CHEMISTRY COMMUNITY

Alexandra Salata 2L

A catalyst will appear in the steps of a reaction mechanism, but it will not appear in the overall chemical reaction (as it is not a reactant or product).
Catalysts can either be in the same phase as the chemical reactants or in a distinct phase.

Alexandra Salata 2L

Zero-order reactions are typically found when a material that is required for the reaction to proceed, such as a surface or a catalyst, is saturated by the reactants. A reaction is zero-order if concentration data is plotted versus time and the result is a straight line.

Alexandra Salata 2L

A catalyst will not appear in the overall rate equation because it isn't chemically used up during a reaction.

Alexandra Salata 2L

J/mol or Kj/mol depending on the question.

Alexandra Salata 2L

Energy must be added to the reactants to overcome the energy barrier, which is recovered when products are formed. The energy barrier is Ea, the activation energy. The activation energy is different from the ΔG, which is the free energy difference between the reactants and products.

Alexandra Salata 2L

An acidic solution has far more positively charged hydrogen ions in it than an alkaline one, so it has greater potential to produce an electric current in a certain situation—in other words, it's a bit like a battery that can produce a greater voltage. A pH meter takes advantage of this and works li...

Alexandra Salata 2L

They are not always at equilibrium. But they work to reach equilibrium which is how a voltage is created.

Alexandra Salata 2L

Have a look at Khan Academy I think they have some good videos explaining the concept thoroughly

Alexandra Salata 2L

It’s found after you balance the redox reactions :)

Alexandra Salata 2L

Kinetic stability basically occurs when the reactants react really slowly. The slower the reaction occurs, the greater the kinetic stability. If you say, "This reaction is kinetically stable," then that implies that the reaction occurs very slowly.

Alexandra Salata 2L

Rechargeable batteries rely on reversible reactions. Non rechargeable reactions do not

Alexandra Salata 2L

A pH meter is an instrument used to measure hydrogen ion activity in solutions - in other words, it measures acidity/alkalinity of a solution. The degree of hydrogen ion activity is ultimately expressed as pH level, which generally ranges from 1 to 14.

Alexandra Salata 2L

In biochemistry, Nernst equation can be used to calculate the potential difference of ion between membranes. Hans H. Ussing, a Danish scientist, used a frog skin to measure the potential difference of sodium and potassium ions across the membranes with his famous invention, the Ussing chamber. This ...

Alexandra Salata 2L

Yes you're right I believe !

Alexandra Salata 2L

An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic cells or galvanic cells and those that generate chemical ...

Alexandra Salata 2L

I believe it should decrease the cell potential.

Alexandra Salata 2L

For standard electrochemical cells: A redox reaction is spontaneous if the standard electrode potential for the redox reaction, Eo(redox reaction), is positive. If Eo(redox reaction) is positive, the reaction will proceed in the forward direction (spontaneous).

Alexandra Salata 2L

Normally the anode is on the left and the cathode is on the right, but it really depends on the diagram, it could be either way :)

Alexandra Salata 2L

It represents the current approaching zero

Alexandra Salata 2L

OIL RIG is the one I used when I learnt high school chemistry. I personally think it’s the best one.

OIL - oxidation is loss of electrons
RIG - reduction is gain of electrons

Alexandra Salata 2L

I really don't think there's a difference, maybe it's just a more specific name ?

Alexandra Salata 2L

The negative value of ΔG° indicates that the reaction is spontaneous as written.

Alexandra Salata 2L

Yes, temperature should always be in Kelvin :)

Alexandra Salata 2L

In Isothermal process the temperature is constant. The internal energy is a state function dependent on temperature. Therefore, the internal energy change is 0.

Alexandra Salata 2L

water has a high heat of vaporization, which is the amount of energy needed to change one gram of a liquid substance to a gas at constant temperature

Alexandra Salata 2L

The delta H vap
of a certain compound is 15.85 kJ·mol−1
and its delta S vap
is 57.91 J·mol−1·K−1.

What is the boiling point of this compound?

