## Search found 50 matches

Sat Mar 17, 2018 3:41 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Pre-Equilibrium
Replies: 2
Views: 155

### Re: Pre-Equilibrium

Either way is fine; technically it is more accurate to write K as k/k', but you can leave the answer in either form.
Sat Mar 17, 2018 3:36 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: wmax
Replies: 4
Views: 333

### Re: wmax

The lecture notes for cell potential state that maximum work = -charge*E. From this, we get that max w=-nFE, and since max w=ΔG (not ΔG°, according to the notes), then ΔG=-nFE.
Sat Mar 17, 2018 3:30 pm
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: Factors Affecting Entropy
Replies: 4
Views: 1419

### Re: Factors Affecting Entropy

The state of the molecule matters more than the weight. So, when you are comparing entropies, you would first look at gas vs. liquid etc., and then if they are in the same phase, then you would calculate the molar mass. Whichever molecule is heavier has the higher entropy.
Fri Mar 09, 2018 11:04 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: Pseudo Rate Law [ENDORSED]
Replies: 3
Views: 199

### Re: Pseudo Rate Law[ENDORSED]

Pseudo rate laws are used essentially when the order is determined by increasing the concentrations of one or more reactants. For example, in the rate law Rate=k[A][B][C], if [B] and [C] are in large excess, then they remain essentially constant, so [A] determines the rate. Since only [A] matters in...
Fri Mar 09, 2018 10:59 pm
Forum: *Nucleophiles
Topic: What is a nucleophile? [ENDORSED]
Replies: 6
Views: 524

### Re: What is a nucleophile?[ENDORSED]

A nucleophile is any electron rich species that donates electrons to elecrophiles to form a chemical bond in a reaction. In substitution reactions, for example, nucleophiles can be electrostatically attracted to a positive central atom, and therefore donate electrons to bond with that central atom. ...
Fri Mar 09, 2018 10:53 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: 15.47
Replies: 3
Views: 179

### Re: 15.47

To identify the intermediates, you would choose the compound that appears as a product in one reaction, and then as a reactant in the next reaction. To find the overall reaction, you would remove all intermediates and simply write the reaction using the reactants and products that are left.
Thu Mar 01, 2018 11:53 pm
Forum: First Order Reactions
Topic: First Order Graph
Replies: 11
Views: 426

### Re: First Order Graph

The purpose of taking the ln of the concentration in the graph for a first order reactant is to have a straight-line graph instead of an exponential curve, as a linear graph is more useful for calculations. Similarly, the graph for second order reactants plots 1/[A] vs. time, and the graph of a zero...
Thu Mar 01, 2018 11:49 pm
Forum: Zero Order Reactions
Topic: Graph of Zero Order vs First and Second Order [ENDORSED]
Replies: 3
Views: 177

### Re: Graph of Zero Order vs First and Second Order[ENDORSED]

It is possible that problems will describe the graph of a reaction and expect you to determine the order. You could also determine the order of the reaction if the problem explicitly states the orders of each reactant, or if the problem gives the units of k.
Thu Mar 01, 2018 11:45 pm
Forum: General Rate Laws
Topic: Determining Order [ENDORSED]
Replies: 8
Views: 303

### Re: Determining Order[ENDORSED]

The order of the reactants should be given in the problem in some form. The problem will either explicitly state "first order reactant" and "second order reactant", or give some other type of information that would help you determine the order. For example, the problem could stat...
Fri Feb 23, 2018 3:05 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 14.29 States of redox couples
Replies: 3
Views: 150

### Re: 14.29 States of redox couples

To determine the states of the molecules in the cell diagram, you can also look at Appendix 2B for the cell potential values. All the half reactions will be listed there with the states of each substance.
Fri Feb 23, 2018 2:23 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Cell diagram [ENDORSED]
Replies: 4
Views: 199

### Re: Cell diagram[ENDORSED]

A single line is used to separate substances in different states, so in this case you would use a single line, instead of a comma, which is used to separate substances in the same state. Furthermore, two double lines are used only to represent a salt bridge, and one single line is used to represent ...
Fri Feb 23, 2018 2:15 pm
Forum: Balancing Redox Reactions
Topic: Reaction E [ENDORSED]
Replies: 5
Views: 249

