CHEMISTRY COMMUNITY

Cassandra Mullen 1E

If external pressure is given in the problem, then it is irreversible so use w=-P(delta)V

Cassandra Mullen 1E

KayleeMcCord1F wrote:Do we have to know the van't Hoff equation for the final?

I think you should be able to derive the Van't Hoff equation with what's given on the equations sheet.

Cassandra Mullen 1E

Your overall rate law for the reaction will be based on the slow step, since the slow step is the rate-determining step. If the slow step is not the first step in the mechanism, then you'll have intermediates that you'll need to get rid of through substitution.

Cassandra Mullen 1E

Michael Cheng 1C wrote:do we need to know nucleophiles and electrophiles for the final? What is the difference between them?

The nucleophile is the molecule/ion that donates electrons and the electrophile is the molecule/ion that accepts the electrons.

Cassandra Mullen 1E

A catalyst is neither a reactant nor a product, so that's why it's not written in the overall chemical reaction.

Cassandra Mullen 1E

Reaction rates can be measured through a spectrometric method, where a spectrophotometer measures light absorbance. A conductometric titration can also be used, which measures the voltage change over time as reactant is added. Titrations work well for slow reactions.

Cassandra Mullen 1E

The unique reaction rate occurs per one mole of the reactant. In this case, you divide by 2 since the stoichiometric coefficient of NO2 is 2.

Cassandra Mullen 1E

You can derive the equation by substituting ∆G˚=-RTlnk into ∆G˚=∆H˚-T∆S˚ to get -RTlnk =∆H˚-T∆S and then dividing both sides by -RT.

Cassandra Mullen 1E

Yes, you are trying to cancel out the electrons to balance out the overall reaction of oxidizing 3 mol of NADH. Since the 2nd and 3rd equations are already balanced, you can also add the ∆Gs, which would correspond to 1 mol of NADH, and then multiply by 3 to get the ∆G value for 3.00 mol of NADH.

Cassandra Mullen 1E

Just like when writing equilibrium constant expressions, the concentration of solids and liquids are essentially constant so they can be omitted in the rate law expression.

Cassandra Mullen 1E

A galvanic cell occurs spontaneously due to the positive voltage. The cell potential is positive since the cathode electrode potential is greater than the anode electrode potential. The negatively charged electrons flow in the cell since they are attracted to the more positive end.

Cassandra Mullen 1E

The uncombined diatomic elements have an oxidation state of 0. You can also observe that there is no superscript indicating a charge on the molecule.

Cassandra Mullen 1E

Remember when writing equilibrium constant expressions, the concentrations of solids and liquids are excluded, since they essentially have constant concentration throughout the reaction. For cell potential, we are concerned with the voltage related to the concentration of aqueous solutions, since th...

Cassandra Mullen 1E

When writing half reactions, I think you only disregard the spectator ions since they don't participate in the actual redox reaction.

Cassandra Mullen 1E

Most of the halogens have a -1 oxidation number, but there are exceptions if the atom is bonded to a more electronegative element. Oxygen usually has an oxidation number of -2, unless it's in oxygen difluoride, where the oxidation number is +2. In peroxides, oxygen has a oxidation state of +1.

Cassandra Mullen 1E

This one's pretty good for understanding how the galvanic cell is set up:
https://www.youtube.com/watch?v=a2v7ph3kLXo

Cassandra Mullen 1E

In the G=H-TS, you can think of Gibbs Free Energy as the free energy left in the system after subtracting energy lost to heat and entropy (-TS term) from H (enthalpy, the total heat content of a system).

Cassandra Mullen 1E

You would use delta s=nRlnV2/V1 to calculate the entropy change of an ideal gas in a reversible isothermal expansion.

Cassandra Mullen 1E

Just to add on, the 12 orientations/3 orientations come from the octahedral shape (6 directions for the ligands), which is the same for both the cis and trans isomers. In the trans isomer, the ligands are across from each other, and in the cis isomer, the ligands are next to each other. Rotating the...

