Search found 52 matches

by Haocheng Zhang 2A
Sat Mar 17, 2018 4:06 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Test 2 Question 8b
Replies: 9
Views: 584

Re: Test 2 Question 8b

I asked my high school chemistry teacher this question, he said we don't have to convert them to the same unit becasue equilibrium constant is unitless, so I am wrong. Sorry about that.
by Haocheng Zhang 2A
Sat Mar 17, 2018 4:05 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 14.39
Replies: 2
Views: 122

Re: 14.39

I asked my high school chemistry teacher this question, he said we don't have to convert them to the same unit becasue equilibrium constant is unitless, so I am wrong. Sorry about that.
by Haocheng Zhang 2A
Sat Mar 17, 2018 3:15 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Pre-Equilibrium
Replies: 2
Views: 124

Re: Pre-Equilibrium

I think it is better to write the forward rate constant over the reverse since the sample question in textbook 15.8 writes in this way.
by Haocheng Zhang 2A
Sat Mar 17, 2018 3:10 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Value of n
Replies: 3
Views: 188

Re: Value of n

write the half reaction and determine how many mole of electrons are transfered in 1 mole of reaction. The mole of electrons is "n".
by Haocheng Zhang 2A
Sat Mar 17, 2018 2:11 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 14.39
Replies: 2
Views: 122

When calculating Q or K, we have to convert concentration to same unit.
by Haocheng Zhang 2A
Thu Mar 15, 2018 12:03 am
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Test 2 Question 8b
Replies: 9
Views: 584

Re: Test 2 Question 8b

You need to convert bar to M. We need to keep unit the same. Think of the idea gas equation PV=nRT, then we have P/RT=n/V. The number of mole divided by volume is molarity, so P/RT=Molarity. That's how we convert bar to M. Hope it helps!
by Haocheng Zhang 2A
Wed Mar 14, 2018 11:57 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Test 2 Q6
Replies: 2
Views: 144

Re: Test 2 Q6

The oxidation number of F- is always -1, so it is not reducing agent. Ag+ is the reducing agent since its oxidation number change from +1 to 0. Hope it helps!
by Haocheng Zhang 2A
Mon Mar 05, 2018 10:00 pm
Forum: Second Order Reactions
Topic: 15.35a
Replies: 1
Views: 110

Re: 15.35a

The final concentration is one-sixteenth of its original concentration, so [A] = (1/16)[A0]. 1/[A] = 1/(1/16)[A0]. Since 1/(1/16)=16, 1/[A] = 16/[A0].
by Haocheng Zhang 2A
Sun Mar 04, 2018 2:31 pm
Forum: Method of Initial Rates (To Determine n and k)
Topic: 15.19(c)
Replies: 2
Views: 121

Re: 15.19(c)

But in the solution manual, the unit of k is L^4*mmol^-4*s^-1, so I think we don't have to convert mmol to mol. Is the solution manual wrong?
by Haocheng Zhang 2A
Sun Mar 04, 2018 2:25 pm
Forum: Second Order Reactions
Topic: Slope of second order reaction
Replies: 3
Views: 171

Re: Slope of second order reaction

For the second order reaction, k is proportional to 1/[A]. As the time of reaction increases, the concentration [A] decreases and 1/[A] increases, so the slope is positive.
by Haocheng Zhang 2A
Sat Mar 03, 2018 10:07 pm
Forum: Method of Initial Rates (To Determine n and k)
Topic: 15.19(c)
Replies: 2
Views: 121

15.19(c)

For homework 15.19 part c, my answer is 2.85(8.7=k*1.25*(1.25^2)*(1.25^2)), but the solution manual says the answer is 2.85*10^12. I notice that the unit of initial rate is (mmol A)*L^-1*s^-1, so (mmol A) means mmol*10^12?
by Haocheng Zhang 2A
Sat Mar 03, 2018 8:48 pm
Forum: General Rate Laws
Topic: Rate constant units
Replies: 2
Views: 110

Re: Rate constant units

Unit of k for zero order: mol*L^-1*s^-1 Unit of k for first order: s^-1 Unit of k for second order: L*mol^-1*s^-1 Unit of k for third order: L^2*mol^-2*s^-1 The unit of reaction rate is always mol*L^-1, so you can derive this by dimensional analysis. Also be careful the order should always be overal...
by Haocheng Zhang 2A
Mon Feb 19, 2018 8:43 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: HW 14.37(C)
Replies: 2
Views: 125

HW 14.37(C)

Determine the potential: Pt(s)|Cl2(g,250 Torr)|HCl(aq,1.0 M)||HCl(aq,0.85 M)|H2(g,125 Torr)|Pt(s)

