Search found 51 matches
- Sun Mar 15, 2020 9:02 pm
- Forum: Arrhenius Equation, Activation Energies, Catalysts
- Topic: catalyst
- Replies: 4
- Views: 363
Re: catalyst
A species is a catalyst if it exists at the start of the reaction, then gets used up by the reaction, but gets reformed at the end of the reaction.
- Sun Mar 15, 2020 8:59 pm
- Forum: Arrhenius Equation, Activation Energies, Catalysts
- Topic: Exothermic reactions
- Replies: 5
- Views: 452
Re: Exothermic reactions
Temperature increases the rate of the reaction regardless of whether it is exothermic or endothermic. The Arrhenius equation, k = Ae^(-Ea/(RT)), shows that increases the temperature increases the rate constant (k), therefore increasing the rate of reaction. Temperature increases the rate of reaction...
- Sun Mar 15, 2020 8:43 pm
- Forum: General Rate Laws
- Topic: Overall Order
- Replies: 6
- Views: 424
Re: Overall Order
To get the overall order of a reaction, add up all the orders of the reactants that are involved in the reaction.
- Sun Mar 15, 2020 8:42 pm
- Forum: Concepts & Calculations Using First Law of Thermodynamics
- Topic: Doing work vs work done
- Replies: 8
- Views: 685
Re: Doing work vs work done
Work is positive when work is done on the system but it is negative when the system is doing work.
- Sun Mar 15, 2020 8:41 pm
- Forum: Balancing Redox Reactions
- Topic: Half rxns
- Replies: 27
- Views: 1477
Re: Half rxns
When balancing basic reactions, balance the reaction just like you would an acidic reaction, but neutralize the H+'s with OH-'s.
- Sun Mar 08, 2020 11:03 pm
- Forum: General Rate Laws
- Topic: Changing the mass of electrodes
- Replies: 6
- Views: 396
Re: Changing the mass of electrodes
Changing the mass of the electrode does not change E(cell), but changing the concentrations of the reactants and products or the temperature of the reaction does change E(cell). You can see this by the variables that participate in the Nernst equation.
- Sun Mar 08, 2020 10:56 pm
- Forum: Arrhenius Equation, Activation Energies, Catalysts
- Topic: A in the Arrhenius Equation
- Replies: 8
- Views: 550
Re: A in the Arrhenius Equation
"A" accounts for how often molecules collide in the proper orientation for the reaction to occur.
- Sun Mar 08, 2020 10:30 pm
- Forum: General Rate Laws
- Topic: units
- Replies: 12
- Views: 645
Re: units
k has different units depending on what order the reaction is. Since rate has units (mol x L)/s, you just set up a unit equation: mol x L/s = k x (mol x L)^(order of rxn) and solve for the units of k.
- Sun Mar 08, 2020 10:22 pm
- Forum: Zero Order Reactions
- Topic: Zero Order Reaction Example
- Replies: 6
- Views: 1359
Re: Zero Order Reaction Example
Professor Lavelle gave an example of zero order reactions as a saturated catalyst reaction, therefore the rate of reaction is no longer dependent on the concentration of the reactants but rather on the speed at which catalysts can interact with the reactants.
- Sun Mar 08, 2020 10:17 pm
- Forum: Method of Initial Rates (To Determine n and k)
- Topic: K
- Replies: 11
- Views: 668
Re: K
K is defined as the ratio of products to reactants when a chemical reaction is at equilibrium. K is dependent on temperature and changes in temperature shift K depending on whether the reaction is endothermic or exothermic.
- Sun Mar 01, 2020 9:07 pm
- Forum: Balancing Redox Reactions
- Topic: When to add H+ or H20
- Replies: 19
- Views: 2488
Re: When to add H+ or H20
In an acidic solution, you would balance oxygen with H2O and then balance the excess hydrogens with H+'s. In a basic solution, you would add get the acidic reaction and then add OH- on both sides to balance out the H+'s.