I keep getting 0.7 degrees Celsius but it's saying it's wrong and I'm not entirely sure why.

Alexandra Salata 2L

For reversible processes (the most efficient processes possible), the net change in entropy in the universe (system + surroundings) is zero.

Alexandra Salata 2L

Hc is specifically enthalpy for combustion. Hf is the standard enthalpy formation in any circumstance basically

Alexandra Salata 2L

The natural form of the elements are most stable - so diatomic molecules are an example

Alexandra Salata 2L

This happens when the system experiences isothermal change(temp remains constant), thus delta U is zero. So, q=-w means that heat loss is equal work done on the system and visa versa is also true.

Alexandra Salata 2L

It’s an oxidation reaction that releases heat. But relatively speaking, the energy released from forming bonds is larger than that of breaking bonds as well.

Alexandra Salata 2L

Vapor and gas are the same thing, so if in a question they say something has been vaporized, it has just turned from a liquid to a gas.

Alexandra Salata 2L

The difference between dH and dU becomes significant only when gases are involved (insignificant in solids and liquids)

dU is the total change in energy and dH is specific to the amount of work

Alexandra Salata 2L

A bomb calorimeter is a type of constant-volume calorimeter used in measuring the heat of combustion of a particular reaction. Electrical energy is used to ignite the fuel; as it's burning, it will heat up the surrounding air, which expands and escapes through a tube that leads the air out of the ca...

Alexandra Salata 2L

Heat capacity is the ratio of the amount of heat energy transferred to an object to the resulting increase in its temperature.
Specific heat capacity is a measure of the amount of heat necessary to raise the temperature of one gram of a pure substance by one degree K.

Alexandra Salata 2L

In an example in the lecture, Dr Lavelle gave us the equation: N2(g)+3H2(g)-->2NH3(g) without adding more reactants how would you increase the yield of NH3? By removing some of the reactant. Surely by removing some of the reactant equilibrium will shift to the left to form more reactants then? I'm c...

Alexandra Salata 2L

we omit them from the equations because their concentrations do not change, therefore they are not involved in the equilibrium expression.

Alexandra Salata 2L

An amphiprotic substance is one which can both donate hydrogen ions (protons) and also accept them. Water is a good example.
The water acts as both an acid (donating hydrogen ions) and as a base (by accepting them).

Amphoteric means that they have reactions as both acids and bases.

Alexandra Salata 2L

There is no specific cutoff, but as people said if the Ka2 is very small, then we can assume that all the ionization is from ka1 and not focus too much on it.

Alexandra Salata 2L

Heat and work are two different ways of transferring energy from one system to another. The distinction between heat and work is important in the field of thermodynamics. Heat is the transfer of thermal energy between systems, while work is the transfer of mechanical energy between two systems.Neith...

Alexandra Salata 2L

It's a quality that is independent of how the substance was prepared.
As an example, the state of an electric battery requires the specification of the amount of electric charge it contains.

Alexandra Salata 2L

Yes, this is right.

Ka (acid disossication constant) * Kb (base dissociation constant) = Kw, which is for the constant for autoprotrolysis of water . (This is also equal to 1x10^-14)

Alexandra Salata 2L

We don’t have to memorise the states of any compounds or elements. Pretty sure they will be given to us in the exam.
(S) solid
(L) liquid
(G) Gas
(Aq) Aqueous

Alexandra Salata 2L

The value will be on a constant sheet that he gives us for the exam! You won’t have to memorise it :)

Alexandra Salata 2L

Hi I'm having some trouble with this question:

N2O4 --->2NO2

I've calculated the equilibrium constant and the direction of the shift and the change in Kc. But I can't seem to work out the final part for the concentrations of NO2 and N2O4 after an extra 1mol.L-1 was added to the solution.

Alexandra Salata 2L

In the example in at the beginning of Lecture 4, Dr. Lavelle said that since Kc is very small we can approximate that the concentrations of the compounds will stay roughly the same, and this is how we would calculate X to figure out the composition of the equilibrium mixture. Would Kc always be smal...

