem phrased like ...

chloewinnett1L

What are some molecular or even atomic factors that influence a compound's heat capacity? For example, why does copper have a higher heat capacity than water, and why does CO2 have a higher heat capacity than CO?

chloewinnett1L

Which topics can we turn in completed problems for for weeks 6 and 7, respectively? Will you accept any, as we will have spent most of both weeks studying previous material?

chloewinnett1L

When can you ignore the second deprotonation of a polyprotic acid? In the solutions manual it says you can ignore when Ka2 is significantly less than Ka1, but how much is "<<"?

chloewinnett1L

In this question, why does decreasing the amount of aqueous glucose cause a shift in the equilibrium concentration? Even though it is a product, it is technically not a gas, and I don't understand how you can include it in a K calculation.

chloewinnett1L

What qualities/characteristics of a molecule or a substance increase entropy?

chloewinnett1L

When calculating ΔS, when should we use R and when should we use Cv and Cp, or (3/2)R and (5/2)R, respectively?

chloewinnett1L

I'm confused as to why the solutions manual doesn't indicate that you use (5/2)R instead of R when calculating the entropy change of non-isothermal irreversible expansion. Isn't the pressure kept constant? And the gas should be assumed ideal?

chloewinnett1L

I was going to ask the same thing, should we always take "ideal conditions" to mean 1 mol of gas and pressure=1 atm?

chloewinnett1L

ΔH is the change in enthalpy, or the amount of heat released or absorbed under conditions of constant pressure, and ΔH* is the standard reaction enthalpy of a reaction in which all reactants and products are in their standard states (for a gas, 1 atm/for a solution, 1 M). ΔH is measured in kJ and ΔH...

chloewinnett1L

We did a problem at a PL session where the number of states wasn't explicitly given, but we were given the chemical formula of the molecule. In this case, you would draw out every possible orientation of the molecule. Think Lewis structures/molecular geometry. For example, the molecule SO2F2 has six...

chloewinnett1L

Also, enthalpy is specifically defined as the amount of heat absorbed or released under conditions of constant pressure. q represents heat absorbed/released, so qsubp is the same thing as enthalpy. qsubv is NOT the same thing as enthalpy.

chloewinnett1L

Make sure to pay attention to the units as well as the context of the problem. For example, if a question asks you to find the work done during reversible expansion, you would use the equation W=-nRTln(v2/v1). Work is always presented in J or kJ, so take a look at the other side: n is in moles, temp...

chloewinnett1L

Reversible expansion occurs when the external pressure of a system's surroundings is very slightly decreased in a series of many, many steps. As the pressure decreases in this manner, the gas inside the system is able to slowly expand due to a difference between internal and external pressures. It i...

chloewinnett1L

Would a molecule with more atoms have a higher specific heat capacity than a molecule with fewer atoms? Or would they have the same specific heat capacity but different molar heat capacities?

chloewinnett1L

How would we go about creating a "perfect system" or at least approximating one? What variables would we need to control? I'm a bit confused as to what this actually means. Are all isolated systems perfect? Sorry this is a lot of questions in one post.

chloewinnett1L

Could someone explain why we need to include the "enthalpy of phase changes" when calculating the amount of heat in a reaction? Could you possibly give an example of how we would apply this concept?

chloewinnett1L

What is the difference between changing pressure by adding an inert gas and changing pressure by decreasing volume when you are trying to use Le Chatelier's principle to calculate reaction dynamics? Why?

chloewinnett1L

What is the molecular reasoning for why the presence of row 1 elements in acids do not affect the pH? I understand that they don't have to be accounted for when calculating, for example, the pH of a solution of nitrous acid and potassium nitrite, but why is that? Just curious.

chloewinnett1L

How do we know when an acid will continue to ionize after an initial ionization? Will every single acid with multiple hydrogen atoms ionize a second/third/etc. time, and if not, how can we tell the difference between those that will and those that will not?

chloewinnett1L

Try thinking about this on a molecular level. When a strong acid is deprotonated, the resulting conjugate base is remarkably stable. This could be due to inductive effects (shifting of negative charges around atoms with large EN differences) or resonant structures (decreased average negative charge ...

chloewinnett1L

The Ka is incredibly small (less than 10^-3), and so we don't need to use the quadratic equation because we assume the molar concentration of the original protonated molecule is going to be essentially unchanged at equilibrium. Though some of the molecule will be deprotonated, it is such a small num...

chloewinnett1L

I understand how to use the formula for Kw to find the molar concentration of hydroxide ions when given the molar concentration of hydronium ions, but I don't understand how, in the solutions manual, #19C is calculated. The 3.1 mol/L is multiplied by a factor of 10^-3, but I don't see why that would...

chloewinnett1L

Think about the math behind what K actually is: a ratio of products to reactants. Therefore, when K=1, that means that the products and the reactants have an equal concentration/partial pressure (for example, 5/5=1). When K>1, that means that the ratio is "top-heavy" and there are more pro...

chloewinnett1L

When writing an expression for the equilibrium constant, does it matter if you use partial pressure instead of concentration or vice versa? And do you use the molecule's coefficient in the chemical formula as an exponent for both partial pressure and concentration?

chloewinnett1L

Under what circumstances do you multiply two K values? I was doing problem #3 in Focus 5H and I'm not quite sure why you're supposed to multiply the K values instead of adding them.

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