CHEMISTRY COMMUNITY

Clarissa Cabil 1I

When calculating q of ice combining with water, I know you would use the equation q = mCΔT + nΔH_fus.
For C or specific heat, do I use the specific heat of water or ice? Why?

Clarissa Cabil 1I

Will catalysts always be in the first step of a reaction mechanism as a reactant?

Clarissa Cabil 1I

On the constants and equations sheet, can someone explain the difference between E = Eº - RT/nF ln Q (Nernst Equation) and E = Eº - 0.05916V/n logQ? Can someone also explain when to use each equation?

Clarissa Cabil 1I

Molecularity describes the number of molecules that come together to react in an elementary reaction (single step), which is why we look at the reactants when describing the molecularity of the forward reaction.

Hope this helps!

Clarissa Cabil 1I

The melting of snow on a sunny day is actually a spontaneous process (ΔG < 0, negative) because when snow melts, it transforms from a solid to a liquid, which means that the entropy is increasing (ΔS > 0). In real life, snow melts by itself, which is also why ΔG < 0.

Hope this helps!

Clarissa Cabil 1I

The integrated rate laws, differential rate laws, and half-life equations for 1st, 2nd, and 0 order reactions are all on the constants and equations sheet. However, it is not specified which equation is which, so I would still have a general understanding of when to apply each equation.

Clarissa Cabil 1I

Given reaction A --> B (concentrations are in mol.L -1 and time is in s) If this reaction is a zero order reaction, the units for its rate constants should be (mol A).L -1 .s -1 If this reaction is a first order reaction, the units for its rate constants should be s -1 If this reaction is a second o...

Clarissa Cabil 1I

The integrated rate law can be expressed in exponential form as [A]=[A].e−kt First order reactions show an exponential decay of reactants as a function of time. Because natural log (ln) was introduced into the integration, we had to use "e" to make calculations easier. In doing that, the ...

Clarissa Cabil 1I

During lecture, Professor Lavelle mentioned how first order reactions produce straight line plots of ln[reactant] vs time with a slope that equals -k. However, he proceeded on with lecture and then mentioned how first order reactions produce exponential curves that are decreasing. Can someone explai...

Clarissa Cabil 1I

A reaction can still occur even if Eº is negative, but it is favorable to have a +Eº because this will make your ∆Gº negative through the equation ∆Gº= - nFEº, and the reaction will occur spontaneously.

If you have a -∆Eº, the reverse reaction will be favorable.

Hope this helps!

Clarissa Cabil 1I

I just wanted to clarify, are the oxidizing and reducing agents in a reaction always going to be part of the reactants? While doing the homework, I found this to be a trend, but I just wanted to be sure.

Clarissa Cabil 1I

Just to clarify, for elements in their natural state (e.g. H₂ (g), O₂ (g), etc.), is their standard Gibbs free energy of formation equal to 0? Does this have anything to do with the standard enthalpy of formation for these elements equaling to 0?

Clarissa Cabil 1I

Dr. Lavelle mentioned this in lecture today and said how the equation is only used when ΔHº and ΔSº are both constant.

Hope this helps!

Clarissa Cabil 1I

We just discussed electrochemistry because we finished thermochem!

Hope this helps!

Clarissa Cabil 1I

Gibbs free energy is the energy associated with a chemical reaction that can be used to do work.
Entropy, however, is the degree of disorder.

I hope this helps!

Clarissa Cabil 1I

Does anyone know if Lyndon moved his workshop to today?

Clarissa Cabil 1I

Aidan Ryan 1B wrote:What are things we have learned that are not state functions?

Work (w) and heat (q) are NOT state functions.

Clarissa Cabil 1I

I am not quite sure yet, but my TA during discussion last week mentioned how there might be thermochem (enthalpy, entropy) and Gibb's.

Clarissa Cabil 1I

I am a bit confused on which units I should use for enthalpy. The book mentions how standard enthalpies of formation are in kJ.mol -1 and standard enthalpies of reaction are in kJ, but some of the answers for the homework switch the two. Can anyone clarify what units we should use for enthalpy, and ...

