CHEMISTRY COMMUNITY

Eva Zhao 4I

For your reference, here are the graphs showing the slope line trends and axes labels for each order of reaction:

Eva Zhao 4I

To add on, the rate constant, k, does have different units depending on what order the reaction is. The general rule is as follows for the units of k: 1/(M^n-1*t^-1), where n is the order of the reaction.

Eva Zhao 4I

To add on, the general rule for rate constant units is:
1/(M^n-1*t^-1), where n is the order of the reaction.

Eva Zhao 4I

Here's a general diagram with the given slope trend lines and axes labels for each type of order:

Eva Zhao 4I

Here's a digram with the given slope line trends and axes labels for each reaction order, if it helps:

Eva Zhao 4I

Here is a diagram of the graphs mentioned above, in case you might find it helpful:

Eva Zhao 4I

As a general rule, anything with the ∆_ r is per mole. Regarding ∆G (according to the reply of a similar question), free energy can have the units of kJ or kJ/mol depending on whether the question refers to the molar free energy of a substance or the free energy of a specific amount of the substance...

Eva Zhao 4I

Free energy is the maximum amount of energy from a system that can be used to perform useful work, in other words, work not associated with the expansion of a system.

Eva Zhao 4I

∆G˚ really tells you the thermodynamic favorability of a reaction using the ratio of the amount of product to the amount of reactant at equilibrium. Also, you are correct in the relationship shown by that equation; if ∆G˚is negative, then K>1 and it may be important to note that E˚ is positive.

Eva Zhao 4I

Gibbs free energy, ΔG, is the maximum amount of energy from a system that can be used to perform useful work, in other words, work not associated with the expansion of a system.

Eva Zhao 4I

To add on, as a general rule, H is +1, O is generally -2 (except for peroxides), and halogens (i.e. F) are typically -1.

Eva Zhao 4I

Yes, you're supposed to use ∆G˚=∆H˚-T∆S˚ with the values provided for ∆H˚ and ∆S˚ within that table. The purpose here is to show that the calculated answer should be pretty close to the actual ∆G˚ provided in that table, but it will likely be off by a few decimals.

Eva Zhao 4I

As a general rule of thumb, try to write out the units as you're working through the problem since problems often give you a value in kJ, but the constants you use involve J. Otherwise unless the problem specifies, you can leave your answer in J or kJ. The answer key likely leaves the answer in kJ w...

Eva Zhao 4I

Just to list a few more, H₂, Cl₂, Br₂, and C(s, graphite) also have enthalpies of formation equal to 0.

Eva Zhao 4I

∆G˚=0 when products and reactants are in their standard state and K=1. Note that this is not common.

Eva Zhao 4I

To add on, the relationship between C_v and C_p is C_p = C_v + 1. The values for C_v are shown below (know that we look at the ideal gas values):

Eva Zhao 4I

To add on, the equation we often use to relate the two is ∆U = q + w. At constant pressure, ∆H = q_p. Thus, we can rewrite the equation as ∆U = ∆H - P∆V when pressure is constant.

Eva Zhao 4I

Something to note is that the work a system can do is greatest in a reversible process, compared to an irreversible process. This is because a reversible process is one that can be reversed by an infinitely small change in a variable, i.e. pressure such that when a gas expands reversibly, the extern...

Eva Zhao 4I

Here's a table that relates the three fairly well, with respect to T in order to determine spontaneity:

Eva Zhao 4I

Something to note is that the work a system can do is greatest in a reversible process, hence the larger area under the curve for the reversible graph. The graphs are attached to this for your convenience. http://chemwiki.ucdavis.edu/@api/deki/files/9998/workdone.JPG?size=bestfit&width=454&h...

Eva Zhao 4I

For this problem, we're using the equation: ∆G˚ = ∆H˚ - T∆S˚. The key point to keep in mind here is that the reaction is spontaneous when ∆G˚ is negative. Thus, we're trying to find the temperature, T, at which T∆S˚ > ∆H˚ such that ∆G˚ is negative. To do so, we can set ∆G˚ = 0 to find the minimum te...

Eva Zhao 4I

To add on, the Boltzmann's constant, k = 1.38 x 10^–23 J·K^-1.

Eva Zhao 4I

As others have said, check for whether the question is asking for ∆S or total entropy. That being said, a useful chart to check for spontaneity with ∆S, provided ∆H is given, is (through the use of ∆G = ∆H - T∆S): https://s3mn.mnimgs.com/img/shared/discuss_editlive/1643930/2012_01_23_14_07_42/FOR_2....

