CHEMISTRY COMMUNITY

Sarah Zhari 1D

Can the rate constant (k) of a reaction change? If so, how?

Sarah Zhari 1D

Dina 2k wrote:how would u determine which elementary step is the limiting?

I think that most likely, they would tell us which one is the slow step. At least that's what the problems in the textbook do.

Sarah Zhari 1D

Enzymes are biological catalysts, and catalysts are mentioned in the outline. Maybe to be safe, you should know "how a catalyst lowers the activation energy of the transition state, how a catalyst increases the rate of a reaction, and also identify a catalyst in a reaction and give an example o...

Sarah Zhari 1D

Conceptually, orders of reactions generally relate to how changing the concentration of a reactant will affect the rate of a reaction.

Sarah Zhari 1D

Yes. As the unit for k1 is s^-1, and the unit of k for first-order reactions is s^-1, the reaction is first order.

Sarah Zhari 1D

I think that an order can be negative if as you increase the concentration of a particular reactant, the overall rate of reaction decreases.

Sarah Zhari 1D

What exactly is A in the Arrhenius equation? From the lecture, I know that it is the "frequency or pre-exponential factor" and it has something to do with particles colliding in the correct orientation, but what does this actually mean?

Sarah Zhari 1D

When you rearrange the equation ln [A]t = -k t + ln [A]o, you get ln [A]t-ln [A]o= -k t. Following the log rules, ln[A]t-ln[A]o is equal to ln([A]t/[A]o). Flipping the fraction to get ln([A]o/[A]t) is simply the negative, which explains the change in sign when you flip the fraction.

Sarah Zhari 1D

i think you're supposed to calculate n by the number of electrons that need to be transferred in the rxn. so you would have to write out the half-rxns of the cathode and anode, and see that only 1 electron is transferred and therefore n = 1 What would the half-reactions be if n=1? Would we still us...

Sarah Zhari 1D

I would assume that pH would only affect oxidizing strength if H+ or OH- ions are involved in the half-reactions. Therefore, for Br, I don't think that pH would affect its oxidizing strength as no H+ or OH- ions are involved in the reaction.

Sarah Zhari 1D

For this question, you would want to find the least reduction potential as that implies that it is better as an oxidizing agent and thus will be better at reducing other compounds. What was your method for trying to solve this problem? For clarification, don't oxidizing agents oxidize other compoun...

Sarah Zhari 1D

Q or K can refer to the equation: $\frac{P_{products}}{P_{reactants}}$ , and $\Delta$ Gº= -RTln(K), therefore pressure can affect $\Delta G$ .

Sarah Zhari 1D

You would look at the potential required to reduce that particular species. For example, H+ and Na+ are present in solution and both are reducible. It takes +0.41V to reduce H+ to H2, and it takes +2.71V to reduce Na+ to Na, therefore H+ would be the species that is reduced, as it takes less potenti...

Sarah Zhari 1D

Why are some standard electrode potentials negative and some positive? What does this mean in relation to the hydrogen electrode?

Sarah Zhari 1D

I would assume that these principles still apply for ∆G.

Sarah Zhari 1D

Firstly, since it is in basic solution, you would add OH- in the reactants side. To balance the O and H atoms, you would then add H2O on the products side, giving the equation OH- + Br2 ----> BrO3- + Br- + H2O. For me, since there are 4 oxygens on the right side, I initially put 4 on OH-. As the num...

Sarah Zhari 1D

I think Dr Lavelle was talking about a bike pump, which is a closed system. For closed systems, you can either heat or cool the system, or expand or compress the system to change the energy.

Sarah Zhari 1D

Since it's a basic solution, you would need to add the OH- as basic solutions have OH- ions.

Sarah Zhari 1D

For example, in the equation 2Fe3+(aq) + Cu(s) -> Cu2+(aq) + 2Fe2+ , would we have to say that Fe3+ is being reduced to Fe2+, or is it enough to say that iron is being reduced?

