CHEMISTRY COMMUNITY

Orrin Zhong 4G

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.

Orrin Zhong 4G

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...

Orrin Zhong 4G

To get the overall order of a reaction, add up all the orders of the reactants that are involved in the reaction.

Orrin Zhong 4G

Work is positive when work is done on the system but it is negative when the system is doing work.

Orrin Zhong 4G

When balancing basic reactions, balance the reaction just like you would an acidic reaction, but neutralize the H+'s with OH-'s.

Orrin Zhong 4G

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.

Orrin Zhong 4G

"A" accounts for how often molecules collide in the proper orientation for the reaction to occur.

Orrin Zhong 4G

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.

Orrin Zhong 4G

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.

Orrin Zhong 4G

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.

Orrin Zhong 4G

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.

Orrin Zhong 4G

Use an inert electrode to conduct the electrons when no solid is present in the reaction.

Orrin Zhong 4G

Kinetics looks at how the reaction progresses over time whereas thermodynamics looks at the beginning and final energy states of a reaction.

Orrin Zhong 4G

Temperature is always Kelvin in PV = nRT.

Orrin Zhong 4G

You will use K when the reaction is at equilibrium and Q when the reaction is not at equilibrium.

Orrin Zhong 4G

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.

Orrin Zhong 4G

I know that potential difference is work/charge but what does that mean?

Orrin Zhong 4G

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...

Orrin Zhong 4G

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...

Orrin Zhong 4G

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...

Orrin Zhong 4G

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...

Orrin Zhong 4G

Expansion does work if there is an external pressure and the volume increases.

Orrin Zhong 4G

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.

Orrin Zhong 4G

Isobaric means constant pressure. Under these conditions, h = deltaH, since deltaH is defined at constant pressure. Also, w = -(Pext)(deltaV) since pressure is fixed.

Orrin Zhong 4G

Yes you can use this equation when you have a gas and need to find unknown parameters of the gas.

Orrin Zhong 4G

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.

Orrin Zhong 4G

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...

Orrin Zhong 4G

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...

Orrin Zhong 4G

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...

Orrin Zhong 4G

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.

Orrin Zhong 4G

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.

Orrin Zhong 4G

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...

Orrin Zhong 4G

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...

Orrin Zhong 4G

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...

Orrin Zhong 4G

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...

Orrin Zhong 4G

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...

Orrin Zhong 4G

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.

Orrin Zhong 4G

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.

Orrin Zhong 4G

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.

Orrin Zhong 4G

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.

Orrin Zhong 4G

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.

Orrin Zhong 4G

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...

Orrin Zhong 4G

The "n" here is in the ideal gas equation, therefore you should plug in the moles of gas.

Orrin Zhong 4G

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...

Orrin Zhong 4G

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...

Orrin Zhong 4G

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...

Orrin Zhong 4G

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.

Orrin Zhong 4G

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...

Orrin Zhong 4G

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...

Orrin Zhong 4G

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...

Orrin Zhong 4G

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.

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