Search found 50 matches

by Kevin Xu 4F
Wed Mar 11, 2020 10:59 am
Forum: First Order Reactions
Topic: Equilibrium constant and elementary reaction rate constants
Replies: 5
Views: 22

Re: Equilibrium constant and elementary reaction rate constants

In an equilibrium reaction, there is a forward reaction and a reverse reaction happening at the same time. The forward reaction's rate can be modeled as rate=k[reactants] and the reverse reaction's rate can be modeled as rate = k'[products]. At equilibrium, since the rates of forward and reverse rea...
by Kevin Xu 4F
Wed Mar 11, 2020 10:54 am
Forum: Zero Order Reactions
Topic: 7.21 HW
Replies: 2
Views: 29

Re: 7.21 HW

I think it is referring [A] to [A]0 because all the equations for the half life depend on the initial concentration of [A]. Like previously mentioned, graphing the half life versus the concentration would refer to the half life equations for a zero and second order reaction, and that tells you wheth...
by Kevin Xu 4F
Wed Mar 11, 2020 10:51 am
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: Pre-Equil Approach
Replies: 2
Views: 42

Re: Pre-Equil Approach

There are two ways to find the combined rate law for an overall reaction, the steady state approach and the pre-equilibrium approach. The steady state approach is based on the assumption that the concentrations of the intermediates remain low and do not change significantly, and that the intermediat...
by Kevin Xu 4F
Wed Mar 11, 2020 10:45 am
Forum: Kinetics vs. Thermodynamics Controlling a Reaction
Topic: HW 7.1 Kinetic control vs Thermodynamic control
Replies: 2
Views: 58

Re: HW 7.1 Kinetic control vs Thermodynamic control

I don't know if it is always true, because there are probably conditions that can be altered that can affect which type of product predominates at a certain temperature given other factors like catalysts, concentrations, pressure, etc. But I think that it generally applies for most cases that at low...
by Kevin Xu 4F
Wed Mar 11, 2020 10:37 am
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: Catalysts
Replies: 3
Views: 24

Re: Catalysts

I think the general rule is that an intermediate is something that is produced as a byproduct of an elementary step and then used as a reactant in another step. For a catalyst, it is incorporated into the reactants of an elementary step and present in the products of another elementary step, indicat...
by Kevin Xu 4F
Mon Mar 09, 2020 1:31 am
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Relationship between Equilibrium Constant and Rate Constants
Replies: 1
Views: 15

Re: Relationship between Equilibrium Constant and Rate Constants

An equilibrium constant is a ratio that reflects the molar activities of all the species within a reaction. Since it is hard to accurately measure the molar activities of substances, the best approximation is the molar concentrations, as the two are directly related. In order to get the equilibrium ...
by Kevin Xu 4F
Mon Mar 09, 2020 1:24 am
Forum: General Rate Laws
Topic: Overall rate law
Replies: 8
Views: 48

Re: Overall rate law

I think this is because this is a very simplified model and way of looking at reactions. Here, we assume an all or nothing approach to reaction processes where the reaction either proceeds to completion or does not proceed at all. This is a result of the activation energies of the reaction, and the ...
by Kevin Xu 4F
Mon Mar 09, 2020 1:21 am
Forum: Method of Initial Rates (To Determine n and k)
Topic: k' vs kr
Replies: 13
Views: 75

Re: k' vs kr

K' represents the placeholder K that is used in psuedo order reactions when the rate law is simplified to only include one species at reaction. For example, if the rate law of a reaction was k[A][B][C] and we wanted to isolate the changes affecting species A, then we could shorten and truncate the r...
by Kevin Xu 4F
Mon Mar 09, 2020 1:17 am
Forum: General Rate Laws
Topic: Second order reaction
Replies: 1
Views: 25

Re: Second order reaction

The equation for the half life of a second order reaction is t(1/2) = 1/ (k* [A]). Thus, this equation shows us that the concentration of A is inversely related to the half life, if the initial concentration of A increases, then the half life decreases. If the initial concentration of A decreases, t...
by Kevin Xu 4F
Mon Mar 09, 2020 1:15 am
Forum: General Rate Laws
Topic: Making one reactant concentration small
Replies: 3
Views: 20