Can someoen show me how to do this question? Thanks!
by Haocheng Zhang 2A
Mon Feb 19, 2018 8:36 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Number 14.35 vs. Number 14.37
Replies: 1
Views: 81

Re: Number 14.35 vs. Number 14.37

The table gives us the value of E for reduction. If the half reaction is oxidation, we should flip the sign of E.
by Haocheng Zhang 2A
Sat Feb 17, 2018 10:22 pm
Forum: Balancing Redox Reactions
Topic: process to balance redox reactions
Replies: 2
Views: 146

Re: process to balance redox reactions

It is useful to read the proceduces of balancing redox equation in textbook page 563. It answers your question in Step4.
by Haocheng Zhang 2A
Mon Feb 12, 2018 9:42 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Internal Energy State Function
Replies: 2
Views: 119

Re: Internal Energy State Function

I think internal energy is a state function. Internal energy does not depend on the path of the process. For example, reversible and irreversible isothermal expansion follow different paths, but the delta U of both expansions are zero.
by Haocheng Zhang 2A
Mon Feb 12, 2018 11:27 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Heat Capacity
Replies: 2
Views: 92

Re: Heat Capacity

You use heat capacity of water for liquid state and heat capacity of ice for solid state.
by Haocheng Zhang 2A
Sun Feb 11, 2018 8:36 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Isothermal Expansion
Replies: 1
Views: 97

Re: Isothermal Expansion

If external pressure is greater than internal pressure, there will be compression instead of expansion.
by Haocheng Zhang 2A
Mon Feb 05, 2018 8:00 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: HW 9.65
Replies: 1
Views: 69

HW 9.65

I think both PCl5 and SO2 become less stable in higher temperature, but the answer says only PCl5 will be less stable. Can someone explain me why?
by Haocheng Zhang 2A
Mon Feb 05, 2018 7:35 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Unstable and Stable Compounds
Replies: 2
Views: 127

Re: Unstable and Stable Compounds

Because the delta G of formation is negative, the formation process is spontaneous. The reverse process, which is the decomposition, will have a positive delta G, so the decomposition is not spontaneous and the compound is stable.
by Haocheng Zhang 2A
Sun Feb 04, 2018 2:59 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: q=-w [ENDORSED]
Replies: 5
Views: 206

Re: q=-w [ENDORSED]

For the first question, we assume the system and its surrounding are at equilibrium, which means they have same temperature. When the system is doing work, it releases energy and its temperature decreases. It is a spontaneous process that the system absorbs heat from its surrounding to reach equilir...
by Haocheng Zhang 2A
Sun Jan 28, 2018 11:02 am
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: 9.7
Replies: 2
Views: 97

9.7

Assuming that the heat capacity of an ideal gas is independent of temperature, calculate the entropy change associated with rising the temperature of 1.00mol of ideal gas atoms reversibly from 37.6℃ to 157.9℃ at (a)constant pressure and (b)constant volume. I have no idea how to do this question. Can...
by Haocheng Zhang 2A
Sat Jan 27, 2018 8:47 pm
Forum: Calculating Work of Expansion
Topic: U for Reversible & Irreversible Expansion
Replies: 2
Views: 108

Re: U for Reversible & Irreversible Expansion

If both systems start and end with same initial and final internal energy, the change of internal energy should be the same, but the work done by the system(W) and the heat abosorbed by system(q) will be different in these two expansions.
by Haocheng Zhang 2A
Sat Jan 27, 2018 8:34 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: units for delta H
Replies: 4
Views: 178

Re: units for delta H

I think the units of both bond enthalpy and standard enthalpy of formation are KJ/mol. When we use bond enthalpy and standard enthalpy to calculate reaction enthalpy, we time the number of mol, so the unit of reaction enthalpy is KJ.
by Haocheng Zhang 2A
Sun Jan 21, 2018 3:28 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: 8.3
Replies: 3
Views: 115

8.3

Air in a bicycle pump is compressed by pushing in the handle. The solution says the work with respect to the air in the pump is positive. But if that's true, then in order to do positive work, doesn't the air in the pump have to expand and release energy? I am confused.
by Haocheng Zhang 2A
Sat Jan 20, 2018 8:26 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Molar heat capacities of an ideal gas
Replies: 2
Views: 125

Re: Molar heat capacities of an ideal gas

Since PV=nRT, in order to increase the temperature and keep constant pressure, the volume has to increase. When the volume increases, the gas is doing work,which requires energy. Therefore Not all of energy is used to raise temperature.
by Haocheng Zhang 2A
Sat Jan 20, 2018 8:14 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: 8.73
Replies: 2
Views: 130

