CHEMISTRY COMMUNITY

Vincent Li 4L

Why is it that in #3 in the reaction mechanisms worksheet we're able to use the pre-equilibrium assumption to substitute and replace [C]? Wouldn't the approximation fail since the first and second steps are both fast, so there's no bottleneck to stop [C] form being consumed instead of forming A and ...

Vincent Li 4L

We were told that the value of A would be given, and if not, to assume that it equals 1.

Vincent Li 4L

Professor Lavelle told us that we would not have to use the direct computation method in the class, but the slides did mention that assuming you perform the steady-state approximation correctly, the final result should be equivalent to the result you get by doing the pre-equilibrium method. Your cho...

Vincent Li 4L

Because the rate law expression is Rate = k[A] (as a first order example), rate is proportional to the concentration of A (or the reactant). As the reaction proceeds, the concentration of A decreases, and as a result, the rate also decreases because there are less reactants available for reaction. H...

Vincent Li 4L

Using the balanced mock chemical reaction, you can use stoichiometry to convert from [B] to [A]. Then, using the concentration of A that you solve in this manner, you can use the integrated rate law of the first order. Hope this helps.

Vincent Li 4L

A reaction profile is just a diagram (2D plot) of energy as a function of time/progression of the reaction. I would say that it's a good idea to know how to draw one (Professor Lavelle drew a few in class today) as it lets you visualize the values of the activation energy, whether or not the reactio...

Vincent Li 4L

Yes, since reactions of higher orders are more uncommon. The textbook and our constant/formula sheet seems to only cover reactions of order 0, 1, and 2, only.

Vincent Li 4L

The dependence of k on temperature and activation energy can be seen in the Arrhenius equation, given as k = Ae^{-E_{a}/RT} . The constant A, is a parameter found through experimentation, similar to the activation energy Ea. As you can see, as the activation energy increases, k approaches 0. As k de...

Vincent Li 4L

The concentrations and orders of the reactants represent the reaction mechanism (how many molecules must collide for the reaction to proceed, which is more clear when we go into elementary reactions), whereas the rate constant k depends on the temperature and activation energy. The higher the value ...

Vincent Li 4L

[A] is assumed to represent the concentration of A at some time t, or the time that elapsed since the reaction began.

Vincent Li 4L

Can someone please explain #4 on the electrochem worksheet? I don't understand why the answer is Ag+ not Fe2+ Since you want copper to spontaneously reduce, then you need to check the oxidation values of elements/options given. In order to have a spontaneous reaction with copper reducing, the cell ...

Vincent Li 4L

Since Gibbs free energy is in terms of joules, it would be more accurate to change the units from V to J/C. You always want to make sure you cancel your units, not just out of necessity, but also because it's a very convenient way to check your work and make sure you're using the correct formula and...

Vincent Li 4L

Since you're given the value of K in this particular problem, rather than look up the Gibbs free energy of formation values, you can just substitute in the -RTlnK for greater convenience in your calculations.

Vincent Li 4L

When a chemical reaction is at equilibrium, the Gibbs free energy is, indeed, equal to zero, but that doesn't mean that the Gibbs free energy under standard conditions is (the little circle to the top right is what you're most likely missing). As you can see from the original equation, \Delta G = \D...

Vincent Li 4L

The problem would either have to specify, or there would be an obvious product being formed (H3O+ for acidic, OH- for basic).

Vincent Li 4L

When calculating S for either reaction, you will use the equation delta S = q(rev) / T. Even though the reaction in b. is not reversible, since entropy is a state function, as long as the initial and final states are the same in both reactions, you can just use that same equation. As for the change ...

Vincent Li 4L

In most cases, if you have to calculate change in entropy and change in enthalpy separately before using the Gibbs Free Equation, entropy tends to be in J/mol whereas enthalpy tends to be in kJ/mol. When calculating each of the constituent thermodynamic values, it doesn't really matter if it's in J/...

Vincent Li 4L

The idea is that the lower the energy of the substance, the more stable it is as a compound. In cases where the formation is negative from the elements to the substance, this means that energy was lost as the elements reacted to create the compound, or that the compound has lower energy and is there...

