CHEMISTRY COMMUNITY

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We use the pre equilibrium approach when the reaction is a fast step followed by a slow step. For pre equilibrium we assume the reaction before the rate limiting step is at equilibrium so we use K. This approach works because there is a bottle neck and a buildup of intermediate which results in the ...

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We can determine which step is the fastest based on the magnitude of the peak, which is the activation energy. The smaller the activation energy, the faster the reaction. The dips represent the formation of intermediates because they show one thing being formed at the end of one dip and then being c...

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Generally, we will not need to know how to find the integrated rate law for reactions with order greater than two because it is rare that 3 molecules will collide together to react so we do not need to calculate rate laws for third order reactions. However, I believe we could test it the same way as...

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Unique rate is rate of formation multiplied by the negative inverse of its coefficient--- aA +bB---> bB + cC would have unique rate= (-1/a) * (d[A]/dt).
This is different from k, which is the rate constant for a reaction.

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Yes, if you are given initial concentrations for every substance in the rate law and you are given the rate, we can use simple math to find k. Example: for A+B---> products, rate=1.5 * 10^-7 mol/L*s and [A]=2.0 * 10^-2 mol/L , [B]=3.7 * 10^-2 mol/L: rate=k[A][B] ------- k=rate/[A][B]-------- k= (1.5...

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To add to this, if we are strictly given time series graphs we can also describe their order based on the plot. For example, if there is a straight line plot for the graph of concentration vs time, it is a zero order reaction. A straight line plot for natural log concentration vs time means a 1st or...

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slope of a tangent line is significant because it tells us the reaction rate at a particular time in the reaction. We are looking for the instantaneous rate as a function of how much concentration we have at a certain time so we must use the differential rate law.

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Molecularity is the number of species in an elementary step. The rate law for each elementary step follows directly from molecularity. For example somehting is bimolecular if the elementary step involves two reactants. A+B ---> P , rate=k[A][B]

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a zero order reaction is one whose rate is independent of the concentration. An example is when nitrous oxide decomposes to nitrogen and oxygen. In the presence of a catalyst, platinum, we see that changing concentration of N2O has no effect on the rate of decomposition so we know the rate only depe...

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There must be a fast step preceding the slow step because the slow step is the rate determining step. Therefore an accumulation of intermediates results in the first step being at equilibrium. The first step must be fastest in order to have this accumulation of intermediates and equilibrium.

caseygilles 1E

n is the moles of the compound given or calculated for; however, do not confuse this with nr, which is a pure number with the unit mol struck out. Nr is just the stoichiometric coefficient in the reaction equation.

caseygilles 1E

We need to know that cell potential is the maximum voltage difference between the two electrodes. We also need to know that maximum voltage potential occurs when very little current flows.

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Max potential is achieved when there is very little current flow because maximum potential difference equals the work done divided by the charge flow. Therefore, when there is very little charge flow, the cell potential increases and is maximized when very little current is flowing.

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We use platinum in the reaction when our half reactions have no conducting solids (such as iron and copper redox) because platinum can be used as an inert conductor, an electrode to transfer electrons.

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When we are given the standard reduction potential, we have to reverse the anode reaction because the anode reaction represents oxidation, so we must reverse it to represent reduction.

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The steps to balance a redox reaction for acids is: Step 1: identify the species being reduced/oxidized Step 2: write the skeletal equation for reduction/oxidation Step 3: balance all elements except H and O Step 4: balance O atoms by adding H2O to a side of the reaction balance H atoms by adding H+...

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We use the Van't hoff equation instead of deltaG=deltaH-TdeltaS when we are trying to relate the change in equilibrium constant, K, to the changing temperature of the chemical reaction and we are given the standard enthalpy change deltaH.

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I do not think we actually need to calculate potential energy. I believe we are just using the term conceptually to understand the relation between entropy and potential energy, since entropy studies the number of different potential microstates, aka states at different energies in a system, somethi...

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For irreversible work we assume a constant pressure, so work= P(external) * delta V. For reversible work, we assume that P external is not constant, so
work=-nRT ln (V2/V1). For constant volume we use the equations U=q=mCdelta T since there is no work done on or by the system.

