CHEMISTRY COMMUNITY

Lucia H 2L

Are all isolated systems rigid like a bomb calorimeter (i.e. volume will never change?

Also, how does qsys = -qsurr if no heat is being exchanged?

Lucia H 2L

since bomb calorimeters have rigid walls, it is impossible for volume to change, so w = -PdV and dV = 0 so w = 0.

Lucia H 2L

The copper wire is the solid in the other one!

Lucia H 2L

The following items are obtained from the stockroom for the construction of a galvanic cell: two 250ml beakers and a salt bridge, a voltmeter with attached wires and clips, 200ml of 0.008M CrCl3 (aq), 200ml of 0.12M CuSO4(aq), a piece of copper wire, and a chrome-plated piece of metal. How would you...

Lucia H 2L

What exactly is the second law of thermodynamics in relation to delta S? I thought it was that delta s universe is always zero, but then a question on a practice final said that delta S total was positive so therefore a reaction was spontaneous. What is delta S total, if not delta S universe? Also, ...

Lucia H 2L

What is the difference was between skeletal reactions that you have to balance by adding H+ and OH- vs skeletal reactions that you can jut copy down from the standard potentials page? In theory, a half reaction is a half reaction, right?

Lucia H 2L

The rate constant of the reaction O(g) + N2(g) --> NO(g) + N(g), which takes place in the stratosphere, is 9.7 x 10^10 L mol-1 s-1 at 800 deg C. The activation energy of the reaction is 315 kJ mol-1. Determine the rate constant at 700 deg C. What is the first step here? Do you need to solve for lnA ...

Lucia H 2L

When do you multiply by a specific rate law when finding pre-equilibrium? For example, in the example we did in class: 2NO + O2 --> 2NO and observed rate = k[NO]^2[O2] step 1 (fast): NO + NO --> N2O2 , rate = k1[NO]^2 step 2 (slow) N2O2 + O2 --> 2NO2 , rate = k2[N2O2][O2] we then went on to remove t...

Lucia H 2L

Hi, can someone explain how to begin solving 15.79? In the reaction of HBr with the reactive intermediate CH3CH==CHCH2 , at low temperatures the predominant product is CH3CHBrCH==CH2, but at high temperatures, the predominant product is CH3CH==CHCH2Br. (where == is a double bond) (a) Which product i...

Lucia H 2L

The problem asks you you to look at a table and determine the order for each reactant. When the concentration of A is 10, rate = 2.0 when the concentration of A is 20, rate = 4.0 (all else equal) Since 2 to 4 could be doubling or squaring, can you tell from this much information what the order is of...

Lucia H 2L

In 20. s, the concentration of nitrogen dioxide, NO2, decreases from 450 mmol/L to 320 mmol/L in the reaction 2 NO2(g) --> 2 NO(g) + O2(g). (a) Determine the rate of reaction of NO2.

How do you know how to calculate rate of reaction if you don't know the order?

Lucia H 2L

I know that stoichiometric coefficients are only supposed to influence rate in the unique rate laws for each substance. But don't moles also affect rate? i.e. if you have NO + NO --> N2O2 Wouldn't the rate be k[NO][NO] aka k[NO]^2, aka second order? In general, how can you tell which rate law to use...

Lucia H 2L

Why does a straight line mean that the reaction is the correct order to match the graph? Is it because a straight line represents the best fit line?

Lucia H 2L

How do you find orders of the reactants?

Lucia H 2L

A unique rate is a rate of appearance/disappearance of any of the species in a reaction divided by its stoichiometric coefficient, so it should be the same for each species. I think Lavelle mentioned that he would explain this in class tomorrow!

Lucia H 2L

How does pressure affect the calculation of Q? Is the equation different than just [P]^n/[R]^n ?

Lucia H 2L

In a cell diagram, is the anode always on the left and the cathode on the right? Is this only the case for galvanic cells? For the nernst equation problems, it has us choose which one is the anode based on the absolute value of Eo for the half reaction...does this mean that sometimes the cathode is ...

Lucia H 2L

A student was given a standard Cu(s) 0 Cu (aq) half-cell and another half-cell containing an unknown metal M immersed in 1.00 m M(NO3)2(aq). When the copper was connected as the anode at 25 C, the cell potential was found to be -0.689 V. What is the reduction potential for the unknown M2 /M couple? ...

Lucia H 2L

AgBr(s) <--> Ag+(aq) + Br-(aq)

anode: AgBr(s) + e- --> Ag(s) + Br-(aq)
cathode: Ag+(aq) + e- --> Ag(s)

Where did these half reactions come from? Shouldn't the anode be losing electrons? Also, where is the AgBr in the cathode equation?

Lucia H 2L

Is there a way to view lectures if we aren't enrolled in lecture 1? That's the only one on bruincast.

