CHEMISTRY COMMUNITY

Helen Struble 2F

Rate constants are always positive. In the plot of ln[A] by time, for first order reactions, the slope is equal to -k. So, while the slope is technically negative, the rate constant, as with any rate constant, is positive.

Helen Struble 2F

Do graphs only show the order with respect to a specific reactant? For example, if we are given a linear plot of 1/[A] by time, we can infer the reaction is second order. But do we know whether other species are involved besides A? Is it possible that there is a species B involved which is first ord...

Helen Struble 2F

If you are given a plot of 1/[A] by time, and the curve is linear, then you know that the reaction is second order with respect to that reactant. The slope is equal to the rate constant.

Helen Struble 2F

Elementary steps show all the individual collisions that lead to a final product, including the formation/consumption of intermediates. The elementary steps represent what is happening on a molecular scale in between individual atoms and molecules.

Helen Struble 2F

Does it matter if the order of the rates? For example if the first step rate is slow and the second step is fast does the slow step control the rate? What happens if the first step is fast and the second step is slow? Slow step determines the rate whether it is preceded by or followed by other fast...

Helen Struble 2F

Half-lives are rarely ever used for second order reactions anyway since they are dependent on the initial concentration. Half-lives are more useful for first order rate laws, because they allow you to deduce certain information about the reaction without knowing the initial concentration at all.

Helen Struble 2F

The rate law shows the order of each reactant. The order of an individual reactant is the power it's raised to (zero if it's not in the rate law at all), the overall reaction order is the sum of all individual reactant orders.

Helen Struble 2F

Since half-lives are not dependent on initial concentration for first order reaction, they allow you to easily figure out certain questions using time and reaction constant alone—how long will it take to decompose by 2x, 4x, etc. So if you're given a time period but no initial concentration, it may ...

Helen Struble 2F

I'm not sure it's necessarily easier, but it is standard and lets us know that we're comparing the same thing when looking at different reactions or different concentrations of reactants. If we didn't have this standard, we would be comparing rates from all different points at a reaction, making it ...

Helen Struble 2F

Depending on the order of the reaction, you will have to plot time by a different y-axis to achieve a straight line with slope kr or -kr. For zero order reactions, simply plot [A] by t. For first order, plot ln[A] by t. For second order, plot 1/[A] by t. If you are given a graph of a straight line t...

Helen Struble 2F

Andrew Pfeiffer 2E wrote:The order of the reactant indicates how much the rate of the reaction is affected if the concentration of (one of) the reactant(s) is changed.

Note that this isn't the same of the overall reaction order, which is the su. of the order of each of the individual reactants.

Helen Struble 2F

I'm still having difficulty understanding what it means if a reaction is spontaneous. How could a reaction be spontaneous but also never proceed in the forward direction (diamonds to graphite example)?

Helen Struble 2F

Corrosion usually happens at the anode because as the metal is oxidized into metal ions, it loses mass. Over time, this builds up until there is noticeable corrosion.

Helen Struble 2F

DHavo_1E wrote:Hello,

How does the voltage difference relate to pH? Thank you :)

The voltage is related to the concentration of hydrogen ions, so reading the voltage across the solution can then be used with the Nernst equation to figure out hydrogen ion concentration.

Helen Struble 2F

99% of the time yes, but it is important to not assume this and always look at the half reactions to figure out which electrode is oxidized (anode/loses electrons) and which is reduced (cathode/gains electrons).

Helen Struble 2F

The cathode is the site of the reduction half reaction. Because metallic cations are being reduced to an uncharged species, the cathode gains mass. The anode is losing mass because the metallic species is losing electrons (being oxidized), and these cations then become aqueous in solution.

Helen Struble 2F

Cell diagrams just help to visualize the half reactions independently because the cathode being reduced and the anode being oxidized are in separate containers with wire and a salt bridge allowing the electrons to flow between the containers. But, the aqueous species and the metallic species involve...

Helen Struble 2F

Do redox reactions happen in gaseous environments? If so, can we still use water to balance reactions? If you could include an example, that would be great.

Helen Struble 2F

Are hydroxide and H+ available to balance redox reactions even in neutral solutions? I know that these species are found in neutral water, but in small, equal amounts. Is there enough of each species present to use them to balance half reactions?

Helen Struble 2F

The main principle is focus on charge first when balancing the redox rxn. When the number of electrons are balanced on both sides, the elements are probably going to be balanced. this is faster than looking at the elements first. hope this helps! This is also the most effective way because in aqueo...

