## Search found 53 matches

Sat Mar 14, 2020 2:51 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Nernst Equation
Replies: 6
Views: 104

### Re: Nernst Equation

No, it is not necessary. Just make sure that the values you use for each variable are in the right units, and if they are not, convert them individually. T should be in Kelvin. R should be in the J/K form. F is the faraday constant in C/mol, and n is in mol. lnQ is unitless. Altogether, the term sho...
Sat Mar 14, 2020 2:44 pm
Forum: Zero Order Reactions
Topic: 0 order
Replies: 6
Views: 123

### Re: 0 order

A zero-order reaction proceeds at the same speed regardless of the concentrations of reactants.
Sat Mar 14, 2020 2:41 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Concentration Cells
Replies: 8
Views: 132

### Re: Concentration Cells

If you have different concentrations of the same species in the sides of the cell, then in the Nernst equation, Eo is zero but lnQ is nonzero. Eo is zero because there is no difference in reduction potential (same species). Since you have more reactants than products (difference in concentration), l...
Sat Mar 14, 2020 2:25 pm
Forum: First Order Reactions
Topic: Linear versus Non-Linear
Replies: 3
Views: 66

### Re: Linear versus Non-Linear

I find the linear form easier to use in general for solving for different variables and basically everything. The linear form is also what you use to graph: you graph lnA versus time and the slope of the line is -k. I can't think of a good use for the nonlinear equation.
Sat Mar 14, 2020 2:15 pm
Forum: Balancing Redox Reactions
Topic: Acid or base?
Replies: 5
Views: 146

### Re: Acid or base?

Yes, we will be given this information. If it is not explicitly stated acidic or basic, they may give the pH of the solution in which the reaction occurs.
Sat Mar 14, 2020 2:09 pm
Forum: Student Social/Study Group
Replies: 1
Views: 90

I have the textbook on Sapling because I used it for chem14A. Anyone else like me having trouble with the answer key in the online textbook? It never fully loads even though I give it A LOT of time. If anyone knows how to fix this, I would really appreciate it. Also, can someone give me the answers ...
Sun Mar 08, 2020 11:02 pm
Forum: Van't Hoff Equation
Topic: What's the purpose of Van't Hoff?
Replies: 5
Views: 114

### Re: What's the purpose of Van't Hoff?

By using the Van't Hoff equation, you can determine K for a reaction at a certain temperature if you already know the K at a different temperature. Likewise, you can find a value for temperature of a K if that is your unknown. Primarily, this equation relates T and K
Sun Mar 08, 2020 10:51 pm
Forum: First Order Reactions
Topic: How does a first order reaction "collide" with itself? [ENDORSED]
Replies: 2
Views: 54

### Re: How does a first order reaction "collide" with itself?[ENDORSED]

I would think that a first-order reaction involves a species that is unstable on its own and transitions to products over time due to its instability. Perhaps it reacts with a solvent whose concentration is so large that is it not significant.
Sun Mar 08, 2020 10:44 pm
Forum: Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)
Topic: Purpose of Nernst Equation
Replies: 5
Views: 35

### Re: Purpose of Nernst Equation

The Nernst equation is useful for relating E of the cell and reaction quotient Q. Given a balanced reaction along with temperature and Eo, Ecell can be calculation using Q and Q can be calculated using E.
Sun Mar 08, 2020 10:33 pm
Forum: General Rate Laws
Topic: Changing the mass of electrodes
Replies: 6
Views: 44

### Re: Changing the mass of electrodes

I believe that nothing changes in the reaction. Equilibrium/rate is not affected because solid electrodes are not components of the expression for Q or K.
Wed Mar 04, 2020 7:57 pm
Forum: Interesting Applications: Rechargeable Batteries (Cell Phones, Notebooks, Cars), Fuel Cells (Space Shuttle), Photovoltaic Cells (Solar Panels), Electrolysis, Rust
Topic: electrolytic cells
Replies: 1
Views: 69

