CHEMISTRY COMMUNITY

romina_4C

If that step is determined to be the slowest step (rate determining step), then yes the molecularity would define the rate law.

romina_4C

There will probably be a lot on kinetics, since that is one of the last topics we learned and have not yet been tested on.

romina_4C

Adding pure water to the cathode or to the anode would decrease the concentration at the cathode/anode, thus increasing the cell potential because theres a bigger difference between the concentration of the anode/cathode (only if you only dilute one, not if you dilute both the same amount).

romina_4C

For those types of problems, you just have to use the integrated rate law equations (specified for which order reaction it is). In that case, your [A] will be (1/16)[A]initial, or whatever fraction of the initial the problem specifies.

romina_4C

Are there any specific equations that we can use to apply to problem the have concentration cells? (ie: test question?) What's the best method to go about solving problems with them?

romina_4C

What exactly about the steady state approach will we have to know for the final?

romina_4C

On the left side of the equation (reactant side) we have 3 moles of reactants and on the right side of the equation (product side) we have two moles of products. Decreasing the volume of a system increases the pressure of the system, and thus the reaction wants to shift to the side with fewer moles ...

romina_4C

Usually if we are given a situation in which we can't ignore the x concentration (cannot follow the assumption), we will be able to solve the equation for x using the quadratic formula. I don't think Dr. Lavelle would give us a cubic equation which we would have to solve without the assumption.

romina_4C

If it is one of the substances being oxidized or reduced, then you would include it on its respective side (anode/left side if it's being oxidized or cathode/right side if it's being reduced).

romina_4C

When do you add an inert electrode (such as platinum) in the cell notation?

romina_4C

When given a balanced chemical reaction, you look at the oxidation numbers of the elements you believe to be involved in redox, and from there obtain the half reactions. Electrons are lost in oxidation and gained in reduction. Once you figure out which element has lost or gained electrons, you can s...

romina_4C

Since in a first order reaction, the coefficient of the reactants is one, and "a" represents the coefficient of the reactant, thus "a" = 1. Thus, the equation becomes -d[A]/dt = k[A]. Normally on the right side of the equation [A] would be raised to the "n" power, but s...

romina_4C

Delta G is the standard change in Gibb's free energy, n is the number of electrons involved in the redox reaction (from the balanced equation), F is Faraday's constant (can be found on equation sheet), and E is the standard cell potential (measured in volts).

romina_4C

The cell diagrams and the location of the cathode, anode, are the same in both an electrolytic cell and a galvanic cell. For both, oxidation still happens at the anode and reduction still happens at the cathode. In a galvanic cell, the anode has a negative charge and the cathode has a positive charg...

romina_4C

The cell diagrams and the location of the cathode, anode, are the same in both an electrolytic cell and a galvanic cell. For both, oxidation still happens at the anode and reduction still happens at the cathode. In a galvanic cell, the anode has a negative charge and the cathode has a positive charg...

romina_4C

I believe you have to calculate for the limiting reactant because you don't actually have the equilibrium value of H2O. Once you find out that graphite is the limiting reactant, you can calculate the concentration of H2O consumed and thus have a value of H2O to use (when making the ice table, otherw...

romina_4C

The electrons flow from the anode to the cathode. Therefore, the cathode must have a positive charge to attract the electrons, and thus the anode must have a negative charge so that the electrons are not attracted to it.

romina_4C

Yes, usually for galvanic cells the anode is on the left and the cathode is on the right. As mentioned above, at the anode oxidation takes place and at the cathode reduction takes place.

romina_4C

Usually a question asking to calculate the change in enthalpy of a reaction will give you a table of the enthalpies of formation, which would require you to use that method to find the change in enthalpy. Sometimes it is easier to use the enthalpies of formation to calculate enthalpy change because ...

romina_4C

In a way it is the charge, but it more specifically is the number of electrons lost or gained by an atom. For example, in KCl, there is no charge on the overall molecule, but K has an oxidation number of +1 because it is donating one electron and Cl has an oxidation number of -1 because it is accept...

romina_4C

The relationship between K and temperature also depends on whether or not the reaction is endothermic or exothermic. If a reaction is endothermic and you are increasing the temperature, it is as if you are increasing the reactants, and thus the equilibrium reaction will favor the products (forward r...

romina_4C

A reaction can only be spontaneous if its delta g is negative, as that is the definition of a spontaneous reaction, so you would be correct.

romina_4C

The half reactions are part of the larger overall redox reaction, which consists of the half reactions of oxidation and reduction. Thus, you are only look at half of the overall equation when you're looking at a half-reaction. That singular electron gained by Ag in a reduction was lost by oxidation ...

romina_4C

In addition to what other people have replied, when K = 1 (which is very rare), delta g equals 0.