- Sun Mar 01, 2020 9:03 pm
- Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
- Topic: cell diagrams
- Replies: 4
- Views: 395
Re: cell diagrams
Use an inert electrode to conduct the electrons when no solid is present in the reaction.
- Sun Mar 01, 2020 9:01 pm
- Forum: Kinetics vs. Thermodynamics Controlling a Reaction
- Topic: thermo or kinetics?
- Replies: 9
- Views: 774
Re: thermo or kinetics?
Kinetics looks at how the reaction progresses over time whereas thermodynamics looks at the beginning and final energy states of a reaction.
- Sun Mar 01, 2020 8:58 pm
- Forum: Thermodynamic Systems (Open, Closed, Isolated)
- Topic: Kelvin or Celsius?
- Replies: 86
- Views: 5674
Re: Kelvin or Celsius?
Temperature is always Kelvin in PV = nRT.
- Sun Mar 01, 2020 8:57 pm
- Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
- Topic: when to use K or Q
- Replies: 18
- Views: 1115
Re: when to use K or Q
You will use K when the reaction is at equilibrium and Q when the reaction is not at equilibrium.
- Sat Feb 22, 2020 1:15 am
- Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
- Topic: Galvanic Cells
- Replies: 8
- Views: 486
Re: Galvanic Cells
Galvanic cells use the energy from spontaneous redox reactions. As the electrons go from the anode to the cathode along a wire, the flow of electrons can be used to do work.
- Sat Feb 22, 2020 1:02 am
- Forum: Work, Gibbs Free Energy, Cell (Redox) Potentials
- Topic: Potential difference
- Replies: 3
- Views: 275
Potential difference
I know that potential difference is work/charge but what does that mean?
- Sat Feb 22, 2020 12:48 am
- Forum: Balancing Redox Reactions
- Topic: Charge of oxygen
- Replies: 15
- Views: 759
Re: Charge of oxygen
Holdup... isn't the oxidation state of O2 equal to zero? O2 has two oxygens, which means that they both have the same electronegativity and as a result, they share the electrons equally among themselves. Therefore the oxygen should not have gained electrons from itself. The oxidation numbers of pure...
- Sat Feb 22, 2020 12:19 am
- Forum: Galvanic/Voltaic Cells, Calculating Standard Cell Potentials, Cell Diagrams
- Topic: How to write Cell Diagrams
- Replies: 5
- Views: 455
Re: How to write Cell Diagrams
I would also like to add that if a porous disc/wall is replacing the salt bridge, the separation between the anode and cathode is denoted by only one "|". Also, I'm not sure if this is a strict rule, but by convention, you place the conducting electrodes on the outsides of the cell diagram...
- Sat Feb 22, 2020 12:02 am
- Forum: Identifying Acidic & Basic Salts
- Topic: Midterm 3C
- Replies: 9
- Views: 838
Re: Midterm 3C
Because NH4Cl is made up of the conjugate acids of a weak base and a strong acid, it is going to be an acidic salt and cause pH to drop when it is added to a solution. Another way to think about it is to write the reaction occuring. NH4Cl disassociates into NH4+ and Cl-, so you could write a reactio...
- Fri Feb 14, 2020 10:33 pm
- Forum: Concepts & Calculations Using Second Law of Thermodynamics
- Topic: DeltaS
- Replies: 2
- Views: 237
Re: DeltaS
In an isothermal, free expansion, the external pressure is 0 since piston is expanding into a vacuum. Since an isothermal expansion has deltaU = 0 = q + w, q = -w. However, since there is no external pressure, the gas is doing no work during expansion. Since the gas is doing no work, there is no hea...
- Fri Feb 14, 2020 10:28 pm
- Forum: Phase Changes & Related Calculations
- Topic: Expansion Work
- Replies: 6
- Views: 465
Re: Expansion Work
Expansion does work if there is an external pressure and the volume increases.
