CHEMISTRY COMMUNITY

Amy Dinh 1A

Can someone explain why cooling a hot cup of coffee after it was brewed spontaneous (delta G < 0) when melting ice is also spontaneous?

Amy Dinh 1A

Whatever is on the syllabus and outline will be tested on.

Amy Dinh 1A

We can assume x is negligible unless stated otherwise.

Amy Dinh 1A

R is the gas constant and it is usually in the units of J/K/mol.

Amy Dinh 1A

k is Boltzmann constant, which is equal to 1.38064852 × 10^-23 m2 kg s^-2 K^-1. K is the equilibrium constant of a reaction, which is calculated by the concentration of the products over concentration of the reactants.

Amy Dinh 1A

You can use either units for the Kp value, but you have to keep it consistent throughout your equation.

Amy Dinh 1A

The initial concentration gets cancelled out when you derive the equation. Thus you end up with the equation t1/2 = 0.693/k

Amy Dinh 1A

Group 1 (alkali metals) elements will always have a +1 charge, Group 2 has a +2 charge, group 7 has a -1 charge.

Amy Dinh 1A

0.0592 comes from changing the equation from terms of natural log (ln) to log base ten.

Amy Dinh 1A

The Cv for diatonic molecules is Cv = 5/2 R. I know they gave the Cv for a monatomic molecule.

Amy Dinh 1A

E is not a state function but through Gibb's free energy, which is a state function, can be broken down into multiple steps.

Amy Dinh 1A

Cathodes will have electrons on its reactants side while anodes will have electrons on its products side when writing out the half reactions.

Amy Dinh 1A

They are! They both mean the same thing. "Galvanic cell" was named after Luigi Galvani, and "voltaic" is named afterAlessandro Volta, but they are both electrochemical cells!

Amy Dinh 1A

Just to add on, charge = moles of electrons transferred times Faraday = nF

Amy Dinh 1A

You have to break this problem up into two steps:

1. Find m * Heat of fusion
2. Find m * C * delta T

When you find those two values, you add them up, and that is the total heat needed.

Amy Dinh 1A

As given, it says to balance the chemical equation first, and then use q = - m * delta T to get q because q reaction = -q calorimeter.

Hope this helps!

Amy Dinh 1A

Cv = 3/2 R This constant is for solving entropy of a system with constant volume
Cp = 5/2 R This constant is for solving entropy of a system with constant pressure

Amy Dinh 1A

Standard enthalpy of formation is the standard reaction enthalpy for the formation of 1 mole of substance from its elements in their most stable form and is measured in units of kJ/moles

Amy Dinh 1A

Degeneracy, W, is the number of possible ways of achieving a given energy state.
It is determined on the number of particles and the number of arrangements.
For example if N particles and each particle can be in one of two states:
W = 2^N

Amy Dinh 1A

Lewis structure will most likely not going to be given on the test since we were expected to know how to do it 14A, but the bond enthalpies are likely going to be given on the test.

Amy Dinh 1A

The values of constant R are different due to the different units shown in each equation.

Amy Dinh 1A

An isolated system is a system that has no contact with its surroundings, but it may or may not have a fixed volume, depending on its conditions.

Amy Dinh 1A

Heat is the transfer of energy between thermodynamical systems, while work is the transfer of energy other than work.

They are related in the equation: delta U = q +w
where U is the change in internal energy, q is the heat transferred in or out of the system, and w is work.

Amy Dinh 1A

Adding on to that, his principle applies to changing physical parameters Pressure and Temperature.

When pressure increases, the reaction favors the side with the least moles of gas.

Amy Dinh 1A

On #4a Worksheet 2, I keep getting pH=4.40, while the answer key says 4.30, when both using the quadratic formula and the shortcut way. I don't understand how to get the answer to

Amy Dinh 1A

For #3 on Worksheet 2, how can you find the Kb value? I'm not quite what is the first step or what the base is.

Amy Dinh 1A

Kc is the equilibrium constant, which is measured using products and reactants in either the gaseous or aqueous state.
Kw is the autoprolysis constant of water at 25 degrees Celsius and is always equal to 1.0 x 10^-14.
pKw is the -log(Kw) as p is shorthand for -log.

Amy Dinh 1A

Aqueous means that the substance is dissolved in water. It is counted when calculating concentrations and the equilibrium constant Kc.

Amy Dinh 1A

No, products are favored but they are not greatly favored. You are right when K>1 there are more more products than reactants at equilibrium.

Amy Dinh 1A

"12. For a chemical reaction that has reached equilibrium which statement is false?"
The question asked for you to select the false statement. You were correct that the forward reaction is the same as the reverse reaction rate, but that statement is not false.

Amy Dinh 1A

For every 1 molecule of X2, it decomposes to 2 molecules of X. That is why there is a coefficient of 2 in front of X.

Amy Dinh 1A

905084274 wrote:Thank you! How do you find the % decomposition? it says (6/11) x 100% but where does 6 come from?

You start out with 11 X2 molecules, and after the reaction reaches equilibrium, you are left with 5 X2 molecules. This means that 6 of the initial 11 X2 molecules decomposed to X.

Hope this helps!

Amy Dinh 1A

No. I think we need to find hybridizations of bonds and atoms only. I don't think we have done an example with the hybridizations of lone electrons yet.

