## Search found 30 matches

Wed Mar 14, 2018 10:35 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: Calculating activation energy from Arrhenius equation
Replies: 1
Views: 140

### Re: Calculating activation energy from Arrhenius equation

Just tried switching k1, k2, T1, T2 and the answer was the same, so I guess the order doesn't matter. Just make sure the you use the correct temperature for their respective rate constants. Mathematically it works out, since a higher temperature gives a larger rate constant. If you use the bigger te...
Wed Mar 14, 2018 10:24 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: 15.109
Replies: 1
Views: 201

### Re: 15.109

You don't need lnA here. You have the rate constant at 800 deg C and the question asks for the rate constant for the same reaction at 700 deg C, so you have these two equations: At Temp 1 = 800 C = 1073 K: lnk_{1}=lnA-\frac{E_{a}}{RT_{1}} At Temp 2 = 700 C = 973 K: lnk_{2}=lnA-\frac{E_{a}}{RT_{2}} C...
Wed Mar 14, 2018 10:11 pm
Forum: Reaction Mechanisms, Reaction Profiles
Topic: 15.49
Replies: 4
Views: 303

### Re: 15.49

I think they're just asking for the rate laws of each individual step, not the overall reaction. For the overall reaction you wouldn't keep the intermediate in the rate law for the slow step and would instead substitute it using the pre-equilibrium approach.
Wed Mar 07, 2018 6:19 pm
Forum: Zero Order Reactions
Topic: Units of K based on the order
Replies: 3
Views: 372

### Re: Units of K based on the order

You can derive the units of k by plugging in [A] = mol.L-1 and t = s into the integrated rate laws and solving for k. (this is basically 15.9 in the textbook) Zero order: [A]_{0}-[A] = kt \rightarrow k=\frac{[A]_{0}-[A]}{t}=\frac{[mol\cdot L^{-1}]-[mol\cdot L^{-1}]}{s} k = mol\cdot L^{-1}\cdot s^{-1...
Wed Mar 07, 2018 5:58 pm
Forum: Arrhenius Equation, Activation Energies, Catalysts
Topic: 15.63 units and reaction order
Replies: 1
Views: 166

### 15.63 units and reaction order

The question gives the rate constant 25 C as 1.5x10^10 L.mol-1.s-1, indicating a second order reaction. The question asks for the rate constant of the same reaction but at 37 C, given an activation energy of 38 kJmol-.1. The answer I got was 2.7x10^10 L.mol-1.s-1, but the answer in the back of the b...
Wed Mar 07, 2018 5:48 pm
Forum: General Rate Laws
Topic: 15.101(a) Overall Reaction
Replies: 2
Views: 228

### 15.101(a) Overall Reaction

a) What is the overall reaction for the following mechanism? ClO- + H2O -> HClO + OH- (fast equilibrium) HClO + I- -> HIO + Cl (very slow) HIO + OH- -> IO- + H2O (fast equilibrium) The answer in the back of book says the overall reaction is ClO- + I- -> IO-. Where did the Cl go? Why isn't it ClO- + ...
Sat Mar 03, 2018 2:37 pm
Forum: General Rate Laws
Topic: Unique Rate [ENDORSED]
Replies: 5
Views: 583

### Re: Unique Rate[ENDORSED]

For a reaction aA + bB -> cC + dD, where the lowercase letters are the stoichiometric coefficients and the uppercase letters are chemical species, the unique rate law is -\frac{1}{a}\frac{d[A]}{dt} = -\frac{1}{b}\frac{d[B]}{dt} = \frac{1}{c}\frac{d[C]}{dt} = \frac{1}{d}\frac{d[D]}{dt} This rate law ...
Sat Mar 03, 2018 2:22 pm
Forum: General Rate Laws
Topic: Half Life
Replies: 3
Views: 224

### Re: Half Life

Half-life is just the time needed for the concentration to reach half its initial concentration. That definition is the same no matter the order of the reaction. To get the half-life equation from each different rate law, just set t=t1/2, and [A]=1/2 {A}0, and then just algebraically separate t1/2 f...
Sat Mar 03, 2018 12:50 am
Forum: Kinetics vs. Thermodynamics Controlling a Reaction
Topic: How to tell difference btwn thermo and kinetics [ENDORSED]
Replies: 7
Views: 553