Re: 8.73

For part (b), we need to consider C-H bond since the reaction requires energy to break it. For part (c), the reactants and products have same number of C-H bond and C-Cl bond, which means the energy abosrbed to break bonds and the energy released to form bonds are same. So the change of enthalpy is ...
by Haocheng Zhang 2A
Sun Jan 14, 2018 10:02 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: standard reaction enthalpy
Replies: 1
Views: 88

Re: standard reaction enthalpy

It's simple. Change of enthalpy = final standard enthalpy - initial standard enthalpy. Just like change of temperature = final temperature - initial temperature. For reaction, initial standard enthalpy is the sum of the standard enthalpy of all reactants and final standard enthalpy is the sum of the...
by Haocheng Zhang 2A
Sun Jan 14, 2018 9:57 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: State Property
Replies: 3
Views: 163

Re: State Property

According to my understanding, state property, like temperature, describes the state of something. A cup of water is currently at 25℃ no matter now the water get to that temperature. Heating that cup of water from 0℃ to 25℃ or from 10℃ to 25℃ require different amount of heat. So heat is not a state ...
by Haocheng Zhang 2A
Thu Jan 11, 2018 12:21 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Catalysts and Enthalpies
Replies: 2
Views: 119

Re: Catalysts and Enthalpies

I think you are right. When we consider state property like enthalpy, the speed doesn't matter. The distance between Royce hall and Young hall is always the same regardless whether we choose to walk or ride a bicycle.
by Haocheng Zhang 2A
Fri Dec 08, 2017 10:21 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Pressure Effect
Replies: 3
Views: 163

Re: Pressure Effect

Increasing pressure means decreasing volume since PV=nRT(P is pressure and V is volume). By increasing volume, you increase the concentration of b and c. But during this process, the equibirium constant does not change because the temperature does not change. Kc=[b][c] for these reaction. We increas...
by Haocheng Zhang 2A
Fri Dec 08, 2017 9:37 pm
Forum: Conjugate Acids & Bases
Topic: basicity strength
Replies: 3
Views: 247

Re: basicity strength

If you get the value of Kb or pKb of bases, the one with higher Kb or lower pKb is stronger. Also, you can see the basicity strength from the stability of base or its conjugate acid. The base that is less stable or have more stable conjugate acid is stronger.
by Haocheng Zhang 2A
Mon Dec 04, 2017 11:18 pm
Forum: Conjugate Acids & Bases
Topic: HW 12.3
Replies: 3
Views: 175

HW 12.3

The answer says the conjugate base of H2SO4 is HSO4-. I wonder whether SO4 2-is also the conjugate base of H2SO4 or it is the conjugate base of HSO4-. I remeber in high school I was told conjugate pair can only have the difference of one hydrogen.
by Haocheng Zhang 2A
Mon Dec 04, 2017 2:33 pm
Forum: Calculating pH or pOH for Strong & Weak Acids & Bases
Topic: when to ignore the original concentration of OH- and H3O+ in neutral water [ENDORSED]
Replies: 1
Views: 136

when to ignore the original concentration of OH- and H3O+ in neutral water [ENDORSED]

Sometime when calculating the concentration of OH- or H3O+, we only count the OH- and H3O+ from the acid or base added to water and ignore the original concentration of OH- and H3O+ in neutral water(10^-7). I know for most of time 10^-7 is really small and we can ignore it. But in what situation we ...
by Haocheng Zhang 2A
Tue Nov 28, 2017 7:40 pm
Forum: Bronsted Acids & Bases
Topic: Bronsted and Lewis acid definitions?
Replies: 3
Views: 228

Re: Bronsted and Lewis acid definitions?

I think Bronsted acids and Lewis acids are just different by definitions. Bronsted acids are proton donators and Lewis acids are electron acceptors. I don't think they are related to strong or weak acids.
by Haocheng Zhang 2A
Mon Nov 27, 2017 10:41 pm
Forum: Naming
Topic: why chloro instead of monochloro
Replies: 3
Views: 172

Re: why chloro instead of monochloro

Also, how to identify a polydentate ligand?
by Haocheng Zhang 2A
Mon Nov 27, 2017 10:00 pm
Forum: Naming
Topic: why chloro instead of monochloro
Replies: 3
Views: 172

why chloro instead of monochloro

The name of [Pt(NH3)5Cl]Br3 is pentaamminechloroplatinum(IV) bromide. I wonder why we write chloro instead of monochloro.
by Haocheng Zhang 2A
Mon Nov 27, 2017 9:51 pm
Forum: Naming
Topic: test 4 [ENDORSED]
Replies: 4
Views: 229

Re: test 4 [ENDORSED]

Thanks you guys!
by Haocheng Zhang 2A
Mon Nov 27, 2017 8:28 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Shifts or Lie to the Right or Left
Replies: 1
Views: 121