- Fri Feb 14, 2020 10:23 pm
- Forum: Ideal Gases
- Topic: initial values
- Replies: 6
- Views: 513
Re: initial values
It depends on whether the other parameters (pressure, temperature) are in their initial states or their final states. Then you would choose the appropriate volume to plug into PV = nRT.
- Fri Feb 14, 2020 10:17 pm
- Forum: Thermodynamic Definitions (isochoric/isometric, isothermal, isobaric)
- Topic: Isobaric systems
- Replies: 16
- Views: 836
Re: Isobaric systems
Isobaric means constant pressure. Under these conditions, h = deltaH, since deltaH is defined at constant pressure. Also, w = -(Pext)(deltaV) since pressure is fixed.
- Fri Feb 14, 2020 10:15 pm
- Forum: Ideal Gases
- Topic: pv=nrt
- Replies: 19
- Views: 1209
Re: pv=nrt
Yes you can use this equation when you have a gas and need to find unknown parameters of the gas.
- Sun Feb 09, 2020 11:26 pm
- Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
- Topic: Specific Heat Capacity
- Replies: 5
- Views: 187
Re: Specific Heat Capacity
Specific heat capacity is used when you want to find the heat change in a substance when given mass (in grams). Molar heat capacity is used when you want to find the heat change in a substance when gives moles.
- Sun Feb 09, 2020 11:19 pm
- Forum: Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)
- Topic: Negative entropies
- Replies: 3
- Views: 172
Re: Negative entropies
I don't think we can have negative entropies, but we can have negative changes in entropy. A system can have a negative change entropy if a reaction increases the order within the system. However, the overall change in entropy of the universe is always positive since another reaction with a higher c...
- Fri Feb 07, 2020 10:38 pm
- Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
- Topic: H=q
- Replies: 4
- Views: 279
Re: H=q
Yeah I would also agree that assuming a constant pressure at standard state is reasonable if a problem does not specify anything. Two other indications that would point to constant pressure would be if the problem gives you ΔH to use because that is specified at a constant pressure. Another indicati...
- Fri Feb 07, 2020 10:21 pm
- Forum: Thermodynamic Systems (Open, Closed, Isolated)
- Topic: question on lecture notes about closed system
- Replies: 5
- Views: 209
Re: question on lecture notes about closed system
A closed system cannot exchange matter with its surroundings but can exchange heat. Therefore, the lack of insulation for the closed system allows for heat exchange between the closed system and the surroundings. If it was completely insulated, it would be an isolated system because it cannot exchan...
- Fri Feb 07, 2020 10:18 pm
- Forum: Concepts & Calculations Using First Law of Thermodynamics
- Topic: state occupation of phases
- Replies: 7
- Views: 291
Re: state occupation of phases
In gases, the kinetic energy is higher than the intermolecular forces between them, allowing them to spread out unlike solids and liquids. Therefore, they take up more space since the individual gas molecules are bouncing all over the place.
- Fri Feb 07, 2020 10:09 pm
- Forum: Phase Changes & Related Calculations
- Topic: -w vs w
- Replies: 15
- Views: 666
Re: -w vs w
Negative work is work being done by the system because the system is losing energy by doing the work. Positive work is work being done on the system because the system is gaining energy from the work.
- Fri Jan 31, 2020 10:00 pm
- Forum: Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions
- Topic: Equilibrium shift by pressure
- Replies: 7
- Views: 269
Re: Equilibrium shift by pressure
That is correct. The equilibrium shift only occurs when the volume is changed (assuming there are gases participating in the reaction), since the partial pressures of the reactants and products will change, modifying their values in the equilibrium calculations. However, if the total pressure is cha...
- Fri Jan 31, 2020 9:54 pm
- Forum: Phase Changes & Related Calculations
- Topic: Enthalpy of sublimation?
- Replies: 8
- Views: 1479
Re: Enthalpy of sublimation?