Clarissa Cabil 1I

A reversible process is one that can be reversed by an infinitely small change in a variable. An irreversible process is an expansion against an external pressure that differs by a finite (measurable) amount from the pressure of the system. In one of the workshops, one of the UAs mentioned how irrev...

Clarissa Cabil 1I

I am not quite sure myself, but I think the midterm covers all of the material until the second and third laws of thermodynamics, which both discuss entropy.

Clarissa Cabil 1I

When it says that K is changing due to a change in temperature, it is referring to the equilibrium constant changing.
When the temperature of a reaction is changed, then you are changing an aspect of the reaction itself, which is why the equilibrium constant, K, and not Q is changing.

Clarissa Cabil 1I

If a certain bond enthalpy is not given, also note if the bond is between elemental molecules at their most natural state at standard conditions (E.g. O₂ (g), F₂ (g)) in which the bond enthalpy is assumed to be equal to 0.

Hope this helps!

Clarissa Cabil 1I

When we need to calculate certain values, the gas/ solution must be at a standard state so that our units can match the quantity we are looking for at the end. Nuances in certain conditions can also affect what constants/units we use.

Hope this helps!

Clarissa Cabil 1I

Bond enthalpies are given as averages because a precise enthalpy value for a certain bond may differ in different molecules.

Hope this helps!

Clarissa Cabil 1I

You can use the ideal gas law when converting between concentrations and partial pressures.

Clarissa Cabil 1I

You can first visualize the reaction by writing a balanced chemical equation: HCl --> H+ + Cl- (HCl is a strong acid so it completely dissociates in water) You can calculate the pH of the desired solution straight away because they give you the concentration, which is 0.025 M pH = -log (0.025M) You ...

Clarissa Cabil 1I

Just to clarify,

the larger the Ka, the stronger the acid

and

the lower the pKa, the stronger the acid?

Clarissa Cabil 1I

For number 3 in chapter 11, the problem asks to find the equilibrium expression for the reaction: 4 NH 3 (g) + 6NO (g) ⇌ 7N 2 (g) + 6H 2 O Isn't this equation unbalanced? There are 10 nitrogens on the left and 14 on the right? However, the solutions manual keeps the coefficients the same? Are there ...

Clarissa Cabil 1I

Are you referring to the change in molarity and not the initial molarity? And if you are given an initial concentration of one of the products, you have to find Q to dictate which direction the reaction is going. If Q<K, the reaction is shifted to the right and favoring the products. So the change i...

Clarissa Cabil 1I

For your first question, I think the solutions manual does not use the equilibrium arrow because the questions asks to "Write a balanced chemical equation for the reaction" and doesn't specifically mention equilibrium. For your second question, they divide the partial pressure of each gas ...

Clarissa Cabil 1I

Setting up the equation to find the equilibrium constant of a reaction, is essentially the same concept for both K C and K P . When given aA + bB ⇌ cC + dD, K C = [C] c [D] d / [A] a [B] b K P = (P C ) c (P D ) d / (P A ) a (P B ) b However, your answer will depend on what the question is asking you...

Clarissa Cabil 1I

You confused reaction rate with the equilibrium constant, K. At equilibrium, the forward reaction occurs at the same rate of the reverse reaction. Therefore, the forward and reverse reaction RATES are equal. However, we can say that the EQUILIBRIUM CONSTANT for the forward reaction is K C . This mea...

Clarissa Cabil 1I

How come water is included in the equilibrium constant when it is in the gaseous state? Is it because all gasses are included in the calculation when finding k? Yes, I believe you are correct. It might also be because in this reaction, water is not acting as a solvent and, therefore, be included wh...

Clarissa Cabil 1I

In Outline 6, it mentions that one of the topics from the book that we need to know is "Box 12.1". Does this refer to Toolbox 12.1 in the book? If so, Toolbox 12.1 discusses how to calculate the pH of a solution of a weak acid. I thought we are only required to know how to calculate the pH...

