Search found 56 matches

Sat Mar 17, 2018 9:48 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Separating electrode and solutions
Replies: 1
Views: 242

Separating electrode and solutions

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!
Thu Mar 15, 2018 5:56 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: delta S for reversible and irreversible reactions
Replies: 1
Views: 300

delta S for reversible and irreversible reactions

Can someone explain why deltaS(tot, rev)=0, but deltaS(tot, irri)>0 ?
Mon Mar 12, 2018 9:30 pm
Forum: General Rate Laws
Topic: 15.99 (e)(f)
Replies: 3
Views: 258

15.99 (e)(f)

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!
Sun Mar 11, 2018 9:38 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: 15.65 (c)
Replies: 2
Views: 227

15.65 (c)

Why k increases more for reaction with higher activation energy?

Thank you!
Sun Mar 11, 2018 9:36 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: 15.53 (c)
Replies: 1
Views: 179

15.53 (c)

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...
Tue Mar 06, 2018 1:07 pm
Forum: General Rate Laws
Topic: Rate of formation/consumption
Replies: 3
Views: 1202

Re: Rate of formation/consumption

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!
Tue Mar 06, 2018 10:46 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Determining Reaction Mechanism
Replies: 2
Views: 193

Re: Determining Reaction Mechanism

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

Hope it helps!
Tue Mar 06, 2018 10:42 am
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Four Cases
Replies: 2
Views: 223

Re: Four Cases

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...
Tue Mar 06, 2018 10:28 am
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: Catalysts
Replies: 5
Views: 486

Re: Catalysts

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!
Mon Mar 05, 2018 8:19 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Molecularity and the Coefficient of an Elementary Step
Replies: 2
Views: 187

Molecularity and the Coefficient of an Elementary Step

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!
Mon Mar 05, 2018 12:19 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: k'
Replies: 4
Views: 288

Re: k'

I think here we were just using the same notation to indicate different concepts that relate to k.
Sun Mar 04, 2018 4:56 pm
Forum: General Rate Laws
Topic: Self Test 15.6B
Replies: 1
Views: 174

Self Test 15.6B

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!
Thu Mar 01, 2018 7:41 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Acidic Cell Diagram
Replies: 1
Views: 167

Re: Acidic Cell Diagram

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...
Thu Mar 01, 2018 7:29 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Basic Cell Diagram
Replies: 1
Views: 171

Re: Basic Cell Diagram

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!
Thu Feb 22, 2018 9:16 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Creating a Cell Diagram with solids
Replies: 2
Views: 246

Re: Creating a Cell Diagram with solids

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...
Tue Feb 20, 2018 8:02 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Surface Area of Electrodes
Replies: 1
Views: 141

Surface Area of Electrodes

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!
Mon Feb 19, 2018 4:39 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: 14.41 (b)
Replies: 2
Views: 247

14.41 (b)

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!
Mon Feb 19, 2018 1:32 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: 14.37(c)
Replies: 1
Views: 167

14.37(c)

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!
Sun Feb 18, 2018 5:50 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 14.13 (d)
Replies: 2
Views: 207

14.13 (d)

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!
Sun Feb 18, 2018 5:45 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 14.13 (b)
Replies: 1
Views: 144

14.13 (b)

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!
Sun Feb 18, 2018 11:55 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 14.11 (e)
Replies: 1
Views: 174

14.11 (e)

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?

Tue Feb 13, 2018 8:27 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: isothermal and deltaU=0
Replies: 3
Views: 350

isothermal and deltaU=0

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!
Wed Feb 07, 2018 8:54 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: 9.47
Replies: 1
Views: 135

9.47

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...
Wed Feb 07, 2018 8:37 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: 9.37
Replies: 1
Views: 162

9.37

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...
Wed Feb 07, 2018 8:23 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Determining Anode/Cathode in a Cell Diagram
Replies: 2
Views: 274

Re: Determining Anode/Cathode in a Cell Diagram

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.
Wed Feb 07, 2018 8:14 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: isothermal reversible expansion
Replies: 1
Views: 168

isothermal reversible expansion

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!
Tue Feb 06, 2018 9:15 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: 9.33 (c)
Replies: 1
Views: 136

9.33 (c)

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...
Tue Jan 23, 2018 10:54 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Work and Volume
Replies: 2
Views: 177

Re: Work and Volume

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.
Tue Jan 23, 2018 7:23 am
Forum: Calculating Work of Expansion
Topic: reversible and irreversible [ENDORSED]
Replies: 2
Views: 134

reversible and irreversible[ENDORSED]

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!
Mon Jan 22, 2018 9:38 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.61
Replies: 1
Views: 197

8.61

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?
Mon Jan 22, 2018 3:52 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: ΔU = ΔH for biological reactions
Replies: 2
Views: 194

Re: ΔU = ΔH for biological reactions

Since w=-P deltaV and there is no change in volume in the biological system, deltaV=0 and therefore w=0.
Mon Jan 22, 2018 3:47 pm
Forum: Phase Changes & Related Calculations
Topic: 8.21 Heat Lost vs. Heat Gained
Replies: 3
Views: 280