Sublimation is the phase change from a solid directly to a liquid. Therefore, the enthalpy of sublimation is the change in enthalpy from a solid to a gas. The enthalpy of sublimation is the sum of the enthalpy of fusion and the enthalpy of vaporization: ΔH(sublimation) = ΔH(vaporization) + ΔH(fusion...
- Fri Jan 31, 2020 9:49 pm
- Forum: Thermodynamic Systems (Open, Closed, Isolated)
- Topic: Textbook problem help
- Replies: 2
- Views: 384
Re: Textbook problem help
Uisng the equation: ΔU = ΔH - ΔnRT, we can just plug in the values to find ΔU (the change in internal energy). Since the equation has a 2 in front of the coefficient for ethanol and we only react 1 mole of ethanol, to find the appropriate ΔH for the reaction, we divide the ΔH they gave us for the eq...
- Fri Jan 31, 2020 9:11 pm
- Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
- Topic: 4A.13
- Replies: 4
- Views: 144
Re: 4A.13
The heat released from a reaction [q(reaction)] must go somewhere, so it is absorbed by the calorimeter. Therefore, if the temperature of the reaction decreases, then the heat from the reaction will go to the calorimeter, increasing the temperature of the calorimeter. Thus, q(reaction) = -q(calorime...
- Fri Jan 31, 2020 9:09 pm
- Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
- Topic: Reversible Process
- Replies: 4
- Views: 178
Re: Reversible Process
This video was really helpful to understanding it: https://www.youtube.com/watch?v=LdM0DzlS3fQ . A reversible system happens when changes to the system and to the surroundings can be reversed. The changes in a reversible system are infinitesimally small, such that the system is always in equilibrium...
- Fri Jan 31, 2020 8:45 pm
- Forum: Equilibrium Constants & Calculating Concentrations
- Topic: Test 1 # 4
- Replies: 10
- Views: 382
Re: Test 1 # 4
Use PV=nRT by plugging in pressure, volume, temperature, and R (0.0821 Latm/(molK)). Isolate and solve for n, which will give you the moles of Xenon present, and you can convert to the grams of Xenon using the molar mass of Xenon.
- Fri Jan 31, 2020 8:29 pm
- Forum: Ideal Gases
- Topic: non ideal gases
- Replies: 8
- Views: 472
Re: non ideal gases
Most gases are non ideal. Ideal gases are used to make it easy to calculate properties of gases using the ideal gas law (PV = nRT) as long as the pressure isn't too high or the temperature isn't too low or else that will cause deviations in the calculations.
- Fri Jan 31, 2020 8:03 pm
- Forum: Heat Capacities, Calorimeters & Calorimetry Calculations
- Topic: qp and qv
- Replies: 8
- Views: 381
Re: qp and qv
If the heat transfer is occurring while pressure is changing, q is not equal to change in enthalpy, since the change in enthalpy is defined at a constant pressure. Therefore, change in enthalpy only equals q when pressure is held constant: qp = change in enthalpy.
- Fri Jan 31, 2020 7:49 pm
- Forum: Phase Changes & Related Calculations
- Topic: Phase Changes
- Replies: 17
- Views: 731
Re: Phase Changes
Phase changes occur at constant temperature because the energy put into the system is breaking the intermolecular forces between the molecules rather than increasing the kinetic energy of the molecules.
- Fri Jan 31, 2020 7:22 pm
- Forum: Ideal Gases
- Topic: R Constant
- Replies: 26
- Views: 1213
Re: R Constant
There are a lot of gas constants, so just fit the units that are involved with the units of the correct gas constant. Generally, I use 0.0821 Latm/(molK) when I'm trying to find volume or pressure and 8.314 J/(molK) when I'm trying to find energy.
