CHEMISTRY COMMUNITY

Justin Quan 4I

Veronica Lu 2H wrote:when using PV=nRT is the temperature supposed to be celsius or kelvin?

To add on, you use kelvin especially since the constant R is in kelvin, not celsius. However, if you use PV=nR∆T, the you may use either celsius or kelvin since the change in temperature would be the same.

Justin Quan 4I

If E˚ is positive, then K>1; if E˚ is negative, then K<1. This will always hold true. It may also be important to note that whenever E˚ is positive, then ∆G˚ is negative, and vise versa.

Justin Quan 4I

A more general rule for the units of k is: 1/M^1-nt^-1 (n is the order of the reaction)

Justin Quan 4I

To add on, the general rate law is a differential equation that describes the change in concentration of reactant over change in time. By using calculus, you can integrate the general rate laws to get an integrated rate equation that relates concentrations with time directly.

Justin Quan 4I

Here's a diagram depicting the graphs of the three rate laws, zeroth, first, and second order reactions.

Justin Quan 4I

To add on, for most electrochemistry problems, you will have to use some form of ∆G = -nFE˚. However, you might have to relate the equilibrium constant K to the system of a electrochemical cell, in which use ∆G = -RTlnK.

Justin Quan 4I

To add on, you can tell that free energy, or ∆G, depends on the equilibrium constant K by observing the equation ∆G˚= -RTlnK. Also, the equilibrium constant will change if the pressure changes, thus ∆G depends on pressure as well.

Justin Quan 4I

The total amount of work a system can do is determine by the change in its free energy, or ∆G. When free energy decrease, or when ∆G is negative, the reaction is spontaneous and work is done during the process.

Justin Quan 4I

To add on, the amount of work a system can do is determined by the total change in free energy. This relates to cell potential because in an electrochemical cell, the system converts chemical energy to electrical energy based on the system's cell potential. The about of work and free energy change d...

Justin Quan 4I

Since the instantaneous rate of change is the rate of change at a particular point or instant in time. This is better than average rate of change because an instantaneous rate will give you the rate of the reaction at that exact instant in the reaction.

Justin Quan 4I

To add on, you can figure out when ∆G = 0 by using the mathematical relationships between ∆G˚= - RTlnK and ∆G = ∆G˚ + RTlnQ. We know that at equilibrium Q=K. If we substitute ∆G˚out, then ∆G = RTlnQ - RTlnK, thus ∆G = RTlnQ - RTlnK = RTlnK - RTlnK = 0 when the reaction is at equilibrium.

Justin Quan 4I

To show why ∆G=0 at equilibrium from a mathematical perspective, use ∆G˚= - RTlnK and ∆G = ∆G˚ + RTlnQ. If we substitute ∆G˚ out, then ∆G = RTlnQ - RTlnK. When the reaction is at equilibrium, the Q=K, thus ∆G = RTlnQ - RTlnK = RTlnK - RTlnK = 0.

Justin Quan 4I

The equation modeling the relationship between work and cell potential is ∆G = - nFE˚_cell, where n is the number of moles transferred, F is faraday constant, and E is the work/charge of the cell measured in volts.

Justin Quan 4I

In a battery, the battery pumps electrons away from the anode, thus making it positive, and towards the anode, making the cation positive. Since an electrolyte is a substance that dissolves in water and creates an electrically conducting solution, the electrolyte helps conduct the current needed to ...

Justin Quan 4I

To add on, an electrolyte is a substance that dissolves in water or any other polar solvent and creates an electrically conducting solution. When the electrolyte dissolves, it spilts into a cation and an anion species.

Justin Quan 4I

To add on, if ∆G is positive, then the forward reaction is non-spontaneous. Whether or not a reaction is endothermic or exothermic is dependent on ∆H. If you were giving ∆G & ∆S, then you can use the equation ∆G = ∆H - T∆S to predict if the reaction is endothermic or exothermic. However, just kn...

Justin Quan 4I

To add on, ∆G = ∆H - T∆S is very useful for predicting if a given reaction is spontaneous or non-spontaneous. When ∆G=0 the system is at equilibrium. When ∆G is negative, the forward reaction is spontaneous. When ∆G is positive, the reverse reaction is spontaneous. The sign of ∆G depends on the enth...

Justin Quan 4I

To add on, when ∆G=0 the system is at equilibrium. When ∆G is negative, the forward reaction is spontaneous. When ∆G is positive, the reverse reaction is spontaneous The sign of ∆G depends on the enthalpy, entropy, and temperature. The table below shows you the relationship between these variables a...