### Re: Reaction E[ENDORSED]

Like density, cell potential is an intensive property. This means that cell potential does not depend on the amount of substance present, so even if you multiply your chemical reaction by a coefficient in order to balance the half reactions, you do not need to do anything to change the cell potentia...
Sun Feb 18, 2018 8:37 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Exercise 14.31
Replies: 2
Views: 156

### Re: Exercise 14.31

Step 1: write the half-reactions Oxidation: Cl2(g) + 2e- --> 2 Cl-(aq) Reduction: 2Br-(aq) --> Br2(l) + 2e- Step 2: calculate the standard cell potential for each reaction by looking at Appendix 2B *you should reverse the anode reaction in the table to get the correct cell potential value for the ha...
Sun Feb 18, 2018 8:32 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Isothermal, irreversible?
Replies: 4
Views: 367

### Re: Isothermal, irreversible?

This only occurs when the reaction is isothermal, because in the equation ΔU=(3/2)nRΔT, if there is no change in temperature, then there will be no change in internal energy.
Sun Feb 18, 2018 8:04 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Concentration
Replies: 3
Views: 203

### Re: Concentration

Since the Nernst Equation is as follows: E=E°-(RT/nF) ln Q, we can see that the cell potential (E) depends on the value of Q, which is the concentration of the products over the concentration of the reactants.
Fri Feb 09, 2018 1:26 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: 9.47 b
Replies: 1
Views: 230

### Re: 9.47 b

For irreversible expansion, ΔS is the same as for reversible expansion, because entropy is a state property. So from part (a) calculations, ΔS=+3.84 J/K. To calculate this value, you would use the equation ΔS=nRln (V2/V1). In free expansion, w=0. This is because in free expansion, the gas expands fr...
Fri Feb 09, 2018 1:17 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: 9.67
Replies: 2
Views: 488

### Re: 9.67

For this problem, you need to find the temperature at which the reaction is spontaneous, so you first have to find ΔG. For part (a), you find ΔG by first finding ΔH and ΔS. Step 1: Find ΔG° by finding ΔH° and ΔS°, and using the equation ΔG°= ΔH°-TΔS°. You can look up the values in Appendix 2A, using...
Wed Feb 07, 2018 9:28 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Needing a conductor or not?
Replies: 2
Views: 142

### Re: Needing a conductor or not?

The products and reactants are never supposed to be in the same solution. They are always kept separate with a salt bridge or porous disc in between. The anode and cathode are always separated. If the substances in the anode or cathode are both aqueous, then a conductor is required in order to trans...
Fri Feb 02, 2018 1:38 pm
Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
Topic: How do we find out if a system is favorable? [ENDORSED]
Replies: 7
Views: 1836

### Re: How do we find out if a system is favorable?[ENDORSED]

To find if a reaction is favorable/spontaneous, you need to calculate ΔG. If ΔG is negative, then the reaction is favorable. There are 2 methods to calculate Gibbs free energy: Method 1: use standard enthalpy of formation and standard entropy values of reactants and products. ΔG°=ΔH°-TΔS° To find ΔH...
Fri Feb 02, 2018 1:30 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: 9.27
Replies: 3
Views: 277

### Re: 9.27

Larger molecules have a larger entropy because they are more complex and contain more molecules, which leads to more vibrational and rotational motions. Keeping this in mind, we can conclude that: (a) HBr has the higher entropy, because Br is larger than F, and therefore is more complex. (b) NH3 has...
Fri Feb 02, 2018 1:25 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Van't Hoff Equation
Replies: 2
Views: 309

### Re: Van't Hoff Equation

From ΔH - TΔS = -RTln(K), you simply divide by -RT to get ln K = -(ΔH/RT) + (ΔS/R). However, this form of the Van't Hoff equation is not that commonly used. To derive the more widespread version of the Van't Hoff equation, you add two Van't Hoff equations in order to calculate the equilibrium consta...
Fri Jan 26, 2018 4:15 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Iron Rusting: Exo or Endo? [ENDORSED]
Replies: 4
Views: 1721