### Re: electrolytic cells

Electrolytic cells are basically galvanic cells run in reverse. In a galvanic cell, electron transfer occurs because the reduction potential of the cathode is greater than the reduction potential of the anode. E of the cell is positive, and electron flow happens on its own. If you provide energy (th...
Sun Mar 01, 2020 11:05 pm
Forum: Kinetics vs. Thermodynamics Controlling a Reaction
Topic: Diamond
Replies: 6
Views: 108

### Re: Diamond

Graphite is thermodynamically more stable than diamond, because it has less total energy. However, the process needed to get from diamond to graphite has a large activation energy (where the "energy barrier" comes from). So, the reaction proceeds so slowly that diamond is said to be stable...
Sun Mar 01, 2020 10:18 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Galvanic and Voltaic Cells
Replies: 7
Views: 75

### Re: Galvanic and Voltaic Cells

They are synonymous. Its sometimes called a voltaic cell because it operates on the principle of voltage, or the difference in potential between the cathode and anode.
Sun Mar 01, 2020 10:06 pm
Forum: First Order Reactions
Topic: first order
Replies: 3
Views: 60

### Re: first order

A first order reaction is a reaction that depends linearly on reactant concentration. This means that if the concentration of one reactant were to double, then the initial rate of reaction would also double. I second this, and also, I think the physical meaning of the "orders" will become...
Sun Mar 01, 2020 10:01 pm
Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
Topic: Salt Bridge
Replies: 12
Views: 107

### Re: Salt Bridge

Since electrons are being transferred from anode to cathode, positive charge is built up in the anode and negative charge in the cathode. The salt bridge allows for ions to pass between solutions and thus maintaining the redox reaction from being slowed.
Sun Mar 01, 2020 9:56 pm
Forum: First Order Reactions
Topic: Slope form
Replies: 2
Views: 40

### Re: Slope form

This is true if you graph ln[A] on the y-axis and t on the x axis
Sun Feb 23, 2020 9:58 pm
Forum: Balancing Redox Reactions
Topic: Basic solutions
Replies: 2
Views: 44

### Re: Basic solutions

You would go through the balancing process (with the half reactions) they same way you would do a reaction in an acidic solution, but at the end, you would add enough OH- to both sides of the equation to effectively change H+ into H20, and OH- that does not pair with enough H+ would remain as OH- In...
Sun Feb 23, 2020 9:55 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Spontaneity
Replies: 8
Views: 83

### Re: Spontaneity

Correct. Negative Gibbs free energy and positive cell potential both mean that the reaction is spontaneous.
Sun Feb 23, 2020 9:52 pm
Forum: Van't Hoff Equation
Topic: Constants in Van’t Hoff Equation [ENDORSED]
Replies: 4
Views: 63

### Re: Constants in Van’t Hoff Equation[ENDORSED]

In the Van't Hoff equation, both deltaS and deltaH are assumed to be constant independent of temperature, however, only deltaH shows up in the most easily usable form of the equation.
Sun Feb 23, 2020 9:48 pm
Forum: Balancing Redox Reactions
Topic: Oxidation Numbers/States
Replies: 8
Views: 93

### Re: Oxidation Numbers/States

Why is oxygen generally -2 oxidation state and hydrogen generally +1? Oxygen forms 2 covalent bonds to complete its octet, and because it has a high electronegativity relative to almost every other element, it will pull these electrons closer to it, effectively owning them and resulting in a -2 oxi...
Sun Feb 23, 2020 9:45 pm
Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
Topic: Conditions of Eo
Replies: 3
Views: 48

### Re: Conditions of Eo

In any case, the little circle above E here denotes standard conditions, which includes constant temperature (0 C) and pressure (1 atm)
Sun Feb 23, 2020 9:42 pm
Forum: Balancing Redox Reactions
Topic: Oxidation Numbers/States
Replies: 8
Views: 93