Sarah Zhari 1D

When is it necessary to use Pt(s) in the skeletal equation of a redox reaction? In the example that Dr. Lavelle gave in class: 2Fe3+(aq) + Cu(s) -> Cu2+(aq) + 2Fe2+ although platinum is part of the redox reaction, platinum is an inert conductor that is transferring e-, as there is no conducting sol...

Sarah Zhari 1D

If you are referring to n in the equation $\Delta G=-nFE$ , then n is the number of moles of electrons in the balanced redox reaction.

Sarah Zhari 1D

Not necessarily, as not all molecules have resonance. It's the number of orientations a molecule can have, and a molecule can have multiple orientations even without resonance.

Sarah Zhari 1D

Another helpful one is OIL RIG - Oxidation Is Loss, Reduction is Gain. Although this may be slightly confusing because this does not mention electrons!

Sarah Zhari 1D

I think that it's important if you only have the equilibrium constant and temperature of a reaction, therefore you wouldn't need to find enthalpy or entropy to measure delta G.

Sarah Zhari 1D

May I ask if the equation you're referring to is pV=nRT? If so, then you can use the equation to find concentration if you have the pressure and temperature. I don't think that you can use this equation to find the concentration/volume of both the reactants and the products, as the pressure or a cer...

Sarah Zhari 1D

In question 4J.7 a) in the homework problems, we are asked to calculate the standard reaction entropy, enthalpy and Gibbs Free Energy for the equation for the decomposition of hydrogen peroxide: 2H2O2 ---> 2H2O + O2 I thought that since we are calculating the standard entropy/enthalpy, we would be r...

Sarah Zhari 1D

I don't think that the equilibrium would shift, because if you add more solid products/reactants, you would not be changing its concentration, thus the value of K would not change.

Sarah Zhari 1D

Yes, I think that you would have to multiply them by their stoichiometric coefficients.

Sarah Zhari 1D

I think that the heat capacity is used in the equation $\Delta S=\frac{q}{t}$ , where the heat capacity is used to calculate the value of q using the equation $q=mc\Delta T$

Sarah Zhari 1D

When calculating work done using the equation: $-P\Delta V$ , what units should we use for pressure to obtain an answer in Joules?

Sarah Zhari 1D

I think that because you are not required to calculate the specific heat capacity, and instead are only asked for heat capacity, mass or number of moles are not needed.

Sarah Zhari 1D

I think that Dr Lavelle mentioned that later we will talk about how we can relate both entropy and enthalpy to determine the spontaneity of a reaction. However, I believe that generally, reactions which cause an increase in entropy tend to be more spontaneous, but I'm not too sure!

Sarah Zhari 1D

We can use degeneracy to calculate entropy, which was given by the equation S=kb*ln(W)

Sarah Zhari 1D

Dr Lavelle was talking about how atoms can have different "microstates" which have the same energy - but what exactly is a "microstate"? Are there any examples?

Sarah Zhari 1D

I think that exothermic reactions tend to be more spontaneous because as the reaction commences, the products end up having a lower energy than the reactants because energy is released. Therefore, the products end up being more stable, therefore this tends to be favored.

Sarah Zhari 1D

Are there any instances where ∆H is not equal to ∆U for a system?

Sarah Zhari 1D

I think it would depend on what information is given to you. For example, if the bond enthalpies are given, use method 2.

Sarah Zhari 1D

I think that there is a correlation, as shorter bond lengths would take more energy to be broken. I found that a C-C bond has a bond enthalpy of 347kJ/mol, whereas a shorter C=C bond has a bond enthalpy of 611kJ/mol.

Sarah Zhari 1D

To add on, bond breaking is an endothermic process and has a positive value, and bond forming is an exothermic process and has a negative value.

Sarah Zhari 1D

Water as a liquid is not included in K expressions, but would we still include H2O gas in K expressions?

Sarah Zhari 1D

On the constants sheet, there are multiple values for the gas constant (R). When would we use each of these values? Specifically, how do we know when to use 8.314 and not 8.314x10^-2?