romina_4C

Honestly, the method that has helped me the most when I am faced with problems I am unsure of how to start (aka many of those on the midterm) is writing down every equation related to what you think the question is asking. Thus, if the question mentions or wants you to find entropy, write down every...

romina_4C

I'm not exactly sure why, but usually for formation reactions you usually want it to be where you only form one mole of the compound in question.

romina_4C

Both q=m c delta T and q = m delta h can be calculated using moles - it just depends on what units either c or delta h are given in. On the equation sheet given by Professor Lavelle, delta H is given in molar units, so you have to convert your mass to moles. Usually c for q = m c delta t is given in...

romina_4C

When writing equations for combustion and formation (and when we need to use their enthalpies/entropies to calculate the entropy or enthalpy of the overall reaction), should I always have the equation forming one mole of the compound in question (other molar coefficients in fractions)?

romina_4C

Why is delta S equal to zero in a reversible reaction?

romina_4C

What are the specific cases when delta U = 0?

romina_4C

Why is there a negative on the w = - P times delta V? Is there any situation where this would not be the case?

romina_4C

When do we use the equation work = -P times the change in volume versus the integration formula?

romina_4C

When you compress the system (with a piston), you're doing work on the system. If you let the piston move out, the system is doing work on the surroundings, and the system loses energy as it is the system that is expending that energy to do the work.

romina_4C

Usually the question will specify. If you are not sure whether to use Kelvin or Celsius, an easy way of checking is to see what the units are for the other numbers or constants in your equation to see if they have Kelvin or Celsius in them. Depending on which one they use, you may or may not have to...

romina_4C

When a salt dissociates, the cation and anion can form a weak acid/base when it reacts with water. For example, when NaCl dissociates, it forms Na+ and Cl-. Na+ reacts with H2O to form NaOH, a strong base. Since it is a strong base, NaOH will fully dissociate in water again to reform Na+ and OH-, th...

romina_4C

The graph that he showed was the heating curve for water. Other people have already explained the steam portion, so I'll talk about the graph itself. The y-axis is temperature and the x-axis is heat absorbed. When the heat added is increasing and the graph is increasing (line has a slope), then the ...

romina_4C

I wasn't sure which topic to put this question under, but does anyone know if test 2 is going to be cumulative or only cover material from test 1 to test 2?

romina_4C

Delta U equals work plus q. I think for this class we do not need to worry about the work involved in a calorimeter reaction/heat reaction. We only need to know the simplified version, when there is no excess work done, where delta h equals q.

romina_4C

The system is the reaction itself, and the surroundings are the "chamber". So if the reaction is endothermic, the surroundings should lower in temperature while the system, the reaction, gains heat. If the reaction is exothermic, the surroundings should increase in temperature while the sy...

romina_4C

Usually in cases like these, since 25 degrees celsius and 1 atm are constants and taken just as constants in the surroundings, you take into account the sig figs for the values that change (moles and liters), thus it being 3.

romina_4C

Le Chatalier's principle states that, when put under stress, the reaction will shift in a way to minimize the stress. For example -adding reactants - reaction shifts to the right to form more products -adding products - reaction shifts to the left to form more reactants -removing reactants - reactio...

romina_4C

When checking the approximation of "x", we learned in class that if it is less than 5% of the initial then the approximation is okay. Is this based solely on the value of "x" or is it impacted by the coefficients of any of the reactants/products (like how we do 2x or 3x for the c...

romina_4C

As long as you also have the equilibrium constant you should be able to.

romina_4C

Most reactions will only lose one H atom at a time, so in one reaction I do not believe you can lose more than one H ion to become a conjugate base. However, polyprotic acids (those with multiple H ions), form conjugate bases in each successive reaction in which they lose an H ion (still within mult...

romina_4C

At 25 degrees celsius, the value of Kw is always 1.0 x 10^-14. You can usually always assume that. If the temperature is different, the problem will usually give you the value of Kw at that temperature.

romina_4C

I don't know if this is even possible, but if you have a reaction with both gases and aqueous solutions and you needed to calculate the equilibrium constant, would you use Kp (ignoring aqueous solutions?) or Kc (convert partial pressure to concentration?)?

romina_4C

I believe it is graded on completion of the 5 problems.

romina_4C

I believe so, since we are still learning equilibrium.

romina_4C

If the volume decreases (pressure increases), the reaction will shift to the side with less moles of a gas. For example, if you had 3 moles of reactants and 2 moles of products and the pressure increased, the reaction would shift to the right since there are fewer moles of gas. If the volume increas...

romina_4C

I remember in high school we learned that when doing ICE tables and having a the final concentration (in the ice table) being something like 3.00 - x, we would just approximate it to 3.00 or something like that. Is there any situation we would apply that approximation to in class or are we only usin...

romina_4C

"Decide whether an aqueous solution of each of the following has a pH equal to, greater than, or less than 7.
e.) AlCl3
f.) Cu(NO3)2