Justin Quan 4I

https://www.brainkart.com/media/extra3/Rtg80Qw.jpg You use the Van 't Hoff equation why you are asked to relate the change in the equilibrium constant, K, of a chemical reaction to the change in temperature, T. The question will likely give you the standard enthalpy change, ΔH, and ask for K, or vi...

Justin Quan 4I

Just to add on, a reversible reaction is a reaction where the reactants form products that will also react together to give the reactants in reverse. If heat is absorbed in the forward process, the same heat will be released out in the reverse process. Also, if work is done by the system in the forw...

Justin Quan 4I

When ∆G is negative, the reaction is spontaneous. When ∆G is positive, the reaction is non-spontaneous. The picture above helps organize how Gibbs free energy changes under different conditions.

Justin Quan 4I

What is the significance of whether or not a reaction is spontaneous? How do we apply this is real-world situations? If a reaction is spontaneous, then the reaction will proceed without intervention. This applies to real-world situations since figuring out if a reaction will proceed or not is impor...

Justin Quan 4I

To add on, a reversible reaction is a reaction where the reactants form products that will also react together to give the reactants in reverse. Furthermore, if heat is absorbed in the forward process, the same heat will be released out in the reverse process. Similarly, if work is done by the syste...

Justin Quan 4I

To add on, ∆U=3/2 nRT comes from the equation ∆U = q + w. Under constant volume conditions, w=0, thus ∆U = q. Since q = n C ∆T and C_v=(3/2) R, we get the equation ∆U=3/2 nRT.

Justin Quan 4I

To add on, ∆U= 0 during an isothermal reaction where temperature is constant. In an isothermal reaction the heat absorbed is equal to the work done on the surroundings.

Justin Quan 4I

To add on, an open system is a system that can exchange both matter and energy with the surroundings. The engine is thus an open system because it takes in matter in the form of gasoline and releases energy in the form of hear to the surroundings.

Justin Quan 4I

To add on, extensive properties are properties that depend in the amount of matter in the substance. For example, the weight, volume, size, and mass are all examples of extensive properties.

Justin Quan 4I

To add on, you usually use q = CΔT when solving a problem with a bomb calorimeter where the mass is not necessary to solve for a value.

Justin Quan 4I

In an isolated system, the system cannot exchange matter nor energy with the surroundings. So basically an isolated system is usually sealed, rigid, and has thermally insulating walls such as a thermos container. The image below visually shows the difference between the three types of thermodynamic ...

Justin Quan 4I

The three different types of thermodynamic systems are an open system, a closed system and an isolated system. Here's an image to help visualize each type of system.

Justin Quan 4I

To add on, the reaction enthalpy is the heat given off or taken up for the reaction; hence, the enthalpy difference between the reactants and products. The enthalpy of formation is formation of a compound from its elements in their most stable states: temperature and pressure.

Justin Quan 4I

KDang_1D wrote:What is heat capacity?

Heat capacity is amount of heat to be supplied to produce a unit change in its temperature. For common species such as water, it's better to be know the value, which is 4.184 Joules.

Justin Quan 4I

Natalie Benitez 1E wrote:How do we tell with enthalpy that a reaction is exothermic or endothermic?

If enthalpy is negative, then heat is lost or released from the system and the reaction is exothermic. If enthalpy is positive, then heat is absorbed or gained into the system and the reaction is endothermic.

Justin Quan 4I

https://s3-us-west-2.amazonaws.com/courses-images/wp-content/uploads/sites/752/2016/09/26195026/ating-20curve-20of-20water.jpeg To clarify, when steam comes into contact with a cool surface such as your skin, it will undergo a phase change from vapor to liquid. As the steam is changing from gas to ...

Justin Quan 4I

Firstly, a state function is a property whose value does not depend on the path taken to reach that specific value. In other words, state functions are variables that define a system. Heat is not a state function because heat is amount of thermal energy that enters or leaves the system during a proc...

Justin Quan 4I

To add on, since Q is the relative ratio of products to reactants at a given instant, you know the reaction shifts right if Q<K and shifts left if Q>K.When Q=K, the reaction is at equilibrium. To summarize, finding Q helps you determine what direction the reaction will proceed based on the ratio bet...

Justin Quan 4I

To summarize, Le Chatelier's is a set of principles or guidelines that predicts the effect of a change to a system under equilibrium conditions. It basically states that when a system experiences a disturbance, it will respond to restore a new equilibrium state. Factors that would change equilibrium...