Eva Zhao 4I

To add on, the only time when a molecule would have no entropy is when you have a molecule at 0 K that’s perfectly ordered. Otherwise, molecules at 0 K would still have entropy due to the concept of residual entropy, which is explained by others above.

Eva Zhao 4I

To add on, I believe the 3/2 in your equation came from 'Ideal gas, C_v = (3/2) R' (which can be found on the formula sheet). Note that this 'C' value varies depending on what you're trying to solve for, i.e. 'Ideal gas, C_p = (5/2) R'.

Eva Zhao 4I

To clarify, adding or removing the amount of substance in a system correlates to changing the energy of an open system. Heating/cooling the system or doing work on the system are examples provided in class for a closed system.

Eva Zhao 4I

No, in an isolated system there can be no exchange of energy with the surrounding. The combustion of glucose in a bomb calorimeter is an example. To clarify, a closed system can have energy exchange with surroundings by heating/cooling or compression/expansion.

Eva Zhao 4I

In a closed system, energy can be exchanged with surroundings, unlike an isolated system in which nothing can be exchanged with the surroundings. The energy of a closed system can be changed by heating/cooling or compression/expansion (i.e. compressing a piston).

Eva Zhao 4I

Just to add on, other examples of closed systems that can be found in the textbook (4A.1) are:

1) A coolant in a refrigerator coil
2) Mercury in a thermometer

Eva Zhao 4I

An example of a closed system (energy can exchange with surroundings) is a sealed beaker of water, in which the beaker does not insulate. An example of an isolated system (nothing exchanges with surroundings) is the combustion of glucose in a bomb calorimeter. These are the examples provided in clas...

Eva Zhao 4I

For evaporation to occur, the temperature does not necessarily need to reach the boiling point. One thing to keep in mind is that evaporation is a natural process (boiling usually isn't) and while similar to boiling, the water can change from liquid to vapor form when there is an increase in either ...

Eva Zhao 4I

To help clarify with the help of the water heating curve, you can see in the diagram that it takes much more energy for liquid water to transition to steam than for ice to transition to liquid water. Thus, steam at 100ºC holds more energy than liquid water at 100ºC, causing worse burns. https://chem...

Eva Zhao 4I

I found this diagram online which will hopefully help clarify things. You can see that it takes some time before the liquid is fully vaporized.

Eva Zhao 4I

Showing the actual diagram, which is the heating curve of water, you can see that it takes a lot more energy to transition from liquid to vapor than it does from solid to liquid. Therefore, steam carries much more heat for worse burns than boiling water. https://chem.libretexts.org/@api/deki/files/6...

Eva Zhao 4I

For a more visual representation through the heating curve of water, you can see that transitioning from liquid to vapor requires a lot more energy than it does from solid to liquid. Thus, steam carries more heat than boiled water. https://chem.libretexts.org/@api/deki/files/61002/6c6b33a839c7613ad3...

Eva Zhao 4I

Q and K are calculated essentially the same way. However, Q expresses the relative ratio of products to reactants at a given instant not necessarily at equilibrium, which is why you can compare Q and K to determine the direction of a reaction. The reaction shifts right if Q<K, shifts left if Q>K, an...

Eva Zhao 4I

My TA said bar would be the most likely unit used. However, it's always a good idea to check your units and ensure the correct cancelation.

Eva Zhao 4I

Q expresses the relative ratio of products to reactants at a given instant not necessarily at equilibrium, which is why you compare Q and K to determine the direction of the reaction. The reaction shifts right if Q<K, shifts left if Q>K, and is at equilibrium when Q=K.

Eva Zhao 4I

Q expresses the relative ratio of products to reactants at a given instant not necessarily at equilibrium, which is why you can compare Q with K to determine the direction of the reaction. The reaction shifts right if Q<K, shifts left if Q>K, and is at equilibrium is Q=K.

Eva Zhao 4I

K_p can only be used when reactants and products are gases. K_c can be used for gases or aqueous substances, but you should check the units involved to ensure you're using the right K.

Eva Zhao 4I

Just to add on, when using PV=nRT, you can rearrange the formula as such: P=(n/V)RT. (n/V) then represents the concentration, mol/L, and using the constant R and the given temperature, you can convert between partial pressure and concentration.

Eva Zhao 4I

Solids have an essentially constant concentration because they are practically incompressible. As a result, it takes enormous pressure to cause even a tiny reduction in volume. As such, solids can be excluded from the equilibrium constant.

Eva Zhao 4I

Liquids and solids have an essentially constant concentration because they are practically incompressible. As a result, it takes enormous pressure to cause even a tiny reduction in volume. As such liquids and solids can be excluded from the equilibrium constant.

