CHEMISTRY COMMUNITY

Junwei Sun 4I

The natural log comes from rearranging the first order rate law: ln[A] = -kt + ln[A]initial

Junwei Sun 4I

And also for problem c) check the unit. The unit difference might be because of unit conversion in this question. The answer key converts everything to mol. But if you have your answers in mmol then the unit will be different.

Junwei Sun 4I

The question is already stating "rate of consumption" of oxygen therefore sign is already taken into account in this case.

Junwei Sun 4I

An intermediate is a substance that is created by one elementary step and further react with other substances to give the final products. The graph shows that there are two intermediates because there're three steps in total, so the first two elementary steps must have produced some forms of interme...

Junwei Sun 4I

What are the differences between observant rate and experimental rate? Will there be circumstances that they don’t match for a given reaction?

Junwei Sun 4I

If you are given the cell potential then the more positive one is the cathode. Since in a galvanic cell Ecell =Ecathode - Eanode has to be positive.

Junwei Sun 4I

I believe the reduction potentials will be given to us on the test.

Junwei Sun 4I

When balancing acidic reactions you balance O using H2O and H with H+
When balancing basic reactions you balance O using H20 and H with H2O then adding OH- to the opposite side

Junwei Sun 4I

What range does a voltmeter need to have to measure pH in the range of 1 to 14 at 25 Celsius degree if the voltage is zero at pH=7.

How do we know if we are solving for Ecell or Enot here? And this might be a silly question but why is that K=[H+]/[OH-]in this question?

Junwei Sun 4I

If a reaction has a negative standard reduction potential what does it mean? For example does it mean that oxidation of the reactants will be favored?

Junwei Sun 4I

Calculate the potential of a cell constructed with two nickel electrodes. The electrolyte in one compartment is 1.0M Ni(NO3)2(aq). In the other compartment,NaOH has been added to a Ni(NO3)2 solution until the pH = 11.0 at 298K. I’m having trouble identifying which compartment is the anode and which ...

Junwei Sun 4I

When we are using the Nernst equation to calculate E, why is it that when calculating Q we can mix concentration and partial pressure together? What should the unit of partial pressure be converted to in this case? For example in 3c, logQ = log (P Cl2)(P H2)in torr / [Cl-]^2[H+]^2in molarity

Junwei Sun 4I

I got that answer too and I think it might be a typo. (I’m not 100% sure)

Junwei Sun 4I

The reaction is Cd(s) + 2Ni(OH)3(s)—->Cd(OH)2(s) + 2Ni(OH)2(s)

The cell diagram is Cd(s)|Cd(OH)2|KOH || Ni(OH)3|Ni(OH)2|Ni(s)

Where does the solid Nickel comes from on the cathode end? Why can’t Ni(OH)2 be the conducting electrode since it’s also a solid?

Junwei Sun 4I

If we have two aqueous for the cathode side does it matter how we write the order of those two aqueous substances?

For example if on the cathode Ce4+ is reduced to Ce3+, does it matter if we write Ce4+,Ce3+|Pt(s) or Ce3+,Ce4+|Pt(s) in the cell diagram?

Junwei Sun 4I

The question gives Au+ (aq) ------> Au 3+(aq) + Au(s) and asks us to write the half reactions and balanced reactions and draw a cell diagram. The solution manual solution is below. Why isn't Au+ both the one being reduced and oxidized in this reaction?

Junwei Sun 4I

It is safer to first write out all the oxidation number of the species in the reaction so that you can clearly see what is being reduced and what is being oxidized.

Junwei Sun 4I

The change in oxidation number is not the change in number of electrons. S2O3 2- actually lost 8 electrons because when you write out the half reactions, you have to make sure the oxidation half and the reduction half reactions have the same number of electrons that can cancel out when adding them t...

Junwei Sun 4I

The question asks to balance the reaction O3 + Br- ——> O2 + BrO3-

I don’t see why oxygen is reduced here since its oxidation number doesn’t change from O3 to O2, could someone please explain?

