[lokifenrir96]

Firstly, on the issue of reversible reactions in dynamic equilibrium, e.g. N2+3H2 -> 2NH3. When pressure increases, I'm confused as to what happens.
                1) Rate of forward reaction increases temporarily to shift equilibrium to right, after which the rate of forward and backward reaction return to the same, hence the yield of NH3 increases (Then what should the option be in multiple choice between rate of forward increases and stays the same, since it increases only temporarily but stays the same afterwards)
                2) But by rate constant k = Aexp(-Ea/RT), the rate of reactions should only depend on activation energy, temperature and orientation of molecules, so rate of forward and backward reaction should not be affected by pressure.
                3) But then again, by kinetic model of matter, when pressure increases, the molecules will collide with each other more frequently, so rate of both forward and backward reaction will increase.
                Some of these contradict each other, so I'm not so sure which one is correct.

Secondly, on the issue of thermal stability of metal compounds. One theory is where the metal cation distorts the electron cloud of the polyatomic anion, hence making it less thermally stable. As positive charge density of the metal cation increases, distortion of electron cloud increases and hence thermal stability decreases. However, I'd like to make a contrast to lattice enthalpy, where the lattice enthalpy is inversely proportional to the size of the cation and directly proportional to its charge, hence as charge density increases, electrostatic forces of attraction should increase, so shouldn't it require more heat energy to decompose?

Thirdly, if Ag is so large and its charge is only +1, it should have a small positive charge density and so it should not distort the electron cloud of the large anion. So, why is its compound, e.g. AgCO3, so unstable?

Fourthly, taking a voltaic cell where the electrolyte is pure water, will any current flow? A multiple choice question said zero current will flow through as water is a simple molecule, but shouldn't there be sufficient H+ and OH- ions to conduct electricity due to auto-ionisation of water?

Fifthly, considering another voltaic cell where the anode is Copper, and the electrolyte contains cations A+, then when oxidation occurs at Cu anode, Cu 2+ ions are liberated into the electrolyte. The A+ should then be reduced at the cathode to become A. But what if Copper is less reactive than A, meaning that the Cu2+ ions that were liberated are more likely to be reduced than A+ instead? Will Cu2+ be reduced instead, or do we have to consider concentrations of each cation? Assuming we do, then after a significant period of time, as more Cu2+ ions are produced and more A+ becomes A, then will Cu2+ start to be deposited as Cu instead? So the cathode will first have a layer of A, then Cu. o.0

Sixthly, in what event is NO3- reduced to NO and NO2, and in what event is it reduced to NH3 instead? I am aware that H2 can act as a reducing agent to reduce NO3- to NH3 in the test for nitrates in unknown solutions, but I was confused when I read elsewhere that it can be reduced to NO and NO2.

Seventhly, in a voltaic cell, a voltmeter is usually added as a component to the external circuit, connected to the wires, to measure the electrode potential. However, doesn't this contradict with the physics concept where voltmeters should be connected in parallel instead? If they are connected in series, wouldn't their extremely high resistance affect the results?

[fledarmus]

I'll take a shot at your first question...

Firstly, on the issue of reversible reactions in dynamic equilibrium, e.g. N2+3H2 -> 2NH3. When pressure increases, I'm confused as to what happens.
                1) Rate of forward reaction increases temporarily to shift equilibrium to right, after which the rate of forward and backward reaction return to the same, hence the yield of NH3 increases (Then what should the option be in multiple choice between rate of forward increases and stays the same, since it increases only temporarily but stays the same afterwards)

The rate of both the forward and reverse reactions increase, because increasing the pressure effectively increases the concentrations of the starting materials more than it does the concentration of the product. Concentration is effectively a measure of the number of collisions of particles that are reacting - when you increase the pressure, the molecules are closer together and collide more frequently. Since there are four moles of gases going to two moles of products, all gaseous, the concentrations of the starting materials are effectively increased more than the concentration of the products, so the forward reaction is increased more than the reverse reaction, and a new equilibrium will be established when the concentrations have adjusted to the point where the forward reaction is once again equal to the reverse reaction.

               2) But by rate constant k = Aexp(-Ea/RT), the rate of reactions should only depend on activation energy, temperature and orientation of molecules, so rate of forward and backward reaction should not be affected by pressure.

The rate constant is not affected, but the rate of the reaction depends on both the rate constant and the concentration of the reagents. The increase in pressure is affecting the concentration of the reagents.

               3) But then again, by kinetic model of matter, when pressure increases, the molecules will collide with each other more frequently, so rate of both forward and backward reaction will increase.

Exactly right. The rate of both reactions will increase, but they will increase by different amounts, and a new equilibrium will be established with concentrations that equalize the reaction rates again.              