Helen Struble 2F

In past chem classes, I was taught OILRIG: Oxidation Is Loss, Reduction Is Gain. Since reduction is gaining electrons, the Fe ion is being reduced because its positive charge decreases, so it is becoming, relative to the original species, more negative. This is because electrons have a negative char...

Helen Struble 2F

Delta U will equal 0, because q = -w. This simply means that any energy transferred from system to surroundings as work will immediately be replaced by a flow of heat from surroundings to system (or vice versa, depending on if it's an expansion or compression). Because there is no net change in ener...

Helen Struble 2F

Delta S and its variants (system, surroundings, total) represent a state function (change in entropy). We can find the value of this change through the equations given to us. However, because a delta valley represents a final value minus and initial value, we know that any system must have a set val...

Helen Struble 2F

Is the delta U of an isothermal irreversible reaction also equal to 0, like it is for an isothermal reversible reaction? If it's not equal to 0, why is that? I'm having trouble wrapping my head around an isothermal irreversible expansion. In isothermal reversible expansions, all the energy transfer...

Helen Struble 2F

In order for work to be done, it has to be done on something—work measures the transfer of energy. In a vacuum, there is no gas to transfer energy to, so no work is done.

Helen Struble 2F

So, for calculating entropy of a system vs. the surroundings vs. the total entropy, should we just assume that the entropy of the surroundings is only affected by heat? Is this based on the assumption that any volume change due to piston expanding or contracting is negligible compared to the volume ...

Helen Struble 2F

In LS7A, two equations are given for Gibbs free energy:

1) deltaG = deltaH - TdeltaS
2) deltaG = G(standard) + RTlnQ

Can someone relate these two equations? If deltaG is a state function (does not change with path), why would the concentrations of products and reactants affect deltaG?

Helen Struble 2F

The first law of thermodynamics is represented as an equation is delta U = q - w. Often simplified to "the energy of the universe is constant." Since U is equal to internal energy, there also must be an energy of the surroundings. Any change in internal energy has the opposite change on th...

Helen Struble 2F

Enthalpy and entropy are state functions, but work is not. For me, I recognize a state function because in equations they usually have a delta symbol...delta H (enthalpy), delta S (entropy), delta U (internal energy), delta G (Gibbs free energy) are all state functions. They are not state functions ...

Helen Struble 2F

The T refers to the temperature of the surroundings, which remains constant. This is because this equation calculates for entropy of surroundings, and entropy increases more at lower temperatures.

Helen Struble 2F

Isothermal systems have delta U = O because the transfer of heat and pressure are both at equilibrium, so all the energy lost by the work of expansion transfers into the system as heat in order to keep the temperature constant. Since work is being done (system losing energy), -w is negative, and sin...

Helen Struble 2F

Is there any scenario in which a closed reaction would do work on the system but still have a positive enthalpy? So, if delta U = q + w, is it possible for a system to lose heat but still expand, doing work on the system? Or is every reaction that causes work of expansion also exothermic?

Helen Struble 2F

Work of expansion indicates work being done by the reaction onto the system. Since the flow of energy is from the reaction to the system, we know the reaction is doing work.

Helen Struble 2F

The system gives you insight into where the energy of the reaction is going. In a closed system, since no heat or energy is lost to the environment, delta H is equal to delta U, the internal energy of the system. For closed or open systems it is much more difficult to quantify how much heat is lost ...

Helen Struble 2F

dtolentino1E wrote:yes!

the change in internal energy (delta U) is usually calculated as heat (q) + work (w), but at constant pressure, q = delta H and w = -P(deltaV)

How would this change for a constant volume set up? Is delta U no longer equal to q_p in that scenario?

Helen Struble 2F

Calorimeters are used to measure temperature changes in constant volume or constant pressure environments. If we can measure the temperature change of a constant pressure or constant volume reaction, we can deduce q p and deltaH. The accuracy depends on the type of calorimeter, more sophisticated, w...

Helen Struble 2F

Elements in their standard state have a standard enthalpy of formation of 0 because they are are not formed, they just exist. They don't require any energy to get to their most stable form if they're already in that form. This is still a little confusing to me because elements in their standard for...

Helen Struble 2F

Theoretically, is the enthalpy of the reaction for the break down of glucose the same whether it is combusted or broken down in a more controlled manner like in cellular respiration?

Helen Struble 2F

What exactly is the difference between enthalpy of a reaction and the energy of the reaction, or is enthalpy just a more specific term for the energy of a reaction?

Helen Struble 2F

It never hurts to draw a Lewis structure to make sure you know exactly which bonds are being broken and which are being formed. For simple molecules, however, you can usually visualize the structure without having to draw the structure on paper.

