CHEMISTRY COMMUNITY

Madeline Ho 1C

For #11, why is Cv used for the change in temp instead of Cp? How do we distinguish when to use what?

Madeline Ho 1C

The equation is always 1/[A] = kt+1/[A]initial. The coefficients are only taken into account when considering concentrations of reactants/products, not rate constants.

Madeline Ho 1C

Why is it that for elementary steps, the rate law can simply be found using molecularity but this does not apply to the overall balanced reaction?

Madeline Ho 1C

The E values we're given are reduction potentials, so the greater the value, the more likely it is to be reduced. Therefore, the species with higher reducing power are the species more likely to be oxidized, i.e. a smaller E value.

Madeline Ho 1C

15.9 Express the units for rate constants when the concentrations are in moles per liter and time is in seconds for (a) zero-order reactions; (b) first-order reactions; (c) second order reactions.
What does it mean for a reaction to have zero-order?

Madeline Ho 1C

The rate of formation is simply the rate of formation of a given product (or rate of consumption for a reactant). This is the value you get when you take the change in concentration over change in time, or d[R]/dt for the instantaneous rate. The unique rate of a reaction is when you take into accoun...

Madeline Ho 1C

14.101 In a neuron (a nerve cell), the concentration of K ions inside the cell is about 20–30 times as great as that outside. What potential difference between the inside and the outside of the cell would you expect to measure if the difference is due only to the imbalance of potassium ions? Does a...

Madeline Ho 1C

Shubham Rai 2C wrote:You would add Pt to the Ce4+ side of the diagram as it has no solids to conduct the charges. We use Pt when there are no solids in the half reaction.

the solutions show Pt(s) on both sides of the cell diagram though

Madeline Ho 1C

14.15 Write the half-reactions and devise a galvanic cell (write a cell diagram) to study each of the following reactions: (b) H+(aq) + OH-(aq) --> H2O(l), the Brønsted neutralization reaction answer: (b) anode: 4 OH-(aq) --> O2(g) + 2H2O(l) + 4 e-; cathode: O2(g) + 4H+(aq) + 4e- --> 2H2O(...

Madeline Ho 1C

Karyn Nguyen 1K wrote:Don't we add water to basic solutions too?

Yes, we add water to basic solutions to balance out the oxygen as well. The difference with basic solutions is that you add the same amount of OH- as there are H+ to both sides of the reaction.

Madeline Ho 1C

How do we know to add Pt(s) to the cell diagram?
example: 14.13 Write the half-reactions, the balanced equation for the cell reaction, and the cell diagram for each of the following skeletal equations:
(b) Ce4+(aq) + I-(aq) --> I2(s) + Ce3+(aq)

Madeline Ho 1C

9.83 Hydrogen burns in an atmosphere of bromine gas to give hydrogen bromide gas. (a) What is the standard Gibbs free energy of the reaction H2(g) Br2(g) S 2 HBr(g) at 298 K? (b) If 120. mL of H2 gas at STP combines with a stoichiometric amount of bromine and the resulting hydrogen bromide dissolv...

Madeline Ho 1C

The calculations for deltaH consider the amount of moles when you multiply the deltaHf by the respective stoichiometric coefficients. So in this case, you would multiply the deltaHf of H2O2 by 2 to account for the fact that the reaction is decomposing 2 moles of H2O2. The end result is kJ/mol.

Madeline Ho 1C

*make sure you're using the correct deltaH values: H2O and H2O2 liquid
deltaH = deltaHf(H2O) + deltaHf(O2) - deltaHf(H2O2) = 2(-285.83kJ/mol) + 0 - 2(-187.78 kJ/mol) = -196.10 kJ/mol

Madeline Ho 1C

Hannah Yates 1K wrote:Should number 4 on the midterm review be when delta T = 0?

Yes, I believe it should be deltaT=0 because temperature must be constant for q=-w.

Madeline Ho 1C

How do you solve for the heat in #6a? Are we supposed to use standard enthalpy of formation or bond enthalpies? The standard enthalpy of formation of HI is not given and the bond enthalpy for C-O is not given.

*edit: nvm, I just realized change in internal energy is given.

Madeline Ho 1C

On Lavelle's solution manual error pdf, it explains how to do 9.13. You assume ideal behavior and 1.00 mol N2 gas.

Madeline Ho 1C

With isothermal reversible expansion, work of expansion is being done by the system so energy is being lost. However, a heat reservoir also adds heat back into the system, so temperature remains constant. This is why q=-w.