Sarah Zhari 1D

Depending on the reaction, changing the temperatures and pressures can also affect the yield product.

Sarah Zhari 1D

You can see from the graph that the partial pressure of A decreases by 10, and similarly the partial pressure of C increases by 10. However, the partial pressure of B only increases by 5, half of A and C. This explains why the molar ratio ends up being 2A ---> B + 2C

Sarah Zhari 1D

You can definitely have an acid with a negative pH, although this would be very rare. Additionally, very strong bases can also have a pH of more than 14.

Sarah Zhari 1D

If we were to use an ICE table, would the values need to be concentrations, or can we use moles when using the ICE table?

Sarah Zhari 1D

If you are looking at a reaction where the reactants form products, the concentrations of the products would increase (a positive value in the C row) whereas the concentrations of the reactants would decrease (a negative value in the C row)

Sarah Zhari 1D

Le Chatelier's principle is when a change is applied to a reaction at equilibrium, the equilibrium adjusts to minimize the effect of the change. When N2 is added, there are more reactants than products, or Q<K. Because of this, the equilibrium will shift towards the right to make more products (in t...

Sarah Zhari 1D

Changing the pressure of a container by decreasing or increasing its volume does not affect K. This is because the partial pressures of all the gases involved in calculating K change by a common factor when a gas is compressed or otherwise, therefore the overall ratio of products to reactants remain...

Sarah Zhari 1D

The textbook states that "if a chemical equation is multiplied through by a factor N, K is raised to the Nth power". Why is this the case? When would it be necessary to multiply a chemical equation by a factor?

Sarah Zhari 1D

For heterogeneous equilibrium equations where there are both gases and aqueous products or reactants, would you write the equilibrium constant in terms of Kp or Kc?

Sarah Zhari 1D

Kp is used when you use partial pressures, which is for gases. Kc is normally used if you use molar concentrations in the equation, which is mostly for substances other than gases, however using the ideal gas equation (PV=nRT), you can convert pressure to concentration.

Sarah Zhari 1D

Chelates form a ring around the central atom, whereas polydentate ligands allow more than one of its binding sites to be occupied. I think that a polydentate ligand can be a chelate, but they are not necessarily a chelate. In the review session i'm pretty sure Matthew said all polydentate ligands c...

Sarah Zhari 1D

From this Lewis structure, I found that 2 of the oxygens in C2O4 bind to the central metal atom. It is bidentate and is a chelate.

Sarah Zhari 1D

Why do small, highly charged metal cations such as Fe3+ act as Lewis acids in water? Is there an equation to demonstrate this?

Sarah Zhari 1D

It depends on which atom is bonded to the metal atom. Cyanido is used when the carbon is bonded to the metal atom, whereas isocyanido is used when the nitrogen is bonded to the metal atom.

Sarah Zhari 1D

The element As is a metalloid, which can form amphoteric oxides. They can react with both acids and bases.

Sarah Zhari 1D

Chelates form a ring around the central atom, whereas polydentate ligands allow more than one of its binding sites to be occupied. I think that a polydentate ligand can be a chelate, but they are not necessarily a chelate.

Sarah Zhari 1D

All of the phosphorus atoms have 3 other phosphorus atoms attached to them. Therefore, the shape of the molecule would be as follows: https://qph.fs.quoracdn.net/main-qimg-f5db806cc5f659130ab2eb07e69972fe.webp There is one lone pair around each phosphorus atom. Therefore, there are 4 regions of elec...

Sarah Zhari 1D

Ka is the equilibrium constant for acids. It's used to determine the level of dissociation an acid in an aqueous solution at equilibrium by finding the ratio between the products and reactants. Because of this, a higher Ka will indicate a stronger acid, as it dissociates more. Inversely, a lower pKa...

