Search found 33 matches

by Juanyi Tan 2K
Wed Mar 14, 2018 9:57 pm
Forum: General Rate Laws
Topic: rate law from elementary step
Replies: 2
Views: 117

Re: rate law from elementary step

Because the slow step kinetically controls the rate of a chemical reaction, the rate law can be determined by looking at the reactants involve in the slow step. That is why we can use the stoichiometric coefficient from slow step to write out the rate law for the whole reaction.
by Juanyi Tan 2K
Wed Mar 14, 2018 9:52 pm
Forum: General Rate Laws
Topic: Unimolecular
Replies: 5
Views: 340

Re: Unimolecular

Just to add on: the order corresponding to the unimolecular or bimolecular should be the overall order of the chemical reaction.
by Juanyi Tan 2K
Wed Mar 14, 2018 9:49 pm
Forum: General Rate Laws
Topic: Rate Law
Replies: 6
Views: 448

Re: Rate Law

Not really. The rate law is not related to the reaction coefficient. A reaction can be zero order such as rate = k, indicating the rate will not be affected by the concentration of reactant. We determine the rate law through experimental data.
by Juanyi Tan 2K
Thu Mar 08, 2018 8:58 pm
Forum: First Order Reactions
Topic: half-lives of first order versus second order
Replies: 3
Views: 238

Re: half-lives of first order versus second order

I am not 100% sure, but maybe you forget to include the initial concentration for second order reaction.
by Juanyi Tan 2K
Thu Mar 08, 2018 8:53 pm
Forum: Zero Order Reactions
Topic: Zero Order Half-Life Calculations
Replies: 2
Views: 292

Re: Zero Order Half-Life Calculations

The safest method is to calculate the rate constant k from the equation t1/2=[A]0/2k, and then use the equation [A]=-kt+[A]0 to find out the time needed to reach a certain concentration.
by Juanyi Tan 2K
Thu Mar 08, 2018 8:32 pm
Forum: General Rate Laws
Topic: 15.17
Replies: 2
Views: 116

Re: 15.17

In this question, you can find out the order for B and C first. After you know their order, you can then compare results from different experiment so that the reaction rate is not squaring when the concentration of A doubles. For example, you can compare experiment 1 and experiment 3 after you deter...
by Juanyi Tan 2K
Thu Mar 08, 2018 8:26 pm
Forum: Kinetics vs. Thermodynamics Controlling a Reaction
Topic: Rate Constants and Temperature
Replies: 3
Views: 200

Re: Rate Constants and Temperature

The equation is written as lnk=lnA-Ea/RT, where R is the gas constant, A is the pre-exponential factor, and Ea is the activation energy. This equation could also represent a negative linear relationship between lnk and 1/T.
by Juanyi Tan 2K
Thu Mar 01, 2018 8:42 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: K2K1/K' Formula
Replies: 3
Views: 128

Re: K2K1/K' Formula

Pseudo-1st-order reaction is one example, which is the circumstance where a second order reaction appears to be first order. That is when one of the reactants in the rate equation is present in great excess over the other in the reaction mixture. So the rate constant of one of the reactants combines...
by Juanyi Tan 2K
Thu Mar 01, 2018 8:35 pm
Forum: General Rate Laws
Topic: nth order reactions [ENDORSED]
Replies: 2
Views: 109

Re: nth order reactions [ENDORSED]

I think in 14B we only focus on those three types of reactions. Knowing the specific reaction order is necessary for a rate law, by which we can calculate the rate constant of a reaction or predict the concentration of reactants or products,
by Juanyi Tan 2K
Thu Mar 01, 2018 8:30 pm
Forum: Zero Order Reactions
Topic: Half-life of Zero Order [ENDORSED]
Replies: 4
Views: 171

Re: Half-life of Zero Order [ENDORSED]

I think we also need to know half lives for second-order reactions.
by Juanyi Tan 2K
Thu Feb 22, 2018 10:26 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: HW 14.117
Replies: 2
Views: 173

Re: HW 14.117

There are examples in the textbook about how to use the equation n=It/F. The page number is 595. Also, the equation is related to delta G and E because delta G= -nFE, which can be written as It=nF=-delta G/E.
by Juanyi Tan 2K
Thu Feb 22, 2018 9:33 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: 14.107 Solution Manual Error?
Replies: 4
Views: 499

Re: 14.107 Solution Manual Error?