Junwei Sun 4I

Hi Junwei Sun 4I! To answer your question, because the reaction quotient value is greater than the value of K at this temperature, the reaction will proceed to form more reactants (I 2 gas). The graph is depicting this instance. As time progresses, the reaction will start to form more I 2 , hence t...

Junwei Sun 4I

In class today Dr. Lavelle gives the expression delta Φ = E as i goes to 0.
Could some one please explain this expression? What does Φ stands for and why it equals to E as current goes to zero?

Junwei Sun 4I

The question asks to draw a graph that depicts the reaction in question 5G.13 which is I2(g) --> 2I(g) at 1200.K (K=6.8) when the partial pressure of I2 and I are 0.13 bar and 0.98 bar respectively. The answer key gives this graph but I still don't quite get what it means. Could someone please expla...

Junwei Sun 4I

Concentrations should be given in mol per liter or Molarity

Junwei Sun 4I

Because the initial and final states are the same. Both internal energy and entropy are state functions, which means they do not depend on the path taken. In this problem eventually pressure and volume are restored to their initial states, therefore deltaU and deltaS are both zero.

Junwei Sun 4I

I think the last question is about finding entropy for reversible and irreversible pathway. For the reversible isothermal pathway deltaS total would be zero and therefore deltaS system plus deltaS of surrounding would be zero. I think for the question we can calculate deltaS for the system by using ...

Junwei Sun 4I

Residual entropy is essentially the entropy due to positions. A perfectly ordered crystal has an entropy of zero at 0K because it has no thermal entropy neither positional entropy.

Junwei Sun 4I

So I understand that if the initial and the final states are the same then change in entropy and internal energy will be zero since they are state functions. But what are the states we are referring to in this case? Is it temperature, volume, pressure, or all of them?

Junwei Sun 4I

I think we can only tell if a reaction is spontaneous or not from Gibbs free energy which has the formula ∆G = ∆H - T∆S. ∆S is only one of the variable in this equation and there are other factors in this equation that affect the value of ∆G.

Junwei Sun 4I

The Boltzmann constant basically relates energy with temperature.More specifically it relates the average kinetic energy of the particles with the temperature. Its unit is energy over temperature which is the same with entropy.

Junwei Sun 4I

The question is: Considering positional order, would you expect a crystal of octahedral cis-MX2Y4 to have the same, higher, or lower residual entropy than the corresponding trans isomer? Explain your conclusion. The solution manual says that cis-MX2Y4 has 12 orientations whereas the trans isomer onl...

Junwei Sun 4I

In class professor Lavelle talked about that given the amount of heat transfer, entropy change is more significant at a lower temperature. Could some please clarify why?

Junwei Sun 4I

The second law of thermodynamics says that change in entropy, ΔS = q/T. Can this equation only be used when energy is transferred reversibly or we can also use this equation when the energy transfer is irreversible?

Junwei Sun 4I

The final enthalpy should be +1560-1300-2(286)=-312kJ

Junwei Sun 4I

Because by definition H is the heat transferred at constant pressure.

Junwei Sun 4I

The equations the solution manual uses are combustion reactions. Because we are told the substances’ enthalpy of combustion we can first write out those combustion reactions as the solution manual did. A combustion reaction happens when a substance reacts with water and produces CO2 and H2O. You got...

Junwei Sun 4I

Yes they need to be on opposite side to be cancelled out.

Junwei Sun 4I

That’s correct because during a phase change energy is used to overcome the intermolecular forced between molecules instead of raising the actual temperature of the substance.

Junwei Sun 4I

Standard reaction enthalpy is the enthalpy of the overall reaction; whereas standard enthalpy of formation is the enthalpy change of forming one mole of a substance from its elements. We can use standard enthalpy of formation to calculate enthalpy change for the overall reaction.

Junwei Sun 4I

In an ICE table we leave about liquids and solids. If the H2O is in gas phase then it will be included in the ICE table as well.

Junwei Sun 4I

It is possible in the context of the Le Chatelier's principle. For example how will the equilibrium shift if temperature is increased/decreased given a reaction is endothermic or exothermic.