Alexandra Salata 2L

Increasing the temperature decreases the value of the equilibrium constant. Where the forward reaction is endothermic, increasing the temperature increases the value of the equilibrium constant.

Alexandra Salata 2L

K will always be the same.
Q would change if you added more products are reactants.

Alexandra Salata 2L

Like other compounds and elements, you do not include water if it is in solid or liquid phase in your expression. Only when H20 is in the gas phase will you be able to include it.

Alexandra Salata 2L

They basically follow the same trend. And as for sits and shifts, they both kinda mean the same thing. For example: equilibrium sits on the left. This would mean that the reverse reaction is favoured and products are converting back into reactants. If you say equilibrium shifts to the left, it still...

Alexandra Salata 2L

They all measure the reaction quotient but specifically, Qp measures partial pressure and Qc measures concentration. SO depending on what the question is asking for, you would substitute one into the equation.

Alexandra Salata 2L

So I know that sp3 hybridization corresponds to 4 regions of electron density. I just don't really understand how 2sp3 3sp3 works? How do you know which region to give what numbers to?

Alexandra Salata 2L

Yes the mean the same thing. An alkaline solution is a basic solution, which has a pH>7.

Alexandra Salata 2L

I honestly think either is fine. Also since I thin our exams are multiple choice, it will be fairly obvious whether they've written the products(salts) in ionic or compound form. I think Lavelle writes them as ions to show the separate cations and anions in the compound.

Alexandra Salata 2L

Bond strength is related to the length of the bond, and because Iodine has a much larger atomic radius than Fluorine, HI has a much longer, and therefore weaker, bond. The hydrogen is removed fairly easily, making HI a stronger acid

Alexandra Salata 2L

the pKa value is constant for each type of molecule. It is unaffected by concentration. Even a chemical considered a base can have a pKa value because the terms "acids" and "bases" simply refer to whether a species will give up protons (acid) or remove them (base). the main diffe...

Alexandra Salata 2L

Anion stability is about the anion product of acid dissociation. If the anion is not stable, it will recombine with H+ and the reverse reaction dominates so there is less dissociation (higher pH). The more stable the anion, the more dissociation "remains" (less reverse reaction and so on).

Alexandra Salata 2L

Cobalt is an essential element for humans, and is found at the centre of vitamin B12. Even though the body contains only 2–5 mg of vitamin B12, its involvement in the production of red blood cells means it is vital to life.

Alexandra Salata 2L

A chelate is a compound containing a ligand (typically organic) bonded to a central metal atom at two or more points.

An example of a simple chelating agent is ethylenediamine
A single molecule of ethylenediamine can form two bonds to a transition-metal ion such as nickel(II), Ni2+.

Alexandra Salata 2L

The oxidation state, commonly referred to as the oxidation number, describes the degree of oxidation (loss of electrons) of an atom in a chemical compound.

Alexandra Salata 2L

With students in different countries, is there a way for us to take our finals at a better time? I'm currently in Hong Kong and the final exam is scheduled for me from 3.30-6.30 am. please let me know. Thank you!

Alexandra Salata 2L

both are accurate but most of the time chloro is used and is more common in textbooks etc. so I would just use chloro to make your life easier.

Alexandra Salata 2L

In chemistry, orbital hybridization is the concept of mixing atomic orbitals into new hybrid orbitals (with different energies, shapes, etc., than the component atomic orbitals) suitable for the pairing of electrons to form chemical bonds in valence bond theory. Hybridization occurs when an atom bon...

Alexandra Salata 2L

it's because the we are only concerned with the 2s and 2p orbitals in this example because we can assume that the 1s orbital is already filled when you look at nitrogen on the periodic table.

Alexandra Salata 2L

So in Dr Lavelle's lecture, he said that the sp3 hybridization orbitals would have four because of the four regions of electron density. but for ethene, why is there sp2 hybridization with an additional 2p orbital, rather than just an sp3 hybridization? I didn't really get his explanation.