I think professor Lavelle has posted the correction online. You can go and check it!
by Juanyi Tan 2K
Sun Feb 18, 2018 12:03 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: salt bridge
Replies: 3
Views: 135

Re: salt bridge

The purpose of the salt bridge is to keep the solutions electrically neutral and allow the free flow of ions from one cell to another. When the electrons flow from the anode to the cathode,the oxidation reaction that occurs at the anode generates electrons and positively charged ions, leaving the un...
by Juanyi Tan 2K
Sat Feb 17, 2018 2:53 am
Forum: Balancing Redox Reactions
Topic: The use of H3O+ versus H+
Replies: 2
Views: 110

Re: The use of H3O+ versus H+

H3O+ is common in acidic solution because we have the equation HA + H2O = A- + H3O-. The number of elements is balanced in this way. When we have the redox reaction, we use H+ to balance the charge. If we use H3O+, we will bring in the unnecessary oxygen element.
by Juanyi Tan 2K
Sat Feb 17, 2018 2:49 am
Forum: Balancing Redox Reactions
Topic: 14.3 Balancing redox reactions
Replies: 2
Views: 117

Re: 14.3 Balancing redox reactions

I agree with Abigail! The two things we should look for when we balance the redox equation are the charge and the number of elements.
by Juanyi Tan 2K
Sun Feb 11, 2018 3:44 am
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: HW 9.19
Replies: 2
Views: 167

Re: HW 9.19

This phenomenon is called supercooling, which means lowering the temperature of the gas below its boiling point without it becoming liquid
by Juanyi Tan 2K
Sun Feb 11, 2018 3:43 am
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: 9.47
Replies: 2
Views: 119

Re: 9.47

Yeah and also, when we look at the question, we won't be able to use the final volume to calculate since the pressure will change at the process.
by Juanyi Tan 2K
Sun Feb 11, 2018 3:36 am
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: Molar entropy v standard entropy of formation
Replies: 3
Views: 676

Re: Molar entropy v standard entropy of formation

Molar entropy is the entropy content of one mole of substance under a standard state, and it is absolute. Normally, we only use enthalpy of formation and you will not see entropy of formation
by Juanyi Tan 2K
Fri Feb 09, 2018 10:22 am
Forum: Van't Hoff Equation
Topic: van't Hoff Equation
Replies: 2
Views: 268

Re: van't Hoff Equation

Also, equilibrium constant, K, is independent of pressure but is dependent of temperature, so the Van't Hoff equation connects the change in equilibrium constant to the change in temperature,
by Juanyi Tan 2K
Sun Feb 04, 2018 7:40 pm
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: Standard State
Replies: 2
Views: 121

Re: Standard State

Normally, the standard state of a chemical substance is 1mole at 25.0 °C and 1 atm.
by Juanyi Tan 2K
Sun Feb 04, 2018 7:25 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Question about Change in Gibb's Free Energy
Replies: 4
Views: 112

Re: Question about Change in Gibb's Free Energy

Just want to add on: enthalpy, entropy, and Gibbs free energy are all state functions, which means they are not dependent on path taken to obtain the current state. Therefore, it is more meaningful to focus on the change between different states.
by Juanyi Tan 2K
Sun Feb 04, 2018 7:17 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Vibrationally active - 9.35
Replies: 1
Views: 91

Vibrationally active - 9.35

In the question 9.35, the difference between container B and container C is that molecules in container C are vibrationally active. I am confused about that Cv for molecules in container B is 5/2 R while that for molecules in container C is 3R. Is there any explanation for the different values of Cv?
by Juanyi Tan 2K
Mon Jan 29, 2018 10:41 am
Forum: Third Law of Thermodynamics (For a Unique Ground State (W=1): S -> 0 as T -> 0) and Calculations Using Boltzmann Equation for Entropy
Topic: Enthalpy of Reaction at lower temperatures [ENDORSED]
Replies: 2
Views: 106

