CHEMISTRY COMMUNITY

Michelle Dong 1F

The two different forms of Van't Hoff are just based on what the question is asking you; usually we use the second one (lnK2/K1 = -deltaH/R(1/T2-1/T1)) to determine if an increase in temperature for a reaction will cause the reaction to form more products or reactants based on whether it is endother...

Michelle Dong 1F

Use the data in Appendix 2B and the fact that, for the half reaction F2(g) + 2H+ (aq) + 2e- --> 2HF(aq), E standard = 3.03V, to calculatethe value of Ka for HF.

Why do you need to take the square root of K to find Ka?

Michelle Dong 1F

A voltaic cell functions only when the electrical circuit is complete. In the external circuit the current is carried by the flow of electrons through a metal wire. Explain how the current is carried through the cell itself.

Can someone explain this?

Michelle Dong 1F

In some reactions, two or more different products can be formed by different pathways. If the product formed by the faster reaction predominates, the reaction is considered to be under kinetic control. If the predominant product is the more thermodynamically stable, the reaction is considered to be ...

Michelle Dong 1F

A 1.0 m NiSO4(aq) solution was electrolyzed by using inert electrodes. Write (a) the cathode reaction; (b) the anode reaction. (c) With no overpotential or passivity at the electrodes, what is the minimum potential that must be supplied to the cell for the onset of electrolysis? How do you know the ...

Michelle Dong 1F

Catalysts are present in the beginning of the reaction and do not get consumed (so it appears in the products side in the next elementary step), while intermediates are formed in one elementary step and then are consumed in the next elementary step.

Michelle Dong 1F

When is the oxidation state for a molecule 0?

Michelle Dong 1F

Can catalysts appear in the experimentally determined rate law? If so, just to clarify, intermediates cannot appear in the experimentally determined rate law, but can appear in the calculation of K (equilibrium constant), but catalysts can appear in both the experimentally determined rate law and th...

Michelle Dong 1F

How do you know if something is more stable based on their standard enthalpies of formation? If a molecule's standard enthalpy of formation is larger, does that mean it's more stable?

Michelle Dong 1F

The proposed 2-step mechanism for a reaction is H2A + B --> BH+ + HA- and its reverse, both of which are fast, followed by HA- + B --> BH+ + A2-, which is slow. Find the rate law with HA- treated as the intermediate and write the equation for the overall reaction. The answer key says k=2k2*k1/k1'. W...

Michelle Dong 1F

Write the overall reaction for the following proposed mechanism and identify any reaction intermediates:
For step 1, why is it written as just Cl and for step 2, why is it Cl- (chloride ion)? Why is the chloride ion the intermediate and not Cl?

Michelle Dong 1F

Dr. Lavelle mentioned how for the integrated rate law, we consider only the reactant (which is why it's usually expressed in terms of loss of [A], a reactant). However, in the book, it says the integrated rate law can be used to find the concentration of reactant or product at any time. Should we ju...

Michelle Dong 1F

This is why we study initial rates, because initially, there is little to no product and everything is in terms of the reactants. That's why for our differential rate law, the species in it are all for reactants.

Michelle Dong 1F

Because we set the other reactants to be very large, they are essentially constant and the concentrations do not change significantly throughout the reaction. So, they don't have much impact on the overall rate. For example, if you have A+ B+ C --> P, and you set A to be really small and B and C to ...

Michelle Dong 1F

Just to clarify, we can't find the differential rate law for a complete reaction, just an elementary reaction? Why is that?

Michelle Dong 1F

The book says the integrated rate law gives the concentration of reactants or products at any time after the start of the reaction. We've only calculated the integrated rate law for the concentration of reactants -- how do we translate it in terms of products?

Michelle Dong 1F

Why does a differential rate law have to be in terms of the initial concentration of the reactant? Why can't it be in terms of products?

Michelle Dong 1F

What does it mean to be kinetically stable and thermodynamically unstable? And vice versa? And what does it mean in terms of the context of C(diamond) --> C(gr) that Dr. Lavelle mentioned in class?

Michelle Dong 1F

If 14.0 g of SO2Cl2 is sealed in a 2500 L reaction vessel and heated to the specified temperature, what mass will remain after 1.5 h?

Why is it okay to only put in 14 g as the initial value ([A]o)? Why do we not need to convert it into concentration?

Michelle Dong 1F

In the chart for #17, the initial rate is referred to as mmol G (which is a product) and in the chart for #19, the initial rate is referred to as mmol A (which is a reactant). Does it matter what the initial rate is of or should I just ignore the fact that one refers to product and the other to reac...

