CHEMISTRY COMMUNITY

Rory Simpson 2F

Your slow step would be the one that determines the rate, so you can compare the rate laws of the steps to whichever rate law is experimentally determined for the complete reaction.

Rory Simpson 2F

Eº is the standard cell potential which in this case refers to a cell under standard conditions (25ºC, 1atm, 1mol/L). E doesn't have these conditions, so you'd be calculating the cell potential for any differences in temperature or concentration, as the equation suggests.

Rory Simpson 2F

Platinum is the most common electrode that you'd use if you need a solid electrode for a cell, so I'd assume that you can use it anytime you need to.

Rory Simpson 2F

Intermediates are any species that are initially formed at some step in the reaction and then are consumed entirely in the reaction process. So, they won't be in the final products but they will be apparent if you write out all of the steps of the reaction.

Rory Simpson 2F

If the slow step is the rate determining step, then in order for a catalyst to speed up the reaction it would need to lower the activation energy of that slowest step. Changing the activation energy of the faster steps wouldn't do anything for the overall reaction because the rate is still determine...

Rory Simpson 2F

A reaction needs to reach the activation energy in order for the reactants to convert to products. You can calculate it numerically with the Arrhenius equation (which shows that it depends on the rate constant k and the temperature of the reaction).

Rory Simpson 2F

I think that you should probably specify if there are multiple rates being dealt with (ie an equilibrium reaction with forward or reverse rates, or multiple reactions occurring) just to be clear as to what reactants are being included in which rate laws.

Rory Simpson 2F

If you look at the initial and final equations, the numerator and the denominator of the ln of concentrations is flipped. That's where the negative sign went (since -ln(x/y) = ln(y) - ln(x) = ln(x/y) ).

Rory Simpson 2F

If you are given the experimentally determined rate law or something like that, you can find an intermediate step that matches that rate law, making that the slowest step.

Rory Simpson 2F

In concentration cells, the same half reaction is occurring but in opposite directions. For the lower concentration, the half reaction would be increasing the amount of the species in solution, while the half reaction for the higher concentration would decreased the amount. The transfer of electrons...

Rory Simpson 2F

You might want to put it so that the reactants are first and the products are second for the half reaction, just so that it is clear what is being oxidized and reduced in the reactions.

Rory Simpson 2F

The Gibbs free energy at any particular stage in the reaction is based on the concentrations of the reactants at that time. For finding the Gibbs free energy at a time, it is the difference of the molar Gibbs free energy of the products and reactants times the number of moles at the time. So, the Q ...

Rory Simpson 2F

You could use the Nernst equation for concentration cells since you could find the reaction quotient Q and then determine the cell potential using the equation. You can also use it for any cells that aren't at the standard potential (with different concentrations of species affecting the cell potent...

Rory Simpson 2F

As the redox reaction continues in a cell, the electrons will move from the anode to the cathode. If the salt bridge weren't there, eventually the reaction would stop as one half reaction gets increasingly positive and the other gets increasingly negative. With the salt bridge, the ion flow countera...

Rory Simpson 2F

The electrochemical series organizes certain ions and compounds based on how well it is able to oxidize or reduce other species. The oxidizing ability of species on the left of half reactions increases as you go up (standard potential gets higher), while the reducing ability of species on the right ...

Rory Simpson 2F

In acidic solutions, you have an excess of H+ which can be used along with H2O to balance H and O in the half reactions. In basic solutions, you have excess OH- which you would use to balance the half reactions. This is necessary to make sure that the charges and elements are balanced in the redox r...

Rory Simpson 2F

Platinum is by far the most common electrode so it can be used if there aren't any solids in the problem that could conduct e-. In the problem in lecture for example, the side with Cu(s) didn't need Pt as an electrode because Cu was the electrode and is a conductive solid. However, the side with Fe3...

Rory Simpson 2F

Basically, the cell potential of a given reaction is dependent on the concentrations on the species in the reaction. At equilibrium in a reaction, the cell potential is zero because the reaction no longer does any work. The Nernst equation gives a quantitative answer as to how different concentratio...

Rory Simpson 2F

Both voltage and electric potential are measured in volts, but voltage is the difference in the electric potentials across two points.

Rory Simpson 2F

By convention in writing out cell diagrams, the anode is typically on the left side and the cathode is typically on the right side. So, the standard cell potential would be the potential of the cathode minus the potential of the anode. You can tell if the left or right side is the cathode or anode b...

Rory Simpson 2F

The changes in $\Delta S^{\circ} and \Delta H^{\circ}$ are generally negligible with a change in temperature so we just assume that they are constant when deriving (delta G, on the other hand, does change significantly with temperature changes).

