CHEMISTRY COMMUNITY

Riya Sood 4G

Entropy was not defined as "disorder" to avoid the negative connotation of the word. Instead entropy can be defined as the measure of randomness.

Riya Sood 4G

Enzymes provide an alternative pathway for the reaction to occur faster. They lower the activation energy, but the change in Enthalpy and Gibbs free energy stays the same because they are state properties independent of the pathway.

Riya Sood 4G

An ion selective electrode can offer several advantages as it allows one to monitor concentrations of ions in solutions, such as monitoring Na+ concentration in cells.

Riya Sood 4G

Think of a reaction mechanism as the sum of all the elementary steps that lead to the overall reaction. Understanding the step by step process of a reaction helps find the slowest rate determining step which can be used to determine the rate law.

Riya Sood 4G

yes, concentration cells can have a positive Ecell value. This happens when the concentration at the anode is smaller than the concentration at the cathode. Keep in mind that Ecell is different from E^standard cell which is always 0 for concentration cells. As long as the concentrations on the anode...

Riya Sood 4G

Yes, we can assume that the reverse reaction of the slow step can be ignored. This is because the reverse step of the slow reaction is considered to be very slow and does not have much effect in calculations.

Riya Sood 4G

The answer key might have a typing error since I even I got 2.85 M^-4 s^-1 for the rate constant for part c.

Riya Sood 4G

We substitute in the rate law when there is an intermediate in the rate law for example. Since we cannot have intermediates in the rate law, we will need to substitute the concentration of the intermediate using the fast step at equilibrium.

Riya Sood 4G

The only time we can use stoichiometric coefficients to determine the rate law is when the question states that the reaction is an elementary reaction. If there is no such indication, then the rate law can be determined based on experimental data and comparing trials to see how changing once concent...

Riya Sood 4G

Yes the units of the rate constant can be used to find the order of the reaction. Depending on the order, the units of the rate constant will be different. For example, for a first order reaction, the units of k will be s^-1. The best way to ensure that you have the correct units is to plug in k and...

Riya Sood 4G

If the rate is given in terms of mass/time then we can use mass in the equation to make calculations simpler. Otherwise if using concentration, it would have to be converted to units of mass for the final answer.

Riya Sood 4G

A is also called the pre-exponential factor or frequency factor. It determines the number of collisions made. It is used to calculate the rate constant in the expression: k=A exp(-Ea/RT)

Riya Sood 4G

Decay is the decomposition of a substance and how its concentration decreases over time. In terms of oxidation and reduction, think about the example of rust. Iron metal is oxidized by H2O and O2 to make rust (Fe2O3 H20). This is an example of decay as the iron is slowly converted into iron oxide.

Riya Sood 4G

For collision theory, remember that the molecules have to have correct orientation and need the minimum energy to reach transition state in order for the collision to be effective and make products.

Riya Sood 4G

We use the equation rate=K[A] when we are solving for the rate or the value of K or [A]. We use the equation ln[A]= ln[A]initial -kt when we are solving for variables like time which are not represented in the first equation. This equation is called the integrated rate law as it allows us to calcula...

Riya Sood 4G

Leila_4G wrote:And what is the units for the rate constant of the zero order rxn?

The units for rate constant for a zero order reaction is mol L^-1 s^-1

Riya Sood 4G

If the reaction is 2A--> Products and is zero order, is the rate constant still K or 2K ?

Riya Sood 4G

The rate law mostly includes reactants, but can also sometimes include products. The rate of a reaction is determined through experimental data which tells us whether the reactant or product is present in the rate law. If the product is in the rate law and present in the denominator then it has a ne...

Riya Sood 4G

Elizabeth Harty 1A wrote:What is the difference between delta g naught and delta g?

Delta G naught is for standard conditions at 25 degree celsius, 1 atm, 1 M. Delta G is for non-standard conditions.

Riya Sood 4G

SHE is the standard hydrogen electrode. It uses a platinum electrode. It is used as a reference and its cell potential is 0V. It is useful since it can take the place of either the cathode or the anode in order for the reaction to be spontaneous.

Riya Sood 4G

Are liquids included in the cell diagram notation?