Hope this helps

[blaisem]

Thirdly, if Ag is so large and its charge is only +1, it should have a small positive charge density and so it should not distort the electron cloud of the large anion. So, why is its compound, e.g. AgCO3, so unstable?

I can only guess at this.  But it may be because Ag2CO3 prefers to dissociate to Ag2O + CO2.  This reaction would remove charge instability.

I checked online, and I found this website that states that in the preparation of Ag2CO3, the pH must be carefully controlled to prevent silver oxide from forming.

Maybe that means something.

Source:

http://www.scribd.com/doc/30122611/Silver-Compounds

Secondly, on the issue of thermal stability of metal compounds. One theory is where the metal cation distorts the electron cloud of the polyatomic anion, hence making it less thermally stable. As positive charge density of the metal cation increases, distortion of electron cloud increases and hence thermal stability decreases. However, I'd like to make a contrast to lattice enthalpy, where the lattice enthalpy is inversely proportional to the size of the cation and directly proportional to its charge, hence as charge density increases, electrostatic forces of attraction should increase, so shouldn't it require more heat energy to decompose?

I'd like to give this one a try as well.

I think it is the lattice energy that is proportional to the charge density.  I can rewrite your quote as:

Secondly, on the issue of thermal stability of metal compounds. One theory is where the metal cation distorts the electron cloud of the polyatomic anion, hence making it less thermally stable. As positive charge density of the metal cation increases, distortion of electron cloud increases and hence thermal stability decreases. However, I'd like to make a contrast to lattice energy, where the lattice energy is inversely proportional to the size of the cation and directly proportional to its charge, hence as charge density increases, electrostatic forces of attraction should increase, so it releases more energy when it decomposes

The lattice energy = -lattice enthalpy.

Sorry my browser is acting up so I can't see what I am typing.  Wikipedia explained this:

http://en.wikipedia.org/wiki/Lattice_energy

[blaisem]

Since I can't edit my last post for some reason, I'll add more here:

Fourthly, taking a voltaic cell where the electrolyte is pure water, will any current flow? A multiple choice question said zero current will flow through as water is a simple molecule, but shouldn't there be sufficient H+ and OH- ions to conduct electricity due to auto-ionisation of water?

From everywhere I have looked, there is a current generated by water; it is just very weak, perhaps too weak to detect by a standard multimeter, depending on the conditions.

Here are several sources I used to help me.  There's definitely a lot more to it that you can read up on here:

http://www.physicsforums.com/showthread.php?t=256409


http://en.wikipedia.org/wiki/Conductivity_(electrolytic)#Weak_electrolytes

http://cn.mt.com/dam/mt_ext_files/Editorial/Generic/5/Paper-THOR-Fundamentals-Cond-Res-08-2004_Editorial-Generic_1161370044608_files/ecs_04.pdf

The last source provides a list of conductivities of water at different temperatures.  At room temperature, 0.055 S/m are cited.  This is proportional to the current you can expect as seen here:

http://en.wikipedia.org/wiki/Siemens_(unit)

[lokifenrir96]

Thanks!

Fledarmus, thank you for clarifying my doubts about the rates of both the forward and backward reactions increasing. It does make sense that since the concentrations of both reactants and products are increased, the rate of reaction will increase (though the rate constant stays the same).

However, I'd just like to make one point a tad clearer: you said that the forward reaction will increase more than the reverse reaction because there are more reactant molecules than product molecules, so the concentration of reactants increases more than that of products. However, it is not true that a new equilibrium will be established. Kp stays the same at a given temperature. It is just that because of the equation of Kp, when you double the pressure of the entire system, the partial pressure of the products increases less than that of the reactants. So, in order to maintain that ratio, the pressure of products must increase more, hence implying that more products will be produced and hence more NH3 produced. However, the equilibrium stays the same. Thanks, though, for clarifying that point!

[fledarmus]

Sorry, perhaps I wasn't clear about what I was trying to say. The equilibrium constant does not change, but the equilibrium concentrations will. That is what I meant by establishing a new equilibrium - changing the pressure effectively changes the concentrations of the reagents and products, the system is now out of equilibrium, so the concentrations of the products and reagents will change until the "new" equilibrium is established, at different concentrations now, but still in accordance with the same equilibrium constant.

[lokifenrir96]

Thank you very much! You couldn't have answered it any better

[lokifenrir96]

*Ignore me, I am impatient* for the other questions!

[Arkcon]

Maybe you can try to solve some of them on your own.  Then, the ones that still give you problems, you can ask again.  One.  Question.  Per.  Thread.  That way, each thread is a self-contained conversation on how to solve a particular problem, for other people to browse for help.  I suppose we could just rename this thread the "Help lokifenrir96 get a A with minimal work" thread, but that's not what this forum is about.