Helen Struble 2F

The method you have to use will usually be obvious based on the information given to you. For example, if bond energies are given to you, you'll have to compute enthalpy using that method.

Helen Struble 2F

pH and pOH are important to consider when solving acid base equilibria. Because pH=-log[H3O+], if you are given the pH either initially or at equilibrium, you can use than to find the concentration of H3O at that instant. Using this, you can create an ICE table and solve for whatever the question as...

Helen Struble 2F

K stays constant, so its talking about which side needs to be produced (either reactants or products) in order to reestablish that constant ratio. The only situation where this isn't the case is when K changes entirely, which is only ever due to a change in temperature.

Helen Struble 2F

The equilibrium either lies towards the reactants or towards the products, depending on how stable the chemical species on either side of the reaction are. More stable chemical species are less likely to participate in the reaction, so equilibrium tends to lie towards whichever side has more stable ...

Helen Struble 2F

What do we need to know about buffer reactions? If given a buffer system, what are potential questions we will be asked?

Helen Struble 2F

How are acid base calculations different than regular equilibria calculations? Or is it all the same concepts just applied in one specific scenario?

Helen Struble 2F

In the same way that K does not tell us about how quickly a reaction reached equilibrium, Q only tells us which direction a reaction will proceed, not how quickly. This is because Q is calculated from the instantaneous pressures or concentrations. Since it's from one instant, we cannot infer any inf...

Helen Struble 2F

The reaction quotient tells us which direction the reaction will proceed to reach equilibrium. If Q<K, then there are less products than would be found in the equilibrium reaction so the reaction proceeds in the forward direction. By the same logic, if Q>K, then there are more products than would be...

Helen Struble 2F

For a reaction in solution, both temperature and system pressure would affect the equilibrium constant. This is because both of these affect the chemical activity of a species. Since concentration is just an approximation of chemical activity, anything that changes chemical activity will change the ...

Helen Struble 2F

As others mentioned, brackets denote concentration (in mol.L -1 ), so you wouldn't use them when calculating partial pressure for Kp. However, just because you are given all gases does not necessarily mean you will be calculating Kp, so pay attention to what the question asks and what values are gi...

Helen Struble 2F

To calculate partial pressure, find the molar fraction of each gas. Then, multiply the molar fraction by the total pressure in the container.

Helen Struble 2F

Do Lewis acids have the same qualities as Bronsted acids, even if there is no transfer of H₊ ion? Would they still taste sour or can we only make this assumption for traditional Bronsted acids?

Helen Struble 2F

Bronsted acids and bases are determined by whether they donate or accept protons. Remember, protons are just H + ions. Since this is just a hydrogen atom with no electrons, it is attracted to lone pairs, almost like a ligand. Since NH 3 has a lone pair, it readily accepts hydrogen atoms, therefore i...

Helen Struble 2F

Since H₂SO₄ is a strong acid with two hydrogens, do both hydrogens ionize in water? If so, does this mean the concentration of H⁺ ions would be double the concentration of H₂SO₄? If only one hydrogen ionizes, why?

Helen Struble 2F

Neutralization brings the pH back to a neutral 7. If the solution is basic (pH greater than 7) then you will have to add an acid to neutralize it. If the solution is acidic (pH less than 7) then you will have to add a base to neutralize it.

Helen Struble 2F

There are only six strong acids, so I've always found it easiest to memorize them. They are: HCl, HNO₃, H₂SO₄, HBr, HI, HClO₃, and HClO₃. Since they're strong acids, these six will completely ionize in water. Be sure to know all the other properties of these strong acids, too.

Helen Struble 2F

Lewis acids can accept electrons because they have an empty orbital. So, to identify a Lewis acid, just look for an incomplete octet. Lewis bases have to have a lone pair (though not every structure with a lone pair is a Lewis base, obviously).

Helen Struble 2F

Colbatate indicates that the entire compound has a negative charge. This means the coordination compound is an anion and can form an ionic bond with a cation to form another compound.

Helen Struble 2F

What is the significance of the coordination sphere? Do all the atoms in a ligand have to be inside the sphere?

Helen Struble 2F

I think the important thing about cisplatin is the way its structure affects its biological function. Because the two chlorine atoms are next to each other instead of across, they can bond to both sides of DNA's double helix, preventing it from "unzipping" and therefore stopping replicatio...

Helen Struble 2F

Why is it that only transition metals form coordination compounds, and not s-block metals?

Helen Struble 2F

A coordinate bond is one in which both shared electrons are provided by a single compound, as opposed to each atom/compound providing one electron to form a bond. The compound which binds to the transition metal by donating two electrons is known as the ligand.