Justin Quan 4I

To add on, generally the rule is that a gas behaves more like an ideal gas at higher temperature and lower pressure. This is because under these conditions, the gas molecules will have little to no intermolecular forces with each other. So it is possible to achieve ideal gas behavior under extreme c...

Justin Quan 4I

To add on, you use Kp when the reactants and products are gases. If your reaction has gases and/or aqueous substances, then use Kc. However, if you're still having trouble deciding, you should check the units to ensure you're using the right K, for example Barr (pressure) vs M (concentration).

Justin Quan 4I

To add on, Le Chatelier's Principle is a set of principles or guidelines that predicts the effect of a change to a system under equilibrium conditions. You use Le Chatelier's Principle when a reaction at equilibrium is disturbed. Factors that would change equilibrium include changes in temperature, ...

Justin Quan 4I

The equilibrium constant K tells you the ratio of products to reactants. If K is a large number, it means that the equilibrium concentration of the products is large. Comparing K to Q will also help you decide which direction the reaction will proceed.

Justin Quan 4I

To add on, for a homogeneous aqueous mixture, we can simply include all the aqueous species when calculating K or Q. However, if there is a heterogeneous equilibria, chances are there is a solid or liquid species in solution. Hence, you will have to be careful to exclude all liquid and solid species...

Justin Quan 4I

Can someone clarify how taking out some of the product can increase the amount of product made without considering temperature or additional input? To clarify, when you remove some of the product, the concentration of the product will decrease, while the amount of reactants will stay the same. Thus...

Justin Quan 4I

To add on, since Q is the relative ratio of products to reactants at a given instant, you know the reaction shifts right if Q<K and shifts left if Q>K.When Q=K, the reaction is at equilibrium. To summarize, finding Q helps you determine what direction the reaction will proceed based on the ratio bet...

Justin Quan 4I

Here's a general set up for the ICE table. Hope this helps.

Justin Quan 4I

To add on, since Q is the relative ratio of products to reactants at a given instant, you know the reaction shifts right if Q<K and shifts left if Q>K.When Q=K, the reaction is at equilibrium.

Justin Quan 4I

In addition, if K=Q, the reaction is in equilibrium because the ratio of products to reactants is equal to the equilibrium constant.

Justin Quan 4I

To add on, for a homogeneous gas mixture, we can simply include all the gas species when calculating K or Q. However, if there is a heterogeneous equilibria, chances are there is a solid or liquid species. Hence, you will have to be careful to exclude all liquid and solid species when calculating K ...

Justin Quan 4I

To add on, generally the rule is that a gas behaves more like an ideal gas at higher temperature and lower pressure. This is because under these conditions, the gas molecules will have little to no intermolecular forces with each other.

Justin Quan 4I

Q is the reaction quotient. If Q>K, then the reaction will proceed backwards (reverse reaction), and if Q<K then the reaction will proceed forward. So if Q is greater than K, that means there are more product than reactant, and the reaction will shift left.

Justin Quan 4I

To add on, dipole moments would only cancel if all the surrounding atoms are the same, ex: CH4 is non polar.

Justin Quan 4I

Lone pairs are not drawn in with the molecular shape, however, they are needed to figure out the exact molecular shape. Lone pairs usually only cancel each other out when the molecular shape is linear.

Justin Quan 4I

To add on, the total number of hydrogen bonding sites is equal to the number of lone pairs on electronegative atoms like N, O, F, plus the number of positively charged hydrogen atoms

Justin Quan 4I

What is the biological significance of EDTA and how does it's structure help with this function?

Justin Quan 4I

What are sigma & pi bond's role in structure and shape in organic and biological molecules?

Justin Quan 4I

To add on, you should count the number of lone pairs on N, O, F atoms, plus the number of positively charged hydrogen atoms. You can tell if a hydrogen atom is positivity charged if it is bonding to another atom with a significant difference in electronegativity, like N, O, or F.

Justin Quan 4I

Andrew Liang 4G wrote:Is there are way to identify a strong/weak acid or base by just looking at the compound?

Personally, I think it is easier to memorize the strong acids and strong bases, then assume all other acids and bases are weak.

Justin Quan 4I

Kristina Rizo 3K wrote:Don't they cancel each other out when the molecule is symmetrical?

To add on, dipole moments will cancel out when the molecule is symmetrical and the surrounding atoms are identical. For example CH4 and CH3Cl are both tetrahedral, but CH3Cl is polar while CH4 is non-polar.

Justin Quan 4I

For this question, you had to be able to identify that all the single bonds had a sigma bond in it and that all the double bonds that a sigma bond and a pi bond in it. When counting the number of hydrogen bonding sites you need to include the H-bonds already present and the lone pairs on N,O, or F....