### Re: Iron Rusting: Exo or Endo?[ENDORSED]

Pure iron metal contains more energy than rusted iron. Therefore, energy would have to be released in order for iron to rust, creating an exothermic reaction. This same principle applies to other oxidation reactions. Since oxygen generally bonds with chemicals that have low-energy bonds, the oxygen ...
Fri Jan 26, 2018 4:08 pm
Forum: Calculating Work of Expansion
Topic: Homework 8.11
Replies: 6
Views: 294

### Re: Homework 8.11

This formula for work is also used only for reversible processes, because temperature is constant, and as volume increases, pressure increases. However, the work formula for an irreversible process is w=-PΔV, because temperature is not constant. Less work is done in an irreversible process compared ...
Fri Jan 26, 2018 4:05 pm
Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
Topic: Irreversible vs. Reversible [ENDORSED]
Replies: 10
Views: 498

### Re: Irreversible vs. Reversible[ENDORSED]

Additionally, reversible systems involved, for example, a bicycle pump with equal pressure inside and outside of the pump. An irreversible system, however, would have differing pressure inside and outside of the bicycle pump example. However, all real/biological processes are highly irreversible, in...
Fri Jan 19, 2018 2:27 pm
Forum: Calculating Work of Expansion
Topic: units for calculating work
Replies: 6
Views: 159

### Re: units for calculating work

If you use the equation w = -P∆V, the units are kg/m⋅s² (m³) ,which is equal to kg⋅m²/s², which is the equivalent of Joules. Alternatively, you could multiply by 101.325 J/L⋅atm, which would result in work with units of Joules.
Fri Jan 19, 2018 2:03 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: q and delta H interchangeable in calculations?
Replies: 4
Views: 249

### Re: q and delta H interchangeable in calculations?

q and ∆H are only equal to each other when pressure is constant. This gives rise to the equation for change in internal energy of a reaction that happens in a sealed container. If you substitute ∆H for q, the change in internal energy equation becomes ∆U = ∆H - w, or ∆U = ∆H -P∆V.
Tue Jan 16, 2018 9:50 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Change in Internal Energy of Calorimeter (HW 8.25)
Replies: 1
Views: 110

### Change in Internal Energy of Calorimeter (HW 8.25)

Question 8.25 in the book states: "A constant-volume calorimeter was calibrated by carrying out a reaction known to release 3.50 kJ of heat in 0.200 L of solution in the calorimeter (q= -3.50 kJ), resulting in a temperature rise of 7.32 C. In a subsequent experiment, 100.0 mL of 0.200 m HBr(aq)...
Fri Jan 12, 2018 12:59 pm
Forum: Phase Changes & Related Calculations
Topic: Heat and work not state functions
Replies: 7
Views: 401

### Re: Heat and work not state functions

Heat and work both depend on the transfer of energy, regardless of the substance. This means that heat and work are not specific to a particular substance, i.e. they are not intrinsic properties so they cannot be state functions. Only properties like volume and pressure that are specific to the subs...
Fri Jan 12, 2018 12:47 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: What type of system is a Bomb Calorimeter?
Replies: 4
Views: 638

### Re: What type of system is a Bomb Calorimeter?

A bomb calorimeter is a closed system because it allows heat to be exchanged. While this system is insulated, an "insulated system" is not one of the main three types of systems: closed, open, and isolated. An isolated system would not allow for any transfer of matter or energy. In contras...
Thu Jan 11, 2018 9:11 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Sublimation of Carbon with Resonance
Replies: 1
Views: 120

### Sublimation of Carbon with Resonance

Question 8.67 asks to find the enthalpy of formation for benzene with and without resonance; how would you differentiate between the two? The problem only gives the chemical compound, so how would we know which chemical equation to use? For example, part (b) of this question asks to estimate the ent...
Fri Dec 08, 2017 4:25 pm
Forum: Conjugate Acids & Bases
Topic: 12.9 d
Replies: 1
Views: 169

### Re: 12.9 d

For part (d), it is helpful to write the net ionic equation: NH4I (am) + KNH2 (am) --> KI (am) + 2 NH3 (l) NH4+ + I- + K+ + NH2- --> K+ + I- + 2 NH3 This simplifies to: NH4+ + NH2- --> 2 NH3 In this case, NH3 is the conjugate acid of NH2-, because when NH2- accepts a proton, it becomes NH3 (Bronsted...
Fri Dec 08, 2017 4:19 pm
Forum: Calculating pH or pOH for Strong & Weak Acids & Bases
Topic: 12.63
Replies: 1
Views: 154