Vincent Li 4L

If this is specifically regarding entropy, that is because in reversible reactions, the total change in entropy (in the universe) is equal to zero. As a result, any change in entropy in the surroundings must be equal and opposite to the change in entropy of the system because they add up together to...

Vincent Li 4L

In general with state functions, if you're asked to find them at a temperature that's not at it's boiling point, etc., you'll always have three separate steps. The first is to calculate, in this case, the entropy required to reach the "normal" temperature at which a process occurs from the...

Vincent Li 4L

We use this equation to find the change in internal energy when we're given a case that deals with an ideal sample of gases.

Vincent Li 4L

The first reaction is intended to give you sufficient information to solve for the calorimeter constant using q = C*deltaT. Then, because the reaction parameters are similar in the subsequent chemical reaction (same amount of solution in the same calorimeter), we can use that same calorimeter consta...

Vincent Li 4L

If the work is negative, how would the isobaric work equation change? Would the integral now be of a positive quantity instead of a negative one? For an isobaric expansion/compression, the pressure is constant (isobaric: deltaP = 0), so the we would use the equation w = -P*deltaV. If the change in ...

Vincent Li 4L

Internal energy is the total store of energy within a system: this would be the sum of the kinetic energy and potential energy of every single particle (electrons, neutrons, protons, etc) within whatever reaction or space we're looking at. The two ways that internal energy can change is through heat...

Vincent Li 4L

Change in entropy can be calculated using the formula: deltaS = q / T. In this case, delta S is change in entropy, q is the heat of a reversible reaction, and T is absolute temperature. Mathematically speaking, the lower your temperature is, the greater delta S will be, thereby allowing a greater in...

Vincent Li 4L

Internal energy is the total energy that is contained in a particular system, and this would essentially be the kinetic energy plus the potential energy of all the particles involved. Now, realistically speaking, it is essentially impossible to calculate the KE and PE of every single little atom, it...

Vincent Li 4L

In the case of converting L * atm to joules, we multiply by 101.325 because 1 joule is equal to 1 kg*m^2*s^-2. This means that we have to convert liters into meters, where 1 m^3 = 1000 L, and we have to convert atm to pascals, which is 1 atm = 101,325 Pa. We convert to pascals because 1 Pa = 1 kg * ...

Vincent Li 4L

For question 6 on worksheet 4, how exactly are we supposed to determine which volume we use to find the pressure for the work component of the internal energy change? To my understanding, the pressure was calculated using PV = nRT, with V = 998L, n = 31.9 mol, R = 0.08206 L * atm / mol * K, T = 311K...

Vincent Li 4L

In this particular problem, you're given the initial conditions of the gas sample prior to any change in volume. This means you can calculate the number of moles in the gas sample, which can then be used in the reversible work equation to find the total work in both cases. PV = nRT -> n = PV/RT. Hop...

Vincent Li 4L

For this problem, think about where the heat is going. Because there is a temperature difference, energy will be transferred from the copper and into the water. Since heat released = - heat absorbed, we can use q = mC \Delta T for both substances, and we solve for T final since we should know all ot...

Vincent Li 4L

The difference really is just a matter of definition. Standard enthalpy of phase change (of formation, bond enthalpy, etc.) or any enthalpy really, is the measurement of energy required to do something to one mole of something, be it breaking a bond, forming the substance from its purest constituent...

Vincent Li 4L

Internal energy is defined as the "total store of energy in a system," which means that to measure it would require us to know the energy of every single individual particle in a system (KE/PE). This is impossible since we would have to individually calculate out these energies for every a...

Vincent Li 4L

Can someone explain how to do number 4b on worksheet 1. I do not seem to understand the steps to take. The Haber process is used to synthesize ammonia gas (NH ) from nitrogen gas ( ) 3 N2 and hydrogen gas ( H ). A system at equilibrium contains 1.85M H2 1.36M N2 and 2.91 x 10 ^−3 NH3 at a constant ...