### Re: How to tell difference btwn thermo and kinetics[ENDORSED]

Thermodynamics predicts the spontaneity, while kinetics predicts the rate/speed of the reaction. If a reaction is spontaneous (thermodynamically unstable) but the reaction doesn't occur at a measurable rate, then it is controlled by kinetics (kinetically stable). For example, C(diamond) spontaneousl...
Thu Feb 22, 2018 10:55 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: 14.15 Parts b and c [ENDORSED]
Replies: 2
Views: 207

### Re: 14.15 Parts b and c[ENDORSED]

Not sure about part b, but it looks like they just took each reactant, put H2O(l) on the product side, and balanced the equation. For part c, KOH is there because you can't put in OH- by itself, so you add in K to make it a solution. NaOH would probably work as well. Although KOH is on the reactant ...
Thu Feb 22, 2018 10:35 am
Forum: Balancing Redox Reactions
Topic: 14.15a
Replies: 3
Views: 603

### Re: 14.15a

I think it's because the oxidation number for bromine doesn't change - for the reactant AgBr, Ag has oxidation no. = +1, and Br has oxidation no. = -1 for an overall charge of 0. So in the anode, Ag(s) + Br- -> AgBr + e (Ag goes from 0 to +1); cathode, Ag+ + e -> Ag(s) (Ag goes from +1 to 0). The ce...
Mon Feb 19, 2018 2:40 pm
Forum: Interesting Applications: Rechargeable Batteries (Cell Phones, Notebooks, Cars), Fuel Cells (Space Shuttle), Photovoltaic Cells (Solar Panels), Electrolysis, Rust
Topic: 14.117
Replies: 2
Views: 437

### 14.117

Here is the question: The body functions as a kind of fuel cell that uses oxygen from the air to oxidize glucose: C6H12O6(aq) + 6O2(g) -> CO2(g) + 6H2O(l) During normal activity, a person uses the equivalent of about 10 MJ of energy a day. Assume that this value represents delta G, and estimate the ...
Wed Feb 14, 2018 5:02 pm
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Why is the change in total entropy zero for a reversible free expansion?
Replies: 2
Views: 295

### Re: Why is the change in total entropy zero for a reversible free expansion?

Did you mean something else? You can't have a reversible free expansion because free expansion is irreversible, since the gas expands in a vacuum. In a reversible reaction P(external) = P(internal), which doesn't work in free expansion where there is no external pressure.
Wed Feb 14, 2018 4:52 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: PV = nRT
Replies: 6
Views: 428

### Re: PV = nRT

Delta nRT is used to calculate the change in internal energy under constant pressure. Delta U = q + w, but under constant pressure it becomes \Delta U=\Delta H-P\Delta V A change in volume is often due to a net change in gas molecules, so P\Delta V=\Delta nRT . Substitute in and you get \Delta U=\De...
Wed Feb 14, 2018 4:41 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Practice Midterm #2C
Replies: 1
Views: 156

### Re: Practice Midterm #2C

Using the equation of broken-formed, we should get (\frac{1}{2}(N\equiv N) + \frac{5}{2}(H-H)+ (O=O)+ 2(Hsub(C))) - (2(N-H) + (N-C)+2(C-H)+(C-C)+(C=O)+(C-O)+(O-H)) Giving you (1/2(944) + 5...
Fri Feb 09, 2018 11:11 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Anode and cathode
Replies: 9
Views: 455

### Re: Anode and cathode

In a galvanic cell, the anode is usually on the right and is the site of oxidation, and generally gets thinner over time as the metal electrode loses electrons and gives off metal ions. The cathode is on the left and is the site of reduction, and gets thicker over time as the metal ions gain electro...
Fri Feb 09, 2018 11:02 am
Forum: Entropy Changes Due to Changes in Volume and Temperature
Topic: Cv,m vs Cp,m vs R
Replies: 4
Views: 4504

### Re: Cv,m vs Cp,m vs R

Cv,m is the molar heat capacity in constant volume, and Cp,m is the molar heat capacity in constant pressure. For liquids and solids, Cv,m and Cp,m are similar. For ideal gases, Cv,m = 3/2 R and Cp,m = 5/2 R.
Fri Feb 09, 2018 10:49 am
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: E and G
Replies: 2
Views: 195

### Re: E and G

Yes, $\Delta G^{o}=-nFE^{o}(cell)$
Where n is the number of moles and F = Faraday's constant = 9.6485 x 10^4 C*mol^-1
Sat Feb 03, 2018 2:12 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: What does it mean for a reaction to be "thermodynamically favored?"
Replies: 6
Views: 14800

