CHEMISTRY COMMUNITY

Hannah Guo 3D

When writing a cell diagram, if there is a solid metal electrode and a solid reactant, do we need to separate them by a single line, or just a comma?
I remember Lavelle mentioned that we should use a comma, but the student solution manual always uses a single line, so I got confused.
Thank you!

Hannah Guo 3D

Can someone explain why deltaS(tot, rev)=0, but deltaS(tot, irri)>0 ?

Hannah Guo 3D

15.99 Which of the following plots will be linear?
(e) k against temperature;
(f) initial rate against [A] for a reaction that is first order in A;

Can someone explain why e is not linear and f is linear?

Thank you!

Hannah Guo 3D

Why k increases more for reaction with higher activation energy?

Thank you!

Hannah Guo 3D

Three mechanisms for the reaction NO2(g) CO(g) + CO2(g) --> NO(g) have been proposed: (c)Step1 NO2 + NO2 ⇌ NO + NO3 and its reverse (both fast, equilibrium) Step2 NO3 + CO --> NO2 + CO2 (slow) Which mechanism agrees with the following rate law: Rate = k[NO2]^2? Explain your reasoning. The answer say...

Hannah Guo 3D

When calculating the rate of consumption/formation, I think we only look at the concentration change of a single reactant or product and then divide that by the time it takes to make the change.

Hope it helps!

Hannah Guo 3D

Based on the practice problems in the textbook, the questions usually indicate the slow and fast elementary steps.

Hope it helps!

Hannah Guo 3D

One way to look at this problem is to plug numbers into the formula ΔG=ΔH-TΔS and try to get the answer through comparing the results. When 1. ΔS +, ΔH - : (T is always >0) thus ΔG<0 -----> spontaneous at all T 2. ΔS +, ΔH + : if TΔS > ΔH, then ΔG<0; so T needs to be large -----> spontaneous at high...

Hannah Guo 3D

The catalyst lowers the activation energy by providing the reaction an alternative energy pathway. With a catalyst, more collisions result in a reaction because the reactants need less energy to get to the transition state (with the highest energy), so the rate of reaction increases.

Hope it helps!

Hannah Guo 3D

Is there any association between the molecularity and the coefficient of an elementary step. For example, when the molecularity is termolecular, does it mean that the coefficients of the reactants in a balanced elementary step add up to be 3?
Thank you!

Hannah Guo 3D

I think here we were just using the same notation to indicate different concepts that relate to k.

Hannah Guo 3D

For the question "Self Test 15.6B" under "Example 15.4" in section 15.4, how come it says for a first-order reaction, rate=kP?
Thank you!

Hannah Guo 3D

We need to first balance the half reaction. If there is oxygen on one side of the half reaction, then add H2O on the other side to balance the oxygen, and then add H+ (if the reaction happens in acidic solution) on the side of oxygen to balance the hydrogen in H20. If this half-reaction is an oxidat...

Hannah Guo 3D

If the reaction happens in OH-(aq) or if the reactants or products can dissociate OH- after they are dissolved in water, then we see if OH- is involved in the reaction. If so, we include OH-(aq) in the cell diagram.
Hope this helps!

Hannah Guo 3D

If the half reaction doesn't have a solid metal, then we need Pt to act as the electrode. Lavelle also mentioned that sometimes people use graphite as an electrode, but generally we still use Pt. In some rare cases, if there is liquid Hg in the half reaction, then we can use it as an electrode. In c...

Hannah Guo 3D

When the surface area of the electrodes increases, the rate of redox reaction is supposed to increase, but will this change affect the values of E and E°?

Thank you!

Hannah Guo 3D

When calculating the concentration of reactants and products in order to get Q for the reaction in which H+ --> H2, do we need to consider the coefficient of the H+? I find it a bit confusing because we consider the coefficient of H+ when calculating the Q in 14.39 (b), but not in 14.41(b). Thank you!

Hannah Guo 3D

I know that when calculating Q, we need to include both the concentration and pressure, but why does the answer calculate as Q=(P(H2)*P(Cl2))/([H^+]*[Cl^-])?
H2 and Cl2 are on different sides of the salt bridge.

Thank you!

Hannah Guo 3D

Based on the original chemical equation, Au^+ was reduced to Au(s) and was oxidized to Au^(3+), but the answer reverses the anode reaction and yield Au(s)-->Au^(3+) + 3e^-. Can someone please explain why we need to reverse the half-reaction? Thank you!

Hannah Guo 3D

Can someone explain why we still need Pt(s) on the left side of the salt bridge even if there is I2(s)? Doesn't I2(s) already act as a conductor?
Thank you!