Vincent Li 4L

Autoprotolysis is the process by which a molecule donates a hydrogen ion to an identical molecule. While we did learn about water in lecture, by this definition, it would also mean that polyprotic acids could potentially undergo autoprotolysis. For example, the hydrogen sulfate ion, or conjugate bas...

Vincent Li 4L

More generally, if delta H is positive, it means that the internal energy of the reaction has increased (endothermic reaction). If delta H is negative, the opposite is true (exothermic reaction). From there, it's simply a matter of seeing how a temperature increase can drive endothermic reaction dir...

Vincent Li 4L

15. The photosynthesis reaction, 6 CO2(g) + 6 H2O(l) ⇌ C6H12O6(aq) + 6 O2(g), is endothermic. What effect will the following changes have on the equilibrium composition. a) Water is added. b) The partial pressure of CO2 is decreased. A. a) Decrease [C6H12O6] and [O2], b) Decrease [O2] B. a) Decrease...

Vincent Li 4L

In order to solve for the hydroxide concentration of the original solution, we must work backwards from the dilution that took place. Using the equation M1 * V1 = M2 * V2, we can plug in the values of the original solution for M1 and V1, and the values for the diluted solution go into M2 and V2. The...

Vincent Li 4L

If the volume decreases, then pressure should increase due to Boyle's Law (PV = c). An increase in pressure then means that the reaction wants to return to equilibrium by minimizing this increase. Therefore, it favors the side that has less moles of gaseous molecules. Another way to think about this...

Vincent Li 4L

The other two acids aren't on the table because they're the conjugate acids of certain bases that can be found on Table 12.2. By using the relationship of Kw = Ka * Kb, you can convert between the acid constant and the base constant to determine the relative strengths.

Vincent Li 4L

In terms of the equilibrium constant, the value of K does not change unless temperature is affected. If the source of the pressure is caused by a change in volume, then the concentrations of the reactants and products change, which may cause either the forward or reverse reaction to be favored to re...

Vincent Li 4L

Water is assumed to be electrically neutral assuming it is pure, so [H3O+] = [OH-]. Given Kw, it is then just a matter of finding the square root of both sides to find the concentration of H3O+ and OH-.

Vincent Li 4L

I believe what was meant by the lecture is that anytime volume is changed, thereby changing the pressure, it's not the number of moles of gas that causes the forward or reverse reaction being favored, it's the change in concentration (which means the reaction quotient is affected). While the conclus...

Vincent Li 4L

The important distinction is that the ratio with the exponents is the equilibrium constant, or some variation of it (one of them is also the reciprocal of K). This means that these specific ratios are unchanged across different samples sizes given the temperature is constant. Because the ratio witho...

Vincent Li 4L

The equilibrium constant will remain the same for a given reaction as long as the temperature remains constant. In this particular case, the equilibrium constant expression is as follows: Kc = [O2]^3 / [O3]^2. Any alteration to the reaction (reversing the direction, multiplying the stoichiometric co...

Vincent Li 4L

In this case, you would use the quadratic equation on what Kate pointed out to be the equilibrium constant expression, Kc = [0.2 + x][x]. Since you have one variable, x, and there is no other way to properly factor it, once you expand out the Kc expression, you use the quadratic formula x = (-b ± √(...

Vincent Li 4L

An ideal gas is one that meets a few criteria (the gas particles do not attract nor repel other particles, their only interaction are in the form of elastic collision, the particles are randomly moving, and the volume of the particles are negligible). Ideal gas assumptions are fine for low pressure,...

Vincent Li 4L

There isn't any mathematical difference in calculating K, Kc, or Kp. The subscripts c and p simply indicate how you're measuring the presence of your reactants and products. Kc is the equilibrium constant calculated from molar concentrations (of the units mol * L^-1), whereas Kp is the equilibrium c...

Vincent Li 4L

The primary concept being tested in this question is the equilibrium constant, K, and what meaning the constant holds. For any given chemical reaction, there is a specific value of K that is dependent on the temperature, and since the problem states that the same temperature is maintained across bot...

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