Helen Struble 2F

Ion-ion is the stronger force. We can infer this from the dramatic difference in boiling point between ionic compounds and compounds with hydrogen bonding. The melting point of water is 0 degrees celsius. The melting point of NaCl, which has ion-ion interactions, is 801 degrees celsius. This shows t...

Helen Struble 2F

^^Lone pairs definitely influence molecular shape. Compared to shapes which do not have lone pairs, shapes with lone pairs usually have smaller bond angles because of the stronger repulsion. However, the actual name ("see-saw" or "trigonal pyramidal") comes from the geometric arr...

Helen Struble 2F

Is the only way to determine a molecule's AXE formula to draw the Lewis structure? Is there any way to determine if there are lone pairs without drawing a Lewis structure?

Helen Struble 2F

Lone pairs have stronger electron repulsion than a bonding atom's electron cloud, so they typically distort bond angles. The bonding atoms are pushed further away from the lone pair. This results in smaller bond angles than if there were only atoms and no lone pairs.

Helen Struble 2F

The example we talked about in class was for the "see-saw," or AX 4 E. This geometry is the same as trigonal bipyramidal except one of the axial atoms is instead a lone pair. Lone pairs have higher repulsion, so the lowest energy structure is one in which the lone pair is in the place of a...

Helen Struble 2F

I think you are confusing the strength of bonds with the strength of the intermolecular forces. The bond is what holds the two atoms together, for F 2 and I 2 , this is a non polar covalent bond. The intermolecular forces are what keep the molecules together as a solid, liquid, or gas. In your quest...

Helen Struble 2F

Dipoles are the partial negative and partial positive charges on molecules which have atoms with a notable difference in electronegativity. We can figure out if a molecule will have a dipole by calculating the difference in electronegativity between atoms.

Helen Struble 2F

The bond type doesn't necessarily contribute to boiling point, but the IMFs associated with certain bonds do. For instance, ionic salts like NaCl have strong ion-ion forces which give it its high boiling point because it requires a lot of energy in the form of heat to overcome these forces. F 2 is a...

Helen Struble 2F

How can we predict the lattice energy of a compound. What forces are responsible for the lattice energy?

Helen Struble 2F

Remember that electrons exist in a cloud around an atom. Though we can use probability and wave functions to get an idea of where they are likely to be, we can never know for sure where electrons are around the atom/molecule at any given moment. Occasionally, the electrons random motion around the a...

Helen Struble 2F

The strength of any bond comes from the bonding electrons attraction to the nucleus. It follows that the more attracted the bonding electrons are to the nucleus of the two atoms involved in a bond, then the stronger the bond. Because the bonding electrons of smaller atoms are closer to the nucleus, ...

Helen Struble 2F

Induced dipoles are caused when a non-polar compound (no dipoles) comes close to a polar compound (with dipoles). Because the polar compound has partial charges, when the electron cloud of the non-polar compound is close, the negatively charged electrons are repelled from the negative pole and attra...

Helen Struble 2F

Given that this is chemistry for life science majors, radicals are particularly important because they can wreak havoc on biological systems. Since they are so reactive, they can react with important biological molecules, even DNA, causing mutations. I'm pretty sure radicals come into play when any ...

Helen Struble 2F

In order to answer this question, we need to look at atomic trends. F, Cl, and Br are all in the same group, but different periods. As you move down a group, atomic radius increases. Why is this important? Increasing atomic radius means increasing bond length. Longer bonds typically require less ene...

Helen Struble 2F

Formal charge allows us to find the most likely molecular structure when there are multiple valid Lewis structures. Among the valid Lewis structure, the structure with FC closest to 0 is the most likely arrangement of atoms. It is super important to note that this is not the same as resonance or ele...

Helen Struble 2F

A resonance hybrid represents electron delocalization. Though the Lewis structures may indicate a different between the single and double bonds, suggesting a difference in bond length, if a molecule or compound is a resonance hybrid then the electrons delocalize and there is no difference in bond le...

Helen Struble 2F

^^It's important to note that this is a shortcut that really only works for the s-block and p-block, when we can assume that most bonding will follow the octet rule. However, there are even exceptions to the octet rule in the p-block—take PCl 5 . So don't always assume a full octet is a full valence...

Helen Struble 2F

Because magnesium is a group 2 element, we can expect it to take on a 2+ charge, making it isoelectronic to neon and thus more stable. Though the p block is a little harder to predict, in an ionic compound, we can make an educated guess that arsenic will take on a 3- electronic charge, making it iso...