Cassandra Mullen 1E

This is an example of an isothermal process (no heat change) where the energy lost by the system is supplemented by the energy gained in the surroundings.

Cassandra Mullen 1E

Gibb's free energy accounts for the enthalpy and entropy of the system. You can think of the H term as all of the the energy in the system and TΔS as the energy lost to heat and entropy.

Cassandra Mullen 1E

The prefix "iso" means equal, so "isothermal" means equal heat, or maintaining the same heat throughout the process.

Cassandra Mullen 1E

To write the formation equation for dinitrogen pentoxide, you want 1 mol of N2O5 to form from its elements in standard state. After writing down the proper elements, don't forget to balance! :)

Cassandra Mullen 1E

There is a mistake in the solution manual as Dr. Lavelle pointed out on his website: the enthalpy of formation of K2S is actually -471.5 kJ/mol but it says -417.5 kJ/mol in the manual. The correct answer is then -38.72 kJ.

Cassandra Mullen 1E

You would use these equations in HW problem 8.31, since you are dealing with ideal gases at constant pressures/constant volumes. The gas constant R is 8.314 J/Kmol.

Cassandra Mullen 1E

Heat constant. When do you use 8.314 and when do you use 0.00821? Looking at the units helps you to determine which R to use. If you're using the ideal gas equation and pressure is in atm and volume is in L, then you would use 0.00821 Latm/molK. Homework problem 8.49 is an example of using the othe...

Cassandra Mullen 1E

I'd also like to ask if someone could tell us how many steps will have to be calculated and how they come together to get the final temperature. (Claire Woolson Dis 1K) The overall goal of the problem is to solve for the final temp. just like in an algebra equation. You'll set up the heat of the wa...

Cassandra Mullen 1E

You'll also have to base your calculations off of the limiting reactant. Since there are 0.130 mol of Zn (8.5 g Zn= 0.130 mol Zn), and twice as many moles of HCl is required, zinc metal is the limiting reactant.

Cassandra Mullen 1E

When determining the bond enthalpy, it helps to draw the molecules or at least visualize the individual bonds within each molecule. The bond enthalpies themselves are defined as the amount of energy to break one mole of the molecule. In this problem, there are four C-H bonds being broken and 4 H-Cl ...

Cassandra Mullen 1E

To start, check out the box on Section 8.7 pg 277. It says that the molar internal energy of a monatomic ideal gas at temp. T= 3/2RT. The average kinetic energy can be found using this expression.

Cassandra Mullen 1E

Conceptually, you can sum all the changes using Hess's Law because enthalpy is a state function. Think of these problems as puzzles where you manipulate the steps and cancel out compounds on opposite sides of the reaction to get to the desired equation.

Cassandra Mullen 1E

If you compare the initial concentrations to the equilibrium concentrations: you will see that on the reactants side, A went from 1.00 M to 2.54 M and B went from 2.00 M to 2.54 M. This means that the reactants increased. C goes from 7.00 M to 5.46 M and D goes from 4.00 to 2.46, so the products dec...

Cassandra Mullen 1E

This reaction goes in the reverse reaction, which is why the x value you calculated is negative. When you plug this value into your equilibrium values in the ice table, you should get the right answer.

Cassandra Mullen 1E

Yes, Kr is stable since it has a filled valence shell, so it holds onto its electrons much more strongly than Mg does. Even though Mg is smaller, it only has 2 valence electrons, and prefers to lose those valence electrons and form a cation that resembles the fully stable octet of neon. Since it's h...

Cassandra Mullen 1E

Fluorine is the most electronegative element and it is tiny compared to bromine. This means that fluorine has a stronger bond with hydrogen than bromine does. Strong acids dissociate completely, and a stronger acid will dissociate more easily. In other words, HF doesn't dissociate as easily HBr does...

Cassandra Mullen 1E

When you make x negligible under the small x assumption, you should check to make sure that you can use this assumption. To do this, calculate that the x value you get is less than 5% of the original concentration.