### Re: Oxidation Numbers/States

How do you use the oxidation numbers/states to get the half reactions? Also do we need to memorize the oxidation numbers for common molecules/elements? You can use oxidation numbers to find out what species what reduced and what was oxidized. This becomes the basis for your half reactions, as there...
Sun Feb 16, 2020 11:02 pm
Forum: Balancing Redox Reactions
Topic: Oxygen
Replies: 9
Views: 74

### Re: Oxygen

The same elements usually have the same oxidation numbers. The periodic table also has trends. Can someone explain the periodic trends please? Oxidation numbers are based off of electronegativity. When we give an atom a negative oxidation number, we are saying that it is more electronegative than t...
Sun Feb 16, 2020 10:56 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: ΔU Equal 0
Replies: 4
Views: 202

### Re: ΔU Equal 0

If you are referring to an isothermal process, a gas can expand (-w) if it is heated (+q) for the same amount of energy. The energy lost by expanding is supplied by heat, and the change in internal energy is 0.
Sun Feb 16, 2020 10:53 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Shift in Equilibrium
Replies: 2
Views: 43

### Re: Shift in Equilibrium

This mainly depends on the substance that you add. For instance, an inert gas wouldn't affect the reaction at all. However, if you add something that reacts with either reactants or products, your system probably wouldn't remain at equilibrium because it would complicate the reaction and change the...
Sun Feb 16, 2020 10:46 pm
Forum: Balancing Redox Reactions
Topic: redox reactions
Replies: 3
Views: 38

### Re: redox reactions

In the clearest cases, ions will change their charge (different charge from reactants to products) and if they lose electrons (more positive) they are oxidized and if they gain electrons (more negative) they are reduced. In other cases, we usually take oxygen to almost always be -2 oxidation state (...
Sun Feb 16, 2020 10:40 pm
Forum: Van't Hoff Equation
Topic: Van't Hoff Eq
Replies: 1
Views: 25

### Re: Van't Hoff Eq

If you are using the Van't Hoff Eq to calculate K (equilibrum constant) for a reaction at a certain temperature, you must have a given K for that reaction in a different temperature, and you must also know the standard reaction enthalpy for that reaction. We assume that the standard reaction enthalp...
Sun Feb 09, 2020 9:34 pm
Forum: Non-Equilibrium Conditions & The Reaction Quotient
Topic: X is to small
Replies: 8
Views: 144

### Re: X is to small

I believe you can check to see if dropping the x is valid. You do the calculation (with x dropped) and then seeing if the resulting concentration value for x is less than 5 percent of the original concentration of solution (what you started with). If it is, then your approximation is valid.
Sun Feb 09, 2020 9:28 pm
Forum: Ideal Gases
Topic: Gas constant
Replies: 10
Views: 115

### Re: Gas constant

Yep, it is based on the units.
R = 8.314 J/(mol * K)
R = 0.08206 (atm*L)/(mol*K)
Sun Feb 09, 2020 9:16 pm
Forum: Phase Changes & Related Calculations
Topic: closed vs isolated
Replies: 10
Views: 110

### Re: closed vs isolated

Isolated systems do not exchange matter or energy with its surroundings (where closed systems can only exchange energy). The universe is said to be the ultimate isolated system because it can't really exchange anything with its "surroundings" (it has no surroundings)
Sun Feb 09, 2020 7:49 am
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Calculating Bond Enthalpies
Replies: 5
Views: 68

### Re: Calculating Bond Enthalpies

Each reaction is different, so I don't think there's a reliable pattern you can follow for bond enthalpies. You can try to visualize the reaction if the molecules are small enough, but in the end, your safest bet is drawing the bonds.
Mon Feb 03, 2020 3:21 pm
Forum: Thermodynamic Systems (Open, Closed, Isolated)
Topic: isolated system example
Replies: 2
Views: 24

### Re: isolated system example

An isolated system is one where neither energy nor matter is exchanged with surroundings. The bomb calorimeter as a whole does not exhange energy or matter with its surroundings, so it is an isolated system.
Wed Jan 29, 2020 4:52 pm
Forum: Concepts & Calculations Using First Law of Thermodynamics
Topic: Deriving Equation
Replies: 3
Views: 33