Riya Sood 4G

Yes, when a battery is dead, the system has reached equilibrium -- though I'm wondering now if it's possible to 'revive' a battery or replace its galvanic cells concentration gradient? yes it is possible to revive a battery by recharging it. This is done by passing an electric current through an ex...

Riya Sood 4G

The n value is called a pure number and is the number of electrons for each half reaction in a redox reaction. the number of electrons for each half reaction should be equal once they are balanced.

Riya Sood 4G

Hi Ami, You can use the Nernst equation to solve for the pH value. One way to do that is to find the H+ concentration and then use the pH formula, -log[H+], to find the pH. Changing lnQ to 2.303logQ can also be a useful way of finding the pH. If Q= [H+] then you can replace that in the equation to g...

Riya Sood 4G

When calculating dela G= -(n)(F)(Ecell), will the units for delta G be KJ/mol or just KJ?

Riya Sood 4G

In an electrolyctic cell, is the anode positively charged or negatively charged ?

Riya Sood 4G

Do galvanic cells ever reach equilibrium where Ecell = 0?

Riya Sood 4G

(d) Pt(s)|O2(g)|H+ (aq)||OH-(aq)|O2(g)|Pt(s)

How would we set up the half reactions for this question ? Is O2 both oxidizing and reducing agent?

Riya Sood 4G

When the value of the electrode potential is given, do we assume it is given for the electrode reduction potential or oxidation potential?

Riya Sood 4G

Voltage is the potential difference between the two electrodes and it is measured to determine if a redox reaction will occur.
Also, voltage is an intensive property meaning that if a reaction is multiplied by a coefficient, the cell potential is not affected.

Riya Sood 4G

The best way to recognize which element is oxidized and reduced is to look at the oxidation number.
The general rule is that:
1. If oxidation number increases= element is oxidized (reducing agent)
2. If oxidation number decreases= element is reduced (oxidizing agent).

Riya Sood 4G

I am not sure which equation you are referring to. If you are asking about the "n" in the nernst equation, then we calculate it based of the # of electrons from the two half reactions once they are balanced. Since the electrons will be the same in the two half reactions in order for it to ...

Riya Sood 4G

Le Chateliers principle can be used to make some predictions. 1. When (reactant) > 1, we assume Ecell > Ecell (standard conditions) 2. When (product)>1, we assume E cell < Ecell (standard conditions) Le-Chateliers principle is not useful because we cannot calculate cell potential from that method. A...

Riya Sood 4G

A salt bridge allows ions transfer. It keeps the 2 solutions on cathode and anode side neutral. In addition in the worksheet that Dr. Lavelle posted on his website he states, "The half cells are connected by a salt bridge that allows the negatively charged spectator ions in solution to move fro...

Riya Sood 4G

According to the third law of thermodynamics, the entropy of a pure crystalline substance at 0K is 0. Does this mean that the substance is in a solid state at 0K?

Riya Sood 4G

What is meant by potential difference and what is it telling us about the cell?

Riya Sood 4G

Do the charges on both sides have to be equal when balancing redox reactions?

Riya Sood 4G

Balance each of the following skeletal equations by using oxidation and reduction half-reactions. All the reactions take place in acidic solution. Identify the oxidizing agent and reducing agent in each reaction. How is the Cl2 the oxidizing agent in part c? (c) Reaction of hydrosulfuric acid with c...

Riya Sood 4G

How can we define residual entropy? And what does it tell us?

Riya Sood 4G

When is delta H written as KJ/mol and when is it written as KJ?

Riya Sood 4G

Is the internal energy of an ideal gas 0 at constant temperature?

Riya Sood 4G

Are state properties also considered to be extensive properties ? Is there any correlation between the two concepts ?

Riya Sood 4G

Delta U= 0 during an isothermal reaction where temperature is constant. As such, the heat absorbed = work done on the surroundings.

Riya Sood 4G

Is Delta H positive or negative for a combustion reaction?

Riya Sood 4G

Under what conditions, if any, does the sign of each of the
following quantities provide a criterion for assessing the spontaneity of a reaction? (a) DeltaG; (b) DeltaH; (c) DeltaS(system); (d) DeltaS(tot).

What is the answer to part (c) and (d) ?

Riya Sood 4G

What is molar entropy? What is it telling us ?

