CHEMISTRY COMMUNITY

Shawn Patel 1I

Hey,
For this question, you would know to find the equations of combustion because the given enthalpies are of combustion. So you would find the combustion reactions, then balance those, and then you could use Hess's Law to find the final reaction enthalpy.

Shawn Patel 1I

Hey, For the cathode, the oxidizing agent is the reactant that gains the electrons. For the anode, the reducing agent is going to be on the left hand side of the reaction, opposite the side that gains the electrons. So for example, in test question 6, H+ gains the electrons so it is the oxidizing ag...

Shawn Patel 1I

Hey,
Each pair has either an oxidizing agent and a reducing agent. For the H+ pair, H+ is oxidizing, but it is also the reduced agent. For Ni, it is the reducing agent, but it is also the oxidized agent.

Shawn Patel 1I

Hey,
The basic versions of the integrated rate laws for each reaction order is given during the tests. Any different versions of the laws are just algebraic manipulations that you can perform yourself if needed.

Shawn Patel 1I

Hey,
The slope of the graph is -k. The graph is baed on ln(a) against time.

Shawn Patel 1I

Hey,
The slope of both 0 and 1st order reactions is -k, second order reactions have a positive k slope. Time is graphed against ln(a).

Shawn Patel 1I

Hey,
I would assume heterogenous catalysts would be included in the rate law because its purpose as a catalyst is still the same. It lowers the activation energy, increasing the reaction speed, so the rate would be affected.

Shawn Patel 1I

Hey,
Most of the time we would not include solids and liquids, only gases and aqueous solutions. Water can be included if it is part of the reaction.

Shawn Patel 1I

The catalyst itself is not consumed during the reaction and so it does not interfere with the reactants turning into the products, aside from decreasing activation energy so that the process is sped up.

Shawn Patel 1I

Hey,
A catalyst is accounted for in the rate law because it speeds up the reaction. It isn't considered in a balanced chemical equation because it doesn't undergo any change, nor does it affect any of the reactants in anyway besides increasing the speed of the reaction.

Shawn Patel 1I

Hey,
Normally you could use that half reaction, but the values for the e cell are not given. Also, the question asks if it disproportionates, which means that the element would act as both an oxidating agent and reducing agent, so you would have to use the two half reactions in the solution manual.

Shawn Patel 1I

Hey,
If you can recall from last year, Q can be calculated from both partial pressures and the concentrations of aqueous solutions. Because Q doesn't have any units, partial pressures and concentrations can be combined.

Shawn Patel 1I

Hey,
When trying to find n in the nerst equation, and there is an uneven amount of electrons on each side, you usually have to balance out the equations so that the electrons cancel. The final amount of electrons after you balance is what you use for n.

Shawn Patel 1I

Hey,
As far as I know, this class isn't bruincasted. The other chem14b class might the one that is bruincasted, so if you need information from a lecture, you would probably have to get it from a classmate or a friend.

Shawn Patel 1I

Hey,
Usually for a cell diagram, the anode would be on the left, and the cathode be on the right. If there is a salt bridge in the cell, two lines are drawn between the cathode and anode. We know an element is reduced when it gains electrons. A vertical line is the interface between phases.

Shawn Patel 1I

Hey,
The standard potentials for strongly oxidizing half reactions are usually positive, while the standard potentials for strongly reducing reactions are negative.

Shawn Patel 1I

Hey, The book explains it best on page 274, but basically the molecules of an ideal gas move at the same average speed as it expands, meaning the total kinetic energy is the same, and since there is nothing acting against the molecules, the potential energy remains the same. Because both of these en...

Shawn Patel 1I

Hey,
A change in temperature is associated with the amount of heat energy transferred or absorbed. So even if temperature is constant, work energy can be done, and this would be the change in internal energy.

Shawn Patel 1I

Hey,
I think STP is supposed to be at 0 degrees Celsius or 273.15K

Shawn Patel 1I

Hey,
Usually qrev will be given as a value in J transferred or absorbed as heat.