### Re: Deriving Equation

This equation holds true for a situation of constant pressure. Professor Lavelle starts with the equation U = q + w, and q = deltaH in conditions of constant pressure. w becomes -PdeltaV because in gas expansions/compressions, work done on the system is equal to -PdeltaV
Wed Jan 29, 2020 4:45 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: internal energy
Replies: 4
Views: 70

### Re: internal energy

Also another derivation of the formula we learned in class today was DeltaU=DeltaH-P(DeltaV) if the pressure is constant. And if the volume is constant, work is equal to 0 because the DeltaV in the work term is 0. So, in that case, DeltaU = q When a question asks you to find change in internal ener...
Wed Jan 29, 2020 4:38 pm
Forum: Phase Changes & Related Calculations
Topic: Calculating Work
Replies: 6
Views: 60

### Re: Calculating Work

Calculate work using the formula work w done on the system = -P \Delta V You mentioned that the work formula you know is w=Fd, and today in lecture, professor Lavelle explained that w=-p \Delta V is based on that formula ! :) Pressure is force/area, so F=pressure*area, and substituting this into w=F...
Wed Jan 29, 2020 4:08 pm
Forum: Calculating Work of Expansion
Topic: Negative sign on work equation
Replies: 3
Views: 29

### Re: Negative sign on work equation

I agree with the above explanations in that the whole term for w is positive/negative depending on whether or not the system is expanding or being compressed. You can think of the negative sign in the formula in a different way: w stands for work being done on the system. In this way, expansion woul...
Mon Jan 27, 2020 11:42 pm
Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
Topic: Constant Pressure Calorimeter
Replies: 1
Views: 24

### Constant Pressure Calorimeter

Why does q = delta H in a constant pressure calorimeter but not in a constant volume calorimeter?
Sat Jan 25, 2020 5:18 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Different methods
Replies: 4
Views: 31

### Re: Different methods

You will be able to tell which method to use based on what information you are given in a problem. If you are given values of bond enthalpies, you will use that method. If you are given standard enthalpies of formation, use that method. If you are given the enthalpy values for certain reactions that...
Sat Jan 25, 2020 1:59 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Standard state
Replies: 3
Views: 21

### Re: Standard state

The standard state of a substance is its most stable form at 1 atm and (usually) 25 degrees Celsius. In standard state, gases are at 1 atm, solutions are at 1 M (and also 1 atm), liquids/solids are assumed to be pure, and elements are in their most stable phase.
Sat Jan 25, 2020 1:52 pm
Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
Topic: Heat
Replies: 1
Views: 22

### Re: Heat

Heat refers to the transfer of thermal energy in a particular instance. If I applied heat to water, then we can't talk about that heat without considering the overall process it is in (heating water). Enthalpy, however, refers to the "state" of the system in that it deals with the total th...
Sat Jan 25, 2020 1:47 pm
Forum: Phase Changes & Related Calculations
Topic: Phase Change
Replies: 4
Views: 41

### Re: Phase Change

The information should be given, possibly in a phase change graph. I believe it would be the difference in y-value between two of the horizontal lines (the height of the diagonal) as this would be the energy into the system to bring the substance from one state up to the next. Yes, it should be giv...
Sat Jan 25, 2020 1:39 pm
Forum: Phase Changes & Related Calculations
Topic: Bond Enthalpies
Replies: 5
Views: 47

### Re: Bond Enthalpies

To add on, bond breaking is an endothermic process and has a positive value, and bond forming is an exothermic process and has a negative value. Also note that the table of bond enthalpies that they give you are going to have all values be positive. You need to change the value to negative for bond...
Fri Jan 17, 2020 4:26 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Strong or weak acid?
Replies: 5
Views: 52