Eva Zhao 4I

Q expresses the relative ratio of products to reactants at a given instant not necessarily at equilibrium. This is mainly useful to compare to K in order to determine the direction of the reaction. When Q=K, the system is at equilibrium.

Eva Zhao 4I

Liquids and solids have an essentially constant concentration because they are practically incompressible. As a result, it takes enormous pressure to cause even a tiny reduction in volume. As such liquids and solids can be excluded from the equilibrium constant.

Eva Zhao 4I

To add on, since Q expresses the relative ratio of products to reactants at a given instant not necessarily at equilibrium, you know the reaction shifts right if Q<K and shifts left if Q>K.

Eva Zhao 4I

Yes, by PV=nRT, pressure (P) can be increased by increasing the temperature (T) or increasing the moles (n) of the substance involved (assuming no other variables are changed for simplicity's sake).

Eva Zhao 4I

As others have mentioned, an ideal gas is theoretical. Most gases do behave closer to ideal with high temperature and low pressure, however. For most problems, unless stated otherwise, ideal gases should be the ones involved such that we can simplify calculations and use i.e. PV=nRT (ideal gas law).

Eva Zhao 4I

Ideal gases have negligible volume, no attractive or repulsive forces, random movement, and perfectly elastic collisions. Most gases behave close to the ideal with high temperature and low pressure; hence an ideal gas is theoretical since PV=nRT shows that pressure typically increases with temperatu...

Eva Zhao 4I

R is the universal gas constant, sometimes known as the Regnault constant. The value of the R constant is 8.3144598 J/mol·K.

Eva Zhao 4I

Dear Dr. Lavelle, Thank you for an incredible winter quarter! I really appreciate all of the resources you provide on your website and the many, many hours of review and study sessions you offer. You definitely help to build a stable foundation for future courses in chemistry. With that, I look forw...

Eva Zhao 4I

The Lewis definition is more consistent in classifying most acids-bases, since a molecule might not have hydrogen to donate for the definition of a Bronsted acid. However, my TA said, at least for now, the Bronsted definition will be primarily used (i.e. for writing chemical equations in acid-base r...

Eva Zhao 4I

Hopefully, this will help you visualize the acid-base pairings in a reaction (if you're a visual learner like me):

Eva Zhao 4I

Just to add on, if you think of a Bronsted acid as donating a proton, it's consistent with the idea of receiving electrons by definition of a Lewis acid (donates H + , gets e- back). Apparently, the Lewis definitions are more consistent for most acids/bases, especially for those without hydrogen whe...

Eva Zhao 4I

I agree! Knowing strong acids and bases would be very helpful in writing out the equation or for calculations, especially for our current level of chemistry which focuses mainly on strong acids and bases. Table 6C.3 in the textbook, page 464, might be helpful.

Eva Zhao 4I

To add on, if the problem asks for the oxidation number of the metal, use:
(# of metal ion)*(oxidation number) + $\sum$ (# if each ligand)*(charge of each ligand) = charge of ion

The charge of the ion should be given, allowing you to isolate and solve for the oxidation number of the metal.

Eva Zhao 4I

Strong acids almost completely ionize in solution. HCl, which dissociates completely, is a strong acid. Therefore, 0.1 M HCl(aq) implies 0.1 M H₃O⁺(aq) and 0.1 M Cl^-(aq).

Eva Zhao 4I

A Bronsted acid is a proton donor (i.e. HCl, HBr), while a Lewis acid is an electron acceptor (i.e. BF₃, H⁺).

Eva Zhao 4I

Bronsted acids are proton donors. I believe that H₂SO₃ is a Bronsted acid because it can donate a proton to become HSO₃^-, its conjugate base.

Eva Zhao 4I

By definition, Bronsted acids are proton donors. HBr is a Bronsted acid because it can donate its H⁺, to which it would become Br^-.

Eva Zhao 4I

To add on, a good rule of thumb is that if the structure is symmetrical, it's typically nonpolar, and if the structure is asymmetrical, it's typically polar. You may have to look at electronegativity differences and dipole-dipole interactions to check.

Eva Zhao 4I

Bent and angular refer to the same shape with VSEPR formulas AX₂E or AX₂E₂.

Eva Zhao 4I

To add on, sometimes you can tell just by looking at the formula where it may be obvious that hydrogen bonds are present, etc. Often times, it's better to draw out the structure to be sure, during which looking at the symmetry of the structure helps to determine whether or not it's polar/nonpolar. N...