### Re: What does it mean for a reaction to be "thermodynamically favored?"

Enthalpy tends to go from high enthalpy to low enthalpy, whereas entropy tends to go from low entropy to high entropy. Delta H -, Delta S + = favored Delta H -, Delta S - = favored at low temperatures only Delta H +, Delta S + = favored at high temperatures only Delta H +, Delta S - = not favored (r...
Sat Feb 03, 2018 2:04 pm
Forum: Gibbs Free Energy Concepts and Calculations
Topic: Why does an exothermic process lead to an increase in entropy?
Replies: 3
Views: 189

### Re: Why does an exothermic process lead to an increase in entropy?

An exothermic process leads to an increase in the entropy of the surroundings, since delta S (surroundings) = - (delta H)/T. For an exothermic reaction, the change in enthalpy is negative, making the change in entropy of the surroundings positive. The entropy of the system will not increase, but the...
Sat Feb 03, 2018 11:53 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: Residual Entropy [ENDORSED]
Replies: 2
Views: 254

### Re: Residual Entropy[ENDORSED]

A greater dipole moment means that the molecules experience more attraction to one another and lie head-to-tail in a more orderly fashion, so their residual entropy is lower. Molecules with smaller dipole moments can be in either direction, resulting in a higher residual entropy.
Fri Jan 26, 2018 10:45 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: reversible and irreversible reactions
Replies: 4
Views: 276

### Re: reversible and irreversible reactions

Not sure if this helps, but in a reversible reaction P(in) and P(ex) is the same, whereas in an irreversible reaction P(ex) differs by P(in) by a measurable amount.
Fri Jan 26, 2018 7:07 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: Formation of Snow in Clouds [ENDORSED]
Replies: 5
Views: 2452

### Re: Formation of Snow in Clouds[ENDORSED]

Snow is formed when temperatures drop low enough for water to change from a liquid to a solid, which is an exothermic process - as the surroundings cool, heat is transferred from liquid water into the surroundings, and the water solidifies into snow.
Fri Jan 26, 2018 6:54 pm
Forum: Concepts & Calculations Using Second Law of Thermodynamics
Topic: Ch. 9 Problems, 5
Replies: 3
Views: 174

### Re: Ch. 9 Problems, 5

For the first reservoir, heat is being transferred out and into the second reservoir, so the change of entropy is negative.
Thu Jan 18, 2018 9:49 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Bond Enthalpies
Replies: 1
Views: 215

### Re: Bond Enthalpies

Br2 was initially a liquid. Since all bond enthalpies refer to species in the gas phase only, Br2 needs to be vaporized (liquid to gas) and that value is added to the overall enthalpy change, since the phase change is endothermic and therefore positive. The bond enthalpy for the formation of C3H6Br2...
Thu Jan 18, 2018 9:02 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Delta U？
Replies: 7
Views: 2399

### Re: Delta U？

Delta U is the change in internal energy due to the transfer of energy as work and/or heat. Delta U = q + w
Thu Jan 18, 2018 8:59 pm
Forum: Calculating Work of Expansion
Topic: The w equation
Replies: 2
Views: 142

### Re: The w equation

The negative sign is there because the internal energy of the system decreases when the system expands, since the system is doing work against the outside pressure and is losing energy. As such, work is negative for expansion (work being done BY the system), and positive for compression (work is bei...
Thu Jan 11, 2018 11:29 pm
Forum: Calculating Work of Expansion
Topic: Irreversible vs Reversible
Replies: 2
Views: 204

### Re: Irreversible vs Reversible

An irreversible process in expansion work is defined as one where external pressure differs from internal pressure by a finite, measurable amount. In a reversible process, the external pressure equals the internal pressure, so it could go either way with an infinitesimally small change.
Thu Jan 11, 2018 11:07 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Molar heat capacity
Replies: 2
Views: 267

### Re: Molar heat capacity

Molar heat capacity is the amount of heat needed to raise one mole of a substance by one degree Kelvin, and is dependent on the intermolecular forces between the particles. For example, a substance with strong intermolecular forces (such as water, which has hydrogen bonds) would have a higher heat c...
Thu Jan 11, 2018 10:59 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Heat and state function
Replies: 2
Views: 175

### Re: Heat and state function

Heat is not a state function because of this equation: q + w = U, where U is the internal energy of the system, q is the heat, and w is the work. Work is not a state function since it depends on the distance an object is moved, or the path, and so therefore the heat must also depend on the path.