Clarissa Cabil 1I

Just to verify, when naming coordination complexes that have a charge, do we always write "ion" at the end of the name? When I went to a review session, we always wrote ion if the coordination complexes had a charge outside of the brackets, but in one of Dr. Lavelle's examples during lectu...

Clarissa Cabil 1I

a) Compound goes into water. It breaks up into Br - and NH 4 + . Br - doesn't do anything. Some of the NH 4 + release a hydrogen and become NH 3 . Releasing a hydrogen makes the solution acidic. pH falls below 7. b) Breaks up in water. The sodium does nothing. The carbonate is the conjugate base of...

Clarissa Cabil 1I

When drawing Lewis structures of coordination compounds, at a review session I attended, the UA mentioned how the transition metal (central atom) will only have single bonds around it. In the example, [HgF2(OH2)2]+, the oxygens only had a single bond with Hg and a lone pair. Why couldn't the oxygens...

Clarissa Cabil 1I

I think we are expected to memorize all of them because we might be getting questions that ask us to write the name of an entire coordination compound, which requires knowing all the ligand names.

Clarissa Cabil 1I

The bond angles for trigonal bipyramidal are 180º, 120º, and 90º and the bond angles for seesaw should be slightly less than 120º and slightly less than 90º because of the lone electron pair. So, I think you're correct, but bond angles do differ according to different molecules.

Clarissa Cabil 1I

For number 13 in chapter 4 in the sixth edition, it asks to write the lewis structure, VSEPR formula, molecular shape, and bond angles for I 3 -. The solutions manual says that the molecular shape is linear, but shouldn't the shape be T-shaped because there are three lone pairs on the central atom a...

Clarissa Cabil 1I

For part c of homework question 19 in chapter 6 of the 6th edition, the question asks to "Account for the following observations in terms of the type and strength of intermolecular forces. (c) The boiling point of pentane, CH3(CH2)3CH3, is 36.1ºC, whereas that of 2,2-dimethylpropane (also known...

Clarissa Cabil 1I

I think we will be tested on determining a molecule's VSEPR formula, molecular shape, polarity, and hybridization. I think it would be okay for you to use the other method as long as you provide them with the correct answers that they are looking for.

Clarissa Cabil 1I

For the lewis structure of NO3-, N is the central atom and there are 3 oxygens surrounding it. One of the oxygens has a double bond with N, and the other two oxygens have a single bond with N because nitrogen is in period 2, so it cannot have more than 4 bonds. All the bond lengths are the same beca...

Clarissa Cabil 1I

Dr. Lavelle mentioned how the different hybridizations describe different shapes. Hybridization doesn't necessarily affected bond angles, but it does describe the regions of electron density, which tell us the shape and from the shape we can deduce the molecule's bond angles.

Clarissa Cabil 1I

When you get your midterm back, you can always go to office hours and ask about the remarks left on your test!

Hope this helps!

Clarissa Cabil 1I

If you have the sixth edition of the textbook, on the syllabus, many questions from the Molecular Shape and Structure section have us practice drawing and analyzing VSEPR structures.

Clarissa Cabil 1I

When doing the homework, the solutions manual sometimes mentions how certain bond angles are "slight less than" 109.5º or 120º etc., but how can you tell if a certain shape is exactly, approximately, or slightly less than a certain degree?

Clarissa Cabil 1I

Electron distortion is when the electrons of the anion are attracted to the cation, which causes the shape of the electron density to change slightly. Polarizability occurs when atoms and ions with electron clouds undergo a distortion. Atoms and ions with large distortions are highly polarizable. El...

Clarissa Cabil 1I

Catherine Kim 3K wrote:You calculate bond lengths by finding the mean/average of the given lengths. You also need to check if it is a resonance structure or not.

In this case, if it has a resonance structure, what would you do?

Clarissa Cabil 1I

Just to clarify, dispersion, induced dipole-induced dipole, london, and van der walls are all the same? Their names are just interchangeable?