Rory Simpson 2F

Half reactions separate the oxidation and the reduction reactions that occur in a redox reaction. You can use it to find out which elements are oxidized and reduced, and also balance redox reactions (since the number of electrons transferred should cancel out in the two half reactions combined.

Rory Simpson 2F

Generally, oxygen will have an oxidation number of -2 and hydrogen will have an oxidation number of +1 across different compounds. However, there are a few exceptions to this (for example, H2O2 where oxygen has an oxidation number of -1.)

Rory Simpson 2F

The Van't Hoff equation gives us a quantitative means of relating the change in temperature to the change in the equilibrium constant (and we can find the new equilibrium constants for a reaction at different temperatures).

Rory Simpson 2F

If you want to find the oxidation states of molecules, you can base it knowing that certain groups of elements tend to have certain oxidation states (Group 1 elements typically have +1, Group 2 have +2, halogens have -1, etc.) with a few exceptions. You can use those to find the oxidation numbers fo...

Rory Simpson 2F

I think they mean that there is a point where the Gibbs free energy of ice and Gibbs free energy of water are equal, so there is no change in it and it is at equilibrium. There is a graph in the section that has a diagram of temperature vs molar Gibbs free energy where the liquid and solid states in...

Rory Simpson 2F

You would used Cv,m and Cp,m if you want to find Cv or Cp in calculating q (using q=C $\Delta$ T). The values using R to calculate C are ways of estimating the heat capacities of certain molecules if you don't know it, but I don't really think he went over that.

Rory Simpson 2F

805097738 wrote:do you have to explicitly state per mole or is that implied already

You should explicitly state it per mole in the case of standard reaction enthalpy or standard formation enthalpy because those are definitionally for one mole of a compound. I guess it's mostly for the consistency of units, though.

Rory Simpson 2F

The number that you multiply by depends on the number of bonds broken or formed. For example, if there are 2 C-C bonds broken in the reaction, then take the bond enthalpy for a C-C bond and multiply it by 2. You should probably know how to do it because certain problems might only give you the bond ...

Rory Simpson 2F

When you have a reversible system, you would want to use the equation that you get from the integral because the changes in volume are infinitesimal. So, use w = -nRTln(V2/V1) for a reversible system, which you can derive from the integral.

Rory Simpson 2F

Basically, during phase changes the temperature of a substance remains constant despite increasing amounts of energy (he showed us that phase diagram in lecture.) So, water at 100 degrees Celsius has much less energy in it than water vapor at 100 degrees Celsius because the vapor has additional ener...

Rory Simpson 2F

Enthalpy for a reaction would typically be given in joules or kilojoules. For molar enthalpy, standard reaction enthalpy or standard formation enthalpy, it would be given in J/mol or kJ/mol.

Rory Simpson 2F

I think that depending on whichever method (I'm assuming for finding delta H) they'll give you the specific info that you need in the problem or just outright tell you. For example, if you need to use Hess's law then the problem will give some equations that you can use to find the enthalpy in the o...

Rory Simpson 2F

I think it will either be given or at least obvious if the system or any energy transfer described in the problem. Just remember that open systems allow matter and energy to transfer with surroundings, closed systems only allow energy, and isolated systems don't allow any transfer with surroundings....

Rory Simpson 2F

Boltzmann's formula in particular is a means of calculating statistical entropy based on W (the amount of different microstates with the same total energy.) The entropy S calculated there is equivalent to the thermodynamic definition of entropy. I think that if using the Boltzmann equation would get...

Rory Simpson 2F

Just make sure that you choose the right R constant depending on the units for pressure and volume, or convert all the units so that you can 0.082 L*atm*mol^-1*K^-1.

Rory Simpson 2F

He drew out the Lewis structures mainly to figure out what bonds were broken and what bonds were formed during the reaction. You want to add the bond enthalpies of the reactant bonds that are broken and subtract the bond enthalpies of the product bonds that are formed (accounting for bond enthalpies...

Rory Simpson 2F

Heat isn't a state function because the amount of energy transferred as heat depends on how the change in energy occurs. On the other hand, enthalpy is a state function because no matter how the energy change occurs, it only depends on the state of the system.

Rory Simpson 2F

The reaction would shift in a way that wants to minimize the resulting increase in pressure, so that reaction would shift to the side with fewer gas molecules because it would decrease the pressure of the system.

Rory Simpson 2F

The standard enthalpy of formation is the enthalpy change that comes from forming one mole of a compound from the elements in its standard states. The standard reaction enthalpy is the change that comes from having all products and reactants in their standard states. You can find the standard enthal...