Re: Making one reactant concentration small

By making all but one of the reactants very large, we can isolate the change in the concentration of one specific compound so that the rate law can be experimentally determined as opposed to testing all different trials and keeping different concentration of species the same. Since the concentration...
by Kevin Xu 4F
Wed Feb 26, 2020 3:02 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 6L 7c
Replies: 1
Views: 24

Re: 6L 7c

The Ni(OH)2 in this case I think is an insoluble salt, whereas in order to function in a galvanic cell, both electrodes have to be solid metals, not just solids. That's why you need solid metal Ni, not just a solid.
by Kevin Xu 4F
Wed Feb 26, 2020 2:59 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Galvanic vs. Concentration Cells
Replies: 6
Views: 40

Re: Galvanic vs. Concentration Cells

Galvanic cells are also typically separated by a salt bridge whereas concentration cells are separated by a porous wall. In addition, the Ecell is typically lower voltage in concentration cell because the reaction is only driven by the difference in concentrations between the anode and cathode react...
by Kevin Xu 4F
Wed Feb 26, 2020 2:54 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Drawing Cell Diagrams
Replies: 1
Views: 25

Re: Drawing Cell Diagrams

Professor Lavelle stated that for him he likes to write it in the direction of reactants to products for the same state of matter, like (aq) of reactants to (aq) of products. However, some textbook problems do switch up the order so as long as you can correctly determine the reactants and products o...
by Kevin Xu 4F
Wed Feb 26, 2020 2:48 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 6.L.3 (d)
Replies: 1
Views: 28

6.L.3 (d)

The question asks for the balanced oxidation and reduction half reactions for the hydrolysis of water, reaction given as Pt(s)|O2(g)|H+(aq) ||OH-(aq)|O2(g)|Pt(s). I thought that the compounds listed to the left of the salt bridge, denoted || are the reactants and products of the anode reaction (oxid...
by Kevin Xu 4F
Wed Feb 26, 2020 2:42 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 6L.1
Replies: 2
Views: 29

Re: 6L.1

The n in the Gibbs Free Energy calculation refers to the total number of moles of electrons transferred in the redox reaction. This can be calculated by balancing the oxidation and reduction half reactions. It doesn't matter whether it comes from the Ce4+ or the I- because after you balance the half...
by Kevin Xu 4F
Wed Feb 19, 2020 8:47 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: molar Gibbs free energy
Replies: 2
Views: 32

Re: molar Gibbs free energy

I think when you are referring to calculating the standard change in gibbs free energy, that is G not, the delta H and delta S in this case are the delta H not and delta G not at standard temperature and pressure, 25 degrees C and 1 atm. Thus, the delta H and delta S remain constant while the temper...
by Kevin Xu 4F
Wed Feb 19, 2020 8:33 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Max Potential
Replies: 5
Views: 38

Re: Max Potential

He meant that the max potential occurs when there is little to no current because that is when the potential difference in the galvanic cell is at its maximum, since it decreases over time when the current flows through the cell. Maximum potential means the most energy available to do work, which ca...
by Kevin Xu 4F
Wed Feb 19, 2020 8:30 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Midterm Q6B
Replies: 5
Views: 110

Re: Midterm Q6B

Remember that delta G = delta H - T*delta S, so the scenario where delta H and delta G are most similar would be the case where delta S is the smallest, so the reaction where the change in entropy is the smallest. The reason it is A is because this reaction has no phase change, hence a very low reac...
by Kevin Xu 4F
Wed Feb 19, 2020 8:23 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Midterm 3B
Replies: 2
Views: 84

Re: Midterm 3B

Since the question asks for the largest change in temperature, it is ideally referring to the change in temperature of the solvent that the reaction mixture is added into. This can be directly calculated by the transfer of heat into the surroundings from the acid base reaction, and assuming constant...
by Kevin Xu 4F
Wed Feb 19, 2020 8:10 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Gibbs Free Energy Equation that relates K
Replies: 4
Views: 48