### Re: 12.63

This problem gives us the % deprotonation of benzoic acid, and asks to find the Ka of benzoic acid. To start this problem, we first need to find [H3O+]. The problem gives us the information that the benzoic acid is in a 0.110 M solution. 1) Find [H3O+]: Since the % protonation is 2.4%, the concentra...
Sun Dec 03, 2017 5:01 pm
Forum: Dipole Moments
Topic: Dipole Moments and Polarity
Replies: 9
Views: 457

### Re: Dipole Moments and Polarity

Dipole moments occur between two bonded atoms of different electronegativities. For example, H20 has a dipole moment between the O-H bond, since the two elements have different electronegativies. And since Oxygen has a greater electronegativity than Hydrogen, the dipole moment would point towards Ox...
Sun Dec 03, 2017 4:53 pm
Forum: Lewis Acids & Bases
Topic: H+ as Lewis acid/base [ENDORSED]
Replies: 2
Views: 506

### Re: H+ as Lewis acid/base[ENDORSED]

A Lewis acid is a compound that accepts an electron pair. So, the H+ ion could accept an electron pair from another compound (base) to form a bond. A Bronsted Acid is a compound that donates a proton. The H+ ion would be accepted by another compound (base) to form a bond. Essentially, H+ can be a Le...
Wed Nov 22, 2017 10:45 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: K and Kc
Replies: 7
Views: 347

### Re: K and Kc

"K" is used to denote Kc or Kp, which you can differentiate by determining if the elements are in a gas phase or an aqueous phase. If the elements are in a gas phase, then you would use the partial pressures to calculate K, which you could also write as Kp. If the elements are in an aqueou...
Wed Nov 22, 2017 10:27 am
Forum: Equilibrium Constants & Calculating Concentrations
Topic: "Phase"
Replies: 5
Views: 266

### Re: "Phase"

"In phase" refers to the physical states of the compounds in chemical equilibrium equations. However, it's important to keep in mind that the equilibrium constant does not include elements in the solid or liquid phase, only aqueous or gas phases. When an element is in the aqueous phase, th...
Fri Nov 17, 2017 3:36 pm
Forum: Hybridization
Topic: 4.35
Replies: 2
Views: 140

### Re: 4.35

4.33 and 4.35 are asking the same thing; the wording is slightly different, but "state the hybridization" and "identify the hybrid orbitals" mean the same thing. The solutions manual answers are in the same format for both questions. You would just answer with the hybridization o...
Fri Nov 17, 2017 3:28 pm
Forum: Sigma & Pi Bonds
Topic: Pi bonds
Replies: 6
Views: 603

### Re: Pi bonds

If we look at C2H4 as an example, there is a pi bond between the two carbons. However, assuming you are looking at the Lewis structure of C2H2, you must think of the pi bonds as having one lobe pointing out of the page towards you, and the second lobe pointing back behind the page. This picture migh...
Thu Nov 09, 2017 9:43 pm
Forum: DeBroglie Equation
Topic: E=hv vs. Work Function?
Replies: 2
Views: 731

### Re: E=hv vs. Work Function?

You would use the equation E=hv when asked to find the energy of the photon. However, if you were asked to find anything related to the emitted electron, you would use the overall photoelectric effect equation, E(photon) - work function = Ek. For example, if you were asked to find the velocity of th...
Thu Nov 09, 2017 9:36 pm
Forum: Resonance Structures
Topic: 3.57 Resonance structures?
Replies: 2
Views: 235

### Re: 3.57 Resonance structures?

Some resonance structures are more favorable than others; this depends on the formal charge of each atom in the compound. The resonance structure that contains the fewest amount of formal charges will be the most favorable, because it is the most stable. Similarly, the resonance structure with the m...
Fri Nov 03, 2017 4:53 pm
Forum: Ionic & Covalent Bonds
Topic: Covalent/ Ionic Bond
Replies: 5
Views: 309

### Re: Covalent/ Ionic Bond

Additionally, ionic bonds are salts that dissociate in water; this is because their electrons and transferred instead of shared (as is the case with covalent bonds), so they compound is much easier to break up/dissolve. In contrast, covalent bonds are molecules that do not easily dissolve in water; ...
Fri Nov 03, 2017 4:46 pm
Forum: Octet Exceptions
Topic: Explaining Octet Rule Exceptions
Replies: 2
Views: 304