Amy Dinh 1A

Ozone's bond lengths are the same because the bond lengths are the averages of the two bonds (the double and the single bond).

Oxygen would have the strongest bond due to having a double bond length, which average is shorter than ozone's average (double + single bond).

Amy Dinh 1A

The Nitrogen atoms have a lone pair, which can accept protons.

Amy Dinh 1A

In a Carbon Carbon double bond, it consists of one pi bond and one sigma bond. Pi bonds are weaker than sigma bonds due to less overlap between the p-orbitals, so therefore, the double bond (1 pi bond and 1 sigma bond) is less than the sum of 2 sigma bonds.
Hope this helps!

Amy Dinh 1A

When the elements are in the same group, the difference in electronegativity isn't that large, so the size of the molecule (London forces) is a more important factor.

Amy Dinh 1A

On question D it tells us to draw the lewis structure of XeF2, name the shape, and list the bond angles. I got the Lewis structure to have two bond pairs and three lone pairs. The key says the molecular shape is linear, but why?

Amy Dinh 1A

Are the angles the same or different whether it is one lone pair or two lone pairs in the bent shape? I presume the bond angles will be different depending on how many lone pairs there are in a bent shape. For sure they are both less than 109.5 degrees, but the more lone pairs there are the smaller...

Amy Dinh 1A

For a tetrahedral shape (4 areas of electron density) with no lone pairs the bond angle is 109.5 degrees, and if there is a tetrahedral shape (4 areas of electron density) with one or more lone pairs, than the bond angle is less than 109.5.

Amy Dinh 1A

How come as the s-character of the hybrids increase, the bond angle between the two hybrid orbitals also increase?

Amy Dinh 1A

Yes! A molecule can have both hydrogen bonds and covalent bond. Hydrogen bonds are INTERmolecular while covalent bonds are INTRAmolecular.

Amy Dinh 1A

I also agree that there's no calculations to determine bond angle; however, knowing how many lone pairs and bonded pair electrons may aid you in knowing the shape of the compound and then the assumed bond angle.

Amy Dinh 1A

That's why it's shape is trigonal planar.

Amy Dinh 1A

I agree with Rachael. It depends on the electronegativity of the atom. Whatever element is the most electronegative in the compound should have the charge. It doesn't have to be specifically the central atom or the outer one.

Amy Dinh 1A

Row three elements and below can hold more than an octet is because they have a d-orbital shell to expand on.

Amy Dinh 1A

An exception to the trend is that the electron affinities of the elements in column 15 are lower than those of column 14.

For example, if we needed to rank C, N, O, F in order of increasing electron affinities, the order would be N<C<O<F.

Amy Dinh 1A

Because radicals have unpaired electrons, they are highly reactive and therefore cause a dangerous amount of damage.

Amy Dinh 1A

The number of degenerate orbitals of an orbital is equal to twice angular momentum plus one (# degenerate orbitals = 2l + 1), so the s-orbital will have 1 degenerate orbital, the p-orbital will have 3, the d-orbital will have 5, and the f-orbital will have 7.

Amy Dinh 1A

When would you use De Broglie's equation (E=h/p) compared to using the Einstein equation (E=hc/lambda)?

Amy Dinh 1A

The mass of the electron is given in the "Constants and Equations" sheet, although just realize when to switch from grams to kilograms when calculating equations calculating energy in general.

Amy Dinh 1A

105002507 wrote:When do we use kg?

We use kg when we are calculating energy, since the units are in jules (kg·m^2·s^-2)

Amy Dinh 1A

The mass and the velocity separately and make you multiply it together to be momentum (p = m x v), but other than that I'm sure the momentum will be given most of the time from the problems I did in the homework.

Amy Dinh 1A

I am a bit confused with the example of today's lecture of the electrostatic potential energy example Lavelle did using carbon as one of the the q's. Can someone explain to me what he was trying to convey?

Amy Dinh 1A

"h" is Plank's constant, and the constant you used for h was for the (h bar) x 1/2.

Amy Dinh 1A

amogha_koka3I wrote:Will we be required to know how to derive De Broglie's Equation?

No, De Broglie's Equation is given on the "Constants and Equation" page, but it is not labeled as De Broglie's equation.

Amy Dinh 1A

For frequency, the standard unit is in s^-1 or Hertz (Hz). For wavelengths, meter is standard in calculations, however, you can change it to nanometers with conversions.

Amy Dinh 1A

You can use 3.00 x 10^8 m/s, but you could use 2.998 x 10^8 or 2.99792 x 10^8 (in Lavelle's constants and equations sheet).

Amy Dinh 1A

For every one mole of epson salt there is 7 moles of H2O, so the multiplication is to match the ratio.

Amy Dinh 1A

I'm assuming that you were given the mass of a substance (6.54 grams) and the density of the substance (0.7857 g/cm^3). Since the equation was D= m/V he substituted in D and m to find volume. I think he was trying to say like the C =n/V problems, you can find any variable of the D = m/V equation as ...

Amy Dinh 1A

We will encounter them in problems eventually, mostly when given a problem where you have to make an experiment or when an experiment is being done. Like the person above me said, it is best to understand them now.

Amy Dinh 1A

Yes, Oxygen is always needed for a combustion reaction to occur, and the products will always have CO2 and H2O.

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