Junwei Sun 4I

I got 8.62 too maybe you forget to convert pKa to pKb since C6H7O2-(aq) is a base. And you first calculate pOH and then subtract and get the pH value.

Junwei Sun 4I

Ka x Kb equals a constant Kw at 25 degree Celsius. Therefore a big Ka value would imply a small Kb value.

Junwei Sun 4I

When do we use the 5% rule? Do we only use it in acid and base equilibrium questions or any equilibrium questions?

Junwei Sun 4I

Molecules tend to be in the lowest energy state to be the most stable. When a bond is formed the molecules become more stable, therefore heat is released so the molecules' energy lowered. Whereas when breaking a bond the molecules become more unstable, which means it is not in its lowest energy state.

Junwei Sun 4I

No removing or adding a solid or liquid would not affect the equilibrium since they are not included in the equilibrium expression K.

Junwei Sun 4I

When we are writing out the equilibrium expression Kp should the partial pressures always be in bar? If given atm do we have to covert that to bar and then calculate Kp?

Junwei Sun 4I

How will the equilibrium shift if pressure is increased by halting the volume?

Junwei Sun 4I

Why is it that when we write the expression for K we don't include solid and liquid? Is it because their concentration does not change or is it that the change is insignificant? Also for solid and liquid do we always write them as 1 in the equilibrium expression?

Junwei Sun 4I

We get the square root of 41 because the stoichiometric coefficients of all molecules in the original equation are all multiplied by 1/2. Therefore we get the value of the new K by taking the square root of the original K value.

Junwei Sun 4I

The coefficients become the power of the molar concentrations in the expression for K and they have to be assigned to the specific molecules as they are in the chemical equation. For example in H2 + I2 --> 2HI,

[H2] and [I2] will have a power of 1
[HI] will have a power of 2

Junwei Sun 4I

This statement is false since even if you start with a higher pressure of reactants, this would only result in production of more products and in the end the equilibrium constant remains the same.The value of K is not affected by the addition of products and reactants as long as the temperature is t...

Junwei Sun 4I

The reaction quotient Q is calculated in the same way with K using the same expression. However the difference is that when you look for Q the concentration/partial pressure values you plugged in are not yet at equilibrium. Reaction quotient Q is important since we can use it to determine which way ...

Junwei Sun 4I

When Q>K, more reactants will tend to be formed so the reaction can reach equilibrium. The reverse reaction in this case is favored.

Junwei Sun 4I

505106414 wrote:What exactly is protonation? What is happening to an acid undergoing protonation?

Protonation is the process in which a proton H+ is added to an atom, ion, or molecule and form a conjugate acid.An acid undergoing protonation gets a proton.

Junwei Sun 4I

The acid gets protonated when pH of the solution is smaller than the acid's pKa value. If pH<pKa, it implies that the solution is more acidic than the acid. In this case the solution will act like an acid and donate a proton. This proton will be picked up by the pKa acid and it will be protonated. I...

Junwei Sun 4I

I don't think we need to know them but I will do it just in case it shows up.

Junwei Sun 4I

It depends on the salt you have. For example, NaCl will not affect the pH of water because it is a neutral salt. However, NH4Cl will lower the pH of water because NH4+ make water acidic.

Junwei Sun 4I

Cl- ion does not affect pH of water because it is a stable ion. It will not tend to bind to hydrogen atoms in water because itself is already stable. Therefore it will not cause any change in pH. And typically group one and group two cations metals are neutral.

Junwei Sun 4I

6C.21 B) asks whether CH3COOH or HCOOH is a stronger acid. How can you tell? I know the solution manual says that -CH3 group has electron donating properties but why is that? Thank you.

Junwei Sun 4I

Homework question 6C.19 d) asks whether HClO4 or H3PO4 is a stronger acid. How can we tell? Thanks.

Junwei Sun 4I

Why does delocalized electrons imply stable anions? In other words why is the anion stabilized by electron withdrawing atoms?

Junwei Sun 4I

Yes this is true. Conjugate base of a strong acid will be a weak base. Conjugate base of a weak acid will be a strong base. The stronger the acid/base, the weaker its conjugate base/acid, and vice versa.