### Re: Strong or weak acid?

Hello. You have to memorize what the strong acids and bases are. If an acid or base is not in the list of strong acids/bases, it is safe to assume that they are weak. I have this picture from Professor Caram's Chem 14A lecture. https://i.imgur.com/4lJtWHA.png You can also look them up on google
Thu Jan 16, 2020 6:41 pm
Forum: Non-Equilibrium Conditions & The Reaction Quotient
Topic: change in temp
Replies: 4
Views: 40

### Re: change in temp

We can treat the heat term when written in the equation in a very similar manner as a standard product or reactant. If the reaction is endothermic, the heat term would be written on the reactants side of the forward reaction, and increasing the temperature has a similar effect as increasing the con...
Thu Jan 16, 2020 3:11 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Ka/Kb of 10^-4
Replies: 3
Views: 28

### Re: Ka/Kb of 10^-4

You can use the assumption to calculate x, and then check to see if x is less than 5% of the original concentration. If it is, then the assumption is valid.
Wed Jan 15, 2020 11:50 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: Le Chatelier overview
Replies: 3
Views: 31

### Re: Le Chatelier overview

Le Chatlier's Principle encompasses changes in all three (concentrations, pressure, temperature).
Mon Jan 13, 2020 3:15 pm
Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
Topic: 5J.9
Replies: 3
Views: 21

### Re: 5J.9

I read it as adding more partial pressure of something means adding more moles of it. If you add more moles of NO, the amount of NH3 increases in order to restore equilibrium ratio of products to reactants. When NH3 is decreased, the reaction shifts so as to increase reactants (to restore equilibriu...
Thu Jan 09, 2020 7:46 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: multiplied reaction
Replies: 3
Views: 41

### Re: multiplied reaction

The above explanations make sense mathematically (thank you for posting them) but I would like to ask:
If each K value is valid for its respective reaction (multiplied or unmultiplied), how can there be 2 different valid K values for what is essentially the same reaction?
Thu Jan 09, 2020 7:41 pm
Forum: Ideal Gases
Topic: What is the Importance of homogeneous vs heterogeneous equilibria [ENDORSED]
Replies: 12
Views: 166

### Re: What is the Importance of homogeneous vs heterogeneous equilibria[ENDORSED]

It's also important to note that for a homogeneous gas equilibrium, we can calculate Kp, but we cannot calculate Kp for a heterogeneous equilibrium that involves both gas and aqueous species.
Thu Jan 09, 2020 7:14 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: How to calculate Q if not given concentrations
Replies: 3
Views: 34

### Re: How to calculate Q if not given concentrations

If concentrations are not explicitly given, there will usually be a way to calculate them. You might be given moles (or mass, which can be converted to moles) and a volume. You can find concentration by dividing moles by volume. If you are given gases and their partial pressures are known and the te...
Thu Jan 09, 2020 4:05 pm
Forum: Non-Equilibrium Conditions & The Reaction Quotient
Replies: 5
Views: 43

### Re: confused about ice table

An ICE table is a method to keep track of initial and final concentrations during an equilibrium reaction. You will write down all your reactants and products and mark their initial concentrations (the I in ICE). Then you will look for the change, which is denoted by x. Reactants will lose concentr...
Thu Jan 09, 2020 3:48 pm
Forum: Non-Equilibrium Conditions & The Reaction Quotient
Topic: calculating reaction quotient
Replies: 3
Views: 30

### Re: calculating reaction quotient

I would assume the balanced reaction is 2As + 3H2 <--> 2AsH3 From there, you would take the mol values for AsH3 and H2 and divide them each by 3.00 L to get the concentrations. I am assuming that As is in solid form and that AsH3 and H2 are gases. Since As is solid, it is not included in the calcula...
Thu Jan 09, 2020 3:14 pm
Forum: Equilibrium Constants & Calculating Concentrations
Topic: Units
Replies: 4
Views: 43

### Re: Units

Yes. K is unitless, but concentrations and pressures always have units. In a problem like you described, your final answer would have units.