Hannah Guo 3D

1. On the left side of the double line (salt bridge), Sn^4+ was reduced to Sn^2+. Does it mean that we can write the reduction reaction on the left side of the salt bridge in cell diagrams?

2. On the right side of the salt bridge, what is the conductor?

Please help! Thank you!

Hannah Guo 3D

In homework 9.47(b) and also from Lavelle's slides, it says when there is an isothermal expansion, deltaU=0. Can someone explain how to lead to this conclusion? Thank you!

Hannah Guo 3D

9.47 Initially a sample of ideal gas at 323 K occupies 1.67 L at 4.95 atm. The gas is allowed to expand to 7.33 L by two pathways: (a) isothermal, reversible expansion; (b) isothermal, irreversible free expansion. Calculate Stot, S, and Ssurr for each pathway. For (a), why since the process is rever...

Hannah Guo 3D

When asked to calculate "standard reaction entropy", when should we calculate the standard reaction entropy of the whole reaction as opposed to calculating standard reaction entropy of per mole of reactants or products? From 9.37 (a) to (c), the answers calculate the standard reaction entr...

Hannah Guo 3D

Lavelle mentioned in class that the side on the left of the double line (salt bridge) represents the anode and the one on the right represents the cathode.

Hannah Guo 3D

In notes from a past lecture talking about isothermal reversible expansion, Lavelle gave an example and explained that in ideal gas, U(tot)=3/2nRT, which leads to deltaU=3/2nR deltaT=0. Can someone help me explain how to get the "3/2" for ideal gas? Thank you!

Hannah Guo 3D

9.33 (c): Without performing any calculations, predict whether there is an increase or a decrease in entropy for each of the following processes: SO2(g) + Br2(g) + 2 H2O(l) --> H2SO4(aq) + 2 HBr(aq). I thought entropy decrease because there are fewer moles of gas on the product side, but why the ans...

Hannah Guo 3D

Not always. Work only occurs when there is an external pressure causing volume change. If the external pressure equals zero, even if the volume changes, work still equals to zero.

Hannah Guo 3D

I'm a little bit confused about formulas for reversible and irreversible work, and formulas to use when P or V is constant. Can someone please explain? Thank you!

Hannah Guo 3D

The question asks to calculate the reaction enthalpy for the synthesis of hydrogen bromide gas. Shouldn't the unit of the answer be kJ/mol instead of just kJ?

Hannah Guo 3D

Since w=-P deltaV and there is no change in volume in the biological system, deltaV=0 and therefore w=0.

Hannah Guo 3D

In this case, q(copper)=-q(water) or -q(copper)=q(water) are the same. The negative sign just indicates that the heat lost by one is gained by the other. As long as q(copper) + q(water) = 0, in another word, as long as the system follows the conservation of energy, the calculation is correct. Hope i...

Hannah Guo 3D

Thank you, but can you please explain more in detailed? I'm still confused. Thank you!

Hannah Guo 3D

I understand everything except the statement in part b: because U is independent of volume for an ideal gas, delta U (step 2) = 0. Please help! The question asks: Calculate the final temperature and the change in internal energy wen 500. J of energy is transferred as heat to 0.900 mol O2(g) at 298 K...

Hannah Guo 3D

Can anyone explain the following statement? It's in Lavelle's class notes. If [H3O+]< 10^-7 then solution is considered neutral because we know autoprotolysis generates 10^ -7 mol/L H3O+. I understand how we get 10^ -7 mol/L H3O+ in neutral solutions, but I don't understand the first part of this st...

Hannah Guo 3D

Cl atoms are more electronegative than H atoms, so electrons are more delocalized in CCl3COOH. We know that the electrons in strong acids are easier to be delocalized, resulting in higher yield of H30+. Another way to think about this is that since CCl3COOH has a lower pKa value and pKa=-log[Ka], CC...

Hannah Guo 3D

Formulas we need: pH=-log[H+] pOH=-log[OH-] 14=pH+pOH pH of 1 M H3O+: -log[1]=0 pOH of 10^-14 M OH-: -log[10^-14]=14 pH of 10^-14 M OH-: 14-pOH=0 Both solutions have very low pH values, so they are very acidic pOH of 1 M of OH-: -log[1]=0 pH of 1 M of OH-: 14-0=14 pH of 10^-14 M H3O+: -log[10^-14]=1...

Hannah Guo 3D

When strong acids or bases are dissolved in water, they are fully protonated, meaning the most of the protons are dissociated with acids and accepted by bases. Since strong acids and bases tend to be fully protonated in water and the reactions tend to shift right, the equilibrium constants of the re...