Shawn Patel 1I

Hey,
In that reaction in a beaker, it is assumed that the beaker is the surrounding because the system (the reaction) is specifically said to be contained inside the beaker. When a system is contained in something, like a piston, that is usually the surrounding.

Shawn Patel 1I

Hey, If J/k mol is given for enthalpy, and there is no value of mol that needs to be accounted for, then the entropy would also be in J/k mol. In some cases, such as when calculating standard reaction entropy, and you are given the amount of moles per product or reactant, then the answer would likel...

Shawn Patel 1I

Hey,
An example of a system doing work would be when a gas is pushing against a piston. When volume and pressure are changing, there can be work being done on the system. If there is a vacuum, work cannot be done because there is nothing to do work against.

Shawn Patel 1I

Hey, You're on the right track. Heat gained by the ice cube is heat lost in the surrounding water. For the ice cube, you have to consider the enthalpy of fusion of the ice, meaning you have to calculate the amount of heat it takes to melt the ice, and then add it to the amount of heat that is gained...

Shawn Patel 1I

Hey,
This means that energy is released in terms of heat.

Shawn Patel 1I

Hey, The question is referring to enthalpy of combustion, so you would create a combustion reaction for each of the reactants and products. Then with the combustion reactions, you would balance them and order them so that if you would add them, you would be left with the original equation. This mean...

Shawn Patel 1I

Hey,
I believe that the value of the standard enthalpy of sublimation of carbon is given as +717 kJ/mol in the problem.

Shawn Patel 1I

Hey,
Table 8.2 uses Joules for specific and molar heat capacity, but it can be converted to kilojoules if heat energy is present in large values.

Shawn Patel 1I

Hey, W is work and Q is heat. Work is motion against an opposing force, and heat is the energy transferred as a result of a temperature difference. When work is done on a system, w is positive, and when work is done by the system, w is negative. If heat enters the system, q is positive, and if heat ...

Shawn Patel 1I

Hey,
Molar heat capacity is usually given by J.K^-1.mol^-1, so I think the book left out the temperature units for 8.43.

Shawn Patel 1I

Hey,
Some of the phase changes that you should know include:
Vaporization: liquid to vapor, Melting: solid to liquid, Sublimation: solid to vapor (Endothermic)
Freezing:liquid to solid, Condensation: gas to liquid, Deposition: gas to solid (Exothermic)

Shawn Patel 1I

Hey, You can find out if there is delocalization in a compound by checking to see if there are any resonance structures. If there are any resonance structures, there is going to be delocalization. Delocalization basically means that when you have a compound with resonance structures, some of the ele...

Shawn Patel 1I

Hey,
The solutions manual doesn't use the approximation, but even if you use the approximation, you will still get the same answer for pH, so its probably okay to use the approximation in this case.

Shawn Patel 1I

Hey,
You use the prefixes bis-, tris-, or tetrakis-, when the ligand is a polydentate.

Shawn Patel 1I

Hey,
Compression will only affect gaseous reactions. Aqueous/liquids and solids in the reaction cannot be compressed, so there would be no change in the equilibrium reaction with molecules in those phases.

Shawn Patel 1I

Hey,
Adding or subtracting water only creates a shift in the reaction when it is in a gas phase. When it is in an aqueous phase, the reaction doesn't change because the water is acting as a solvent on both sides.

Shawn Patel 1I

Hey, For part A, you have the correct reasoning, but on the right side of the reaction, you have I2(s), which is a solid. You only have to check for the most amount of gas moles in the reaction, not including solids or liquids. For part B, when the reaction is exothermic, and the heat is decreased, ...

Shawn Patel 1I

Hey,
In SO4 2-, we see that the sulfur is bonded to four oxygens. This means it has four bonding densities, and therefore the hybridization would be sp3. You would have a tetrahedral shape with 4 atomic orbitals and 4 hybrid orbitals.

Shawn Patel 1I

Hey,
When writing the VSEPR formula, you should count the number of bonding densities or regions instead of counting the number of bonded atoms. The double bond between the C and O is considered to be one bonding region, meaning you will have an AX3 formula.