I am a little confused as to why the pH in both of these problems is less than 7 and why both Cu and Al can bond with H2O.

romina_4C

The coordination compound is an anion, therefore we need to add the suffix "-ate" to the end of the transition metal name to indicate that.

romina_4C

I think for titrations, since he mentioned that they were 14B and 14BL material, we will most likely just have to know the stoichiometric point and what it means (when moles of the acid = moles of the base) and whether or not the resulting solution will be basic or acidic/if the pH at the point of t...

romina_4C

We have to add the -ate to the transition metal since the coordination compound is an anion, and when we do this with iron, we use the latin name of iron (ferrum) and change it to add -ate to it.

romina_4C

Chloro and chlorido are the same; chlorido is just the newer naming system, so I don't think it matters which one you use (hopefully) as they mean the same thing.

romina_4C

Do the conjugate acid/base pairs change depending on whether or not a molecule is a bronzed acid/base or lewis acid/base?

romina_4C

Organic bases are bases that can donate an electron pair (usually include nitrogen). Inoragnic bases are those which dissociate into hydroxide.

romina_4C

Strong acids have weak bonds holding the hydrogen atom to the anion is weak (weaker, longer bond between the two). Weak bonds are usually unstable, as you can easily break them apart, which is why strong acids dissociate 100% in water. They prefer to be in a more stable state, which is with the anio...

romina_4C

I think so, as water is considered an amphoteric substance (can act as both an acid and a base). I believe you would only be able to classify it as specifically an acid or base when it reacts with another molecule, and not solely its own dissociation.

romina_4C

Strong bases are those that are group one or two elements bonded with either hydroxide or oxide (example: NaOH, BaOH2, LiO, BeO2).

romina_4C

What is more important when determining the polarizing power of a cation - its size or its charge?

romina_4C

You're correct about the melting and boiling point increasing with stronger intermolecular forces. The difference between the two is that when melting, a substance goes from a solid to a liquid and in boiling, a substance goes from a liquid to a gas. Usually boiling requires more energy than melting.

romina_4C

Can a molecule be polar if it only has non-polar bonds? How do lone pairs factor into the polarity of a molecule?

romina_4C

EDTA is often added into solutions because it binds to the metals and thus "removes" them from reacting in the solution.

romina_4C

Each lone pair on an N, O, of F is considered a Hydrogen bonding site.

romina_4C

This is where lone pairs come into play. A molecule with four regions of electron density (lone pairs and bonded atoms) is considered tetrahedral, but if there are 3 bonded regions and one lone pair, the shape is trigonal pyramidal. Basically, when you count the regions of electron density, that giv...

romina_4C

A little "cheat" you can follow is that all of the VSEPR models that do not have lone pairs (linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral) and where all of the bonds are the same (central atom bonded to the same type of atom/atoms of the same element, ie. CCl4, CH...

romina_4C

When molecules have stronger IMF, they are more strongly attracted to each other and therefore do not have as much motion as molecules that are not as attracted to each other. Therefore, molecules with a stronger IMF will have a higher viscosity than molecules with low IMF. The same thing applies to...

romina_4C

I think so. In order to have a dipole-dipole force, you must have polar bonds within a molecule that don't cancel out (essentially the molecule is polar). Dipole-induced dipole, however, exists among polar and non-polar molecules.

romina_4C

Since it wasn't on the midterm, will polarizing power/polarizability be included on test 2?

romina_4C

As mentioned above, the overall charges should add up to the charge of the molecule. However, you usually want more of the atoms to have a formal charge of zero, so if you can make a structure in which the overall formal charges add up to the charge of the molecule and have the atoms where it's poss...

romina_4C

Yes, elements in period 3 and on are exceptions to the octet rule (can have an expanded octet) because they have a d-orbital that is not filled and that can accept electrons (this is because the d orbital starts existing/being found when n=3, as in you can have a 3d orbital but not a 2d or 1d orbita...

romina_4C

When you're given the amount of molecules produced (they usually won't give you percentages of molecules since the molecules represent the products), with molecules being something like H20 or CO2, you convert the grams of the molecules to moles of the molecule. Then, you need to use the molar ratio...

romina_4C

When you are already given the calculated molar mass of the compound, you just need to multiply the percentages by that molar mass to get the grams of each element and then divide that by the molar mass of the element to get the moles. You do not need to divide each mole of each element by the small...

romina_4C

I believe the only thing we need to know about bond dissociation energy for now is that stronger bonds have a larger dissociation energy (requires more energy to break them) and weaker bonds have a smaller dissociation energy (requires less energy to break them).

romina_4C

I think you might need to know if a bond is going to be ionic or covalent based on the electronegativities (above 2 = ionic, below 1.5 = covalent) along with the trends in electronegativity. But since we haven't really done ay problems calculating exact electronegativities yet, I doubt that'll be on...