Justin Quan 4I

Intramolecular forces are the forces holding atoms together within a molecule, while intermolecular forces are forces that exist between molecules. Some examples of intermolecular forces include hydrogen bonds, dipole-dipole, induced dipole-dipole, and London dispersion forces. Intramolecular forces...

Justin Quan 4I

To add on, a Cis- molecule is a polar molecule that has polar bonds with dipoles that do not cancel, while a Trans- molecule must have zero electric dipole moment which is possible if it contains non -polar bonds, or polar bonds with dipoles that cancel.

Justin Quan 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. This is because different atoms have different electronegativity. Thus, each atom surrounding the central atom will have a slightly different attraction to ...

Justin Quan 4I

Sigma and pi bonds are chemical covalent bonds formed by an overlap in atomic orbitals. These bonds are important because knowing if a bond has sigma, pi, or both bonds helps us determine behavior of molecules in molecular orbital theory.

Justin Quan 4I

To add on, if two molecules have similar or identical dipole-dipole forces, then LDF will have a significant difference on relative boiling points of two molecules. If, for example, the only difference between two molecules is their central atom, just the difference in size between the central atom ...

Justin Quan 4I

To add on, if the atoms surrounding the central atom is not the same, it will have dipole moment because the different surrounding atoms will have various electronegativity, therefore the dipole forces will not completely cancel out.

Justin Quan 4I

Lone pair-lone pair > Lone pair-bonding pair > bonding pair-bonding pair

Lone pair-lone pair have the most repulsion force because of the shape of the electron density. The lone pair e- are more closer together, so they have stronger forces.

Justin Quan 4I

To add on, induced dipoles forms when a polar molecule induces a dipole movement in another atom or a non-polar molecule by disrupting the electron arrangement in that non-polar molecule. This is why it's call induced dipole.

Justin Quan 4I

To add on, a bigger atomic radii would mean they have more electrons and more opportunities to create dipole-dipole forces. The larger the atomic radii basically means larger atomic size, which would therefore create more London-dispersion forces. Since bigger atomic size means stronger intermolecul...

Justin Quan 4I

To add on, non polar molecules have all their electrical charges evenly distributed, therefore there are no dipole-dipole forces. However, since all molecules experience London-dispersion forces, London-dispersion is the only intermolecular forces for non polar molecules.

Justin Quan 4I

Due to the shape of the electron density, lone pairs have greater repulsion force than bonding pairs of electrons. The shape of the orbitals that hold lone pairs is shorter and rounder, much like a balloon, whereas bonding pairs are more elongated, thus they have less repulsion force.

Justin Quan 4I

To explain this break in the periodic trend, we need to take a look at the electron configuration of nitrogen and oxygen. The electron configuration of nitrogen is 1s2 2s2 2p3, which means it has a stable, half-filled 2p shell. Oxygen's electron configuration is 1s2 2s2 2p4, one electron beyond stab...

Justin Quan 4I

To add on, double and triple bonds have shorter bond lengths than single bonds because there are more electrons shared between atoms in a double and triple bond. These additional electrons will have a stronger attraction to the positively charged nucleus, therefore, the distance between the two atom...

Justin Quan 4I

To add on, you can think of dipole moments as vectors, in which the vector points at the direction towards the more electronegative atom. If all the vectors cancels out, then the molecule does not have a dipole moment.

Justin Quan 4I

Due to the fact that elements in the third period elements have a d-orbital shell, they can exceed the octet rule and hold the additional electrons

Justin Quan 4I

To add on, if you are comparing ionic and covalent bonds in air or space, then ionic is stronger. However, since water dissociates ionic bonds, for example salt in water, covalent bonds are stronger in aqueous environments.

Justin Quan 4I

If you see that all the dipole forces cancels out, then the molecule is nonpolar. However, if there is a net dipole moment that's not zero, then you can tell that the molecule is polar.

Justin Quan 4I

To add on, the electronegativity is based on a scale on measurable quantities like electron affinity and ionization energy. While electronegativity itself is not an intrinsic physical property that we can directly measure, it is based off of other measurable quantities.

Justin Quan 4I

Essentially yes, having multiple resonance structures means that there are several different combination of electrons shared between atoms. These electrons that you are "moving" around can go on to create chemical bonds at different sites in the molecule.

Justin Quan 4I

When we draw lewis structures, should we always assume to draw the resonance structures even if the problem didn't say to do so?

Justin Quan 4I

To add on, electron distortion causes the shape of the electron density to change due to the electrons in the anion to attract to the cation. This causes ionic bonds to have covalent bond characteristics because the the electrons will be slightly closer to the cation.

