CHEMISTRY COMMUNITY

Caroline Beecher 2H

I think that since Dr. Lavelle went over the derivations in class it is important to understand the concept but they should all be on the equation sheet so I wouldn't worry about memorizing! It is more important to understand the differences and know which equation to use for different ordered react...

Caroline Beecher 2H

Temperature and activation energy affect the rate constant. When temperature increases, more reactants collide and therefore k increases too. On the other hand, when Ea is high, there is a larger energy barrier and therefore k is smaller.

Caroline Beecher 2H

I was also confused on this! k is the rate constant, while k' is the rate constant for the reverse reaction. On the other hand, K (capital) is the equilibrium constant, from earlier in the quarter.

Caroline Beecher 2H

I believe that coefficients determine the order of reactants in the rate law when we are dealing with elementary reactions. Elementary reactions are when chemical species react directly to form products with a single reaction step and single transition state.

Caroline Beecher 2H

I think a better mindset to have when considering a reaction is to think that thermodynamic analysis tells us whether the forward or reverse reaction is favored (spontaneous or not spontaneous). However, thermodynamics does not tell us about the pathway, which is where kinetics comes in. From kineti...

Caroline Beecher 2H

Yes, I believe so. Adsorption is when reactants sit on the surface of a catalyst versus absorption, where you can think of water soaking into a sponge (reactant breaching surface of the catalyst).

Caroline Beecher 2H

I think that all of the basics that we will need for the test will be covered on the test, but it might be necessary to use the Van't Hoff equation, which is derived from deltaGo = -RTlnK. The Van't Hoff equation is ln(K2/K1) = deltaHo/R * (1/T1 - 1/T2)

Caroline Beecher 2H

I don't think we have to have it memorized or anything, but the electrochemical series is a way elements are arranged according to their standard reaction potentials. It is more important to understand the magnitude of reaction potentials can either make a half reaction more likely to reduce or oxid...

Caroline Beecher 2H

From what I have seen the problem will always tell us what kind of solution (basic or acidic) we are dealing with. There wouldn't really be a way to know which redox process otherwise so I would just worry about being able to know how to do the problems for the different solutions.

Caroline Beecher 2H

Elizabeth Harty 1A wrote:What is the difference between delta g naught and delta g?

The units when you are talking about delta G naught would be kilojoules per mole, whereas under nonstandard conditions the units for delta G would be just kilojoules.

Caroline Beecher 2H

I was also confused on this! N in this sense is the moles (or amount) of electrons being transferred in the redox reaction. You must balance your reduction half-reactions and make sure there are the same amount of balanced electrons in each half-reaction, and this will be your N value to use in this...

Caroline Beecher 2H

bloodorangefield wrote:How do you tell the difference between when it is an anode vs when it is cathode?

An anode represents the oxidation half-reaction and a cathode represents the reduction half-reaction. The anode is usually on the left and the cathode is on the right.

Caroline Beecher 2H

Since K is unitless, I think that is part of why using both partial pressures and concentrations is okay in the Nernst Equation. The redox equations in these cases have both gases with partial pressures and solutions with different concentrations, so we want to use all components of the reaction whe...

Caroline Beecher 2H

In general, when looking at the overall redox reaction use the half reaction that will cancel what needs to be cancelled (since there are multiple half reactions for the same elements). A lot of time I think the problem will make it pretty clear which half reaction should be used based on the compon...

Caroline Beecher 2H

I also got K = 107, maybe there is a mistake in the solutions for the textbook.

Caroline Beecher 2H

Because cell potential is sensitive to [R] and [P], we can make concentration cells. Concentration cells have two half cells (like galvanic cells) with the same electrode and components but differing concentrations. In concentration cells the electron flow is caused by the difference in concentratio...

Caroline Beecher 2H

How do we find n in the Nernst Equation? It says in the book that it is a pure number and signifies molar convention, but I am confused on how this changes for different redox reactions.

Caroline Beecher 2H

It is more simple to know that the reducing agent is what is being oxidized, and the oxidizing agent is what is being reduced. A reducing agent loses an electron (oxidizes), which reduces another compound in the reaction. An oxidizing agent gains an electron (reduces), which oxidizes the other compo...

Caroline Beecher 2H

Making sure your electrons are balanced and cancel out is the most important thing. Everything else should be fine to be included in the final equation.

Caroline Beecher 2H

I think that the process is the same for both redox reactions of acidic and basic solutions. The difference will come in balancing the equation and whether you are adding H+ (acidic reactions) or OH- (basic reactions).

Caroline Beecher 2H

I don't think there is a specific list to know or memorize. I think it is just important to understand the biological examples Dr. Lavelle mentions in class and on the homework problems. Usually the problems explain the biological concept too.

Caroline Beecher 2H

When calculating delta S(total) of an irreversible system, it will equal delta S(system). Since no work is done in free expansion, w = 0. And because delta U = 0, and delta U = w + w, q also = 0. With this, we know that no heat is transferred into the surroundings, so delta S(surroundings) = 0.

Caroline Beecher 2H

Additionally, when you calculate delta S(total) for isothermal reversible expansion in this problem, the value ends up being 0 because delta S(surroundings) = -delta S(system). However, for isothermal irreversible expansion in this problem, the total entropy was equal to delta S(system), as no heat ...

Caroline Beecher 2H

Also, Dr. Lavelle mentioned in class today that we will be given those equations or the heat capacities for those specific molecules. I think we will mainly use this concept for monatomic molecules.