Sarah Zhari 1D

Why is this the Lewis structure of sulfuric acid? https://www.fishersci.com/us/en/products/chemicals/acids-bases/acids/inorganic-acids/sulfuric-acid/jcr%3Acontent/browse-results/browse_content/imageandrte_66e2.img.jpg/1535483076248.jpg I initially thought that sulfuric acid consisted of hydrogen cat...

Sarah Zhari 1D

How do some compounds have both basic and acidic character?

Sarah Zhari 1D

Normally, electron acceptors (Lewis acids) are positively charged, and electron donors (Lewis bases) are negatively charged (this may not be the case all the time). For me, drawing the Lewis structure helps determine which species has lone pairs of electrons, which helps to determine the Lewis base....

Sarah Zhari 1D

Would we always need to draw out Lewis structures to determine if a molecule can form chelate complexes?

Sarah Zhari 1D

Cisplatin can form coordination compounds with DNA by binding to the lone pair of electrons on nitrogen in the DNA base guanine, whereas transplatin is unable to do so.

Sarah Zhari 1D

I think you would probably need to know how to determine what kind of hybridized orbitals are present in a molecule, as well as the concept behind hybridization, and how it is caused by the merging of orbitals to form a hybridized orbitals with the same energy.

Sarah Zhari 1D

Yup, when you draw dipoles the arrow is pointing towards the more electronegative atom. The dipoles cancel out because there is no net dipole, because the two dipoles are going in opposite directions.

Sarah Zhari 1D

What are the intermolecular forces present in CO2? Does a non-polar molecule with polar bonds have dipole-dipole interactions?

Sarah Zhari 1D

Electronegativity is the ability of an atom to attract electrons. Polarizability refers to how much an atom's electron cloud will be distorted when it is bonded to another atom.

Sarah Zhari 1D

Why and how are London forces present in all molecules?

Sarah Zhari 1D

I think that visualizing the molecules in 3-D really helps, especially when you can't remember the names of the shapes.

Sarah Zhari 1D

Why are bond angles smaller when there are lone pairs of electrons present? For example, why are the bond angles in a tetrahedral structure larger than the bond angles in a trigonal pyramidal structure?

Sarah Zhari 1D

If you know the formula, you would be able to draw the Lewis structures, and you would know the number of bonding and non-bonding pairs of electrons around the central atom, so, I think it would be easier to apply VSEPR theory in that case.

Sarah Zhari 1D

What exactly determines the polarity of a particular molecule? Is there a specific difference in electronegativity between two elements that is needed in order for a molecule to be polar?

Sarah Zhari 1D

To add on, sigma bonds are a result of direct overlaps between orbitals, whereas pi bonds are a result of an overlap between two adjacent parallel p-orbitals.

Sarah Zhari 1D

Hydrogen bonds are intermolecular forces, whereas ionic and covalent bonds are bonds between atoms. Therefore, hydrogen bonds are weaker than ionic and covalent bonds.

Sarah Zhari 1D

What is the difference between a dipole-dipole force and an induced dipole-induced dipole intermolecular force? Are molecules that experience dipole-dipole forces polar?

Sarah Zhari 1D

Butanol has hydrogen bonds, where the electronegative oxygen pulls in electrons, causing a slight negative charge on the oxygen, and a slight positive charge on the hydrogen. The slight positive charge on the hydrogen in one molecule is attracted to the lone pair of electrons on oxygen in another mo...

Sarah Zhari 1D

Yes, this trend continues for elements below copper and chromium, because a half-filled d orbital is more energetically stable.

Sarah Zhari 1D

Elements from the third period onwards generally can have an expanded octet because they have an empty 3d orbital which can accommodate for additional electrons.

Sarah Zhari 1D

When the 3d orbitals contain electrons, the 3d orbitals have a lower energy than the 4s orbitals. This is caused by electron-electron repulsions, which lower the energy.

Sarah Zhari 1D

Additionally, radicals are also highly reactive. You can identify if a species is a free radical by looking at its Lewis structure - instead of a pair of valence electrons there should be an unpaired electron on the atom or molecule or ion. For example, this is what a methyl radical would look like:...