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Degeneracy represents the number of ways of achieving a given energy state, and represents the minimized energy at equilibrium. Entropy is at equilibrium when degeneracy is a maximum value.

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Work will have a negative sign if the the system does work on its surroundings, it will be releasing energy to the surroundings. Work will have a positive sign if the surroundings are doing work on the system, aka energy is being gained by the system.

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Entropy is a measure of the number of potential microstates/orientations something could be in. Therefore changes in entropy are changes in the number and position of possible microstates.

caseygilles 1E

I think the difference between reversible and irreversible work is that infinitesimally small changes in pressure are considered insignificant in irreversible systems and pressure is considered constant; whereas, in reversible processes, external pressure is not constant and infinitesimal changes wi...

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Hess's law will yield a more accurate example because the numbers are more individualized and discrete for each molecule measured, whereas bond enthalpy numbers are averages from many different molecules and are less accurate. The third method, which measures standard enthalpies of formation of mole...

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The bond enthalpy method is the least accurate because bond enthalpies are averages of numbers for many different molecules, whereas the other methods are more precise and individualized for each specific molecule. However, bond enthalpies of diatomic molecules are accurate because they are measured...

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No, we do not always assume change in U to be positive. Change in U can either be negative or positive depending on whether the internal energy of the system increased (energy was added to the system, aka surroundings did work on the system, positive change in U) or decreased (energy was released fr...

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Heat is the energy transferred as a result of a temperature difference. Therefore, we know that heat is a way that energy is transferred specifically from a high-temp to low-temp region. This is different than enthalpy. Enthalpy is a state function that allows us to keep track of energy changes at a...

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Yes, the total can change signs depending on whether it is endothermic or exothermic. A negative change in enthalpy indicates a release of heat through forming more bonds than breaking and a positive change in enthalpy indicates that heat was used to break bonds. Remember that when products are form...

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The equilibrium only changes if the change in pressure is due to a changing volume. If the volume decreases, the pressure will increase and the reaction will try to stabilize by decreasing pressure and the system will shift to the side with less moles of gas. However, if pressure changes due to an i...

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The conjugate seesaw says that the stronger the acid, the weaker that acid's conjugate base. The stronger the base, the weaker that base's conjugate acid. But here's why... We know that Ka * Kb =Kw, and we know that Kw is a the ionization constant of water= 1* 10^-14. Therefore if we have a large Ka...

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Yes it is safe to assume this, although checking the percent ionization at the end is always smart just to make sure. To do this use the formula: % ionization= ((molarity of conjugate base formed ) /(initial molarity of acid)) * 100%. If this number is <5% then we can use approximate for the change ...

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Reducing the partial pressure of CO decreases the pressure of CO2 because the system looks to minimize the effect of changes. Therefore, if pressure of CO is decreased, the reaction will shift to increase CO back to its pressure and so the reaction shifts left, thus the products are being consumed t...

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Changing the concentration doesn't change K because the system will settle back to the same ratio of K even if the concentrations are different. The system looks to minimize the effect of changes so when equilibrium is perturbed it slowly settles back to its stable equilibrium ratio. For instance, i...

caseygilles 1E

We calculate Q the exact same way we calcuate Kc or Kp, that is
Kc= [products]^a/[reactants]^b
Kp= (P products)^a / (P reactants)^b
The products are always in the numerator.

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The reactant side will have a negative change in concentration when we initially have only reactants that are forming products. However, the reactant side could have a positive change in concentration if we initially start with products that are being consumed to form reactants. Generally the negati...

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Kc = equilibrium constant: concentration (mol / L) Kp= equilibrium constant: pressure (bar) Q = reaction quotient, calculated the same way as Kc or Kp but can be calculated at any time during the reaction (at equilibrium or not) The value of Q gives us vital information as to the future nature of th...

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The concentration of Pure solids and liquids are not used in calculating the equilibrium constant because their concentrations stay constant throughout the reaction. We can see this because their concentrations are calculated by density/ molar mass. The activity ion pure solids and liquids is theref...

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a heme complex is a coordination complex consisting of an iron ion coordinated to a porphyrin. Porphyrin acts as a tetra dentate ligand.