Cassandra Mullen 1E

As volume increases, pressure decreases. Conversely, if volume decreases, pressure increases. Pressure and volume are on the same side of the equation to show this inverse relationship.

Cassandra Mullen 1E

When converting your pressure units, remember that 1 bar=1 atm. Most of the time we have been using atm as our unit of pressure, especially for the ideal gas law, but it's good to remember the conversion. :)

Cassandra Mullen 1E

If you're given the temperature in celsius, you can easily convert it to Kelvin by adding 273.15

Cassandra Mullen 1E

When the complex has an overall negative charge, you add the suffix -ate to the metal's name. This is the case for part b, since the anion has an -2 charge. Hope this helps! :)

Cassandra Mullen 1E

The k is kappa and the chemical symbol written next to it denotes which atom is linked to the metal atom, just to avoid ambiguity. If you do choose to use this method, you can also underline the element.

Cassandra Mullen 1E

Acid rain also affects the biodiversity of certain lands and their ecosystems. Some plants are sensitive to acidic environments and may die as the soil/water becomes more acidic. A certain plant species could perish and be replaced by another species. This impacts the entire food web, since plants a...

Cassandra Mullen 1E

An example of a linear shape with a triple bond on one side and a single bond on the other side is hydrogen cyanide (HCN). A triple bond connects C to N and a single bond connects H to C. It has no lone pairs and 2 VSEPR regions, so the molecular shape is linear.

Cassandra Mullen 1E

^ Just to add on, a molecule composed of two of the same atom has a nonpolar bond, since the electronegativities are the same (electronegativity difference is 0) and the electrons are shared equally.

Cassandra Mullen 1E

With access to the 3d orbital (10 extra electrons), the third energy level can technically hold 18 electrons max.

Cassandra Mullen 1E

Remember that the possibility of finding the electron would be in the space of the atom (electron is confined to within the diameter of the atom), so you use the diameter.

Cassandra Mullen 1E

How would you know which electron has a higher electron affinity than the other? You can examine the trends in the periodic table... electron affinity increases across a period and decreases down a group. Also, an atom with less valence electrons is less likely to gain electrons, so it has a lower ...

Cassandra Mullen 1E

Hi, Dr. Lavelle mentioned that he looked up the bond lengths that were in the powerpoint. The bond lengths were experimentally determined, so we will most likely be given a table with all the bond lengths/bond energies and then be asked to do calculations using these given values.

Cassandra Mullen 1E

Coulomb's law can then be related to ionization energy since the inner shell electrons are closer to the nucleus and harder to remove. The inner shell electrons have a higher attractive force (greater numerator) and the distance is less (smaller denominator), which means IE is higher.

Cassandra Mullen 1E

Remember that this process happens at a 1:1 ratio, where one photon excites/interacts with one electron.

Cassandra Mullen 1E

For Part C, you need to combine several equations to find the frequency. By combining KE= E-threshold, E=hv, and KE=1/2mv^2, you get frequency= (1/2mv^2 + threshold)/ h (Planck's constant) Remember to use the mass of an electron: 9.11 x 10^-31 kg for the mass and to square velocity when plugging in...

Cassandra Mullen 1E

For Part C, you need to combine several equations to find the frequency. By combining KE= E-threshold, E=hv, and KE=1/2mv^2, you get frequency= (1/2mv^2 + threshold)/ h (Planck's constant) Remember to use the mass of an electron: 9.11 x 10^-31 kg for the mass and to square velocity when plugging int...

Cassandra Mullen 1E

For Part C, you need to combine several equations to find the frequency. By combining KE= E-threshold, E=hv, and KE=1/2mv^2, you get frequency= (1/2mv^2 + threshold)/ h (Planck's constant) Remember to use the mass of an electron: 9.11 x 10^-31 kg for the mass and to square velocity when plugging int...

Cassandra Mullen 1E

What exactly is happening at the quantum level when we see color? Does this have to do with the "excess" energy leftover when an electron moves up an energy level?

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