Junwei Sun 4I

They are talking about the same thing. Lewis acid is just one of the definitions of acids. All acids can be said in two different definitions, which are bronsted definition and lewis definition.In bronsted definition the acts is the proton(H+) donor, and in lewis definition the acid is the electron ...

Junwei Sun 4I

Acids are the proton donor and bases are the proton acceptor (Bronsted definition).

Junwei Sun 4I

In homework problem 9C.3 there's a complex name containing "bisoxalato." What's the difference between using bi-, tri-, tetra- and using bis-, tris-, and tetrakis- and when do we use each set of prefixes? Thanks!

Junwei Sun 4I

In the textbook it writes iron as "ferrate" in the complex with an overall negative charge. For example hexacyanidoferrate(II) ion. I'm just curious if there is such thing as "ironate" or it is a rule to write iron as "ferrate" when we are naming a complex with overall ...

Junwei Sun 4I

Transition metals having many oxidation states make them good for electron transfer.

Junwei Sun 4I

According to my notes, a chelate complex is a complex containing ligands that form a ring of atoms that include the central metal atom. Chelating ligands can bind cations really tightly, therefore such complexes such as EDTA, is good at removing metals from solution because the metal ion is bounded ...

Junwei Sun 4I

Ligands directly attacked to central ion make up the coordinate sphere.

Junwei Sun 4I

In class Dr. Lavelle talked about transition metal cations in solution form complex with H20, and when NH3 or K+CN- are added those species will replace the H20 ligands. Why does that happen and how? Thanks!

Junwei Sun 4I

Basically cisplatin forms a coordinate compound with the DNA strands by binding its two Chlorine atoms to DNA. Since there are two places that are bounded cisplatin bounds to DNA really tightly. When cell division happens cisplatin blocked the enzyme from going through, therefore stopping DNA replic...

Junwei Sun 4I

Yes lone pairs are counted as areas of electron density just like the bounding electrons.

Junwei Sun 4I

No the shape does not vary. Just like resonance structures does not matter in VSEPR model.

Junwei Sun 4I

A represent the central atom.
X represents the bonded atoms.
E represents the lone pairs.

For example if you have NH3 you got central atom N with three atoms bonded to it and one lone pair, therefore the notation should be AX3E.

Junwei Sun 4I

Could someone please clarify how we can tell if a molecule is polar or non-polar from looking at its lewis structure? What are some factors in a lewis structure that will make a molecule polar?

Junwei Sun 4I

VSEPR model only takes in consideration of region of electron density. Therefore having a double bond or a single bond indicate the same, one region of electron density.

Junwei Sun 4I

I agree with the above posts of doing more practice problems.Also if you remember the repulsion strength order in which lone pair - lone pair repulsion is bigger than lone pair - bonding pair repulsion is bigger than bonding pair - bonding pair repulsion it also helps. Since when you draw out the le...

Junwei Sun 4I

All non-polar molecules have London Dispersion forces but be careful sometimes in the presence of a polar molecule they can also have dipole-induced dipole forces.

Junwei Sun 4I

SO2 molecules have dipole-dipole interactions because they are polar. When drawing out the lewis structure for SO2 the central atom S has one lone pair, which means it's polar.

Junwei Sun 4I

Yes dipole moments can happen in ionic bonds. For example when there's an obvious difference in electronegativity between ions there can be dipole moments.

Junwei Sun 4I

A dipole moment basically occurs whenever there is a distinction between negative and positive charge. When in a chemical bond atom on one side of the bond has more negative charge than atom on the other side, we can say there's a dipole moment, since there is a separation of the positive and negati...

Junwei Sun 4I

Aluminum does not have an expanded octet. Aluminum is an exception just like boron where it can be octet deficient. Just like in BF3 boron is stable with only six electrons, in AlCl3 aluminum also does not fill an octet.

Junwei Sun 4I

It will be more clear to look at the actual electronegativity value but in this case you can also look at their position on the periodic table and predict the electronegativity difference with periodic trend. C and S are located really close to each other, so it is reasonable to assume they have sim...