Michelle Dong 1F

In 20 s, the concentration of nitrogen dioxide, NO2, decreases from 450 mmol/L to 320 mmol/L in the reaction 2NO2(g) --> 2NO(g) + O2(g).
(c) What is the unique rate of the reaction?

How do you find the unique rate of the reaction? I don't really get what the solutions manual says about this part.

Michelle Dong 1F

So the standard potential in general states how likely a half reaction will become reduced. So, the higher the standard potential, the more likely the half reaction will reduce (and thus act as the oxidizing agent because it oxidizes the other half reaction). Therefore, the more negative the standar...

Michelle Dong 1F

Usually, Pt(s) is used when there are no conducting solids in the half reactions, which is why it's usually added to half reactions where it's only aqueous or gas.

Michelle Dong 1F

A line signifies changes in physical state or the interaction of one physical state with another. A comma means both substances are in the same physical state.

Michelle Dong 1F

What's the effect of changes in concentration of reactants and products for cell potential?

Michelle Dong 1F

Write balanced half reactions for the redox reaction of an acidified solution of potassium permanganate and iron(II) chloride. Are we supposed to look in appendix 2B for this or are we supposed to somehow figure out the skeletal half reaction? Also, how do we know which half reaction to use if we us...

Michelle Dong 1F

Are we supposed to use appendix 2B to find the half-reactions?

Michelle Dong 1F

Is the units for standard delta G J/mol or just Joules? When using standard delta G = -nFE standard, the units seem to cancel out to only leave Joules but the solutions manual has the units written as J/mol.

Michelle Dong 1F

Does standard Gibbs free energy stay the same for the molecule at whatever condition? I thought it changed based on temperature due to the equation delta G standard = delta H standard - T*delta S standard. So when you plug in any temperature, you get a delta G standard based on the temperature. Howe...

Michelle Dong 1F

If delta G of formation of a compound is negative, then it is thermodynamically stable with respect to decomposition because it is more likely to NOT decompose into its elements and instead favor the formation of products (spontaneous in forward direction). If delta G of formation of a compound is p...

Michelle Dong 1F

I'm pretty sure Dr. Lavelle said that he would accept either unit. However, for enthalpy of formation or standard molar entropy or Gibbs free energy of formation, it must be per mole because that's the definition of "formation".

Michelle Dong 1F

We don't know what delta S is (we can't assume entropy is increasing). If delta S is negative, then no matter how much you increase temperature, the reaction still won't be spontaneous/favorable because delta G will never be negative, so the answer is false because we can't assume that delta S is po...

Michelle Dong 1F

Pressure is usually infinitesimally different between the interior and exterior of the system, and because it is always infinitesimally changing in order to change the volume, that's why we assume that pressure is not constant in a reversible, isothermal reaction. That's why we use the ideal gas equ...

Michelle Dong 1F

When calculating partial pressures from equilibrium constant K, does it have to be in bar? So if we got numbers for the partial pressures but the question asked for them to be in kPa, would we have to convert them to kPa from bar because they're automatically in bar?

Michelle Dong 1F

Is internal energy a state function? Why?

Michelle Dong 1F

Is enthalpy of formation an intensive property because it's only for one mole of substance?

Michelle Dong 1F

Why/how does free energy depend on pressure and the equilibrium constant?

Michelle Dong 1F

A technician carries out the reaction 2SO2 (g) + O2 (g) --> 2SO3 (g) at 25 degrees celsius and 1.00 atm in a constant-pressure cylinder fitted with a piston. Initially, 0.030 mol SO2 and 0.030 mol O2 are present in the cylinder. The technician then adds a catalyst to initiate the reaction. (c) Assum...

Michelle Dong 1F

Do all isolated systems have constant volume, or is that only for sealed calorimeters?

Michelle Dong 1F

Calculate the equilibrium constant at 25 degrees celsius and at 150 degrees celsius for each of the following reactions, using data available in appendix 2A. Why can't we just use the standard Gibbs free energy of formation to calculate the standard Gibbs free energy and use that for both temperatur...

Michelle Dong 1F

If standard Delta G is negative, does that mean it's spontaneous for the forward reaction? Because the equation standard Delta G = -RTln(K) represents equilibrium, so when standard Delta G is negative, technically the reaction is at equilibrium, so would it still be spontaneous in the forward direct...

Michelle Dong 1F

If delta G standard is positive, does that mean it's spontaneous in the reverse direction?

Michelle Dong 1F

I know that if a reaction is exothermic and the temperature is raised, the formation of reactants will be favored because K1 is smaller than K2. However, if a reaction is exothermic and the temperature is lowered, would the formation of products be favored? So does the favorability of either product...