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If the question asks to find the minimum value for uncertainty in position or uncertainty in momentum, then the , >= sign acts as an equal sign because the delta x or delta p will be greater than or equal to the answer, indicating a minimum value for the indeterminacy.

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Yes, when given mass of an object as having the same energy as momentum of a gamma ray we must use E=pc to solve for energy. Then we equate this energy of a gamma ray to the kinetic energy of the object, inserting our mass in. KE = 1/2 mv^2. We ca solve for the velocity of the object in this way.

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A= central atom. X= bound atom. E= Lone pair of electrons. AXE could potentially be on test 3 if we are talking about the VSEPR notation of an end atom. For example, OCN- will have a triple bond between C and N and then the N atom will have a lone pair, so the VSEPR notation of the N atom will be AX...

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The reason why a carbon carbon double bond is weaker than the sum of two carbon carbon single bonds is because a Sigma bond is much stronger than a pi bond, therefore two Sigma bonds are stronger than a Sigma bond plus a pi bond. This also has to do with rotation because Sigma bonds are allowed to r...

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We do not need to worry about the charge if they all have the same charge. If they have different charges we name them in: neutral, negative, positive. When naming same charge ligands we only care about alphabetical order of them. Ligand names always come before naming the metal ion.

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Hydrates form around coordination complexes in order to form bridges between crystals wanting to form ionic compounds to be more stable but don't want to alter their chemical being.

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for ligands that are anions, change the endings as follows: -ide -o; -ate -ato; -ite -ito.

For neutral ligands, use the regular name of the molecule. Exceptions to the rule include: water = aqua, ammonia=ammine, carbon monoxide= carbonyl, and the N2 and O2 = dinitrogen and dioxygen.

caseygilles 1E

In cations and anions, there is a chance that excess lone pairs could be necessary in the Lewis structure of the atom, thus distorting the bond angle from its standard number. For example, a lone pair on the Carbon of CH4 causes the bond angle to drop slightly below its normal 109.5 degrees for the ...

caseygilles 1E

Distortion, which is proportional to the polarizability of electrons, depends on the number of electrons and the size of the atom or molecule. Therefore, distortion affects the interaction potential energy because the larger the distortion in electron clouds, the greater the potential energy due to ...

caseygilles 1E

So we learned in class that larger atoms that bond together have a larger bond length and thus a weaker bond, so why would I2 be a solid in its natural form and Br2 be a liquid when they are larger atoms bonding together and F2 and Cl2 are gases but are smaller bonds? Is this talking about intramole...

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Due to the strength of double and triple bonds, they affect the bond angle of the molecule by acting like nonbonding electron pairs and skew the normal bond angles. In general, multiple bonds (double, triple) in the VESPR model exert a greater repulsive force on adjacent electrons than the single bo...

caseygilles 1E

I believe we only have to know the 5 most basic molecule shapes involving just two elements in a compound. The 5 shapes are: linear (180 deg), trigonal planar (120 deg), tetrahedral (109.5 deg), trigonal bipyramidal (3 at 120 deg, 2 at 90 deg), and octahedral (90 deg). The specific bond angles also ...

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Elements themselves are not polar or non polar. The bonds they form with other elements are classified as polar or non polar covalent bonds based on the orientation of electrons, shape of molecule, and electronegativity of the elements in the molecule. Generally elements with high electronegativity ...

caseygilles 1E

The question is trying to get us to mention that the more we know about one, the less we know about the other. So, if we know the uncertainty in position is a very specific discrete value, we will know less about the value in the uncertainty of momentum.

caseygilles 1E

They are exceptions because they do not have enough orbitals or electrons to fill an octet and therefore can only fill a duplet. Other exceptions to the octet rule are elements in the third period and below, which can have expanded octets because of their higher energy orbitals (example: SF6 and PCl...

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No, we do not need to know the discrete values; however, we must understand the relationships between elements and what makes them more or less electronegative than others. Electronegativity is more conceptual rather than calculated in this class, ie being able to understand that electronegativity i...

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Atomic Absorption Spectroscopy is a technique the light than ions will absorb when electrons get excited in heat (aka a flame). It is used to determine the atoms and concentration of a sample. Atoms of the light are excited over the flame and the energy difference is tested, because energy differenc...

