CHEMISTRY COMMUNITY

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Can someone please explain how to interpret a reaction profile graph for reactions with multi-step mechanisms? How can you determine which step of a mechanism is the fastest from the graph? Why do the dips represent the formation of intermediates? Why do the number of peaks represent the number of s...

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Problem 7.29 in the seventh edition textbook asks us to determine whether the two steps in a suggested mechanism for the destruction of ozone by chlorofluorocarbons are initiating, propagating, or terminating. It also asks to write a chain terminating step for the reaction. What do the 3 terms mean ...

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In the seventh edition textbook, problem 7.21 lists a number of plots and asks us to determine which are linear (for example, [A] vs t for a 0th order reaction in A is linear). The solutions manual states that the option (f). initial rate against [A] for a reaction that is first order in A, is linea...

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Radioactive decay is a first-order reaction since the decay rate is proportional to the initial amount of radioactive atoms present to the first power. They are proportional through a decay constant. Hope this was helpful!

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The solutions manual's answer is in terms of moles instead of mmol (1 mmol = 1x10^-3 mol). They converted all the units from mmol to mol before solving for the units of k. Since the whole problem is in terms of mmol, I think you are okay leaving the answer in terms of mmol as well.

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You do not consider C because when [A] and [B] are constant and [C] changes, the rate does not change. Thus you can conclude that the reactant C is zero order and the rate of the reaction is independent of [C] (and it does not show up in the rate law). Hope this helped!

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In a concentration vs time graph of a reaction, the slope or derivative at a certain time is the rate of the reaction (how fast or slow a reaction proceeds), and kinetics is the study of the rates of chemical reactions. Initial rates of reactions can be calculated but it becomes more difficult the f...

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When using Q in the Nernst Equation, how come we are allowed to mix values for partial pressure and concentration of the products and reactants?

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Why is it that anytime the Nernst equation is used to solve for K, K is written with only one significant figure (for example: K = 1x10³)?

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If the change in enthalpy is positive the reaction is endothermic (it requires heat to form products) and so raising the temperature will cause the equilibrium to shift toward the right (i.e., more product will be formed since more heat is available). If the change in enthalpy is negative the reacti...

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What is the standard hydrogen electrode? I thought electrodes were made of metal to conduct electricity, so what does it mean for the electrode to be hydrogen?

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Can someone please explain the difference between current, charge, volt, and potential? I am having a hard time understanding what they all mean.

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I need help thinking through how a raise in temperature affects the stability of compounds with respect to their elements. I know you can use the G f and H f of the compound to solve for the -TdeltaS f term in the gibbs free energy equation, and that if T increases than the whole -TdeltaS f term inc...

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If it is negative, the reaction is spontaneous.

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Since you have a reaction you can use the enthalpies of formation of the products and reactants to find the reaction enthalpy. Look up the enthalpies of formation for both SO3 and SO2 and then multiply them by their molar coefficients from the reaction. Then subtract the enthalpy of formation for SO...

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Enthalpy is heat released or absorbed in chemical reactions and physical changes. Entropy, on the other hand, is needed to describe the likelihood of a system being in a particular state. The textbook refers to entropy as a measure of disorder within a system, although Dr. Lavelle has decided to avo...

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The reference point affects the sign of q: q (cal) = -q. The experiments were conducted in the calorimeter so the heat capacity of the calorimeter must be calculated (which is calculated from the first reaction that was used to calibrate the calorimeter). The equation for this is C (cal&...

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Can someone explain statistical entropy and how it differs from residual entropy? Dr. Lavelle's lecture slides indicated that statistical entropy had a large error in W. What does that mean in this context?

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The work by expansion equation $w=-P_{ex}(\Delta V)$ gives an answer in the units atm * L not J or kJ (the unit used for work). How do you convert atm * L to J?

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In class, Dr. Lavelle talked about the small fluctuations in volume that occur when a system is at equilibrium, and how the sum of these infinitesimal changes contribute to reversible reactions doing more work than irreversible reactions. However, how do these infinitesimal changes in volume occur i...

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Yes, the cooling curve is the heating curve but in the opposite direction (going from right to left). Temperature is on the y-axis, so reading a heating curve from right to left shows a decrease in temperature which shows the cooling process. Hope this helped!

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Yes, the cooling curve is the heating curve but in the opposite direction (going from right to left). Temperature is on the y-axis, so reading a heating curve from right to left shows a decrease in temperature which shows the cooling process. Hope this helped!