Clarissa Cabil 1I

Molecules are the most stable when they have a formal charge of 0. After drawing the lewis structure, to check if the element is at it's most stable state, calculate its formal charge. To calculate formal charges, use the equation: FC = V - ( L + S/2 ). FC : formal charge V : valence electrons L : #...

Clarissa Cabil 1I

A coordinate covalent bond is when another atom provides an electron in order to complete an octet. In lecture, Dr. Lavelle displayed this by showing how Lewis bases donate electron pairs to Lewis acids, forming a coordinate covalent bond.

Clarissa Cabil 1I

When determining Lewis structures, there are exceptions to the octet rule. So, not all atoms will follow the octet rule due to expanded valence shells. In lecture, however, Dr. Lavelle mentioned how atoms are the most stable when they have a formal charge of 0.

Clarissa Cabil 1I

n denotes the possible orbits/energy levels/shells we can have. n can range from 1, 2, 3, 4, 5, etc. If n = 5, the possible l values are 0, 1, 2, 3, and 4 because l = 0, 1, 2, ... (n-1). However, since it is specified in the problem that we are in the d orbital, l MUST equal 2 (since l=0 for s, l=1 ...

Clarissa Cabil 1I

The Photoelectric Effect, Atomic Spectra, Wave Properties of Electrons and the De Broglie Equation, and Heisenberg Uncertainty Principle Modules are recommended to do before Test 2.

Clarissa Cabil 1I

Hi, I was wondering if chapter 1F will be included on the test on Tuesday because in class, Dr. Lavelle mentioned the test would only include information taught up to Friday's lecture, but the outline on his website shows that we should also know 1F. One of the TA also said we have to memorize the ...

Clarissa Cabil 1I

First, calculate the energy at level 4 using the E = - hR/(n^2). Then, do the same when you calculate the energy at level 2. Once you find the energy at both levels, find the difference in energy, which is E final - E initial . Then use the frequency = energy/h (Planck's constant) to find frequency....

Clarissa Cabil 1I

Does Dr. Lavelle prefer that we use the difference in energy equation, and then the Bohr frequency condition, and then the c = hv equation to calculate an electron's wavelength when it transitions from a higher energy level to a lower energy level as opposed to the Rydberg equation? In his lecture a...

Clarissa Cabil 1I

For a worked example during his atomic spectrum lecture and module, the question asked us to calculate frequency of light emitted by a hydrogen atom when an electron makes a transition from the 4th to 2nd principal quantum level. I understand that we first need to calculate the energies of both the ...

Clarissa Cabil 1I

For mass percent composition problems, if percentages are given to each element, the elements' percentages should all add up to 100%. However, if the masses, or grams, of each element are given, then the masses of the reactants should be equal to the masses of the products. You then convert the mass...

Clarissa Cabil 1I

According to my TA, during every test/quiz, we are provided a periodic table. In a lecture, Dr. Lavelle said that he put a link to his periodic table on the class website, however, I cannot find it either. If anyone knows where it is, please let me know! My TA said that as long as we use the correct...

Clarissa Cabil 1I

I think we can do questions from either section, but doing the questions from the fundamentals section might be more beneficial for the test this week.

Clarissa Cabil 1I

What does the concept "the moles of solute stay the same"? Can someone give me an example and how I would apply it? In terms of dilution, "the moles of solute stay the same", which means that the moles in the beginning of the experiment, will equal the moles at the end of the ex...

Clarissa Cabil 1I

Jordanmjones13 wrote:Do you know if the problems will specifically tell us how many significant figures to use?

No, it is up to us to determine how many significant figures to use. Always use the least amount of significant figures that were given in a problem.

Clarissa Cabil 1I

Due to the numerous factors that can contribute to losing product in the beakers, such as side reactions, impurities, and substances adhering to the container walls, it will be increasingly difficult to not only determine how much product was lost, but also how the product was lost. Therefore, calcu...

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