CHEMISTRY COMMUNITY

peteryim

Professor Lavelle will release more information on the final shortly. Please stay calm and do not worry too much about it.

peteryim

I think the method you're describing works. Basically, if you need to balance hydrogens on one side of the reaction, you add water to the side that needs hydrogens and an equal amount of hydroxide to the other side of the reaction. The hydroxide will cancel out everything in the water molecule excep...

peteryim

Activation energy is specific to a reaction. molecules will need to overcome an energy threshold to undergo the reaction. Free energy of activation refers to how much energy is available to cause a reaction to take place.

peteryim

Kinetic control v thermodynamic control refers to what process is dominating the reaction process, kinetics or thermodynamics. Kinetic control infers that the fastest reaction will be taking place, whereas thermodynamic control means the reaction that is the most energy conserving, I think.

peteryim

I think Prof. Lavelle will give more details in days to come. I wouldn't worry about it too much though.

peteryim

Professor Lavelle said that the order of the reaction indicates how many molecules are colliding in the reaction. How does a first order reaction work, since one molecule cannot collide with itself?

peteryim

The right side of the diagram will be the cathode; it will be a reduction half reaction. Take whatever ions or substances are given, and make a reduction half reaction from them. The left side will be an anode; it will be an oxidation half reaction. Take whatever ions or substances are given, and ma...

peteryim

The Nernst equation is used to find the cell potential of an electrochemical reaction that is not in equilibrium, but is in transit. Since you can plug in reaction quotient Q, you can determine the "weaker" cell potential, since the reaction has been going on for some time.

peteryim

Is there a specific problem you are asking about?

The size of the rate law will generally tell you how fast the rate is relative to other rates.

peteryim

Use the pH to find concentration of H+ ions. That equals the concentration of conjugate base. You divide this concentration by the total concentration to get the percent ionized. To get Ka, use the formula [A-][H+]/[HA].

peteryim

You can always look it up on google.

peteryim

Concentration cell is a different type of cell from the galvanic cell, since the same ion is on both side. However, the due to the concentration difference, which creates a different cell potential on each side, there is a flow of electrons, a current.

peteryim

In a cell diagram, you always put the cathode on the right side of the double lines, and the anode on the left side of the double lines. any phase change is separated by a single line. If there is no phase change, use commas.

peteryim

Use the Nernst equation to calculate cell potential in nonstandard conditions. Also, while the redox reaction is taking place.

peteryim

Remember that Q is the reaction quotient. (quo-tee-ent)

peteryim

When the reaction is reversible expansion, delta S total is zero. Thus, delta S surroundings = negative delta S system.
When reaction is irreversible free expansion, delta S total equals delta S of system, and delta S of surroundings is zero.

peteryim

Electrons are too small and fast moving to be quantified, so they cannot be directly counted.

peteryim

You can think of Gibbs free energy as the energy level of reactants/products. If you think of a reaction curve where the reactants sit at a certain G and the products sit at another G, the difference between those energies will be delta G. If the reactants G are lower than products G then it will re...

peteryim

Also, just adding onto the above point, if the solution is basic, then you can add OH- to the reactants. If acidic, you can add H+. It's really just trial and error until you make it work.

peteryim

Concentration tells you what direction the reaction will go in. If it is not going forward, then the cell will not be going forward.

peteryim

Adding on, the salt bridge consists of salts that will balance out the building charge of the anion and cation.

peteryim

Examples of conducting solids are all transition metals and some metalloids. Non conducting solids are everything that is not in those two categories.

peteryim

Just adding on, for the overall redox reaction, you usually wouldn't write the electrons, but in the reduction and oxidation half reactions you would.

peteryim

Hi Janet, I'm sorry you felt that way about the midterm. Honestly, sometimes tests are more about test taking than knowing the material. I know you probably did a lot of practice tests, but the only advice I can give is to do practice test. Finding previous midterms can be difficult, but is possible...

peteryim

at equilibrium, delta G = -RTlnK

peteryim

For the delta G equation at equilibrium, you take the natural log of K. If K is less than 1 but greater than 0 (so a fraction) then the value of the natural log will be a negative number. That's why it will be negative.