Re: 8.21 Heat Lost vs. Heat Gained

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...
Tue Jan 16, 2018 11:19 am
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Example 8.6 (b)
Replies: 2
Views: 208

Re: Example 8.6 (b)

Thank you, but can you please explain more in detailed? I'm still confused. Thank you!
Mon Jan 15, 2018 3:50 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Example 8.6 (b)
Replies: 2
Views: 208

Example 8.6 (b)

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...
Sat Dec 09, 2017 3:34 pm
Forum: Calculating pH or pOH for Strong & Weak Acids & Bases
Topic: Confused about What Lavelle Mentioned in class
Replies: 1
Views: 205

Confused about What Lavelle Mentioned in class

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...
Wed Dec 06, 2017 12:15 pm
Forum: Calculating pH or pOH for Strong & Weak Acids & Bases
Topic: Relative Acidity
Replies: 3
Views: 640

Re: Relative Acidity

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...
Sun Dec 03, 2017 11:00 pm
Forum: Calculating pH or pOH for Strong & Weak Acids & Bases
Topic: Very acidic and Very basic
Replies: 1
Views: 146

Re: Very acidic and Very basic

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...
Sun Dec 03, 2017 10:51 pm
Forum: Properties & Structures of Inorganic & Organic Acids
Topic: Weak Acids and Bases
Replies: 2
Views: 275

Re: Weak Acids and Bases

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...
Tue Nov 28, 2017 10:16 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: HW 11.43
Replies: 1
Views: 153

HW 11.43

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...
Thu Nov 23, 2017 10:28 am
Forum: Determining Molecular Shape (VSEPR)
Topic: HW 4.43
Replies: 1
Views: 133

HW 4.43

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...
Thu Nov 23, 2017 10:23 am
Forum: Determining Molecular Shape (VSEPR)
Topic: HW 4.15 (c)
Replies: 1
Views: 139

HW 4.15 (c)

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!
Wed Nov 22, 2017 12:13 pm
Forum: Naming
Topic: HW 17.29 (a)
Replies: 2
Views: 144

HW 17.29 (a)

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"?
Wed Nov 22, 2017 11:50 am
Forum: Shape, Structure, Coordination Number, Ligands
Topic: Determine the Charge on Ligands
Replies: 4
Views: 3592

Determine the Charge on Ligands

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?
Sun Nov 19, 2017 3:54 pm
Forum: Lewis Structures
Topic: Lewis Structure of CO
Replies: 2
Views: 234

Lewis Structure of CO

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...
Sun Nov 12, 2017 8:41 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Trigonal Bipyramidal with 1 lone pair and 4 bonds
Replies: 3
Views: 321

Trigonal Bipyramidal with 1 lone pair and 4 bonds

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...
Sun Nov 12, 2017 8:24 pm
Forum: Electronegativity
Topic: Determine Electron Affinity
Replies: 2
Views: 287

Determine Electron Affinity

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.
Sun Nov 05, 2017 8:20 pm
Forum: Polarisability of Anions, The Polarizing Power of Cations
Topic: HW 3.83
Replies: 6
Views: 1838

HW 3.83

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 ...
Sun Nov 05, 2017 7:31 pm
Forum: Lewis Structures
Topic: HW 3.59 PART C
Replies: 1
Views: 117

HW 3.59 PART C

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...
Fri Oct 27, 2017 9:56 pm
Forum: Lewis Structures
Topic: Lewis Structures- resonance [ENDORSED]
Replies: 4
Views: 317

Re: Lewis Structures- resonance[ENDORSED]

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.
Fri Oct 27, 2017 9:46 pm
Forum: Resonance Structures
Topic: Delocalized Structures [ENDORSED]
Replies: 2
Views: 193

Re: Delocalized Structures[ENDORSED]

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...
Tue Oct 17, 2017 1:08 pm
Forum: Photoelectric Effect
Topic: Post Module #23 [ENDORSED]
Replies: 2
Views: 224

Re: Post Module #23[ENDORSED]

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.
Mon Oct 16, 2017 9:39 pm
Forum: DeBroglie Equation
Topic: λ=c/v VS. λ=h/mv
Replies: 9
Views: 6141

λ=c/v VS. λ=h/mv

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?
Thu Oct 12, 2017 9:50 pm
Forum: Properties of Light
Topic: Question 1.33
Replies: 1
Views: 137

Re: Question 1.33

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
Thu Oct 12, 2017 9:14 pm
Forum: Einstein Equation
Topic: Experimental determination of wavelength/energy
Replies: 1
Views: 212

Re: Experimental determination of wavelength/energy

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 ...
Thu Oct 05, 2017 6:50 pm
Forum: Balancing Chemical Reactions
Topic: Percent yield [ENDORSED]
Replies: 6
Views: 726

Re: Percent yield[ENDORSED]

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...
Thu Oct 05, 2017 12:52 pm
Forum: Accuracy, Precision, Mole, Other Definitions
Topic: Question about E.35
Replies: 1
Views: 255

Re: Question about E.35

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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