Alexandra Salata 2L

Many covalent compounds contain multiple bonds (double or triple bonds). A difference between single and multiple bonds is that single bonds only have a sigma bond, whereas multiple bonds have both sigma and pi bonds.

Alexandra Salata 2L

Yes, these rules will always be followed provided the structures have the right number of bonding electrons and lone pair electrons. so in the example you gave, any molecule with one lone pair of electrons and three bonding pairs of electrons will always be trigonal pyramidal, regardless of differen...

Alexandra Salata 2L

In tetrahedral molecular geometry, a central atom is located at the center of four substituents, which form the corners of a tetrahedron. In square planar molecular geometry, a central atom is surrounded by constituent atoms, which form the corners of a square on the same plane . Example of tetrahed...

Alexandra Salata 2L

does it matter which molecules you put on each wedge or in the plane? Let's say there was a molecule with different types of atoms, like PF3Cl2.

Alexandra Salata 2L

they are all around the same strength, but as they said, the dipole-dipole intermolecular forces are permanent. induced dipole-induced-dipole are caused by correlations in the fluctuating polarizations of nearby particles, therefore they are constantly changing.

Alexandra Salata 2L

In Lavelle's lecture, he gave the example of CCL4 and HCl4, I get that CCl4 is a bigger molecule. why does it have greater electron distortion? Is it just because cl has more e-? Also why would this mean that CCl4 has a much higher boiling point than methane?

Alexandra Salata 2L

Lone Pairs of electrons on neighboring atoms weaken the bond because of electron repulsion. Neighboring atoms in a molecule that have lone pair electrons will not be held together in their bond as tightly because the lone pair electrons of both atoms repel each another.

Alexandra Salata 2L

there are a number of factors affecting covalent bond strength: 1)Extent of atomic orbital overlap - Dependent on the size of the atom ,smaller atoms have better overlap of orbitals ,larger atoms tend to have more diffused orbitals, resulting in less effective overlap 2)Number of bonds between atoms...

Alexandra Salata 2L

Van der Waals forces include attraction and repulsions between atoms, molecules, and surfaces, as well as other intermolecular forces. Interactions between ions and molecules are ion-dipole interactions. Note that Van der Waals forces are induced dipole-dipole forces. These forces exist between all ...

Alexandra Salata 2L

In a water molecule, both O-H covalent bonds are of the 'normal' or 'conventional' type because each H atom contributes 1 electron to be shared between the two atoms. So no, it isn't a coordinate covalent bond.

Alexandra Salata 2L

boron is in group 13, therefore it only has 3 valence electrons and can only form three bonds to have an FC=0.

Alexandra Salata 2L

reva_bajjuri wrote:do expanded octets usually correspond with resonance lewis structures or are those two things unrelated?

I'm pretty sure they're unrelated:)

Alexandra Salata 2L

Free radicals are formed from molecules via the breakage of a chemical bond such that each fragment keeps one electron, also via redox reactions in the body. I'm not sure what specifically would happen if you were vitamin deficient, but oxidative stress can arise when cells cannot adequately destroy...

Alexandra Salata 2L

Resonant structures are simply, as Lavelle said in his lecture, when one Lewis structure has multiple bonds at equal locations. Delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond. This is in contrast to localized electr...

Alexandra Salata 2L

Resonant structures are simply, as Lavelle said in his lecture, when one Lewis structure has multiple bonds at equal locations. Delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond. This is in contrast to localized electr...

Alexandra Salata 2L

The simplest way to draw the nitrate ion I think would be either one of the three resonant structures. You can draw either of the 3 unless they ask you specifically to draw out all three. The reason why we can do this for benzene is because it is in a closed, hexagonal shape, therefore the circle in...

Alexandra Salata 2L

When one or more of the groups is a lone pair of electrons (non-bonded electrons), the experimentally-observed geometry around an atom is slightly different than in the case where all groups are bonds. The actual bond angles are similar, but not exactly the same, as those predicted based on the tota...