Re: Gibbs Free Energy Equation that relates K

Delta G not is the change in gibbs free energy given conditions under standard pressure (1 atm) and with stoichiometric coefficients corresponding to the balanced chemical reaction. Hence, the equation to calculated it is delta G not = -R * T * ln K. The K here represents concentrations at equilibri...
by Kevin Xu 4F
Wed Feb 12, 2020 11:44 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Units for enthalpies
Replies: 2
Views: 47

Re: Units for enthalpies

I think it depends on what the question is asking for. If the question is asking for the change in enthalpy per 1 mole of reaction, hence the standard enthalpy of reaction, then you would give the answer in kJ/mole. But if the question was asking you for the change in enthalpy given a certain mole o...
by Kevin Xu 4F
Wed Feb 12, 2020 11:42 am
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Reversible vs Irreversible
Replies: 5
Views: 113

Re: Reversible vs Irreversible

Yes, those equations can be used to find the change in entropy but also keep in mind what reversible and irreversible reactions mean. A reversible reaction is one that has infinitesimal changes that can be reversible by addition or subtraction at any given point in time, whereas irreversible changes...
by Kevin Xu 4F
Wed Feb 12, 2020 11:39 am
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: Different forms of entropy
Replies: 2
Views: 73

Re: Different forms of entropy

Also the Stotal is used to determine the spontaneity of a reaction, as it refers back to the second law of thermodynamics, which states that the entropy of an isolated system increases in the course of any spontaneous change. Thus, if Stotal is greater than 0, then the reaction occurs spontaneously....
by Kevin Xu 4F
Wed Feb 12, 2020 11:33 am
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: 3/2R & 5/2R
Replies: 9
Views: 135

Re: 3/2R & 5/2R

Heat capacity at constant volume for an ideal gas is used whenever the volume is kept constant, in the case solving for q, delta U, and delta S. The only exception is delta H, which uses Cp because delta H does not equal q at constant volume. Heat capacity at constant pressure for an ideal gas is us...
by Kevin Xu 4F
Wed Feb 12, 2020 11:30 am
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Pizza Rolls #7
Replies: 3
Views: 66

Re: Pizza Rolls #7

Question 7 asks you to find the change in internal energy when the reaction occurs with 1 mole of CO2 being produced. The equation for internal energy is q + w. In order to find q, remember that q equals delta at constant pressure. Using the enthalpy of reaction (-245 kJ per mole of rxn) convert tha...
by Kevin Xu 4F
Fri Feb 07, 2020 4:51 pm
Forum: Phase Changes & Related Calculations
Topic: relationship between heat capacity and heating curve
Replies: 6
Views: 37

Re: relationship between heat capacity and heating curve

The slope of a heating curve represents the rise/run, in this case the change in temperature over the change in energy as heat. The heat capacity of a substance is equal to the change in enthalpy (heat) divided by the change in temperature. Thus, the heat capacity is inversely related to the slope o...
by Kevin Xu 4F
Fri Feb 07, 2020 4:41 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Conceptualizing Cv, Cp
Replies: 2
Views: 30

Re: Conceptualizing Cv, Cp

At a constant volume, there is no expansion work, so all energy that enters the system is converted in the form of heat. At a constant pressure, all energy that enters the system is converted into both heat and work through expansion of gasses, and both contribute to a greater increase in thermal an...
by Kevin Xu 4F
Fri Feb 07, 2020 1:51 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Adiabatic System
Replies: 3
Views: 48

Re: Adiabatic System

I think professor lavelle just included adiabatic because he stated that some textbooks/questions use that term. It just means there is no heat flow in the system, therefore q = 0. In regards to reversible/irreversible reactions, reversible reactions are isothermal which means there is no change in ...
by Kevin Xu 4F
Fri Feb 07, 2020 1:49 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: 4f.1
Replies: 2
Views: 22

Re: 4f.1

I don't know if it is incorrect to but if the units of entropy are in Celsius, that would affect the calculations of other equations that use temperature. The temperature would have to be in C instead of K, and could produce mathematical errors if the temperature is 0 degrees Celsius (divide by zero).
by Kevin Xu 4F
Fri Feb 07, 2020 1:47 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: 4f.7
Replies: 1
Views: 24