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The second ionization of alkali metals is dramatically higher than the first ionization energy because after the first electron is removed, the atom drops energy levels. For example, for potassium, when the first electron is removed, the energy level drops from the 4s level to the 3p level; therefor...

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Multi electron atoms are all the atoms except for hydrogen, which only has one electron. Multi electron atoms are more complex because of the reactions that multiple electrons have with each other that effect electron energy. For example, in multi electron atoms, there is something called electron s...

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A wave function is basically describing the quantum state of a system. Wave function (psi) is how the wave behaves in a system, such as amplitude of wave. The wave function squared (psi^2) is the probability of finding an electron in a specific area of the atom. This is the electron density probabil...

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The second shell for an atom has two subshells. It contains the 2s orbital and 2p orbitals. The s-type has only one orbital, holding two electrons. As we move up in shells, the p-type has three available orbitals, which can hold a total of six electrons. Therefore, when n=2, the atom has one s-type ...

caseygilles 1E

This equation is actually not Coulomb's law. Coulomb's law is applied when finding the force of attraction or repulsion between electrons (F=k*Q1*Q2 / r^2). Although it may derive or relate to Coulomb's law, Q1*Q2 / r measures the electrostatic potential energy of the electrons, not their forces.

caseygilles 1E

This equation is actually not Coulomb's law. Coulomb's law is applied when finding the force of attraction or repulsion between electrons (F=k*Q1*Q2 / r^2). Although it may derive or relate to Coulomb's law, Q1*Q2 / r measures the electrostatic potential energy of the electrons, not their forces.

caseygilles 1E

Can someone explain to me the reason nodal planes exist where they do. Why do some areas of atoms have no electron density? Also why do p and d orbitals have non symmetric electron density but the s orbital has symmetric electron density?

caseygilles 1E

No, photons do not gain mass as they move faster and faster. They do however have a relativistic mass based on their energy. Since a massless particle like a photon has energy it therefore also has momentum even though it has no rest mass. This does NOT mean that as particle move closer to the speed...

caseygilles 1E

Well, the change in momentum can be replaced with mass times the change in velocity. Momentum is not mass times position (m*x) as you are referring to it. So, the proper way to substitute for uncertainty in momentum would be m*delta v, not m*delta x. Therefore, the total equation would be (m*delta V...

caseygilles 1E

According to quantum mechanics, there can never be a particle with zero energy, but rather it must possess an energy. Why is it that a particle can never be completely still confined between two walls? Why must every particle have some kinetic energy? Can a particle not transfer all of its energy to...

caseygilles 1E

The mass of an electron is the same in each problem regardless of the metal that the election is emitted from. The mass of an electron is 9.10938356 × 10^-31 kilograms.

caseygilles 1E

Although the threshold energy is zero, the detector for the experiment would still be able to detect when an electron is being emitted. The energy is constantly changing due to electrons being emitted, thus the metal will have a slight positive potential due to loss of electrons (even at 0 kinetic e...

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Yes, the Rydberg formula allows us to find wavlengths which correspond to each series. Spectral series are critically important in astronomy. So yes, each wavelength is contained in a series: Lyman series= UV band, Balmer series=visible band, Paschen series=infrared band, and so on and so forth. Eac...

caseygilles 1E

The textbook explains "competing reactions," a reaction taking place at the same time as the one in which we are interested in and using some of the reactants. I'm confused about how we are able to tell when a competing reaction is occurring in a chemical equation when we notice actual yie...

caseygilles 1E

Yes, it is much easier to write out units using dimensional analysis because it allows us to see what units are canceling and what are not. Even though it might take extra time it could save silly mistakes that pay off in the future.

caseygilles 1E

Molarity is the moles of solute per volume of solution. I think the textbook is trying to say that with molarity we are concerned more with how much solute and solution there is, but not how much solvent there is.

caseygilles 1E

The delta sign above the equation just means there is a change in heat which means a change in energy. I think this symbol is merely being given at the moment to introduce us more to thermodynamics and the actual physical changes in a reaction. It does not change how we balance the equation.

caseygilles 1E

Do molecules that are hydrates have any effect on the way we normally balance chemical equations?

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