Michelle Dong 1F

What are standard conditions? For pressure, temperature, etc?

Michelle Dong 1F

Is it possible for the change in entropy of the universe to be negative? Or does it have to be either zero or positive?

Michelle Dong 1F

In lecture, Dr. Lavelle derived the equation to find the change in Gibb's free energy for the reaction quotient: Delta G= standard delta G + RTln(Q) Later, he mentioned something about how if you wanted to make a reaction move right, start off with very little product. Can someone explain what this ...

Michelle Dong 1F

Why does reaction free energy depend on pressure and the equilibrium constant?

Michelle Dong 1F

Write the balanced chemical equation for the formation reaction of (a) NH3 (g) For each reaction, determine delta H (standard), delta S (standard), and delta G (standard). How do we know if the question is asking for delta S of the reaction or if it's asking for standard molar entropy? The solutions...

Michelle Dong 1F

Initially a sample of ideal gas at 323 K occupies 1.67 L at 4.94 atm. The gas is allowed to expand to 7.33 L by 2 pathways: (b) isothermal, irreversible free expansion. Calculate deltaS total, deltaS system, and delta S surroundings for each pathway. Why is delta S total not 0 J/K anymore? What are ...

Michelle Dong 1F

If SO2F2 adopts a positionally disordered arrangement in its crystal form, what might its residual molar entropy be? Why do we raise the number of microstates to Avogadro's number? How do we know it's talking about one mole of SO2F2? Why isn't it just raised to the power of 1 because it's one molecu...

Michelle Dong 1F

Can the change in internal energy of a closed system be 0? In this way, q=-w, so if a system does work (expands its volume), heat would move into the system and keep the change in internal energy 0, but would it happen under a closed system? And would the system be expanding reversibly or irreversib...

Michelle Dong 1F

Use data in Table 9.3 or Appendix 2A to calculate the standard reaction entropy for each of the following reactions at 25 degrees celsius. For each reaction, interpret the sign and magnitude of the reaction entropy. (d) The decomposition of potassium chlorate: 4KClO3 (s) --> 3KClO4 (s) + KCl (s) Why...

Michelle Dong 1F

Without performing any calculations, predict whether there is an increase or a decrease in entropy for each of the following processes: (a) Cl2 (g) + H2O (l) --> HCl (aq) + HClO (aq) The answer says that entropy decreases because there's not as many moles of gas on the right side of the equation. Sh...

Michelle Dong 1F

Which substance in each of the following pairs has the higher molar entropy at 298 K?
(a) HBr (g) or HF(g)

Can someone explain why HBr (g) has higher molar entropy? How can you tell if something has higher molar entropy in general? Is there a trend?

Michelle Dong 1F

Are all calorimeters isolated? Or is the only isolated calorimeter the bomb calorimeter?

Michelle Dong 1F

What's the difference between a constant volume and a constant pressure calorimeter? Are there any calculation differences? For example, for #25 in chapter 8, it's a constant-volume calorimeter and we first found the specific heat of the calorimeter and then found q. In the example we did in class (...

Michelle Dong 1F

Benzene is more stable and less reactive than would be predicted from its Kekule structures. Use the mean bond enthalpies in Table 8.7 to calculate the lowering in molar energy when resonance is allowed between the Kekule structures of benzene. What does it mean in terms of enthalpy when it says ben...

Michelle Dong 1F

65. Calculate the standard enthalpy of formation from dinitrogen pentoxide form the following data, 2NO (g) + O2 (g) --> 2NO2 (g) delta h = -114.1 kJ 4NO2 (g) + O2 (g) --> 2N2O5 (g) delta h = -110.2 kJ and from the standard enthalpy of formation of nitric oxide, NO. How do you go about solving this?

Michelle Dong 1F

I'm pretty sure it's because when writing out the standard reaction equation for each of the compounds, carbon's standard state is C(gr) which is a solid, but in order to use bond enthalpies, carbon must be in a gaseous form, so the enthalpy of sublimation for carbon is to convert C(gr) -- solid -- ...

Michelle Dong 1F

In what case would a standard reaction enthalpy be equal to the standard enthalpy of formation?

Michelle Dong 1F

When a molecule is diatomic, does that mean they have the same atoms (eg: H2 (g)) or do they also have different atoms? If they have different atoms, is the standard enthalpy change still 0?