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Can someone please explain what it means for a state property to be independent of the path taken to obtain that state? I understand state properties are only concerned with the current state of a substance, but what does the "path taken" refer to? Would that have to do with heating, cooli...

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Heat and work are two ways in which energy can be transferred. The more energy a system has, the greater is capacity to do work and the more heat it can produce. Hope this was helpful!

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A closed system has a fixed amount of matter and can exchange energy with its surroundings. An example would be a cold pack that it used for athletic injuries since it absorbs heat from its surroundings to become cold. An isolated system can exchange neither matter nor energy with its surroundings a...

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Hi. The problem 6D.15 part (b) asks to calculate the pH of 0.055M AlCl3. I was confused on how to set up a reaction since hydrogen isn't a part of AlCl3. The solutions manual sets up the following reaction: Al(H2O)6^+3(aq) + H2O(l) <---> H3O^+3(aq) + Al(H2O)5^+2(aq). I understand why the AlCl3 had t...

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Use the formula Ka*Kb = Kw to convert all to Ka values. Then compare the values: the smaller the value, the weaker the acid. Hope this helped!

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I also ran into the same problem. However, using the value 0.07379 from my calculation, the equilibrium concentrations come out the same as those in the solutions manual. The discrepancy is taken care of by rounding and sig figs.

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Pure solids and liquids are not included when doing an ICE table calculation. Their concentrations do not change, and if they do, the change is so minimally it is insignificant anyway. Hope this was helpful!

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Multiply the right hand of the equation out. Doing so gives you 2x=3.60x10^-4 - 3.16x10^-3x (multiply 3.16x10^-3 in the first parenthesis with both terms in the second parenthesis). To solve for x add the 3.16x10^-3x from the right hand side of the equation to the 2x on the left. This gives you 2.00...

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Concentration is equal to moles/liters. By decreasing the volume by compression (decreasing liters), the concentration of the substances is increased (since the amount of moles are initially constant before the equilibrium adjusts). To adjust, the equilibrium will shift in the direction of the least...

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Can someone please explain why water is sometimes written after the ligand in a formula (in alphabetical order) and sometimes before (not in alphabetical order)? For example, in the 7th edition textbook in section 9C, problem 3 asks you to write the formula of the coordination compound given the nam...

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Many homework problems ask you to determine the pH of a solution before and after it was diluted to fill a larger volume. Can someone please explain why you must multiply [H30+] by (initial volume/volume after dilution)?

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In the seventh edition textbook, problem 9 in section 6B asks you to determine [H30+], [OH-], pH, and pOH given just one of these things. For one of the problems the [H30+] given is 1.5 mol/L and thus the pH = -log(1.5) = -0.18. The solutions manual uses this value for pH but disregards the negative...

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A single bond is made up of one sigma bond. A double bond is made up of one sigma bond and one pi bond. A triple bond is made up of one sigma bond and two pi bonds. All bonds will have at least one sigma bond and then depending on whether it is a single, double, or triple bond it will have either no...

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The bond lengths in SO3^2- are not the same. The most stable structure has one double bond and 2 single bonds. However, SO3^2- has resonance and thus the actual SO bond length in this molecule is not determined by any one individual bond but rather a blend of all 3. Thus the SO bond length in SO3^2-...

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The solutions manual correctly states that b is non-polar and that d is polar. Try taking another look!

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The number of hybridized orbitals of any one atom is the same as the number of regions of electron density around the atom. This regions of electron density give us the electron arrangement which is different than the molecular shape (molecular shape does not take lone pairs into consideration). Thu...

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To solve problems such as this one, draw the lewis structure of the compounds and then from there you can deduce how many regions of electron density surround the atom in boldface. The total number of regions of electron density will be the hybridization of the atom. For example if there are 5 regio...

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There is an understood 1 after the s in the sp3 hybridization of methane (s1p3). Methane has four regions of electron density and uses 1 s-orbital and 3 p-orbitals, making a total of 4 hybridized orbitals. Thus, compounds or molecules with 4 regions of electron density (such as methane) have an sp3 ...

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Molecules that have lone pair electrons on the central atom will have bond angles that are slightly less than the value they would be if the lone pair were a bonded atom. The reason for this is that repulsion of a lone pair of electrons is greater than the repulsion of a bonding pair, thus the lone ...