Shawn Patel 1I

Hey,

Once you draw out the lewis structure, the question is asking you for the hybridization of the B and N atoms. Since there are three bonding regions around both the B and N atoms, the orbitals will be sp^2 hybridized.

Shawn Patel 1I

Hey, In methanol, the CH3 group is attached to OH. Oxygen is more electronegative than the rest of the molecule, so there is a slight negative charge on the OH end of the molecule, and this leaves a slight positive charge on the CH3 group. Hydrocarbons are always non-polar, so there is nothing to ca...

Shawn Patel 1I

Hey, Free radicals affect the shape of other molecules that they interact with. The atom with a free radical does not have a full octet set, so it tries to fix this by stealing an electron from another molecule. When they steal an electron from another molecule, particularly an organic molecule, it ...

Shawn Patel 1I

Hey, The radical electron, or the lone electron, would be placed on the least electronegative atom because the more electronegative atoms in the molecule would rather have the full set. The more electronegative an atom is, the more it pulls away other electrons, so the least electronegative atom is ...

Shawn Patel 1I

Hey, There is a trigonal planar arrangement because there are actually 3 electron densities. There is a lone pair on the chlorine and two oxygens double bonded to it. Because there are 3 electron densities, and one of them is a lone pair, this makes the shape of the molecule angular. The electron pa...

Shawn Patel 1I

Hey, In response to question 61 part b, you have to first count the total number of electrons in the molecule. Once we find that, and then draw the basic lewis structure, we can start adding on electrons to the chlorines. All the chlorines will come out with full valence shells, but we are still lef...

Shawn Patel 1I

Hey, Yes, the method you use is a simple way to figure out if a bond is ionic or not. This trend is because ionic bonds form when there is a nonmetal and a metal reacting, and both of these are usually on opposite sides of the periodic table. Covalent bonds form between nonmetallic elements, which a...

Shawn Patel 1I

Hey,
An atom thats in lower right group would still be smaller than a lot of the atoms on the left that are higher up. This is easily comparable through a chart that shows you the relative size of each atom in the periodic table.
For example, Radon is still slightly smaller than Sodium.

Shawn Patel 1I

A cation is an ion that has a positive charge because it gave away its electron, such as Na+. An anion is an ion that has a negative charge because it took an electron, such as Cl-. It might be helpful to memorize a mnemonic, like that an anion is (A) (N)egative (ION), and a CATion is PAWSitive.

Shawn Patel 1I

Hey, The reason we show the 2px^1 and 2py^1 instead of just writing 2p^2 is to outline the Aufbau principle. The Aufbau principle states that electrons will occupy the lowest energy levels. Instead of having those two electrons forced into 2px, the electrons would rather occupy one orbital each, all...

Shawn Patel 1I

Hey,
I'm pretty sure Dr. Lavelle stated in class that the concept is good to know and understand, but you wouldn't get points marked off if you chose to not label px or py.

Shawn Patel 1I

Hey, The series consist of the lines that are grouped together based on their lower energy principal quantum level. Each series has the same principal quantum number associated with them, so the Lyman series has a lower energy level of n=1. The series are just a convenient way to associate the lines...

Shawn Patel 1I

Hey, Basically, the photoelectric effect states that electrons will be ejected from the surface of a metal when exposed to electromagnetic radiation. However, in order for an electron to be ejected, the radiation has to have a frequency above the threshold value for that particular metal. The photoe...

Shawn Patel 1I

Hey, In part A, we found that the limiting reactant was ClO2. This means that the amount of product that is created is based on the amount of moles of ClO2 present. We have 12 moles of ClO2, and the ratio of ClO2 to Br2 is 6:1. So that means for every 6 moles of ClO2 there is, 1 mole of Br2 can be m...

Shawn Patel 1I

Hey, You have all your numbers correct, but I think the reason you aren't getting the right answer is because the ratio of the oxygen to the product, phosphorus(V) oxide is 2:1. This means that when calculating for the final product, you have to divide the moles of oxygen remaining by two. This shou...

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