CHEMISTRY COMMUNITY

Emily Oren 3C

I think the rate determining step would be the slowest of the slow steps overall.

Emily Oren 3C

Catalysts increase the k value in the rate law by lowering the activation energy of the reaction. They do not show up when writing the overall reaction and are not directly included in the rate law, only indirectly by changing the k value.

Emily Oren 3C

Adsorption is when the catalyst is a solid and the reaction takes place on its surface. This image is an example to help visualize:

Emily Oren 3C

H2O cannot be an intermediate because it is one of the reactants in the overall reaction.

Emily Oren 3C

This can be seen in the Arrhenius equation: ln(k)=-Ea/RT+ln(A) where Ea is the activation energy. The slope of the line plotted on a ln(k) vs 1/T graph is -Ea/R. This means that for higher activation energies, the slope is more steeply negative so the ln(k) value, and also the k value, changes more ...

Emily Oren 3C

We use pseudo first order reactions when we are studying a reaction with multiple reactants with concentrations that could affect the rate law. For example if the reaction in question is A+B-->P we would design an experiment to find out how the rate changes with the concentrations of each reactant. ...

Emily Oren 3C

Yes, the straight line is a best fit line to the data. When trying to determine the order of a reaction from the data, you need to plot the data and see what operation you need to do to make the best fit curve be a straight line instead of an exponential decay or other nonlinear graph. For example, ...

Emily Oren 3C

Some reactions, including concentration cells, use porous discs in the center instead of a salt bridge. This would be drawn as a single line, not a double line, in the center of the cell diagram.

Emily Oren 3C

What does it mean for there to be a fractional (for example 1.5) reaction order and how does this happen? Will we have to know/work with reactions like this on exams? Also, is having a fractional reaction order common?

Emily Oren 3C

I think you can't just do the half-life times four because the half-life is 50.5 s when [A]0=0.84 mol/L, but the half-life changes as the concentration changes, so later in the reaction it is a different value.

Emily Oren 3C

There is one in the appendix of the textbook on page A17 if you want an example.

Emily Oren 3C

With the equations we have so far (as of end of week 7) I'm pretty sure that we need to be given the concentrations in order to calculate a numerical value for n and m.

Emily Oren 3C

Concentration cells can be used to make a simple battery (using only a difference in concentration not substance) but not a very strong one

Emily Oren 3C

Can you please clarify? In what context do you mean?

Emily Oren 3C

The Nernst equation allows us to calculate E based on the standard electric potential, temperature, and concentrations of a redox reaction.

Emily Oren 3C

Yes, n is referring to the number of moles of electrons transferred in the redox reaction, and this number can be found in the balanced equation.

Emily Oren 3C

In nature, and in the lab, all redox reactions involve both oxidation and reduction. This is due to the conservation of electrons. They have to go somewhere!

Emily Oren 3C

When the oxidation number increases, it is being oxidized, and if the oxidation number decreases, it is being reduced.

Emily Oren 3C

A vial filled with an organic solvent is a closed system (if the vial has a lid) because no matter can enter of leave the vial, but energy can enter it by heating or cooling it.

Emily Oren 3C

I'm not really sure what you mean by minimum temperature (minimum temperature of what?), but delta G has to be zero at phase changes (ex. at boiling point and melting point) because this is the point at which the process goes from being spontaneous (negative delta G) to not spontaneous (positive).

Emily Oren 3C

I'm pretty sure Dr. Lavelle told us in lecture that we did not have to be able to mathematically derive the equations for the test but that being able to do so is helpful for understanding what the equations mean so that we can apply them correctly.

Emily Oren 3C

Yes, a change in temperature makes the reaction not reversible because all reversible reactions are isothermal. However, it is also possible to have an irreversible isothermal reaction.

Emily Oren 3C

According to the textbook the Boltzmann equation calculates "statistical entropy," which I think takes both positional and thermal entropy into account.

Emily Oren 3C

I think that either this equation is assuming that the substance is a perfect crystal or that it is an approximation and that the positional entropy of the substance at 0K is negligible when compared with the deltaS term.

Emily Oren 3C

My TA writes it as kJ/mole of reaction, but said it doesn't really matter if you use kJ or kJ/mol. I think just make sure to keep track of your units in your work and use that.

Emily Oren 3C

Using standard enthalpies of formation and using bond enthalpies are actually two different methods of calculating the enthalpy of the reaction. If you are given standard enthalpies of formation (SEF), do the sum of the SEF of the products minus the sum of the SEF of the reactants to get the enthalp...

Emily Oren 3C

Heat and work are not state functions because they depend on the path taken (like distance traveled), while enthalpy is a state function because it does not depend on what happens between the initial and final states (like elevation gain). For example, if there is a reaction in which there is no cha...

Emily Oren 3C

An extensive property depends on the amount of a substance there is, while an intensive property is the same no matter how much of a specific substance there is. Heat capacity depends on the amount of a substance because it takes more heat to raise the temperature of a larger sample so it is an exte...

Emily Oren 3C

Also, Dr. Lavelle said that standard reaction enthalpy is at 1 atm, but my TA said it was at 1 bar. Which is it?