Caroline Beecher 2H

That is such a helpful graphic for reversible versus irreversible reactions! Since reversible reactions use infinitesimal steps along the reaction pathway, it remains very close to equilibrium and therefore can be reversed easily. This also means that reversible reaction pathways require more work. ...

Caroline Beecher 2H

Thank you, this is such a helpful summary! I didn't realize that when you divide two extensive properties it ends up being an intensive, but now that makes sense.

Caroline Beecher 2H

There is also a chart on page 266 of the 7th Edition textbook that explains the use of these ratios and shows the differences between them for atoms, linear molecules, and non-linear molecules.

Caroline Beecher 2H

Also, reversible systems use infinitesimal steps to calculate change of variables. These small steps make it possible for it to be reversible, as it will always be close to equilibrium.

Caroline Beecher 2H

State functions and properties are important for use in thermochemistry problems because their value is determined by its current state and is not dependent on the path taken to obtain that state.

Caroline Beecher 2H

The system is the part of the universe we are studying (ex: reaction, gas, closed, open, or isolated systems). The surroundings include everything else (universe) but usually we are just looking at particular surroundings.

Caroline Beecher 2H

Internal energy, U, is a state property and functions from its final and initial states. For example, if a closed system is changed by heating (+q) and compression (+w) then the equation for internal energy is: delta U = q + w. If volume is constant, delta U = q(v).

Caroline Beecher 2H

Yes, a closed system can be changed by changing temperature (heating or cooling), pressure, or volume. For example if you have mercury in a thermometer, outside temperature will cause a change in energy of the system.

Caroline Beecher 2H

What section of the book or lecture is this from? I am also confused on the reasoning behind it.

Caroline Beecher 2H

The three things Dr. Lavelle was talking about are heat capacity, molar heat capacity, and specific heat capacity (also called specific heat). Heat capacity is the heat required to raise the temperature of any object by 1 degree Celsius and is a more general term than the others. Molar heat capacity...

Caroline Beecher 2H

The rule of thumb is that when delta H is positive, it is endothermic (it takes up heat and its' surroundings become cooler) and when delta H is negative, it is exothermic (it releases heat and its' surroundings become warmer).

Caroline Beecher 2H

Since enthalpy is the amount of heat released or absorbed at a constant pressure, what are the units of measurement for this value? Is delta H always measured in Joules or are there other units as well?

Caroline Beecher 2H

Additionally, it is good to note that smaller pKa or pKb values mean that the acid or base is stronger, while smaller Ka or Kb values mean that the acid or base is weaker.

Caroline Beecher 2H

Additionally, it is good to note that smaller pKa or pKb values mean that the acid or base is stronger, while smaller Ka or Kb values mean that the acid or base is weaker.

Caroline Beecher 2H

It is also good to note that when a reaction requires heat (endothermic) while forming product, then heating will favor product formation. When a reaction gives off heat (exothermic) while forming product, then heating will favor reactant formation.

Caroline Beecher 2H

It might be good to know what polyprotic acids are for the future but I don't think they are something to worry about for this test.

Caroline Beecher 2H

What is protonation or deprotonation referring to when talking about acids and bases? How do we calculate percent protonation/deprotonation?

Caroline Beecher 2H

You can use approximation for weak acids and bases problems when the K value is less than 10^-3. This means it is small enough to not make a huge difference when calculating concentrations, so you wouldn't need to use the quadratic formula. The 5% rule refers to evaluating whether the approximation ...

Caroline Beecher 2H

Kw refers to the equilibrium constant for water and/or the ionization constant. Also as Dr. Lavelle discussed further today in class, for acids you can call it the acidity, ionization, or dissociation constant.

Caroline Beecher 2H

Yes, when the partial pressure of NH3 is decreased in this reaction, the reverse reaction will be favored. This will cause and increase in partial pressure for O2 as it is part of the reactants and will increase when the reaction shifts to the left.

Caroline Beecher 2H

Yes, when the partial pressure of NH3 is decreased in this reaction, the reverse reaction will be favored. This will cause and increase in partial pressure for O2 as it is part of the reactants and will increase when the reaction shifts to the left.

Caroline Beecher 2H

I was also curious about this question! Applying Le Chatelier's principle makes sense also for part B of this question. In part B, if the partial pressure of CO is decreased, then the partial pressure of CO2 will decrease because the reverse reaction will be favored and there will be less products.

Caroline Beecher 2H

K, being the equilibrium constant, itself does not have units but when writing in values for the concentration or pressure it is definitely good to add them in. For example it might be molar (M) for concentration or atmospheres (atm) for pressure.

Caroline Beecher 2H

I'm not 100% sure but I think stick to one form, either concentration or pressure. Since pressure can be converted to concentration I think it is safe to use that form!

Caroline Beecher 2H

Kp and Kc are measuring different relationships within a reaction, but are both equilibrium constants. This is just dependent on the nature of the reactants and the products. The K constant in both cases will have no units anyway, so there isn't much to worry about there but more about the concept i...

Caroline Beecher 2H

An example of using Q in comparison to K we did as an example in class with the reaction: N2O4(g) == 2NO2(g). Partial pressures of the two gases were given, and the goal was to see if the system was at equilibrium at this specific point in the reaction. The Qp ended up being less than the Kp, meanin...

Caroline Beecher 2H

Thanks so much for the advice! So nice to hear that these courses helped you with your career and path in medicine - and that there are so many helpful resources!

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