## Search found 33 matches

Wed Mar 14, 2018 9:57 pm
Forum: General Rate Laws
Topic: rate law from elementary step
Replies: 2
Views: 138

### 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.
Wed Mar 14, 2018 9:52 pm
Forum: General Rate Laws
Topic: Unimolecular
Replies: 5
Views: 365

### Re: Unimolecular

Just to add on: the order corresponding to the unimolecular or bimolecular should be the overall order of the chemical reaction.
Wed Mar 14, 2018 9:49 pm
Forum: General Rate Laws
Topic: Rate Law
Replies: 6
Views: 498

### 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.
Thu Mar 08, 2018 8:58 pm
Forum: First Order Reactions
Topic: half-lives of first order versus second order
Replies: 3
Views: 260

### 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.
Thu Mar 08, 2018 8:53 pm
Forum: Zero Order Reactions
Topic: Zero Order Half-Life Calculations
Replies: 2
Views: 326

### 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.
Thu Mar 08, 2018 8:32 pm
Forum: General Rate Laws
Topic: 15.17
Replies: 2
Views: 135

### 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...
Thu Mar 08, 2018 8:26 pm
Forum: Kinetics vs. Thermodynamics Controlling a Reaction
Topic: Rate Constants and Temperature
Replies: 3
Views: 222

### 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.
Thu Mar 01, 2018 8:42 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: K2K1/K' Formula
Replies: 3
Views: 157

### 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...
Thu Mar 01, 2018 8:35 pm
Forum: General Rate Laws
Topic: nth order reactions [ENDORSED]
Replies: 2
Views: 126

### 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,
Thu Mar 01, 2018 8:30 pm
Forum: Zero Order Reactions
Topic: Half-life of Zero Order [ENDORSED]
Replies: 4
Views: 204

### Re: Half-life of Zero Order[ENDORSED]

I think we also need to know half lives for second-order reactions.
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: 190

### 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.
Thu Feb 22, 2018 10:17 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Variations of Nernst Equation
Replies: 1
Views: 102

### Re: Variations of Nernst Equation

2.303 is the value to convert lnQ to logQ
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: 529

### Re: 14.107 Solution Manual Error?

I think professor Lavelle has posted the correction online. You can go and check it!
Sun Feb 18, 2018 12:03 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: salt bridge
Replies: 3
Views: 165

### 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...
Sat Feb 17, 2018 2:53 am
Forum: Balancing Redox Reactions
Topic: The use of H3O+ versus H+
Replies: 2
Views: 128

### 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.
Sat Feb 17, 2018 2:49 am
Forum: Balancing Redox Reactions
Topic: 14.3 Balancing redox reactions
Replies: 2
Views: 143

### 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.
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: 187

### 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
Sun Feb 11, 2018 3:43 am
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: 9.47
Replies: 2
Views: 135

### 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.
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: 831

### 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
Fri Feb 09, 2018 10:22 am
Forum: Van't Hoff Equation
Topic: van't Hoff Equation
Replies: 2
Views: 296

### 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,
Sun Feb 04, 2018 7:40 pm
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: Standard State
Replies: 2
Views: 130

### Re: Standard State

Normally, the standard state of a chemical substance is 1mole at 25.0 °C and 1 atm.
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: 131

### 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.
Sun Feb 04, 2018 7:17 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Vibrationally active - 9.35
Replies: 1
Views: 103

### 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?
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: 131

### 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...
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: 206

### 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.
Mon Jan 29, 2018 10:28 am
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Calculating Entropy Change [ENDORSED]
Replies: 2
Views: 97

### 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.
Mon Jan 29, 2018 10:20 am
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Closed vs. Isolated
Replies: 8
Views: 378

### 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...
Sat Jan 20, 2018 2:56 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: What does R stand for?
Replies: 13
Views: 787

### Re: What does R stand for?

The one that we most commonly use in thermochemistry is R = 8.314 J·K-1·mol-1
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: 118

### 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!
Sat Jan 20, 2018 2:42 am
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: 8.103
Replies: 2
Views: 117

### 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
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: 114

### 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)
Thu Jan 11, 2018 10:45 pm
Forum: Phase Changes & Related Calculations
Topic: Superheating
Replies: 6
Views: 485

### 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...
Thu Jan 11, 2018 10:31 pm
Forum: Phase Changes & Related Calculations
Topic: constant pressure
Replies: 3
Views: 163

### 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...