Justin Quan 4I

To add on, comparing the electronegativity between atoms is the general way to tell if what type of bonds the molecule will form. Typically a greater difference in electronegativity would mean an ionic bond will form. A small difference, for example around 0.5, would result in non-polar covalent bon...

Justin Quan 4I

The electron affinity trend increases from left to right and upward for groups because atoms towards the right side of the periodic table have more valance electrons, therefore they want to complete their valance shell by accepting electrons rather than losing them. For example, F and Cl, both on th...

Justin Quan 4I

To add on, electron affinity is basically the likelihood of an atom to gain an electron. Electron affinity increases upward for the groups and from left to right across periods of a periodic table. Hopefully this image helps you visualize the trend. https://upload.wikimedia.org/wikipedia/commons/thu...

Justin Quan 4I

To add on, if you compare ionic bonds and covalent bonds in space, ionic bonds are stronger. However, if you compare the two bonds in water, covalent bonds are stronger because water dissociates ionic bonds. For example, if you put NaCl in water, it dissolves , the Na+ ion and the Cl- ion will separ...

Justin Quan 4I

To add on, Li and Be are metals, and so they are like to lose electrons and become cations. After they lose their electrons they revert to a noble gas configuration. Also hydrogen doesn't need to fill its valance shell when forming bonds. Hydrogen just likes to give away its lone electron.

Justin Quan 4I

If the problem asked us to figure out the sign of the electron spin, and the electron is alone in its orbital, how do we know whether the electron has a positive or negative spin?

Justin Quan 4I

Will there be a difference in our calculations if you used 3.0*10^8 verses 2.998*10^8? I would think using sig figs at the end of our answer our account for any difference in the answers.

Justin Quan 4I

Just to add on, a good trick you may use to better figure out which equations you need is to look at the units. For example, if the problem gives you m/s, you probably have to use c=lambda(v) because c is measured is m/s. Sometimes you need to use several equations to convert the units into the unit...

Justin Quan 4I

I had a bit of trouble with this problem at first, but I looked at the problem on pg.7&8 in the textbook as an example. It goes from n1 and n2 to the wavelength of the emitted radiation. My only trouble in going back was how do we know n1=1? Otherwise it was easy to use the equations c = lambda...

Justin Quan 4I

LNgo 14A wrote:Quanta is the general term for particles or pieces that make up a whole, like how quantum mechanics refers to the mechanisms behind very small objects. Photons are a quantum measurement of light.

Based on this definition,is quanta a smaller unit or measurement than photons?

Justin Quan 4I

Knowing how to convert to angstrom is helpful, however I just wanted to add on that on the last test, the conversation was given, so I wouldn't worry too much about it.

Justin Quan 4I

What type of atoms prefer ionic bonds and what type of atoms prefer covalent bonds? Ionic bonds will form when when an atom transfers its electron to another atom, whereas covalent bonds form when the electrons are shared between atoms. Atoms that like to transfer there electrons are usually anions...

Justin Quan 4I

I found that the most efficient method is to start with the largest or most complex molecule. For example, since Cu is alone, I would balance that last.

Justin Quan 4I

Why is Avogradro's number based on Carbon 12? Chemists chose to use Carbon 12 because Carbon 12 has 6 protons, 6 neutrons, and 6 electrons.This is interesting because the mass of an electron is about 1/12th the mass of Carbon 12. Chemists also found that when you have Avogadro's number of Carbon 12...

Justin Quan 4I

I'm still a little confused as to when I should use scientific notation or not. For example, the textbook solution for Fundamentals E.23 part a is 0.0134 mol Cu^2+, while the solution for E.23 part b is 8.74x10^-3. Why did they decide not to use scientific notation for part a? Using scientific nota...

Justin Quan 4I

Is light transferring energy to the electrons through photons?

Justin Quan 4I

For this problem, you do not use M1V1=M2V2 because there’s no initial or final molarity or volume. It’s simply just moles/(total volume). The total volume being 125ml.

Justin Quan 4I

Does anyone know if the TAs will deduct points on the first test for wrong sig figs?

Justin Quan 4I

Montana James 4G wrote:What is an angstrom and do we need to know it for test?

Luckily for us, if you refer to the bottom of the formula sheet, it’s showed that angstrom was equal to 1*10^-10m.

Justin Quan 4I

In space or air, ionic bonds are much stronger than covalent bonds. However, since molecules with ionic bonds can be dissolved in water, covalent bonds are stronger in water than ionic bonds.

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