Junwei Sun 4I

Polarizing power normally indicates the ability of the cation to attract the valence electrons of an anion, which is the ability of an atom to cause electron cloud distortion. Whereas polarizability means the atom's ability/tendency to be polarized, in order words the atom's ability to form dipoles....

Junwei Sun 4I

Because if the size of the cation is smaller, the closer the distance between the cation's nucleus with the electrons, which means the electrons will experience a higher attraction force from the cation's nuclear charge. As a result, electrons are more likely to be pulled towards this small size cat...

Junwei Sun 4I

Highly distorted electrons are described as highly polarizable because they are more likely to cause dipoles. As electrons are pulled toward one side of the ions they are really likely to create dipole since the charge now is uneven. The more the number of distorted electrons being pulled into the b...

Junwei Sun 4I

BaBr2 is more ionic compare to BeBr2 and BBr3 because the electronegativity difference between Ba-Br is the greatest compares to electronegativity difference between Be-Br and B-Br. Electronegativity increases across the period and decreases down the group. Therefore the electronegativity difference...

Junwei Sun 4I

Do atoms that are more electronegative always the central atom in a lewis structure? In order words does the central atom of a lewis structure always has the highest electronegativity compare to the other atoms in the structure?

Junwei Sun 4I

Electronegativity indicates HOW WELL/TENDENCY an atom attracts electrons to itself. Whereas electron affinity measures the exact amount of energy released when an electron is added to an atom in the gas phase.

Junwei Sun 4I

Electronegativity indicates how well an atom attracts electrons to itself. It increases across the period since atomic radius decreases across the period. With valence electrons being closer to the nucleus they experience more attraction forces. Therefore the atom attracts electrons better. Electron...

Junwei Sun 4I

Formal charge of the central atom is not always zero. It is better to look at formal charges of all atoms inside a structure. With more atoms having a formal charge of zero the structure is the most stable. And in the case when the formal charges are not zero, the more electronegative atom should ha...

Junwei Sun 4I

Formal charge for the same atom in resonance structures should be the same. For example in SO4 2-, when you draw out the lewis structure you get S as the central atom with two Os attached to it by double bonds and two Os attached to it by single bonds. No matter how you orientate the double bonds in...

Junwei Sun 4I

Radicals are simply compounds with unpaired electrons. Radicals are exception of the octet rule because a radical might not have a filled octet with 8 electrons. For example Dr. Lavelle talked about CH3 in class. When we draw out the lewis structure for CH3, the central atom C have 7 valence electro...

Junwei Sun 4I

Yes it is necessary. It's always good to draw all the resonance structures since the actual structure of the molecule is the average of all the resonance structures.

Junwei Sun 4I

Phosphorous violates the octet rule since it can have an expanded octet. P is in the third period on the periodic table, which means it has d-orbital that can accommodate extra electrons.For example when P bonds with Cl forming PCl5, the central atom P would have a total of 10 electrons, which viola...

Junwei Sun 4I

There are no specific rules on where to start putting dots but when dots are put around an atom it has to reflect whether the electrons are paired or not. Therefore when you put dots around an atom you always put four separate dots on four sides first.

Junwei Sun 4I

Yes besides H, He, Li and Be there are other exceptions to the octet rule. For example boron. When boron bounds with three fluorine atoms boron only gets six electrons instead of eight. And there's also something called expanded octet, which is when a central atom in a lewis structure can have more ...

Junwei Sun 4I

PranaviKolla3G wrote:In terms of electrons, what is the difference between a triple bond, a double bond, and a single bond?

Triple bonds represent three pairs of electrons. Double bonds represent two pairs of electrons. Single bonds represent one pair of electrons.

Junwei Sun 4I

Your understanding is correct. Basically ionic bonds are formed between metals are nonmetals, and covalent bonds are formed between nonmetals and nonmetals. In an ionic bond electrons are transferred and ions are formed. In a covalent bond electrons are shared.

Junwei Sun 4I

It's always good to know the commonly known compounds but I think when you are asked to draw the lewis structures the formulas will be given.

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