Riya Sood 4G

If a compound is thermodynamically unstable, does it decompose into its constituent elements?

Riya Sood 4G

What are some key indicators in a question to identify whether pressure is constant or not to calculate the work when the question does not explicitly use the words "constant external pressure"?

Riya Sood 4G

4H.1 Which substance in each of the following pairs has the higher molar entropy at 298 K: (a) HBr(g) or HF(g); (b) NH3(g) or Ne(g); (c) I2(s) or I2(l); (d) 1.0 mol Ar(g) at 1.00 atm or 1.0 mol Ar(g) at 2.00 atm? For part (d), why does a lower pressure result in a higher entropy? The answer key stat...

Riya Sood 4G

When calculating enthalpies, using Hess's Law, do we use the units kj to denote delta H?

Riya Sood 4G

is the delta H positive or negative when ice turns to liquid?

Riya Sood 4G

Reversible expansion means when we increase the volume of an ideal gas in very small amounts at a constant temperature. Also the external pressure is approximately equal to pressure inside the system in a reversible expansion. I am guessing we should know how to solve the integral since it is part o...

Riya Sood 4G

Does increasing the temperature, increase or decrease the entropy of a system? Since increasing the temperature should put the molecules in an excited state to move around more freely

Riya Sood 4G

Gasses have the largest entropy because they can exist in many microstates. Molecules in a liquid and solid can move around less and are more rigidly bound. Therefore, they have lower entropy compared to gasses. When there is a phase change, going from solid to liquid and liquid to gas, always incre...

Riya Sood 4G

Why is calculating enthalpy from bond enthalpies considered to be not as efficient as other methods?

Riya Sood 4G

When calculating from the specific heat capacity using the formula q=C(m) (delta T), do we use units for temperature as degree celsius or Kelvin?

Riya Sood 4G

Do we have to calculate enthalpy from DSC curves?

Riya Sood 4G

Are enthalpies always expressed as positive? Can they ever be negative?

Riya Sood 4G

For this question, we need to take into account both the mass of the water and copper and calculate the heat required (q) for both of them and add it together. Your equation should be set up as: (400g)(4.18 J/cg)(100-22C)+ (500g) (0.38 J/Cg)(100-22C)= 1.4 x 10^2 KJ. In the solutions manual, they con...

Riya Sood 4G

Will we have to draw the Lewis structure to calculate the bond enthalpies or they were just used as an example in lecture?

Riya Sood 4G

PranaviKolla2B wrote:How do we calculate the pH of a buffer solution? Are there any good videos explaining this?

We only have to calculate the pH based on the ice table example used in the notes.

Riya Sood 4G

How are the delta H and delta U connected for reactions with ideal gasses?

Riya Sood 4G

Delta Hc with the degree sign refers to the standard enthalpy of combustion.

Riya Sood 4G

6C.5 Write the chemical formula for the conjugate base of formic
acid, HCOOH and calculate its pKb from the pKa of formic acid
(see Table 6C.1).

what is the answer to this problem? Cannot find it in the solutions manual

Riya Sood 4G

Do we have to use the Henderson-Hasselbach Equation when calculating the pH of a buffer solution? or we just use the ICE table ?

Riya Sood 4G

Does adding a liquid or solid to a reaction at equilibrium affect the concentrations of the reactants or products?

Riya Sood 4G

What does an equivalence point tell us about a titration?

Riya Sood 4G

1. Identify the following systems as open, closed, or isolated: (a) coffee in a very-high-quality thermos bottle; (b) coolant in a refrigerator coil; (c) a bomb calorimeter in which benzene is burned; (d) gasoline burning in an automobile engine; (e) mercury in a thermometer; (f) a living plant. How...

Riya Sood 4G

The way to ensure that you get -4.92, is by first simplifying the denominator. Once you do that you can multiply to the 0.56 (Kc value). At this point you should be at -3.92. And then we would subtract the equation we get from 3+x. And then -3.92-1= -4.92

Hope this helps.

Riya Sood 4G

Does a large Ka value mean that that the acid is more stable or unstable?

Riya Sood 4G

Calculate the pH of (a) 0.19 m NH4Cl(aq); (b) 0.055 m
AlCl3(aq).

How do we do part b of this question? What will be the dissociation reaction?