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Does a single electron on a radical count as an area of electron density as much as an electron pair counts? Is the repulsion less for a single electron than for a pair, and does this mean that the bond angles of a molecule with a single electron will be slightly larger than the bond angles of a mol...

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Anions do have polarizing power. The trend is that that larger, less electronegative anions will have a greater polarizing power. For cations the trend is that smaller, more highly charged cations have more polarizing power. Hope this helps!

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Permanent Dipole Moments occur when the difference in the electronegativity of two atoms in a molecule is large. The more electronegative atom will attract more electrons, thus creating a partial negative charge around that atom and a partial positive charge around the atom with the lower electroneg...

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The center atom will be the atom with the lowest ionization energy. Ionization energy increases as you go across from left to right on the periodic table and decreases as you go down a group. Hope this was helpful!

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Chlorine is more electronegative because it has more protons and thus exerts a stronger pull for electrons (the effective nuclear charge is greater than that of Oxygen). Additionally, Chlorine is only one electron away from completing its octet while Oxygen needs two electron to complete its octet, ...

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The value .112 is equal to 1/n2^2. By rearranging the formula: frequency (v) = R(1/n1^1 - 1/n2^2) we get 1/n2^2 = 1/n1^1 - v/R. Thus, 1/n1^1 - v/R = .112 which is what you solved for. You can write 1/n2^2 = .112 and so 1/.112 =n2^2. 1/.112 is approximately 9 so we can say n2^2 = 9 and thus n2 = 3. H...

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Oxygen is diatomic so the O2 molecule has a molar mass of 32.00g/mol (twice the amount of O which is 16.00g). Since the problem refers to 1 molecule of Oxygen (not a mole of Oxygen), then you need to figure out the weight of 1 molecule of Oxygen. To do this you can use dimensional analysis: 32.00 g ...

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Problem 5 a) on chapter 2C 7th edition: the problem calls for you to draw the lewis structure for ClO. The solutions manual shows it drawn with a single bond, but I was wondering why it would be incorrect to draw the structure with a double bond instead (since the formal charge would then be 0 for b...

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The shell n=3 is lower in energy than the shell n=4 and thus it is written first to get the lowest energy configuration. The shells in the configuration should be in order of numerical value (which makes sense because it is in order of increasing energy). Since the d-block is included after Argon, a...

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In class, Dr. Lavelle mentioned that Copper and Chromium are strange exceptions and they fill up (or half fill up) their d-subshell before the s-subshell because that is the configuration with the lowest energy. I was wondering if this also applies to Ag and Mo as well? Thank you!

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I am trying to do the electron configuration for W. Since W is in a row that breaks into the f block I was wondering where I would write the f-electrons. Would they come before or after the d-electrons in an electron configuration? So far I have it written as: [Xe]6s2 5d4 4f14 Is this correct? If no...

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Use the formula when you are asked to calculate an electron's change in energy when it transitions from different energy levels (e.g. n=4 to n=2). You can also use the change in energy to calculate the frequency and wavelength of the light emitted due to the loss of energy when an electron transitio...

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Hi. I have a pretty clear understanding of what nodal planes are but I am confused as to what nodes are. Are they the same as nodal planes or are as lobes, or are they something completely different?

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The question states that the lamp is rated 32W meaning 32J of energy are emitted every second. In 2 seconds 64 J of energy would be emitted by the lamp. Solve for the frequency using the speed of light and lambda and then use the frequency in the formula E=hv to calculate the energy per photon. Sinc...

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I am very confused as to what is meant when a problem asks you to find the uncertainty in the kinetic energy of an electron. I have solved for the uncertainty in velocity using the Heisenberg equation and the question is asking me to use it to find the uncertainty in the kinetic energy of the electr...

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I know that wavefunctions describe the distribution of an electron in an atom but I am having trouble understanding that concept. For one, why is it called a wavefunction and does it have anything to do with an atom's energy levels?

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From my understanding, polyatomic ions are just one of those things you have to memorize. Once you use them enough you will become very familiar with them. I would suggest memorizing them as you come across them, and the more you are asked to use them in problems the easier they will become. Also, p...

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You would need to find the mass composition of each element and then convert those to percentages based on the entire mass of the compound. For example, the mass of C in the compound is 84.077g. To find the mass percent composition of C in the compound divide the mass of C in the compound by the tot...

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I know that to determine the amount of formula units in a compound, but I am having a hard time understanding the difference between molecules or atoms and formula units. How would you know when to use which terminology?

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