Re: Shifts or Lie to the Right or Left

I personally think we use "shift to the left or right" when the reaction is not at equilibrium and goes toward either direction. We use "lie to the left or right when equilibrium constant is quite small or large so the reaction strongly favors reactants or products.
by Haocheng Zhang 2A
Mon Nov 27, 2017 8:19 pm
Forum: Ideal Gases
Topic: I deal gas law formula
Replies: 3
Views: 360

Re: I deal gas law formula

PV=nRT
P is the pressure of the system. V is the volume. n is the total number of moles of gases in system. R is a constant. T is temperature.(be careful the unit of temperture must be Kelvin)
by Haocheng Zhang 2A
Sun Nov 26, 2017 9:46 pm
Forum: Naming
Topic: test 4 [ENDORSED]
Replies: 4
Views: 229

test 4 [ENDORSED]

Do we need to comprehend naming of complex compounds for upcoming test?
by Haocheng Zhang 2A
Sun Nov 26, 2017 9:31 pm
Forum: Ideal Gases
Topic: Conversions- Gases
Replies: 3
Views: 198

Re: Conversions- Gases

I don't think so. I think we will be given different values of constant R for different units of pressure so we don't have to convert the unit.
by Haocheng Zhang 2A
Fri Nov 17, 2017 6:27 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: electron configuration for Fe2+
Replies: 3
Views: 5237

electron configuration for Fe2+

For midterm question Q5C, why the electron configuration for Fe2+ is not [Ar]3d^5 4S^1?
by Haocheng Zhang 2A
Wed Nov 08, 2017 10:36 pm
Forum: Trends in The Periodic Table
Topic: 2.93
Replies: 3
Views: 207

Re: 2.93

The size of atom in neutral state decrease from right to left, so A should be Cl and B should be Na. After the reaction, Na loses an electron and becomes Na+. It no longer have eletron in 3s orbit, so Na+ is actually smaller than Cl-.Thus, C should be Cl- and D should be Na+.
by Haocheng Zhang 2A
Sun Oct 22, 2017 8:33 pm
Forum: Trends in The Periodic Table
Topic: List of all trends?
Replies: 5
Views: 416

Re: List of all trends?

I think these videos from Khan Academy can help you better understand all the periodic table trends.

https://www.khanacademy.org/science/che ... -trends-ap
by Haocheng Zhang 2A
Sun Oct 22, 2017 8:25 pm
Forum: DeBroglie Equation
Topic: When to use De Broglie Equation [ENDORSED]
Replies: 11
Views: 1159

Re: When to use De Broglie Equation [ENDORSED]

I think we can use de Broglie equation to find kinetic energy if we know the wavelength and either mass or velocity of the obeject.
by Haocheng Zhang 2A
Sun Oct 15, 2017 12:53 pm
Forum: Einstein Equation
Topic: Planck's Constant
Replies: 3
Views: 257

Re: Planck's Constant

For a formula, the units of both sides of equation should be same. For example, v = at, the unit of accerleration is "m*s^-2", the unit for t is "s", for a*t, the unit is m*s^-1, which is same as the unit of velocity. For E = hv, the unit of E is joules, the unit of v(frequency) ...
by Haocheng Zhang 2A
Sun Oct 15, 2017 12:36 pm
Forum: Properties of Light
Topic: Understanding the Rydberg Formula [ENDORSED]
Replies: 6
Views: 388

Re: Understanding the Rydberg Formula [ENDORSED]

When using the Rydberg formula to calculate the frequency, we actually calculate the energy gap between two energy levels. Getting a positive value means the electron gains energy and it needs absorb light. Getting a negative value means the electron loses energy and emits light. Whether the light i...
by Haocheng Zhang 2A
Thu Oct 05, 2017 10:33 pm
Forum: Empirical & Molecular Formulas
Topic: Chemical Principles F.24 HELP!
Replies: 2
Views: 1174

Re: Chemical Principles F.24 HELP!

According to the description, we get the reaction equation MgCO3 + CaCO3 → MgO + CaO + CO2. Then we balance it: MgCO3 + CaCO3 → MgO + CaO + 2CO2. According to the law of conservation of mass, the mass of products has equal to the mass of reactants, which is 9.66g. The products are Mgo, CaO, and C02....
by Haocheng Zhang 2A
Thu Oct 05, 2017 9:55 pm
Forum: Molarity, Solutions, Dilutions
Topic: G. 21
Replies: 3
Views: 311

Re: G. 21

When you dissovle ionic compounds into water, they exist in form of ions. For example, when you dissovle 1 mole of KCl into water completely, you get 1 mole of potassium ions and 1 mole of chlorine ions. The number of mole of potassium ions is that of potassium. For this question you only have to ca...

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