CHEMISTRY COMMUNITY

Harshita Talkad 4L

Yes, Dr. Lavelle said we are supposed to assume A is 1 unless another value of A is given. We did not cover how to calculate A in lecture so I don't think we have to worry about any questions that would ask us to do so.

Harshita Talkad 4L

A catalyst doesn't get used up in a reaction, so even if it is used up in one of the steps, it'll be regenerated in another step. Intermediates are the products of one step that are then used as reactants in a following step.

Harshita Talkad 4L

They are not included, since only reactants are included in the rate law.

Harshita Talkad 4L

The K used in the Van't Hoff equation is the equilibrium constant. It is useful for finding the equilibrium constant at different temperatures.

Harshita Talkad 4L

The fast steps, in comparison to the slow step, are negligible because they occur so quickly. The time that the slow step takes is more representative of how long the entire reaction takes.

Harshita Talkad 4L

The integrated rate laws are given on the formula sheet, but I think you should still have a general idea about how to integrate to get them.

Harshita Talkad 4L

I think it's ok to add them in redox reactions because the half reactions are still parts of the same overall redox reaction, as opposed to adding together the E values of two completely separate reactions.

Harshita Talkad 4L

It is spontaneous when E is positive, which means Gibbs free energy would be negative because when E is positive in delta G=-nFE, delta G will be negative, which once again shows that the reaction is spontaneous.

Harshita Talkad 4L

Why is delta G naught positive when K<1? Wouldn't the forward reaction be favored and therefore spontaneous, making it negative?

Harshita Talkad 4L

Either J or kJ should be fine for the final answer, as long as you're consistent with units while you're doing the calculation so that the number ends up being correct.

Harshita Talkad 4L

I did mostly Gibbs free energy questions and a few of the electrochemistry questions from the beginning of outline 5.

Harshita Talkad 4L

It is the energy available to do work, which determines spontaneity of a reaction because a negative value of delta G means the reaction is spontaneous, while a positive value means it is not spontaneous.

Harshita Talkad 4L

Yes, it would be 298K because T is in Kelvin in the Gibbs free energy equation.

Harshita Talkad 4L

Changing the temperature can make a reaction spontaneous, since increasing temperature will increase entropy. At a certain temperature, the value for Gibbs free energy will become negative and the reaction will be spontaneous.

Harshita Talkad 4L

It uses enthalpy, temperature, and entropy to determine whether or not a reaction will be spontaneous.

Harshita Talkad 4L

Since work is the force applied on the object multiplied the distance it has moved, if volume is constant, the distance is zero, which means no work is done.

Harshita Talkad 4L

There is a link on the Chem 14B website called "Constants and Equations" that has the formula sheet.

Harshita Talkad 4L

Matter and energy can be exchanged in an open system, like a reaction in an open beaker. Only energy, but not matter, can be exchanged with the surroundings in a closed system, like when there is a piston. No matter or energy is exchanged with the surroundings in an isolated system, like in a closed...

Harshita Talkad 4L

I think we are supposed to use atm for pressure, and since there's a very small difference between atm and bars, the value with either of these units will be pretty much the same.

Harshita Talkad 4L

Forming bonds releases energy, so the negative sign represents the loss of energy from the system.

Harshita Talkad 4L

Delta H is the change in enthalpy, while q is just heat, so it doesn't represent a change.

Harshita Talkad 4L

The reaction is endothermic, since a positive value of delta H means that the heat of the products is greater than the heat of the reactants, which means the reaction required the absorption of heat.

Harshita Talkad 4L

Enthalpy of a reaction with multiple steps can be found by adding the enthalpy of each step together because it is a state function, while the same can not be done with heat.

Harshita Talkad 4L

Since the positive and negative indicates whether energy is being absorbed or released for the reaction to take place, if the reaction absorbs heat in one direction, the reverse reaction will release heat since the bonds that were initially being broken are now being formed and vice versa.

Harshita Talkad 4L

The number of sig figs in the H3O+ concentration is the how many numbers there should be after the decimal place in the pH value.

Harshita Talkad 4L

x needs to be smaller than 10^(-3) to be ignored. This means it's small enough to be insignificant, which connects to the approximation being valid if it's within 5% because it means the x was small enough.

Harshita Talkad 4L

Since water is neutral, the concentration of H3O+ ions and OH- ions has to be equal, and since water is neutral at any temperature, Kw has to be the same.

Harshita Talkad 4L

K is calculated with the concentrations of the products and reactants when the reaction is at equilibrium, while Q can be calculated at any point in the reaction, which means the concentrations of the products and reactants can vary at different times in the reaction. Q is compared to K to determine...

Harshita Talkad 4L

Yes, the homework is due in discussion so it'll be due on Tuesday for you.

Harshita Talkad 4L

K is calculated with the concentrations of the reactants and products when the reaction is at equilibrium, which means K will always be constant as long as it is the same reaction at the same temperature. Q can be calculated at any point in the reaction, so there will be different concentrations of ...

Harshita Talkad 4L

Lewis acids accept lone pairs and Lewis bases donate lone pairs. Bronsted acids donate protons and Bronsted bases accept protons.