- Fri Jan 31, 2020 7:19 pm
- Forum: Ideal Gases
- Topic: ideal gases
- Replies: 14
- Views: 982
Re: ideal gases
To be an ideal gas, a gas must have no volume, have no intermolecular forces between each other, move randomly, and have perfectly elastic collisions with the walls of the container (no energy loss). I don't think ideal gases exist in reality, but they are a good model to use when calculating values...
- Fri Jan 31, 2020 7:14 pm
- Forum: Ideal Gases
- Topic: Determining N
- Replies: 9
- Views: 426
Re: Determining N
The "n" here is in the ideal gas equation, therefore you should plug in the moles of gas.
- Fri Jan 31, 2020 7:06 pm
- Forum: Ideal Gases
- Topic: pressure
- Replies: 10
- Views: 405
Re: pressure
Partial pressure is the pressure of an individual type of gas molecule within a gas mixture. The total pressure is the combined pressure of all the gases making up the gas mixture. Therefore, the total pressure is equal to the sum of all of the partial pressures for each type of gas in the gas mixtu...
- Fri Jan 31, 2020 6:59 pm
- Forum: Ideal Gases
- Topic: Temperature
- Replies: 17
- Views: 788
Re: Temperature
Most of the time, temperature has to be changed to Kelvin to be plugged into a formula. However, when an formula requires the change in temperatures (such as in q = mcdeltaT), the use of Celsius for the initial and final temperatures are acceptable because the scaling factor from Celsius to Kelvin c...
- Fri Jan 31, 2020 6:56 pm
- Forum: Ideal Gases
- Topic: Inert Gas
- Replies: 12
- Views: 706
Re: Inert Gas
Inert gases are just another term for noble gases. They are inert because they don't react with anything due to their full valence shells. In regards to class material, they say that when noble gases are added to a reaction, they don't shift the equilibrium because all they do is increase the overal...
- Fri Jan 31, 2020 6:44 pm
- Forum: Equilibrium Constants & Calculating Concentrations
- Topic: autoprotolysis
- Replies: 7
- Views: 299
Re: autoprotolysis
From Kw = [H+][OH-], you can negative log both sides => -log[Kw] = -log([H+][OH-]), and using the log rules, you can get =log[Kw] = -log[H+] + -log[OH-]. Since "p-" in chemistry denotes "-log" (as in pH = -log[H+]), you can get the equation pKw = pH + pOH.
- Fri Jan 31, 2020 6:40 pm
- Forum: Equilibrium Constants & Calculating Concentrations
- Topic: ICE
- Replies: 20
- Views: 932
Re: ICE
You use an ICE table when you need to find missing concentrations (most likely trying to find equilibrium concentrations given an equilibrium constant). You have an initial concentrations row, where you put your initial concentrations for each species in the reaction. The change is how much the init...
- Fri Jan 31, 2020 6:34 pm
- Forum: Equilibrium Constants & Calculating Concentrations
- Topic: Negative pH
- Replies: 9
- Views: 327
Re: Negative pH
Although the concentration of H+ ions cannot become negative, pH can become negative since pH = -log[H+]. Therefore, when the H+ concentration is greater than 1 M, the pH can become negative. However, we don't see negative pHs all that often because most reactions don't have H+ concentrations as hig...
- Fri Jan 31, 2020 6:29 pm
- Forum: Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)
- Topic: Hess' Law
- Replies: 7
- Views: 263
Re: Hess' Law
Enthalpy is a state property, so only the final and initial states matter. The process by which you get from an initial state to a final state does not matter for state properties. Therefore, if you want to find the enthalpy value of a reaction through Hess' Law, you can just add the enthalpy values...
- Fri Jan 31, 2020 6:25 pm
- Forum: Phase Changes & Related Calculations
- Topic: Units
- Replies: 16
- Views: 846
Re: Units
When calculating change in temperature, you can use either Celsius or Kelvin because Celsius = Kelvin + 273. Therefore, when you subtract two Celsius temperatures, they the +273 will cancel out and will equal the different between those two temperatures as Kelvin.