Rory Simpson 2F

Concentration and pressure only affects Q and not K; the reaction will shift accordingly to return to a ratio of reactants and products equal to K. Temperature actually changes K because of the changes in energy that are involved in the reaction.

Rory Simpson 2F

Could someone explain how autoprotolysis of water leads to Kw? need help on this too; also, does anyone know how much we need to know about the topic of autoprotolysis for the test? In the autoprotolysis reaction 2H2O(l) \rightleftharpoons H3O+(aq) + OH-(aq), we get an equilibrium constant Kw = [H3...

Rory Simpson 2F

I think the textbook is using K to refer to Kp, so the K value uses partial pressures. There's an equation to convert Kp to Kc in section 5.H, but I don't think we're expected to know it since he didn't go over it in lecture. Basically it's derived using the Ideal Gas Law to substitute in the K equa...

Rory Simpson 2F

When the reaction is endothermic, heat energy is absorbed from surroundings. Because of this, K would increase with increasing the temperature since mean that there is greater heat energy to absorb. This would mean that the reaction would shift towards the products.

Rory Simpson 2F

When you have a very small value of x it is essentially negligible when compared to the initial concentration, so we can approximate to make the calculations much simpler as compared to using a quadratic equation.

Rory Simpson 2F

The acceptable answers would be any that has it so that the equilibrium concentrations or partial pressures of the reactants or products are positive.

Rory Simpson 2F

In the table I think that K is just referring to Kp based on the fact that all of the equations only have gases. So, use "K" in the table if you are solving using partial pressure and use "Kc" in the table if using concentrations.

Rory Simpson 2F

Le Chatelier's principle is useful in describing how a chemical reaction shifts in response to changes in conditions. Changes in concentration of reactants or products, pressure, and temperature would result in shifts in the equilibrium, which is consistent with Le Chatelier's principle.

Rory Simpson 2F

You'd use the tables if you have an unknown change in the concentrations (represented by X). In the equation you said, the initial concentrations of each of the reactants would be changed by -X, while each of the products would be changed by +X. You would find the final equilibrium concentrations in...

Rory Simpson 2F

Decreasing the pressure would cause the reaction to shift towards whichever side has more moles. Because of the pressure decrease and the volume increase, the concentrations would decrease resulting in the shift.

Rory Simpson 2F

NH3 has one lone pair if you draw out its Lewis structure, meaning that it can form one coordinate covalent bond with a central metal.

Rory Simpson 2F

If you're drawing it out, the nitrogen with one H bonded together would be central and bonded to two carbons (you can pretty much figure out the order of the chain based on the order of the formula, especially if its written out at NH2CH2CH2NHCH2CH2NH2). The formal charge would be zero for everythin...

Rory Simpson 2F

The strong bases that I think we have to know are the Group 1 and 2 hydroxides and oxides.

Rory Simpson 2F

The reaction between HCl and CaO would be a neutralization reaction between an acid and base that results in the formation of a salt and water. The balanced reaction would be 2HCl + CaO -> CaCl2 + H2O. Basically once you have the equation you want to find out how much of either HCl or CaO is leftove...

Rory Simpson 2F

The molecular shape for CCl2H2 would be tetrahedral using VSEPR, so the dipoles wouldn't actually cancel each other out and there would be a net dipole.

Rory Simpson 2F

Amphoteric compounds are compounds that can react both as acids and bases and have both acidic and basic character. Amphoteric oxides are certain oxide compounds that have those characteristics, and the metals that form them are in a diagonal line just left of the metalloids in the periodic table (F...

Rory Simpson 2F

The calculations are more complicated and involve chemical equilibrium which is something that I don't think gets covered until 14B, so I think you just need to know the basics for now.

Rory Simpson 2F

If you look at the equation for chemical equilibrium, a larger Ka value would result if there is a greater amount of product than reactant at equilibrium (and so there would be a lower pKa value because pKa is -log Ka). Since strong acids almost entirely dissociate, there would be a much greater con...

Rory Simpson 2F

Generally the conjugate base of an acid is the species remaining after removing a proton, and the conjugate acid of a base is the species after accepting a proton.

Rory Simpson 2F

It's possible for there to be odd numbers of ligands, though rarer. The textbook says that the most common coordination numbers for the central metal atom are 6 and 4, so those are probably the ones that you would see the most as examples.

Rory Simpson 2F

You want to always name the ligands in alphabetical order based on the root (not the prefix), regardless of the formula order.