Helen Struble 2F

I think that Lavelle really only expects us to know the chromium and copper exceptions, but from the little bit of research I've done, it seems that rubidium and silver also have abnormal electron configurations because it is more stable to have a half-filled or filled d orbital. I'm not sure if thi...

Helen Struble 2F

Period 3 elements have access to the 3d orbital even though they fill the 3p orbital first. Electrons in the d orbital still can function as valence electrons. This allows them to form more bonds/have more lone pairs than the octet rule would typically allow.

Helen Struble 2F

The spin of an electron is only relevant when filling out electronic configurations. The quantum number ms in no way determines how many orbitals exist—that is all based on the principle quantum number (n), angular momentum quantum number (l), and magnetic quantum number (ml). ms comes into play whe...

Helen Struble 2F

Let's walk through the possible quantum values, knowing that if n is given, l can be = 0, 1, 2...n-1 and m can be = l, l-1, l-2....-l. An orbital is any valid set of unique quantum numbers. A) n=2, l=1. ml is not given, and can have the possible values of -1, 0, and 1. Therefore, since we can make t...

Helen Struble 2F

The second quantum number, l, describes shape because it clues us into how many radial and angular nodes are in the subshell. With the principle quantum number, n, we can figure out how many radial and angular nodes are in a subshell. N=sum of radial and angular nodes. N = n - l - 1. So, for the 3p ...

Helen Struble 2F

The use of the variable psi denotes a wave function in the same way that the use of the variable x and y denotes a cartesian function or r and theta denotes a polar function. Psi itself does not really have an equivalent in the physical system, however, as it only represents the amplitude of the wav...

Helen Struble 2F

Remember, the angular momentum quantum number, l, determines the subshell. n can have integer values greater than 0, in this case n=4 is given. l can have integer values from 0 to n-1, so if n=4, l can = 0,1,2,3. So, in a ground state there are a possible of four subshells: l=0 would be the s subshe...

Helen Struble 2F

I find that I keep visualizing light as a wave of photons now. Does the wave-particle duality mean that light is like a stream of photons in a wave? Or does it mean that every individual photon also individually behaves as a wave? If the latter, is the wavelength of light the same as the wavelength ...

Helen Struble 2F

The photoelectric effect actually studies the particle-like qualities of light, more so than electrons. If light were truly a wave, then by the standard wave model increasing the intensity of the light should eject electrons from the surface of a metal. It was assumed that more intensity = more ener...

Helen Struble 2F

The diffraction patterns exhibited by electrons prove that electrons exhibit wave-like properties as well as typical particle-like properties. Diffraction is a phenomenon which occurs in waves which are directed at an obstacle or slit which is similar in size to the wavelength. Because diffraction i...

Helen Struble 2F

Instead of using the Rydberg formula, I prefer to simply find the change in electron energy between each energy level. We know the E n = -(hR)/n 2 . This gives us the energy of an electron at every energy level in the hydrogen spectra. Knowing this, we can set up an equation for Delta E, knowing tha...

Helen Struble 2F

We only need to describe light as quantized when we are describing interactions between individual photons and electrons/atoms/m'cules. This is why we need to use the quantized model to explain the photoelectric effect—because it is individual photons exciting individual electrons, it is only the en...

Helen Struble 2F

Because you know how many moles of H 2 O are in the coffee, you can use molar ratios to find the solution. Because it is H 2 O, we know that there are two hydrogen atoms in each water molecule, therefore for each mole water, there are two moles hydrogen. Then, we can set up a conversion factor: 2 mo...

Helen Struble 2F

Once you have the moles available of each reactant, you must compare to the molar ratios given in the chemical equation. Take the balanced equation: N 2 + 3H 2 = 2NH 3 . You have already calculated moles of each reactant as 3.0 moles of N 2 and 8.0 moles of H 2 . I would proceed by picking one of th...

Helen Struble 2F

For me, the easiest way to approach this problem was simply as a dimensional analysis problem. As with any dimensional analysis, I started by establishing conversion factors. It is important to note that the problem gives the radius of an Ag atom, but since we are thinking of a string of atoms lined...

Helen Struble 2F

Keep in mind, significant figures are not only about decimal places. Leading zeros and trailing zeros are still not significant. For example, .0000000078 actually still only has two significant figures. Likewise, 78000000 only has two. Decimal places do denote significant figures when a number is wr...

Helen Struble 2F

Mols are not the same as g*mol or g*mol-1. A mole is a unit of amount—in chemistry, it is usually used to represent how many particles are in a certain mass of compound (this is much easier than doing calculations using the actual number of atoms/particles). If you need to know exactly how many atom...

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