peteryim

That's my understanding of it. Max work only occurs in a theoretical scenario, as does most of thermodynamics and chemistry, because real life situations are too complicated.

peteryim

Basically, you don't have to worry about the process of determining the heat capacity. Just worry about the heat capacity itself, which usually functions as a constant.

peteryim

This is the equation for change in enthalpy at constant pressure. At constant pressure, delta H is equal to the heat.

peteryim

It depends on the units of the other measurements given.

peteryim

So the q of the copper is equal to the - q of the water. Just set them equal to each other, then plug in all the variables you know. Since you don't know the final temperature, plug that in as a variable. Since the final temperature will be the same for both materials, you use -q(cu) = q(h2o) to sol...

peteryim

Heat capacity is intensive. It does not matter what quantity is there, the heat capacity stays the same.

peteryim

You're right, integrals are used to quickly find the sum of an infinite number of sums. I don't think Lavelle has given us any problems that require us to use it yet...

peteryim

If it says the surroundings are at standard temp and pressure it would be 1atm. If it's not specified, then you probably don't need the specific pressure to solve the problem.

peteryim

State functions can be contrasted with path functions, where the pathway to the current state does matter. Compared to those, state functions do not rely on the pathway, so yes, can be added or subtracted simply.

peteryim

Degenerate describes states that have the same energy level, but different positions/configurations. For example, the three p-orbitals of a given energy level are degenerate. Calculating it is complicated because it depends on the number of particles and the types of particles in the system.

peteryim

An open beaker has constant pressure because you are assuming the only pressure acting upon it is air pressure, which will be uniform on the contents of the beaker.

peteryim

It's any quantity that is independent of how the substance was prepared.

peteryim

Pressure, volume, temperature, enthalpy.

peteryim

Also, keep in mind that although:

[Ka][Kb] = Kw and [H3O+][OH-] = Kw

[Ka] != [H3O+] and [Kb] != [OH-]

peteryim

Yes, if 2 bonds of C-H were formed, multiply by 2. Coefficients do matter, a lot. Make sure the equation is balanced.

peteryim

Enthalpies should be given. It is hard to determine them on your own. And you will need to know number of bonds broken and formed for the second method that Dr. Lavelle mentioned in class.

peteryim

Technically, since energy is conserved, the change in overall free energy would be the same. However, there are many other factors that come into play, such as entropy.

peteryim

Enthalpy of formation will depend on the substance you are working with. But yes, it is standard for each substance.

peteryim

Conjugates are also good to know for weak acids and bases; the conjugates tell you which reactant/products are related in the equation.

peteryim

In summary of the above comments, using Ka or Kb is situational. However, many times the question will give you the constant that you need to answer the question.

peteryim

The solution manual is correct for these. Make sure you are using the equation 1.0 x 10^-14 = [H3O+][OH-]

peteryim

I'm not really sure what this question is asking, but I don't think you need Ka or Kb. It looks like the questions just wants you to convert pH into concentration of [H3O+].

peteryim

As for the first part of the question, photosynthesis is not an exothermic reaction in the forward direction, because exothermic implies that the reaction is spontaneous, and the products have a lower free energy than the reactants. However, CO2 and H2O have less free energy than a large molecule li...

peteryim

To clarify further upon Anna's comment, when the forward reaction is favored, more products will be created, and the concentration of products will grow. If the reverse reaction is favored, more reactants will be created, and the concentration of reactants will increase. Q is relevant because it com...

peteryim

Le Chatelier's Principle applies fundamentally to changes in temperature, pressure, or concentration. Changing other physical parameters will matter when they affect these three factors. For example, changing the volume of the container in which the reaction takes place effects the concentration of ...

peteryim

Professor Lavelle mentioned this reaction in class. Apparently, nitrogen dioxide is the gas that gives smog its yellowish color, because as a gas it appears yellow to brown. Interesting!

peteryim

For substances that are aqueous (aq), simply plug the concentration (n/V) of that substance at equilibrium into the equation for K.

peteryim

What exactly is the difference between an equilibrium constant and a reaction quotient? The textbook said that the reaction quotient can be used for any stage of the reaction, and it accounts for free energy, but I am confused as to how exactly it does this.

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