Rory Simpson 2F

I think that a Bronsted definition of acids means that acids require a hydrogen (because a proton H+ needs to be released), whereas a Lewis acid means that some compounds without hydrogen can still be acids in certain reactions.

Rory Simpson 2F

A polydenate is essentially a ligand that has multiple bonds with the central metal atom in coordinate complexes. Polydenates are able to form chelates, which are a type of coordinate complex that has a ring-like structure.

Rory Simpson 2F

The denticity refers to the number of lone pairs of a ligand that are bonding to the central metal atom. For example, a monodentate ligand would have one available lone pair to make a coordinate complex with a metal.

Rory Simpson 2F

You should think of this molecule in the 3d form. In a tetrahedral, no matter where you put the atoms the Chlorine atoms will always be next to each other. I can't Draw a picture on here, but just look one up it might be helpful. The lewis structure looks like u can orient the chlorines to be oppos...

Rory Simpson 2F

I think that by definition ligands are those that bind to the central atom of the coordination complex, so they can't be the central atom.

Rory Simpson 2F

The strength of the Van Der Waal's forces in the atom increases because of the greater number of electrons, which would increase the polarizability and thus the strength of IMFs.

Rory Simpson 2F

It mainly depends on the VSEPR structure. Since lone pair-lone pair repulsion forces are the strongest, you want the lone pairs to be the farthest away from each other and have the greatest bond angle in considering the position of each of the electron density regions (bonds and lone pairs).

Rory Simpson 2F

You can generally predict the bond angle based on the VSEPR molecular structure (eg a tetrahedral molecule would have bond angles of 109.5 degrees). If a central atom has lone pairs, using VSEPR you can approximate the bond angle just knowing that the lone pair-atom repulsions are greater than atom-...

Rory Simpson 2F

If you mean arrangement as in VSEPR, then it relates to hybridization because the hybridization is according to the number of bonds (and thus electron density regions which affects molecular shape).

Rory Simpson 2F

Rod shaped molecules have a larger surface area that are close to each other, meaning that there is less distance between molecules and stronger interaction of Van Der Waal's forces. The stronger intermolecular forces causes an increase in the boiling point for rod-shaped molecules as compared to sp...

Rory Simpson 2F

Resonance structures are just all of the possible Lewis structures that work for the atom when it has resonance, representing that the bonding of the resonance hybrid is an average of the resonance structure.

Rory Simpson 2F

905416023 wrote:So hydrogen bonds make the molecule harder to break apart? So then they have a higher boiling point?

Yes, the strength of hydrogen bonds causes the molecules to have a greater intermolecular attraction which increases their melting and boiling points.

Rory Simpson 2F

I think it would be a lower energy orientation because the molecule would have the positive and negative ends of adjacent atoms would be near each other, since it would be favorable to be attracted and have the molecules stay in that orientation longer.

Rory Simpson 2F

If you want to find the most stable Lewis structure, you would want to minimize formal charge and then use expanded octets if possible. Remember that expanded octets only apply for atoms in the 3rd period and after because they have d-orbitals available in their valence shell.

Rory Simpson 2F

For the photoelectric effect, if the energy of the photons is not enough to pass the threshold energy, then electrons won't be ejected regardless of intensity. This is important because it goes against the wave model since electrons wouldn't be ejected regardless of intensity, since a wave model wou...

Rory Simpson 2F

In looking at a coordinate covalent bond specifically, where two shared electrons are coming from the same atom, it's probably best to look at the Lewis structure and look at how the bond formed in the first place from reaction (where by looking at the valence shells you can see if a single atom is ...

Rory Simpson 2F

The lone pair electrons of fluorine would be interacting with each other because of the small size of fluorine atoms, causing a repelling force that would weaken the overall bond. That's why fluorine has a fairly small dissociation energy because of these lone pair electron interactions between the ...

Rory Simpson 2F

Anions would be larger because the extra electrons would cause greater electron-electron repulsion in the atom and would decrease the effective nuclear charge. Cations would be the opposite, since less electrons means less electron-electron repulsion.

Rory Simpson 2F

The more Bonds means a stronger pull so consequently you also get a smaller length because of the increased attraction. So the smaller the bond, the stronger it is? A stronger bond will typically be shorter and have a larger dissociation energy because there are more electrons acting in the bond, m...

Rory Simpson 2F

I think that if you're looking at the amount of expanded octet bonds or lone pairs for those elements you would want to consider formal charge. For example, PCl5 would have phosphorus with 5 single bonds, and phosphorus has 5 valence electrons, so the P atom would have a formal charge of 0.