Hannah Guo 3D

11.43 Consider the reaction 2 NO(g) <---> N2(g) + O2(g). If the initial partial pressure of NO(g) is 1.0 bar, and x is the equilibrium concentration of N2(g), what is the correct equilibrium relation? (a) K = x^2/(1.0 - x); (b) K = x^2; (c) K = x^2/(1.0 - 2x)^2; (d) K = 4x^3/(1.0 - 2x)^2; (e) K = 2x...

Hannah Guo 3D

4.43 Noting that the bond angle of an sp3 hybridized atom is 109.5 and that of an sp2 hybridized atom is 120, do you expect the bond angle between two hybrid orbitals to increase or decrease as the s-character of the hybrids is increased? The answer is: As the s-character of a hybrid orbital increas...

Hannah Guo 3D

When writing the VSEPR for COF2, I put AX4, but the correct answer is AX3. Do we consider that there is only 1 bonded atoms around C in CO double bond? Please explain. Thank you!

Hannah Guo 3D

The correct naming for [Fe(CN)6]^4- is hexacyanoferrate(II) ion, but according to the Table 17.4, the name of ligand CN^- is "cyanido". Why the correct answer is "cyano" but not "cyanido"?

Hannah Guo 3D

How do we determine the charge on ligands? I know we are supposed to draw the Lewis structure first, but then how can we go from that?

Hannah Guo 3D

I drew the Lewis structure of CO as there is a double bond in between, so there are 2 lone pairs around the oxygen and 1 lone pair around the carbon, but the correct Lewis structure of CO has a triple bond in between and 1 lone pair around each atom. Isn't the formal charge of the first Lewis struct...

Hannah Guo 3D

Prof. Lavelle talks about when there is one lone pair of electrons and 4 bonds in a trigonal bipyramidal molecule (AX4E), the lone pair is more likely to be in the equatorial plane because the lone pair in this way has less repulsion and is more stable. Can anyone explain it in detail that why the l...

Hannah Guo 3D

I'm a little confused about why from Ni to Pd, the electron affinity decreases, but from Cu to Ag, electron affinity increases. In both of the cases, the second element is in the same group as the first element.

Hannah Guo 3D

3.81 Arrange the cations Rb+, Be2+, and Sr2+ in order of increasing polarizing power. Give an explanation of your arrangement. 3.83 Arrange the anions Cl-, Br-, N3-, and O2- in order of increasing polarizability and give reasons for your decisions. According to HW 3.81, smaller, more highly charged ...

Hannah Guo 3D

3.59 Write the Lewis structure of each of the following reactive species, all of which are found to contribute to the destruction of the ozone layer, and indicate which are radicals:(c) chlorine nitrate, ClONO2 (the central O atom is attached to the Cl atom and to the N atom of the NO2 group). I dre...

Hannah Guo 3D

In the real world, nitrate ions are always changing between the three resonance structures because when electrons move to different locations, one structure can easily change to another one. I wouldn't say there is a preferred structure, but we should look at what the questions ask us to do.

Hannah Guo 3D

Delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond. Delocalized electrons form partial double bonds, meaning the distance between electrons and atoms is further than the one in double bonds. In other words, the delocali...

Hannah Guo 3D

You are on the right track, but light only has wavelike and particle-like properties. Photon is the subunit of light, but it's not one of the light properties.
So the answer is wave and particle.

Hannah Guo 3D

What's the difference between λ=c/v and λ=h/mv (De Broglie's wave equation λ=h/p)? Do we use λ=c/v when calculating the wavelength of photons and λ=h/mv when calculating the wavelengths of other moving particles?
Please help. Thank you!

Hannah Guo 3D

From b, we know E(work function)= 1.66* 10^-17 J.
For c, E(photon)=E(kinetic)+E(work function)=2.25*10^-17 J; and then, λ=hc/E(photon)=8.8 nm

Hannah Guo 3D

When a photon hits a metal surface with high enough energy, the metal surface will emit an electron, which is then detected and calibrated for its energy. The scientist can calculate the energy (E) of the photon from the electron's kinetic energy and the thermal energy released in the air. When the ...

Hannah Guo 3D

You don't need to care about other solutes that are not involved in the reaction since they have no influence on the reaction. The following steps are what you need to solve the problem: 1. converting 35 g ammonia to moles 2. calculating the expected moles of Hydrazine in the product 3. converting t...

Hannah Guo 3D

percentage mass increase = change of mass/initial mass
So what you understand is correct, the calculation should be (504.3-502.31)/502.31*100%

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