Madeline Ho 1C

For irreversible processes, deltaU=q+w, so deltaU could be zero if q=-w. While it's possible, it's not ironclad like it is with reversible processes.

Madeline Ho 1C

Can someone explain how to do #3?
I got deltaHrxn= 1mol(1560 kJ/mol) - [1mol(-1300 kJ/mol)+2mol(-286 kJ/mol)] = 3432 kJ

Madeline Ho 1C

katherinemurk 2B wrote:Can someone explain how to do #1. I found the heat gained by the water but without the final temperature of the metal how would I find the specific heat??

The final temperature of the metal is the same as the final temp of the water.

Madeline Ho 1C

https://lavelle.chem.ucla.edu/forum/viewtopic.php?t=17644 I found this thread that explains how to do the problem well. Essentially, you have to find the combustion reaction for each element of the given reaction and then use method 1 for solving for enthalpy to find the reaction enthalpy for the hy...

Madeline Ho 1C

A 50.0-g ice cube at 0.0 C is added to a glass containing 400.0 g of water at 45.0 C. What is the final temperature of the system (see Tables 8.2 and 8.3)? Assume that no heat is lost to the surroundings. For this question, I found the deltaH for the phase change from ice to water, the heat absorb...

Madeline Ho 1C

For part a, you want to find the work so you would use w=-PV. V=π(r^2)(h), so you would use the values given for the diameter of the pump and the distance the pump is depressed to find the volume. For part b, if work is being done on the system (the air in the pump), work is positive. If work the sy...

Madeline Ho 1C

I'm having trouble with this too... I did the sum of the standard enthalpy of combustion for products - reactants but I got +312 kJ instead of -312 kJ. I know it should be negative because it's a combustion reaction, thus exothermic, but that math doesn't match up.

Madeline Ho 1C

I still used 101.325 J/L*atm for 8.9. And then you have to convert it into kJ to solve for the change in internal energy.

Madeline Ho 1C

If work is done on a system, the value is positive. If the system is doing work, the value is negative.

Madeline Ho 1C

What does it mean when the standard enthalpy of formation of one molecule is more negative than another? For example, in class the standard enthalpy of formation for CO2 was -394 kJ/mol and for CH4 it was -75 kJ/mol. Does this mean CO2 is more stable?

Madeline Ho 1C

You can also double check by calculating the percent ionization. If it's less than 5%, the approximation is valid.

Madeline Ho 1C

For part a, you're given that the pH of 0.10 M HCLO2 is 1.2. pH is -log[H3O+], so to solve for [H3O+] you have to do 10^-pH. In this case, it would be 10^-1.2=0.06 M H3O+. Then, you solve for the Ka (acidity constant), which is (0.06^2)/(0.1-0.06). The pKa is the -log of your Ka value. Part b is the...

Madeline Ho 1C

With partial pressure, you treat the equilibrium composition the same as you would a change in concentration. If the partial pressure of something is increased on the products' side, for example, the reaction will shift towards the reactants. An overall increase in pressure usually is in response to...

Madeline Ho 1C

K is the equilibrium constant while Q is the reaction quotient. So when you calculate Q, the reaction is not necessarily at equilibrium, though the equation to solve for Q and K are the same. You can then compare the values of Q and K to determine which direction the reaction will go.

Madeline Ho 1C

From the graph, you can see that B increases by 5, C increases by 10, and A decreases by 10. So you can think of it as +x, +2x, and -2x, if x=5. That's why the stoichiometric coefficients are 2A / B / 2C

Madeline Ho 1C

Why is $X_{2}\rightleftharpoons 2X$ an endothermic reaction? Shouldn't it be exothermic because enthalpy decreases/it is a catabolic reaction?

Madeline Ho 1C

The balanced equation for decomposition is simply $X_{2}\rightleftharpoons 2X$ because in the (forward) reaction, the diatomic dissociates into two. The quantity of X2 and X in the flask is irrelevant for the equation.

Madeline Ho 1C

If I understand it correctly, products will be favored at equilibrium if you increase the amount of reactant when the reaction is at equilibrium in order to minimize the effect of adding more reactant; the forward reaction is favored, producing more product. Initial concentrations of reactants or pr...

Madeline Ho 1C

You also want to look at flask #3, when the reaction has reached equilibrium. I used the number of X2 and the number of X over the total number of molecules (17) for the mole fraction when calculating the partial pressures for X2 and X (mole fraction times total pressure) to find K. Hope that helps!

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