Eva Zhao 4I

To add on, typically speaking, a molecule with a tetrahedral shape is only nonpolar if all four atoms bonded to the central atom are the same.

Eva Zhao 4I

Referencing back to the seesaw shape, the lone pair in the equatorial plane is removed first to minimize repulsion. By doing so, the more repulsive lone pair interacts with only 2 bonds at 90° instead of three bonds if the axial lone pair was removed. Hence, removing the lone pair in the equatorial ...

Eva Zhao 4I

VSEPR stands for Valence-Shell Electron-Pair Repulsion. The VSEPR Model explains the experimentally observed shape of molecules and can predict distortions qualitatively but not quantitatively. It may be helpful to memorize the shapes and general angles, though you probably don't need to memorize ex...

Eva Zhao 4I

Lewis structures are 2-D representations of molecular shape and indicate the approximate location of bonding e- and lone pair e-. The VSEPR Model, or the Valence-Shell Electron-Pair Repulsion Model, explains the experimentally observed shape of molecules. Note that the VSEPR Model can predict distor...

Eva Zhao 4I

If you're a visual learner like I am, here's the image for SF₄, AX₄E with 4 bonding pairs and 1 lone pair:

Eva Zhao 4I

Like what others have said, it will probably be helpful to memorize the angles. As a side note, trigonal planar angles are 120 degrees. The 107 degrees was for, i.e. NH₃, a molecule with 1 lone pair and 3 bonding pairs.

Eva Zhao 4I

While not always the case, symmetrical shapes typically indicate the molecule is nonpolar; there are dipole-dipole interactions, but they cancel out for a nonpolar molecule. Asymmetrical shapes typically indicate the molecular is polar since the dipole-dipole interactions can't cancel out.

Eva Zhao 4I

Just to add on, for the more in-between electronegativity differences, where it's uncertain whether the bonds are covalent or ionic, it's based on properties. For example, if the substance can dissociate in water, it may be more ionic in character.

Eva Zhao 4I

m_s is usually pretty arbitrary unless the question specifies that the electron is +1/2 or -1/2. The most important thing to remember is that no two electrons in an atom can have the exact same four quantum numbers, so if one electron is +1/2, the other in the pair is -1/2.

Eva Zhao 4I

n = Principle Quantum Number l = Angular Momentum Quantum Number m l = Magnetic Quantum Number m s = Spin Magnetic Quantum Number n defines l and m l and as such is the principle quantum number. For l, I tend to think of it as the odd one out; it's not the principle and it's not magnetic so it must ...

Eva Zhao 4I

As mentioned, m_s can be either +1/2 or -1/2, and it's pretty arbitrary unless the question specifically provides the spin. The most important thing to note is that no two electrons in the same atom can have the exact same four quantum numbers.

Eva Zhao 4I

To add on, the angular momentum quantum number (l), or the secondary quantum number, describes the shape of the orbital that an electron occupies with allowed values of n-1.

Eva Zhao 4I

The Angular Momentum Quantum Number (l) describes shape by corresponding with a certain sub-shell, each of which with a unique arrangement. I hope this diagram will help you better picture what the second quantum number describes; note that not all possible variations of each sub-shell are shown in ...

Eva Zhao 4I

Magnetic Quantum Number (m l ) labels different orbitals or sub-shells with allowed values of l, l-1,..., 0,..., -l. For example: l=2; m l can be -2, -1, 0, 1, 2 Spin Magnetic Quantum Number (m s ) denotes the spin of an electron with values of +1/2 or -1/2, depending on if the electron is spin up o...

Eva Zhao 4I

As said previously, the spin magnetic quantum number (m_s) can be +1/2 or -1/2. It's usually pretty arbitrary, so long as you're aware that no two electrons in the same atom have the same four quantum numbers.

Eva Zhao 4I

You would only say 5 if the question asks for the number of possible m_l values. Otherwise, given that l=2, you can't say that 5 is a possible value of m_l since the allowed values are l, l-1,..., 0,..., -l.

Eva Zhao 4I

The spin number, or the spin magnetic quantum number (m s ), is +1/2 or -1/2 since an electron can be spin up or down. The significance is that no two electrons in the same atom can have the same four quantum numbers. It tends to be pretty arbitrary as long as the combination of the four quantum num...

Eva Zhao 4I

Here's a visual aid for Hund's Rule, which may help you understand a bit better (if you're visual learner like I am):

Eva Zhao 4I

An important thing to note with the inclusion of the spin quantum number is that no two electrons in the same atom can have exactly the same four quantum numbers. The spins are indicated, as everyone said, by +1/2 or -1/2 since the electron can spin up or down.