Emily Oren 3C

You can only use the products-reactants method if you are given the heats of formation and this problem gives heats of combustion instead. The answer book manipulates (adds, reverses, multiplies by a constant) the given combustion equations until getting the equation: C2H2 + 2H2 --> C2H6. The same o...

Emily Oren 3C

Yes the equilibrium constant will change if the stoichiometric coefficients are halved. For example for the reaction 2A <--> 3B K1 = [B]^3 / [A]^2 which is not the same as the equilibrium constant of the reaction A <--> 3/2B K2 = [B]^3/2 / [A] because the coefficients become the exponents in the K e...

Emily Oren 3C

Can someone please explain the sig fig rules for logarithms? And why do they make sense? Why can't we just use the number of sig figs in the given numbers in the question?

Emily Oren 3C

I found a past post that might help: "Re: particular d-block metals form amphoteric oxides? Postby Chem_Mod » Sun Aug 21, 2011 10:46 am We covered metal oxides and hydroxides (e.g., NaOH) which are basic compounds. We also covered non-metal oxides (e.g., H2SO4, H3PO4, etc.,) which are acidic co...

Emily Oren 3C

Also, the Lewis definition is more general than the Bronsted definition (not all Lewis acids are Bronsted acids, but all Bronsted acids are Lewis acids)

Emily Oren 3C

[Co(NH3)4(H20)2] has an overall charge of +3 and each bromide ion has a charge of -1, so three bromide ions are needed to balance the charges in this ionic compound, hence the subscript 3.

Emily Oren 3C

The answer book says that the "arrangement of electron pairs will be trigonal bipyramidal ... the actual structure is described as a seesaw" for this problem (page 98). When classifying the shape of a molecule we do not count lone pairs, so we get seesaw, but if we did count lone pairs as ...

Emily Oren 3C

I looked online and could not find dimethylmercury being referred to as dimethylmercury(0). Where did you see it called that? But you are right that it is Hg(II).

Emily Oren 3C

This question is actually really misleading because while it looks like you can place the Cl atoms opposite each other and therefore cancel the dipole moments if you just use the Lewis Structure, if you look at the molecule's shape you can see that this doesn't work. CH2Cl2 has four regions of elect...

Emily Oren 3C

The non-central atoms in a trigonal bipyramidal molecule don’t have to be the same. For example PF3Cl2, where P is the central atom and both F and Cl surround it.

Emily Oren 3C

F is the smallest atom among F, Cl, and Br, meaning that the internuclear distance between the fluorine and carbon would be the smallest even while ignoring any effects by the relative electronegativities. This means the F-C bond is the shortest of the three.

Emily Oren 3C

Here's some example images to help you visualize the difference between trigonal planar and trigonal bipyramidal: Trigonal Planar: https://upload.wikimedia.org/wikipedia/commons/6/6c/AX3E0-3D-balls.png Trigonal Bipyramidal: https://upload.wikimedia.org/wikipedia/commons/3/3a/Trigonal-bipyramidal-3D-...

Emily Oren 3C

Also, some molecules with polar covalent bonds are overall nonpolar due to their symmetry. For example, CO2 is a nonpolar molecule even though the C to O bond is polar covalent (electronegativities: 3.5-2.5=1 which is between .5 and 1.5). This is because it is symmetrical, so the partial negative ch...

Emily Oren 3C

What if there are multiple of the atom with the lowest ionization energy (for example 4 carbon atoms)? Which one would be central and how would they be arranged? Can we predict the shape of the Lewis structures for complicated molecules?

Emily Oren 3C

To elaborate on orbitals, there are three orbitals in the p-subshell (px, py, and pz) that are determined by the ml quantum number. You can think of the orbitals as the different "orientations" that the subshell can have. The d-subshell has five orbitals. Each orbital can have two electron...

Emily Oren 3C

The cartesian axes are the x-,y-, and z-axes representing the three physical dimensions. In other words they are directions in space (forward-backward, left-right, and up-down). The orientations of the orbitals are defined along these axes in order to make the math work. There is no "universal&...

Emily Oren 3C

If an electron is in its ground state, for example 1s, and is excited, is there a difference in energy between being excited to the 2s vs 2p orbitals? We were told in class that the first quantum number is the one that determines energy but also that electrons fill the s orbitals before the p orbita...

Emily Oren 3C

Does that mean that electrons have different wavelengths? How would you find the wavelength or frequency of an electron?

Emily Oren 3C

I'm pretty sure that this is a trick question and none of the answers are correct (but I think the wording is open to the possibility of there being no correct answer) A) This isn't true because all EM radiation travels at a constant speed in a vacuum and they did not say that there was a medium B) ...

Emily Oren 3C

If the problem has both addition and multiplication, do we use the addition/subtraction rule or the multiplication/division rule for determining the number of sig figs in the final answer?

Emily Oren 3C

A formula unit is a group of ions. You can think of it as a "molecule" of an ionic compound such as NaCl (table salt). When the textbook asks for the amount of a compound in formula units it is asking for a count of formula units or "How many formula units are there in this sample.&qu...

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