Sarah Zhari 1D

Elements in the p-block in the third period have valence electrons in the n=3 energy level. Yes, there are no electrons in the 3d orbital, and this is why these elements can have an expanded octet, because additional electrons can accommodate the empty 3d orbitals.

Sarah Zhari 1D

Atoms in period 3 or higher have d-orbitals in their valence shell which can accommodate more than 8 electrons, which is why these atoms are exceptions to the octet rule.

Sarah Zhari 1D

If a question asks you to "give the number of valence electrons (including d electrons)" for manganese, how many valence electrons are there? I thought that the electron configuration of manganese is [Ar]3d5 4s2, therefore should the number of valence electrons be 2 or 7?

Sarah Zhari 1D

Valence electrons are the number of electrons in the outer shell of an atom. The valence of an atom, however, refers to the "number of bonds the atom can form by sharing electron pairs". This determines how many bonds an atom can make when covalent bonds are formed.

Sarah Zhari 1D

Electron affinity is defined as the energy released when an electron is added to an atom in the gas phase. Generally, the elements on the right side of the periodic table have higher electron affinities. This is because the electrons are attracted to the higher effective nuclear charge, therefore en...

Sarah Zhari 1D

Additionally, in resonance structures, the bond lengths are equal due to the delocalization of electrons between the atoms. The molecule, as a result, has a lower energy, and this helps to stabilize the molecule.

Sarah Zhari 1D

In the textbook, it states that in a hydrogen atom, "all the orbitals of a given shell are degenerate (have the same energy)". Why do all the orbitals in a hydrogen atom have the same energy?

Sarah Zhari 1D

A wave function is a mathematical function that can tell us the shape of an orbital. If you square the wavefunction, you can find the probability density of an electron being found within that particular region.

Sarah Zhari 1D

If you are using the equation wavelength=h/p, you need to convert the wavelength to metres, but you do not have to convert the mass of the neutron to grams, and you have to leave this mass in kilograms because the units for momentum(p) is kgm/s

Sarah Zhari 1D

If you look at the "Constants and Equations" sheet (which can be found on the class website), you will find the conversion for 1eV, which is equal to 1.602x10^-19J.

Sarah Zhari 1D

Silver atoms were chosen because they have one unpaired electron. Therefore, the atom acts as a single unpaired electron. As a stream of silver atoms are passed through a magnetic field, they split into two streams, one where the unpaired electron is spin-up, and the other where the electron is spin...

Sarah Zhari 1D

Light sources with short wavelengths have high frequencies, and therefore high energy. Electrons are only ejected from a metal surface if the frequency or energy of the incoming light is equal to or is higher than the threshold energy of that particular metal. The reason why increasing the intensity...

Sarah Zhari 1D

If an electron moves to a lower energy level, light is emitted. Therefore, the energy change of the electron has decreased or is negative, however, the energy of the photon emitted is positive.

Sarah Zhari 1D

Do you mean the Balmer and Lyman series? In the emission spectrum, the Balmer series refers to the visible light spectrum, which occurs when electrons move to n=2. The Lyman series occurs when electrons move to the first energy level, and this is in the UV spectrum.

Sarah Zhari 1D

Photons act as particles of electromagnetic radiation, therefore they have particle characteristics

Sarah Zhari 1D

I am not entirely sure of how the experiment demonstrating the photoelectric effect works... How and why are electrons ejected when light of a certain frequency is directed towards a metal surface?

Sarah Zhari 1D

When an electron gets excited and moves up higher energy levels, light is absorbed. Electrons can get excited by heat.

Sarah Zhari 1D

If you look under the "Solution Manual Errors 7th Edition" on the class page, you will find that there is an error in question L35, and that the compound should be Fe3Br8.

Sarah Zhari 1D

Generally, if a question is asking for you to add or subtract, you round your answer based on the value in your working with the least number of decimal places. For example, 33.567+2.1 = 35.7. For multiplication and division questions, you round off your answer based on the value in your working wit...

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