Re: Enthalpy of Reaction at lower temperatures [ENDORSED]

I find an explanation in the textbook to be helpful: just imagine how sneezing in a quiet environment will attract attention more than in a noisy environment. The energy transfer to a cool system where there is little thermal motion will result in a more noticeable change, leading to a greater entro...
by Juanyi Tan 2K
Mon Jan 29, 2018 10:34 am
Forum: Third Law of Thermodynamics (For a Unique Ground State (W=1): S -> 0 as T -> 0) and Calculations Using Boltzmann Equation for Entropy
Topic: Spontaneous Processes for Entropy
Replies: 2
Views: 184

Re: Spontaneous Processes for Entropy

Just want to add on: if the entropy is negative, the reverse process will be spontaneous. When the entropy is zero, the process has no tendency to proceed in either direction.
by Juanyi Tan 2K
Mon Jan 29, 2018 10:28 am
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Calculating Entropy Change [ENDORSED]
Replies: 2
Views: 90

Re: Calculating Entropy Change [ENDORSED]

When using the equation ∆S=q/T, we should be clear about the entropy we want to calculate. Do we want to calculate the change of entropy of a system or it surrounding? The negative sign will depend on the situation.
by Juanyi Tan 2K
Mon Jan 29, 2018 10:20 am
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Closed vs. Isolated
Replies: 8
Views: 310

Re: Closed vs. Isolated

An isolated system could not exchange energy or matter with surrounding, like bomb calorimeter and thermos. A closed system allows energy transfer but does not allow matter exchange. An example is a normal water bottle since the matter in it could not exchange with surrounding, but the energy transf...
by Juanyi Tan 2K
Sat Jan 20, 2018 2:56 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: What does R stand for?
Replies: 13
Views: 653

Re: What does R stand for?

The one that we most commonly use in thermochemistry is R = 8.314 J·K-1·mol-1
by Juanyi Tan 2K
Sat Jan 20, 2018 2:50 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Problem 8.65
Replies: 3
Views: 107

Re: Problem 8.65

For the question, we are looking for the standard enthalpy of formation for N2O5 (1 mole), so we want the coefficient of N2O5 in the reaction to be 1. Hope this helps!
by Juanyi Tan 2K
Sat Jan 20, 2018 2:42 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: 8.103
Replies: 2
Views: 108

Re: 8.103

Just want to add on: R is gas constant that has different value when its unit varies, but the most common one we use in thermochemistry is 8.314 J·K-1
·mol-1
by Juanyi Tan 2K
Sun Jan 14, 2018 10:50 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Internal Energy
Replies: 2
Views: 108

Re: Internal Energy

Just want to add on. Internal energy is the sum of the kinetic and potential energies of the particles that form the system and enthalpy is defined as the heat absorbed or released when a system does work. That is way the enthalpy is given by the equation H (enthalpy) = U (internal energy) + PV (work)
by Juanyi Tan 2K
Thu Jan 11, 2018 10:45 pm
Forum: Phase Changes & Related Calculations
Topic: Superheating
Replies: 6
Views: 371

Re: Superheating

Hi Abby! What I learn is that superheating describes the phenomenon when a liquid is heated to a temperature above its boiling point,without boiling and vaporizing. When the temperature of a liquid is below the freezing point but the liquid doesn't become solid, this phenomenon is called supercoolin...
by Juanyi Tan 2K
Thu Jan 11, 2018 10:31 pm
Forum: Phase Changes & Related Calculations
Topic: constant pressure
Replies: 3
Views: 149

Re: constant pressure

What I understand about this part is that the reaction is exposed to the atmosphere, which has a constant pressure of 1 atm. So when the system expand or contract, we can consider the work is done on the system or done by the system. Either way the work is done at a constant pressure since the chemi...

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