Harshita Talkad 4L

HBr has a greater bond length because Br has a greater atomic radius than F, which means it is easier for HBr to donate a proton, so it is a stronger acid.

Harshita Talkad 4L

Polarity is determined by the unequal sharing of electrons, so a molecule would be polar if the atoms that it is made of do not have the same electronegativity and do not cancel each other out with dipoles in opposite directions.

Harshita Talkad 4L

Not all parts of the body can handle low pH levels the same way, which is why it hurts to get vinegar in a cut that is more sensitive to the acidity.

Harshita Talkad 4L

It depends on what the molecule is being used for, because cis will usually be polar and trans will usually be nonpolar.

Harshita Talkad 4L

You would find the oxidation number by figuring out how many electrons it would lose or gain.

Harshita Talkad 4L

Hybridization occurs because hybridized orbitals have lower energy than when they are unhybridized, so the compounds that they form are more stable.

Harshita Talkad 4L

A single bond is a sigma bond, a double bond is one sigma and one pi bond, and a triple bond is one sigma bond and two pi bonds.

Harshita Talkad 4L

Dr. Lavelle said he writes it as sp^3d, but the textbook writes it as dsp^3, so either is fine.

Harshita Talkad 4L

A molecule is nonpolar if its atoms have the same electronegativity or if the dipole moments cancel each other out, and it is polar when the dipole moments do not cancel out.

Harshita Talkad 4L

How would you be able to tell whether a bond is a sigma or a pi bond?

Harshita Talkad 4L

Cis is when atoms of the same element are on the same side of the molecule and trans is when atoms of the same element are on opposite sides of the molecule.

Harshita Talkad 4L

The repulsion strength between lone pairs is greater than the repulsion between a lone pair and a bonding pair, and a lone pair and bonding pair have a greater repulsion than two bonding pairs. As repulsion increases, the size of the bond angle increases.

Harshita Talkad 4L

We also need to know that the dissociation energy for longer bonds is lower than the dissociation energy of shorter bonds because they are easier to break.

Harshita Talkad 4L

They are tetrahedral when there are four areas of electron density because the atoms will spread out as far apart from each other as possible.

Harshita Talkad 4L

Nodal planes are where there are no electrons in an atom. Because s-orbitals are spherical, they have no nodal planes. P-orbitals have 1, d-orbitals have 2, and so on.

Harshita Talkad 4L

The first part is explaining how a combination of all the resonance structures is a more accurate representation of the bond lengths than each individual resonance structure on its own.

Harshita Talkad 4L

We are supposed to calculate formal charge when drawing every Lewis structure and try to minimize it as much as possible. I don't think you have to draw multiple resonance structures unless the question specifically asks for them.

Harshita Talkad 4L

It doesn't matter which side the electrons are drawn on, but they are usually spread around the atom evenly, so they don't start pairing until there is at least one electron on each side.

Harshita Talkad 4L

When he drew the Lewis structure of the entire molecule, he drew the SO₄ molecule between the NH₄ molecules, without including any of the lines that indicate bonding.

Harshita Talkad 4L

Also, if the molecule is charged, you need to account for those electrons. For example, if the molecule has a charge of +1, you need to subtract one electron, or if the molecule has a charge of -2, you need to add two electrons.

Harshita Talkad 4L

Problems will sometimes give the indeterminacy in position (delta x) and mass of the object and ask you to solve for delta v. This can be done because (delta p)=(delta v)*mass, so you can use the information you are given to calculate delta v.

Harshita Talkad 4L

The intensity of light in the wave model is associated with the amplitude of a wave because a higher amplitude correlates with higher intensity light. When thinking about light as a particle, the intensity of the light is based on the number of photons.

Harshita Talkad 4L

The greek letter nu is the curly v that represents frequency (in Hertz or s^-1), while the regular v represents velocity (in m/s or m*s^-1).

Harshita Talkad 4L

During Friday's lecture, Professor Lavelle said that 1 J= 1 kg.m^2.s^(-2). In the equation, the units are (J.s)/(kg.m/s), so if you plug in the units for J, all the units except for m will cancel out, which gives wavelength in meters.

Harshita Talkad 4L

We don't need to worry about knowing their significance in this course because they are only for hypothetical situations.

Harshita Talkad 4L

We would use kg in the equation because it is the standard SI unit. Most equations will use base SI units.

Harshita Talkad 4L

Any number of grams can technically be used to find the empirical formula, but because percentages are given as parts of 100, it makes it easier to assume that it is a 100 gram sample.

Harshita Talkad 4L

Another example is if you are measuring the temperature of a substance and consistently get 39 degrees Celsius during 5 separate trials, but the substance is actually 35 degrees Celsius. This would mean that the measurements are precise, since you repeatedly get the same result, but they are not acc...

Harshita Talkad 4L

Molarity is also important for dilution problems because it can be used in the equation (M_initial)(V_initial)=(M_final)(V_final) to determine concentrations, volumes, or moles of the solute before or after the solution has been diluted.

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