Lucia H 2L

What exactly are all of the values that equal zero in an isolated system (or bomb calorimeter) vs in a reversible isothermal system?

I know that:

isolated
w = 0

reversible
deltaH = 0
deltaU = 0
w = 0?

what happens to q, and delta S, in these? /anything else important that I am forgetting?

Lucia H 2L

You have a system consisting of 0.60 moles of an ideal gas contained in a 50.0L container at 1.0 atm. You just love chemistry to a fault, so you perform a series of steps to the system. First, you perform an isobaric compression of the container to 20.0L. Then, you pressurize the system to 8.0 atm u...

Lucia H 2L

Thank you! How do you reduce the pressure from 8 atm back to 1 atm in the isothermal expansion stage?

Lucia H 2L

You have a system consisting of 0.60 moles of an ideal gas contained in a 50.0L container at 1.0 atm. You just love chemistry to a fault, so you perform a series of steps to the system. First, you perform an isobaric compression of the container to 20.0L. Then, you pressurize the system to 8.0 atm u...

Lucia H 2L

When looking at reactions and deciding whether entropy will increase or decrease from reactants to products, what are all of the contributing factors? I know that more moles, more gas relative to other states, dissolution, and complex molecules with a lot of positions will generally have more entrop...

Lucia H 2L

I know that the 2nd law of thermodynamics states that in an isolated system, entropy is always increasing.

How does this change in a non-isolated system? In normal problems, is it legal for deltaS surroundings or deltaS total to have negative values?

Lucia H 2L

A certain enzyme-catalyzed reaction in a biochemical cycle has an equilibrium constant that is 10 times the equilibrium constant of the next step in the cycle. If the standard Gibbs free energy of the rst reaction is 200. kJ mol 1, what is the standard Gibbs free energy of the second reaction? So k2...

Lucia H 2L

Can someone please show the math they did in 11.19c?

Calculate k for CaCO3(s) --> CaO(s) + CO2(g) at 25 degrees C

k = 1x10^-23 but my calculator keeps rounding to zero and I want to see if I did it right.

Thanks.

Lucia H 2L

You use the gaseous phase because it is listed as gas in the problem. Aqueous means that it's dissolved in water, so the problem would specifically mention that its an NH3 solution if it wanted you to find the enthalpy of the aqueous form.

Lucia H 2L

I think when we discuss salt bridges we use them in general terms to mean that the system continues transferring voltage through the circuit, instead of just stopping when the reactant has been fully oxidized. So I think we can assume that no matter what it's made of, it will serve to transfer enoug...

Lucia H 2L

How do you find the standard molar entropy of the reaction of NH3(l) --> NH3(g)? The tables in the book show the standard molar entropy for the gas, but nothing for the liquid. Is it assumed we can use the aqueous solution?

Lucia H 2L

We learned that in constant temperature conditions, internal energy is constant. Why is this? Can heat and work change if T is constant?

Lucia H 2L

If a reaction is irreversible, is it spontaneous? Is there any relation between these or are they calculated separately?

Lucia H 2L

Can someone explain why we keep finding derivatives and integrals when deriving work equations?

I understand that reversible isothermal expansion involves changing energy at infinitesimal increments but that is about it.

Lucia H 2L

They are listed on the formula sheet!
Ideal gas, Cp = (5/2)R
Ideal gas, Cv = (3/2)R
Both are constant because heat capacity for gases isn't dependent on temp or volume.

Lucia H 2L

I know that breaking bonds is endothermic and making bonds is exothermic, so total avg enthalpy is bonds broken - bonds formed, aka (bonds in reactants ) - (bonds in products ). However, in general, total change in enthalpy is (enthalpy of products ) - (enthalpy of reactants ). This seems opposite o...

Lucia H 2L

For questions on bond enthalpies, should we expect to have to know which bonds are broken and formed for each molecule/draw Lewis structures? I took 14A a while ago and I want to know if I should spend time reviewing that content for 14B tests.

Lucia H 2L

I know Prof. Lavelle mentioned in class we didn't need to know the isothermal expansion equation on p. 266, but there are several boxes later on that talk about gases that we haven't learned about in class (all the stuff with PV = nRT, 3/2 RT, etc.). Can we expect to learn this on Friday or is this ...

Lucia H 2L

When a problem specifies an "initial temp" they are talking about the system. 25 degrees is is the standard temperature for the surroundings, not the system.

Lucia H 2L

There are different equations for heat capacity in the cases of standard pressure vs standard volume. Does the same apply to enthalpy? Or do we always assume standard pressure for equations outside of a calorimeter?

Lucia H 2L

Follow up question - what is isothermic expansion?

Lucia H 2L

The back of the book does have standard enthalpy of formation for several compounds in multiple states. It is definitely listed for water in solid, liquid and gas forms (they just did the phase conversion for us).

Lucia H 2L

What is the difference between delta H and delta U? and q for that matter

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