Eva Zhao 4I

The angular momentum quantum number (l) describes "shape" and is also known as the sub-shell. The allowed values are l = 0,1,2,...,n-1. Note that n is the principle quantum number which determines energy and size, or shells.

l = 0 s-orbital
l = 1 p-orbital
l = 2 d-orbital
l = 3 f-orbital

Eva Zhao 4I

Hello! Just to add on a little more information, certain energy formulas will not work for photons due to photons having zero mass. E=mc^2, for example, will not work for the photon as it would for an electron; photons have energy, yet the formula E=mc^2 would result in an answer of 0 J for the phot...

Eva Zhao 4I

Just to add on, destructive interference occurs when waves come together in a way that they cancel each other out. While usually in between for a result of smaller amplitude, destructive interference can produce zero amplitude in the right conditions. Note that when two waves interfere destructively...

Eva Zhao 4I

Just to add on: the work function is the energy needed to remove an electron, also known as the threshold energy. The unit for energy is J (joule); as such, the unit for work function is J.

Eva Zhao 4I

The SI units for Joules (J) are kg * m^2 * s^-2. Not sure if this will be the best method to help you, but my TA explained how he remembers the SI units for Joule: W = F * d; where W is work (unit is J), F is force, and d is distance = [m(a)] * d; since F=ma = [kg(m*s^-2)] * m; where kg is SI unit f...

Eva Zhao 4I

I agree with the previous post! Just to make it clearer (if you're a visual learner like I am): \frac{2.26*10^-46 m}{1} * \frac{1 nm}{1*10^-9 m} Since meters (m) is in both the numerator and denominator, you can cross them out. Then you'll be left with nanometers (nm), the unit you want. Again, conv...

Eva Zhao 4I

Just to add on to previous replies, the spin magnetic quantum number was deemed necessary after the Stern and Gerlach Experiment, where the electrons were found to have two different spins (hence +1/2 or -1/2). It's important to note that no two electrons in the same atom have the same four quantum ...

Eva Zhao 4I

Just to add on, the Balmer Series corresponds to visible light, n1=2; Lyman series corresponds to ultraviolet light, n1=1. This is good to know for some problems that just provide what type of electromagnetic radiation is involved, i.e. 1A.15.

Eva Zhao 4I

The unit for joules (J) in SI units is kg⋅m^2⋅s^−2. Like what others have said, meters and joules are units of measure for different criteria; meters are units of length and joules are units of energy. A possible equation that relates the two is: Kinetic energy = 1/2 * mass * velocity ^2.

Eva Zhao 4I

To keep your work as accurate as possible, try not to round until the final answer. On the basis that you do round, keeping a certain number of significant figures should be enough to result in the final answer. For example, if the answer needs to have two significant figures, keeping four significa...

Eva Zhao 4I

Formula units work the same way as do molecules and atoms through Avogadro's number. According to the book, the key difference is that formula units pertain to ionic compounds, whereas molecules are of molecular compounds and atoms are of elements. If you want to check it out yourself, the informati...

Eva Zhao 4I

Just to add onto the topic of combustion, O2 (oxygen gas) is reacted with the given fuel for the products of CO2 (carbon dioxide) and H2O (water). Depending on the fuel, you may need to balance the equation.

Eva Zhao 4I

There is a list of practice problems, for fundamentals and further topics, on the class syllabus. Just scroll down to the last pages.

Here's the link: https://lavelle.chem.ucla.edu/wp-conten ... SYLL_1.pdf

Eva Zhao 4I

Thanks!

Eva Zhao 4I

Avogadro's number, 6.022 x 10^23, represents the number of "particles" within one mole of a substance. These particles can be, for example, the number of atoms per mole of a given compound.

Eva Zhao 4I

It's best to only round at the end of the calculations. If you round throughout the series of calculations, the answer could be different depending on the number of significant figures needed. Generally speaking, using 7 instead of 6.94 for Lithium can likely get you the right answer for say multipl...

Eva Zhao 4I

Theoretical yield is the amount of product that can be obtained if a chemical reaction has 100% efficiency, the maximum amount of yield possible. Actual yield is the amount of product actually produced by the reaction. Due to side reactions, impurities, some of the product sticking on to the sides o...

Eva Zhao 4I

E.16 The molar mass of the metal oxide M2O is 231.74 g.mol^-1. What is the molar mass of the chloride of this metal?

For this question, what exactly is the chloride within M2O, and how would you go about finding the molar mass of said chloride?

Search found 101 matches