Riya Sood 4G

One possible way to tell is if they gave us the concentrations of products. Then we assume that reactants would be formed and the reverse reaction would take place. Also if Q is given, then we can compare it to the K and see which direction the reaction would proceed in.

Riya Sood 4G

This happens because the answer key is using the 6th edition of the textbook where the value of H3O+ is 3.2 mmol/L. But in the 7th edition of the textbook the value is 3.2 mol/L. This is why the answer key gives 3.2 x10^-3 in the denominator.

Riya Sood 4G

Yes the denominator will just be 1. This is because the reactant is a solid and we do not include solids and liquids in our calculations for the equilibrium constants.

Riya Sood 4G

In class he showed us three bar graphs that compared K and Q. When the bar for Q was higher than K (Q>K), we assume that the reaction favors formation of reactants. When the bar for K and Q are equal (K=Q), than the reaction is considered to be at equilibrium. Lastly, when the Q is a smaller bar tha...

Riya Sood 4G

When do inert gases have no effect on the equilibrium reaction?

Riya Sood 4G

What units for pressure do we use when calculating using the formula, pv=nRT? Is it bar or atm?

Riya Sood 4G

Why are equilibrium constants useful? A. One number that gives us information about the relative concentrations of reactant and product. B. One number that gives us information about the relative production rates of reactant and product. C. One number that gives us information about the relative act...

Riya Sood 4G

Also remember that when a system is at dynamic equilibrium, its composition stays the same.

Riya Sood 4G

Do we add the equilibrium sign if we are writing the dissociation of a strong acid/strong base ?

Riya Sood 4G

Acid rain can be reduced by reducing burning of fossil fuels. Also, neutralizers like limestone can be used

Riya Sood 4G

When do we add the word "ion" to the end?

Riya Sood 4G

What is the formula for bond order and what is the bond order of Cl2?

Riya Sood 4G

How can we determine which atoms are in the same plane and which can undergo rotation ?

Riya Sood 4G

Is T shape polar or nonpolar?

Riya Sood 4G

How many hydrogen bonding sites can H2O form? Do the 2Hs also act as potential hydrogen bonding sites ?

Riya Sood 4G

It also helps to draw out the lewis structures to help associate them with the names of ligands and remember their charge.

Riya Sood 4G

It is important to know that cisplatnin is a chemotherapy drug. It forms a coordination bond with DNA which stops cell division since the Cls are on the same side in a cis compound.

Riya Sood 4G

How can we determine which atoms stay in the same plane in a compound?

Riya Sood 4G

Do we have to take into account the lone pair electrons when finding the coordination number ? for instance, what is the coordination number for the shape triagonal pyramidal?

Riya Sood 4G

Yes the s orbital comes before the p orbital. But I don't believe that the order matters. For instance, both dsp^3 and sp^3d are considered the same thing.

Riya Sood 4G

How does the bond strength play a role in determining the strength of the acid? Is there a trend?

Riya Sood 4G

Increasing the number of oxygens increases the strength of the acidity because the additional O helps to pull out the atom easily. For example in HClO2 and HClO, the extra oxygen of HClO2 makes it stronger and easier to pull out the H atom from the H-O bond. Also, the oxidation state of the Cl is gr...

Riya Sood 4G

Decide whether an aqueous solution of each of the following salts has a pH equal to, greater than, or less than 7. If pH . 7
or pH , 7, write a chemical equation to justify your answer.
(a) NH4Br; (b) Na2CO3; (c) KF; (d) KBr; (e) AlCl3; (f) Cu(NO3)2.

How do we solve this problem?

Riya Sood 4G

How is H20 (water) both monodentate and bidentate?

Riya Sood 4G

Write the chemical formula for the conjugate acid of the base pyridine,
C5H5N, and calculate its pKa from the pKb for pyridine.

How do we solve this problem?

Riya Sood 4G

Can the value of pH every be negative ?

Riya Sood 4G

Does the placement of the ligands matter when going from the name of the coordinate compound to the formula? How do we know which ligand comes first after the metal?

Riya Sood 4G

What is the name of the coordinate compound [Fe(en)₂(NO₂)₂]SO₄ ?

Riya Sood 4G

Is there a formula to follow while naming coordination compounds that tells us what comes first and so on?

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