Alexandra Salata 2L

Electron affinity is the energy change that results from adding an electron to a gaseous atom. (EG) when a fluorine atom in the gaseous state gains an electron to form F⁻(g), the associated energy change is -328 kJ/mol. Unlike ionization energies, which are always positive for a neutral atom because...

Alexandra Salata 2L

why do elements with a low ionization energy form cations?

Alexandra Salata 2L

No, they would not be the same size due to their difference in nuclear charges. depending on the number of protons and neutrons in the nucleus, this determines the overall nuclear charge. and this means that the greater the nuclear charge, the smaller the atomic radii/ionic radii.

Alexandra Salata 2L

1m = 1x10^12pm

Alexandra Salata 2L

Yeah I think using the molar masses on the periodic table that he's given us for the midterm would be the most accurate. As for the sig figs, I think once you've calculated the answer, 3 sig figs will do. In your calculations, however, I would probably use the number exactly as it's given in the per...

Alexandra Salata 2L

This happens because effective nuclear charge, which is a measure of what the net positive charge felt by the electrons is, increases. This implies that the atomic size of carbon will be a little bigger than that of nitrogen, which in turn will be a little bigger than that of oxygen

Alexandra Salata 2L

I heard that there will be around 10-15 questions. All of them will be multiple choice. I also asked my TA about how partial credit is going to work. Some of the answers will be very similar, in the sense that, for example, A) is 100% correct and B) is 75% correct because you missed a calculation or...

Alexandra Salata 2L

I've seen quite a few topics on the Schrodinger's equation and I understand what all the elements represent... apart from H(Hamiltonian). I have no idea as to what it does even after reading the textbook and watching Dr Lavelle's lecture. Anyone have any easy and straightforward explanations for it?

Alexandra Salata 2L

Dr Lavelle mentioned in his worked example that a wavelength such as 1.64x10^-38 m is too small to be measured for wave-like properties.

what's the smallest detectable wavelength?
and also does this mean the object has only particle-like properties?

Alexandra Salata 2L

E=h/v is the relationship showing the energy per photon, where the h represents Planck's constant (6.626x10^-34) and v represents the frequency. The equation E= hc/v does not exist, but I think you're referring to E= hc/ λ, which is the relationship between Planck's constant, the constant speed of l...

Alexandra Salata 2L

Dr Lavelle drew a diagram when he was explaining the energy levels in H-atoms. he said that all spectral lines showing up in the UV region involve the energy gap n=1 and n=2. Why is this? Is it because it has the highest energy difference?

Alexandra Salata 2L

Dr Lavelle specifically said that black body radiation is more of a physics subject and that we won't need to know what it is or any detail about it for Chem14A.

Alexandra Salata 2L

It really depends on the problem and if they specify It or not. When they do not specify, I just like to go with 3 sig figs because its the most standard way of rounding up figures and in the past I have been taught to do this by teachers

Alexandra Salata 2L

Increasing the frequency would result in the photon having more energy (E=hv). Increasing the intensity would result in an increase in the number of photons, but this has no change in their individual energy. No matter how bright the light is (or how many photons there are) none of the photons will...

Alexandra Salata 2L

I just watched Dr Lavelle's lecture on the photoelectric effect and i was confused about the intensity of the light. so I know that changing the intensity of the light source doesn't change the energy per photon, but changing the frequency does. Does an increase in frequency mean a higher number of ...

Alexandra Salata 2L

Avogadro's number is 6.022×1023 molecules. With Avogadro's number, scientists can discuss and compare very large numbers, which is useful because substances in everyday quantities contain very large numbers of atoms and molecules. It is used in the equation number of moles = number of particles/avag...

Alexandra Salata 2L

what I find most useful is when I relax a bit the night before. instead of cramming through the night you should try and get a good night of sleep. then in the morning, you should look through topics that you are the least confident in or compile some quick exam questions just to go through to boost...

Alexandra Salata 2L

Is there a copy of the official periodic table we should use that I could print off and have in my folder because I would find it much easier to use than the periodic table on the sapling learning website. If there is, where could I find it?

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