Re: 4f.7

The entropy change of an ideal gas through raising the temperature is found by the equation delta S = n*Cp * ln (T2/T1). Cp in this equation can be converted by the formula Cp = Cv + nR. Heating capacity at constant pressure is higher than heat capacity at constant volume because more temperature is...
by Kevin Xu 4F
Tue Jan 28, 2020 2:23 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Standard enthalpy of formation
Replies: 3
Views: 21

Re: Standard enthalpy of formation

In addition, keep in mind that enthalpy of formation for elements is always zero because they are naturally occuring, and it takes no energy to form a naturally-forming compound, such as O2 or Cl2 gas.
by Kevin Xu 4F
Tue Jan 28, 2020 2:15 pm
Forum: Phase Changes & Related Calculations
Topic: Cv and Cp
Replies: 9
Views: 91

Re: Cv and Cp

I think both Cv and Cp can be used to calculator enthalpy change in the formula q= nCT. Just know that Cv means the molar heat capacity of a gas at a constant volume whereas Cp means the molar heat capacity of a gas at a constant pressure.
by Kevin Xu 4F
Tue Jan 28, 2020 2:07 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Calorimetry
Replies: 4
Views: 47

Re: Calorimetry

A bomb calorimeter is specifically used to measure the heat released of a substance at constant volume. A substance is placed inside a bomb vessel and it is heated to combustion with ample oxygen, and the temperature change in that reaction is measured by heat sensors within the calorimeter. Then, t...
by Kevin Xu 4F
Tue Jan 28, 2020 2:00 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Delta T
Replies: 4
Views: 39

Re: Delta T

The definition of an exothermic reaction is one that gives off heat. In the case that delta T is positive, that is looking at the change in temperature with respect to the surroundings, because sometimes there is no appropriate way to measure the delta T of the system. An exothermic reaction as a sy...
by Kevin Xu 4F
Tue Jan 28, 2020 1:58 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Neutralization Reactions
Replies: 2
Views: 24

Re: Neutralization Reactions

I think so because when calculating the heat given off by an exothermic system of the neutralization reaction, it would always equal the negative of the heat absorbed by the system or solution; in this case water. Therefore, the specific heat of water can be used to calculate that change in heat val...
by Kevin Xu 4F
Thu Jan 23, 2020 1:12 am
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Changes in pressure
Replies: 5
Views: 50

Re: Changes in pressure

A system will react/shift to a change in pressure when volume decreases specifically because it changes the concentrations of the gasses. And changing the concentration of the gasses may put the reaction out of equilibrium (if the moles of gasses are unequal) so therefore the reaction shifts to acco...
by Kevin Xu 4F
Thu Jan 23, 2020 1:09 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: 6D 15
Replies: 1
Views: 21

Re: 6D 15

The Ka values are listed earlier in the section in a chart. You can use the Kb value of NH3 to find the Ka value of NH4+, and you can use the value of Ka given in the positive cation chart to find the pH of AlCl3.
by Kevin Xu 4F
Thu Jan 23, 2020 1:07 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: 6 C.21 part b
Replies: 1
Views: 27

6 C.21 part b

To explain the differences between the acid strength of acetic acid versus strength of formic acid, I put that acetic acid is stronger because it has a higher inductive effect that pulls the electrons away from the hydrogen in the alcohol group. However, the textbook states that formic acid is sligh...
by Kevin Xu 4F
Thu Jan 23, 2020 1:00 am
Forum: Non-Equilibrium Conditions & The Reaction Quotient
Topic: 6D.15 part b
Replies: 1
Views: 34

Re: 6D.15 part b

The Ka of metal cations is given in in a chart in 6.D.1 and the value of Ka for Al3+ when forming the Al((H20)6)3+ compound is 1.4 * 10^-5. You can then use that Ka in a normal ICE box equation, with dissociation into H+ ion and then use it to solve the pH value.
by Kevin Xu 4F
Wed Jan 22, 2020 1:44 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: polyprotic acids
Replies: 3
Views: 37

Re: polyprotic acids

You would need to calculate the dissociation separately, first using Ka and then finding the concentration of the second acid and using the Ka2 to find the remaining H+ concentration. You then add the two H+'s together to find pH. Also, for some problems, you don't have to calculate the second disso...
by Kevin Xu 4F
Fri Jan 17, 2020 11:04 pm
Forum: Acidity & Basicity Constants and The Conjugate Seesaw
Topic: Autoprotolysis
Replies: 1
Views: 76