### Re: Explaining Octet Rule Exceptions

Elements in period 3 and below contain an empty d-subshell (since the d-block starts at n=3), which creates room for extra electrons to exceed the octet rule. Elements in periods 1 and 2, however, only have s- and p-orbitals, therefore they cannot have more than 8 valence electrons.
Thu Oct 26, 2017 11:20 pm
Forum: Hybridization
Topic: Exceptions to Electron Orbitals
Replies: 3
Views: 846

### Re: Exceptions to Electron Orbitals

The two particular exceptions we need to know are Copper and Chromium. Copper would have an electron configuration of [Ar]3d9 4s2, but notice that the d subshell is one electron away from being completely full. In order for the atom to become more stable, an electron would be needed to complete the ...
Thu Oct 26, 2017 11:13 pm
Forum: Electron Configurations for Multi-Electron Atoms
Topic: Electron Configuration Exceptions [ENDORSED]
Replies: 3
Views: 245

### Re: Electron Configuration Exceptions[ENDORSED]

Yes, the exceptions would apply to all elements in the same groups as Chromium and Copper. Elements in groups 6 and 11 would have an electron configuration that includes nd4 or nd9, therefore it would be more stable if the d subshell was half full or completely full. In order to achieve this, one el...
Thu Oct 19, 2017 10:32 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: Nodal Planes/ Subshells
Replies: 1
Views: 626

### Re: Nodal Planes/ Subshells

n is the principle quantum number, and describes the energy level and shell size of the electron. l, the angular quantum number, describes the shape of the orbital. For reference: when l=0, the orbital is an s-orbital when l=1, the orbital is a p-orbital when l=2, the orbital is a d-orbital when l=3...
Thu Oct 19, 2017 10:23 pm
Forum: Determining Molecular Shape (VSEPR)
Topic: Quantum Mechanics
Replies: 1
Views: 133

### Re: Quantum Mechanics

You would write the subshell notation by first writing the energy level n as the coefficient. Next, you would determine the type of orbital based on the angular quantum number, l. How to determine the type of orbital: When l=0, the orbital is an s-orbital When l=1, the orbital is a p-orbital When l=...
Fri Oct 13, 2017 1:56 pm
Forum: Wave Functions and s-, p-, d-, f- Orbitals
Topic: lines in electromagnetic spectrum
Replies: 1
Views: 165

### Re: lines in electromagnetic spectrum

When a light source is shined on a hydrogen sample, it has the potential to excite the electron from its ground state to an excited state. Or, the incoming energy from the photon can bring an electron down from an excited state to a less excited state (ex. n=4 energy level to n=2 energy level). An e...
Fri Oct 13, 2017 1:47 pm
Forum: Bohr Frequency Condition, H-Atom , Atomic Spectroscopy
Topic: why is Bohr Frequency equation negative
Replies: 4
Views: 586

### Re: why is Bohr Frequency equation negative

The equation E=-hR/n^2 represents the change of energy level that an electron goes through when there is a light source with enough energy to move the electron. An important thing to note: as the energy level approaches infinity, E approaches 0 (plug in infinity for the "n" in the equation...
Wed Oct 04, 2017 10:43 pm
Forum: Limiting Reactant Calculations
Topic: Page F109 from book (Self-Test M.2A)
Replies: 2
Views: 205

### Re: Page F109 from book (Self-Test M.2A)

To find the limiting reactant, you would first convert the given masses into moles using the molar masses for each reactant. Then, using the balanced equation, you can see that 1 mole of Al2O3 requires 6 moles of Na. When you calculate the moles of Al2O3 and multiply by 6 (to find the moles of Al2O3...
Wed Oct 04, 2017 10:29 pm
Forum: Molarity, Solutions, Dilutions
Topic: L7 Help
Replies: 4
Views: 409

### Re: L7 Help

You must use 454g for part (b), because in order to find the mass of oxygen necessary to oxidize the fat, you must relate the moles of C57H110O6 to moles of O2 using their molar ratios from the balanced equation. In order to compare the moles of fat and the moles of oxygen, you must first use the ma...