Michelle Dong 1F

The reaction of 1.40 g of carbon monoxide with excess water vapor to produce carbon dioxide and hydrogen gases in a bomb calorimeter causes the temperature of the calorimeter assembly to rise from 22.113 C to 22.799 C. The calorimeter assembly is known to have a total heat capacity of 3.00 kJ/C. (b)...

Michelle Dong 1F

We should probably know that everything in the halogens group's standard states are in gases, except for Br2, whose standard state is in liquid form, and I2, whose standard state is in solid form.

Michelle Dong 1F

In an adiabatic process, no energy is transferred as heat. Indicate whether each of the following statements about an adiabatic process in a closed system is always true, always false, or true in certain conditions (specify the conditions): (d) delta U=q Why is this true when w=0? I thought it's alw...

Michelle Dong 1F

Determine the reaction enthalpy for the hydrogenation of ethyne to ethane, C2H2 (g) + H2 (g) --> C2H6 (g), from the following data: delta Hc (C2H2,g)= -1300 kj/mol, delta Hc (C2H6,g) = -1560 kj/mol, delta Hc (H2,g)= -286 kj/mol. Can someone explain how to do this? Why is it that delta Hc(C2H6,g) bec...

Michelle Dong 1F

I think that if it's asking for the standard reaction enthalpy, it's the enthalpy change for the entire equation so it doesn't matter how many moles of the substance itself because it's kind of like one mole of the entire reaction. I'm assuming this is why it can be in kJ and not kJ/mol. However, if...

Michelle Dong 1F

For #59, the answer writes the standard reaction enthalpy in kJ/mol. However, in #63, the answer key for standard reaction enthalpy is in kJ only. Why are some reaction enthalpies in kJ/mol and some in just kJ?

Michelle Dong 1F

Since delta U = q + w, if delta U is 0, then q would equal w. This means that the heat transferred to and from a system is equal to the work done by or on a system. I may be wrong, though; correct me if I am!

Michelle Dong 1F

Constant volume would mean that work (w) equals 0 because of the equation w= external pressure * change in volume. This means that the change in energy at constant volume would only equal q (delta U = q), which is the amount of heat given off or absorbed in a reaction.

Michelle Dong 1F

In lecture, Dr. Lavelle talked about how the temperature doesn't necessarily change in an exothermic reaction, even though an exothermic reaction suggests a net release of heat. Can someone explain this?

Michelle Dong 1F

I know that the more polar a bond is, the stronger the acid. However, HF has a greater polar bond than HBr, but HBr is still a stronger acid? Can someone explain this?

Michelle Dong 1F

Can the electron configuration for Br- be written as [Kr] or does it have to be written as [Ar]3d10 4s2 4p6?

Michelle Dong 1F

What does stable conjugate base mean? Is a stable conjugate base the same as a weak base because its acid is strong? Does a stable acid mean it's a stronger acid?

Michelle Dong 1F

Suggest an explanation for the different strengths of
(b) acetic acid and formic acid
Why is formic acid a stronger acid than acetic acid?

Michelle Dong 1F

In the question, it says to write the equilibria for the following acids, suggesting that H2PO4^- is acting as an acid when you write it in this case.

Michelle Dong 1F

I believe it's like calculating M2 using the M1V1=M2V2 equation. M1=0.025 and V1=0.2 L. M2=? and V2=0.25 L. In order to find M2, you divide 0.025*0.2 by 0.25, and that gives you 0.02 M. Since HCl dissociates all the way and creates H3O+ ions, we can assume that the concentration of H3O+ is also 0.02...

Michelle Dong 1F

We only assume X is small because the K equilibrium constant is small enough to consider the change in concentration for the products negligible.

Michelle Dong 1F

How many places can we be off by (according to the official answer) to still get full credit? For example, if my answer is 115*10^-9 m, but the answer in the answer key is 116*10^-9 m, will I still get full credit?

Michelle Dong 1F

How do you calculate bond order of a molecule?

Michelle Dong 1F

State whether the following oxides are acidic, basic, or amphoteric:
(a) BaO
(b) SO3
(c) As2O3
(d) Bi2O3

How do you know which oxide is acidic, basic, or amphoteric? How would you go about answering this problem?

Michelle Dong 1F

How do you know which reaction can be classified as a Bronsted acid and base reaction?

Michelle Dong 1F

Can someone explain the relationship between Lewis acids and Bronsted acids, and Lewis bases and Bronsted bases? Eg: is a Lewis acid the same as a Bronsted acid (the electron acceptor is the same as the proton donor)?

Michelle Dong 1F

In lecture, Professor Lavelle talked about the order of repulsion strength: lone-lone pair> lone-bonding pair> bonding-bonding pair.
Can someone explain this more in depth?