Rory Simpson 2F

when do you know to turn into a double bond instead of leaving the electrons on the other side of the element when drawing a lewis structure? It would depend on the total number of electrons you have to work with (valence electrons of every atom, adjusting for net charge if there is any). Otherwise...

Rory Simpson 2F

The lower ionization energy means that the s-block metals are very likely to lose their valence electrons. This tendency makes them highly reactive.

Rory Simpson 2F

As long as you follow the octet rule (besides any exceptions), then it wouldn't matter where the additional electron is since you'll have satisfied the octet rule for every atom. For your example of BrO-, just make sure that you have the right amount of electrons. So, count out the valence electrons...

Rory Simpson 2F

Electronegativity is a value that is calculated from an formula. In the textbook section 2D.1, it mentions that the values were first calculated with an expression that determines the differences in electronegativity between bonds by using dissociation energies of that bond. All of the other periodi...

Rory Simpson 2F

If a Lewis structure has resonance it doesn't mean that there are actually different structures, but it means that the actual structure has bonds that are intermediate in properties. For example, Dr. Lavelle used nitrate as an example in class, which has 2 N-O single bonds and 1 N-O double bond. In ...

Rory Simpson 2F

The Pauli exclusion principle says that there can be no more than 2 e- per orbital; that's why the arrows are grouped in pairs with opposite spins. If you're filling out diagrams like that, you'd start by adding electrons with a maximum of 2 e- for each orbital, and adding up spin electrons first if...

Rory Simpson 2F

The total magnetic spin for the shell would add up to zero if it was full. Because of the Pauli exclusion principle (in that no two electrons can have the same exact quantum number configurations), an atom would need paired electrons with a positive and negative spin value in an full orbital.

Rory Simpson 2F

You basically just have to familiarize yourself with the conditions for the values of each quantum number. So, n = 1,2,3,...; l = 0,1,2,...,n-1; m_l = l, l-1, l-2,..., -l; and m_s = +1/2 or -1/2. 2,0,0 should theoretically be possible values for n, l, and m_l.

Rory Simpson 2F

The wave function is a way that we can model the wave-like properties of atomic particles. The orbital is the same thing, as a math function that represents the wave function nature of electrons in atoms, where orbitals can be made into a three-dimensional model for the space where certain electrons...

Rory Simpson 2F

Ionization energy refers to the minimum energy needed to remove an electron in its gas phase specifically, whereas the threshold energy is seen in the photoelectric effect (typically seen in solid metals).

Rory Simpson 2F

Each of the spectral line groupings ends with the electron at a certain energy level. For example, the Lyman series is a reduction to an energy level of n=1 (ground state), so the spectral lines result from transitions from n \geq 2 to n=1. The Balmer series is a reduction to an energy level of n=2,...

Rory Simpson 2F

In the case of atomic spectra, the electron is not ejected and is still attached; a photon is emitted because of the transition from the higher to lower energy states for the electrons, and the atomic spectra lines are showing that. That's why the spectral lines correspond with the specific energy l...

Rory Simpson 2F

For a mole of photons, it would just be the energy of a single photon (so, using a formula like E=hv) multiplied by Avogadro's number.

Rory Simpson 2F

The uncertainty principle definitely isn't noticeable and has no real practical effect on a larger object because the relative uncertainty is negligible compared to the macroscopic object; the principle mainly applies to subatomic particles where measurements have to be precise. Additionally, it wou...

Rory Simpson 2F

It's true that hydrogen is the simplest to analyze because it only has a single electron. The substitution for other elements refers to the fact that any element could theoretically be substituted as long as it only has a single electron (for example, He+ or Li2+). Otherwise, the interactions betwee...

Rory Simpson 2F

Another way would be to compare the moles of reactants that you are given to the molar ratios of the stoichiometric coefficients. For each reactant, divide the moles by the stoichiometric coefficient of the reactant, and whichever is the least is the limiting reactant :)

Rory Simpson 2F

For H7a, the unbalanced equation would be Ca(s) + H2O(l) -> H2(g) + Ca(OH)2(aq). Generally for balancing equations you would just go through each element step-by-step (starting with the least occurring element) and make sure that they are all balanced individually. Ca is already balanced, so we can ...

Rory Simpson 2F

Avogadro's constant would be used if you want to find the number of molecules or atoms of something, since one mole of a substance has 6.0221*10^23 molecules of that substance. For example, if you are asked to find the number of individual atoms of an element in some amount of moles, then you would ...

Rory Simpson 2F

pm is picometers, and 1 pm = 10^-12 m. So, 1.73*10^26 pm (10^-12 m/ 1 pm) = 1.73 * 10^14 m

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