romina_4C

If the molecule does not have a charge associated to it (a charge like $SO_4{}^{-2}$ ), then the formal charges should cancel out to zero. In the example of the molecule I mentioned the overall formal charges of the molecule should equal -2, since the molecule has a charge of -2.

romina_4C

If a problem wants you to find the wavelength of an ion or an atom given the speed its traveling and no other information, how do you get the mass of the singular atom? If it was potassium for example, would you use the molar mass of potassium, change it to kilograms, and then divide by avogadros nu...

romina_4C

Do you only use this equation to find the wavelength or velocity of something that is not light (electrons, atoms, objects, etc.)?

romina_4C

Can you only use the equations c=wavelength times frequency and energy = h times frequency for light (and not for atoms for example)?

romina_4C

705198479 wrote:how do I know how many bonds or "dots" go on the ion in the lewis structures

The number of dots on an atom is the same as the number of valence electrons that the atom has. So for carbon, for example, you would draw four dots as it has four valence electrons.

romina_4C

Can someone explain to me why the answer for part D is 4?

romina_4C

Adding on to the other answers, the molecule has resonance, and the more resonance structures a molecule has, the more stable it is.

romina_4C

You're correct! You can also compare the electronegativities of the of the atoms involved in the bond. If the difference between their electronegativities is greater than 1.6, then the bond is ionic.

romina_4C

Also, cations are smaller than their originating atom because they have lost an electron, and thus the attraction between the fewer electrons and the positive charge from the nucleus is greater, resulting in a smaller ion (more attraction). Anions are larger than their originating atom because they ...

romina_4C

I think when an e- goes from n=3 to n=1 the change in energy is negative because its releasing energy (going from a higher energy state to a lower one, and the change in energy is measured as En(final) - En(initial)). Going from n=1 to n=3 should be a positive change in energy since you have to abso...

romina_4C

From what I understand, when we mention "math function" it relates to the probability of finding and e- in a certain area (at least for the Schrodinger equation), which is represented by the math function (sin or cos squared, where the x-intercept represents a zero probability of finding a...

romina_4C

I believe so. The electron would not have any velocity (through the kinetic energy equation KE = 1/2 mv^2) to fully remove itself from the metal, but it is considered "ejected".

romina_4C

I think he mentioned in class that we will not actually have to draw out the orbitals and that they're just a visual tool.

romina_4C

I am confused as to why, for part c when it asks for the wavelength of the radiation, you can't just use the relation that c = frequency times wavelength, seeing that they give you the frequency of the radiation in part b? Why do we have to use the equation we learned in class instead?

romina_4C

I guess graphing calculators allow you to store equations and they usually are programmable, so it's there way of preventing us from cheating by storing equations they want us to memorize.

romina_4C

In class when we completed the Vitamin C problem, I remember that we converted the grams of each element into percentage using the total grams of vitamin c. In other problems (such as F13), it is not required to convert to percentage to do the problem. I guess my question is when do we have to conve...

romina_4C

Are "number of molecules" and "formula units" considered the same unit? As in, do we use avogadro's number to get the answer when its required to be in those units?

romina_4C

The Balmer series refers to the UV light emissions. Essentially, UV light is created whenever an electron jumps to the energy level of n=1. n refers to the energy levels. It does not matter whether an electron jumped from n=2 to n=1or n=4 to n=4 - whenever an electron jumps to n=1, the light emitted...

romina_4C

To calculate the moles of an ion in a molecular compound, you see how many moles of that are present in the compound and use that number as the multiplication factor. For example, to see how many moles of Na+ are in Na2CO3, you would find the total number of moles of Na2CO3, and you know that for ea...

romina_4C

Will we have to know how to calculate accuracy/precision (I vaguely remember something about a number plus or minus another number but do not remember how to get to that point)?

romina_4C

Starting from the beginning you can use the MiVi = MfVf equation. Thus, you have the original molarity of the solution times 0.01 L (convert milliliters to liters) = the new/unknown concentration (can give it variable x) times 0.09 L (90 ml converted to liters). Thus you find the molarity by finding...

romina_4C

The process that helps me the most with the mole to mole ratio is this. 1. Convert grams of reactant to moles of the reactant using molar mass. 2. Convert the moles of the reactant to moles of the reactant using the stoichiometric coefficients given in the balanced chemical equation (for example, if...

romina_4C

Usually the problem will specify which units to leave the answer in. Just make sure to convert to liters when doing the molarity calculations! :)

romina_4C

In the first problem, since there are 3 sig figs in 10.0 (the last zero still counts as it shows to which decimal it is accurate), you need your answer to have 3 sig figs (0.0981, as the zero after the decimal and before the 9 is not significant). On the second one I'm not actually sure because ther...

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