Michelle Dong 1F

I know that we add "-ate" to the end of a metal name if the complex has a negative charge (eg: [Ni(CN)4]2- is ...nickelate (II)), but why is it that in some cases, we still use "-ate" even though the complex does not have an overall negative charge? An example would be K3[CoF6], ...

Michelle Dong 1F

Thanks everyone!
I had another question: if a reactant was a solid and amount of solid was increased, would the reaction move to products side as well?

Michelle Dong 1F

15. The photosynthesis reaction, 6 CO2(g) + 6 H2O(l) ⇌ C6H12O6(aq) + 6 O2(g), is endothermic. What effect will the following changes have on the equilibrium composition. a) Water is added. b) The partial pressure of CO2 is decreased. I thought that liquids have no effect on the equilibrium compositi...

Michelle Dong 1F

29. A researcher fills a 1.00 L reaction vessel with 1.84 x 10-4 mol of BrCl gas and heats it to 500 K. At equilibrium, only 18.3 % of the BrCl gas remains. Calculate the equilibrium constant, assuming the following reaction is taking place. 2BrCl(g) ⇌ Br2 (g) + Cl2(g) I keep getting 1.68 x 10^-4, b...

Michelle Dong 1F

An organic compound distilled from wood was found to have a molar mass of 32.04 g/mol and the following composition by mass: 37.5% C, 12.6% H, and 49.9% O. (a) Write the Lewis structure of the compound and determine the bond angles about the carbon and oxygen atoms. Why is the bond angle about the o...

Michelle Dong 1F

(a) Draw a Lewis structure for each of the following species: CH3+, CH4, CH3-, CH2; CH2 2+, CH2 2-. (b) Identify each as a radical or not. (c) Rank them in order of increasing HCH bond angles. Explain your choices Why does CH2 have a smaller bond angle than CH3+? Is it because the lone pair pushes d...

Michelle Dong 1F

A 0.10 mol sample of pure ozone, O3, is placed in a sealed 1.0-L container and the reaction 2O3 (g) --> <-- 3O2 (g) is allowed to reach equilibrium. A 0.50 sample of pure ozone is placed in a second 1.0-L container at the same temperature and allowed to reach equilibrium. Without doing any calculati...

Michelle Dong 1F

Write the equilibrium expression K for each of the following reactions: (a)2C2H4(g) + O2(g) + 4HCl(g) --> <-- 2C2H4Cl2(g) + 2H2O(g) How do we know whether to write the equilibrium Kp or Kc? In the answer key, K was written as the equilibrium expression for partial pressure (Kp), but how would we kno...

Michelle Dong 1F

Axial would basically be the line the runs from top to bottom, and equatorial would be anything that goes around the vertical line. Most of the time, when taking out bonds and replacing them with lone pairs, we take bonds from the equatorial position because the bond angles would be greater, giving ...

Michelle Dong 1F

Write the Lewis structure, VSEPR formula, molecular shape, and bond angles for each of the following species:
(d) N2O

The most stable Lewis structure should a triple bond between N and N -- why does the answer key show a not-as-stable Lewis structure?

Michelle Dong 1F

Using Lewis structures and VSEPR, give the VSEPR formula for each of the following species and predict its shape: (b) Iodine Trichloride When I drew out the Lewis structure, there turned out to be 2 lone pairs on the Iodine atom. I understand that the name of the shape should be influenced only by t...

Michelle Dong 1F

Is there a relationship between a molecule having a dipole moment and the atoms having formal charges that aren't 0? For example, the Lewis Structure of CO shows that it's linear, but the formal charges on C and O are -1 and +1 respectively -- does that create a dipole moment?

Michelle Dong 1F

How does CO have a dipole moment if the Lewis structure shows that the shape is linear?

Michelle Dong 1F

The hydrogen atom has a radius of approximately 0.05 nm. Assume that we know the position of an electron to an accuracy of 1 % of the hydrogen radius, calculate the uncertainty in the speed of the electron using the Heisenberg uncertainty principle. Comment on your value obtained. Would you have to ...

Michelle Dong 1F

Is electron affinity similar to electronegativity in the sense that both describes how likely it is for an atom to accept an electron?

Michelle Dong 1F

Does silicon also have a higher electron affinity than phosphorus because phosphorus is already in its stable half-filled subshell? Does this mean that the elements with stable, half-filled subshells always have lower electron affinity levels than the elements next to them due to them already being ...

Michelle Dong 1F

Any element in group 11 follows a similar electron configuration of Cr, in which the d subshell is full and the s subshell misses an electron.

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