Re: Autoprotolysis

The autoprotolysis or autoionization of water occurs when one water molecule donates one of its protons to another water molecule, forming the compounds H3O+ and OH-. This reaction occurs naturally with an equilibrium constant of 1.0 * 10^-14. I think this is experimentally determined with condition...
by Kevin Xu 4F
Fri Jan 17, 2020 10:56 pm
Forum: Acidity & Basicity Constants and The Conjugate Seesaw
Topic: Conjugate Acids/Bases
Replies: 2
Views: 57

Re: Conjugate Acids/Bases

An acid is a compound that readily donates a H+ proton. The base that receives that H+ proton subsequently becomes the conjugate acid.
A base is a compound that readily accepts an H+ proton. The acid that gives that H+ proton becomes the conjugate base.
by Kevin Xu 4F
Fri Jan 17, 2020 1:54 pm
Forum: Phase Changes & Related Calculations
Topic: 4A. 1 Identifying open and closed system
Replies: 11
Views: 195

Re: 4A. 1 Identifying open and closed system

A calorimeter is designed to be a closed system because it is supposed to measure the gain or loss of heat. A car engine is an open system because it is not contained or isolated in a container, even if it might be physically enclosed, for example in a car.
by Kevin Xu 4F
Thu Jan 16, 2020 5:26 pm
Forum: Non-Equilibrium Conditions & The Reaction Quotient
Topic: Temperature Change
Replies: 9
Views: 172

Re: Temperature Change

A reaction just by itself being endo or exothermic does not tell anything about whether or not it is product/reactant favored. However, when the temperature is raised or lowered, that would in effect shift the reaction in the direction that minimizes the gain/loss of heat appropriately. For example,...
by Kevin Xu 4F
Thu Jan 16, 2020 5:23 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Cubic functions
Replies: 3
Views: 21

Re: Cubic functions

I think that professor Lavelle mentioned in class how he would not test us on cubic equations unless there was someway to simplify or approximate the expression. So the example you wrote would not show up on a test. However, to solve it you would need to use a graphing calculator as a scientific one...
by Kevin Xu 4F
Mon Jan 13, 2020 1:35 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Stability
Replies: 5
Views: 125

Re: Stability

I think the stability of a reaction also has to do with how naturally favored it is to incline to either the products or the reactants. Say the products are more stable than the reactants. Then that means that it would require less energy to form the products than the reactants, hence why the reacti...
by Kevin Xu 4F
Mon Jan 13, 2020 1:24 am
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Excess Reactant in Le Chatelier's Principle
Replies: 6
Views: 69

Excess Reactant in Le Chatelier's Principle

Let's suppose we have a reaction N2 + 3H2 --> 2NH3. If we add more N2, the reaction by le chatelier's principle shifts to the right. But do we always assume that we have excess reactants? Because what if H2 is the limiting reactant, adding more N2 would not shift the reaction to the right because th...
by Kevin Xu 4F
Mon Jan 13, 2020 1:18 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: emitting solids and liquids
Replies: 2
Views: 13

Re: emitting solids and liquids

Another way to think of it is that there is no such thing as the concentration of a solid (concentration matters because it is directly related to a molecule's activity level), and most of the time liquids are so much in excess (pure liquids are most of the time the solvent) that the small relative ...
by Kevin Xu 4F
Mon Jan 13, 2020 1:13 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: ICE
Replies: 3
Views: 23

Re: ICE

ICE stands for Initial, change, and equilibrium molar concentrations of a reaction at equilibrium. It is useful because it is an easy and quick way to store what concentrations are known and unknown with respect to a change that can be directly plugged into the equilibrium constant expression and th...
by Kevin Xu 4F
Mon Jan 13, 2020 1:08 am
Forum: Ideal Gases
Topic: ICE Table
Replies: 11
Views: 108

Re: ICE Table

An ICE table shows the initial, change, and equilibrium concentration values for a reaction at equilibrium. To solve it, you would need to find the equilibrium expression K for the reaction and the starting/ending values of the initial or end concentrations. To solve for the unknown, plug the known ...

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