CHEMISTRY COMMUNITY

Chem_Mod

The computer stated that the optimal hybridized C-C-C angle value was 60º. However, we know that cyclopropane is sp3 hybridized and therefore should have an optimal value of 109.5º. Which value should we use?

Chem_Mod

1) We treat Z as cis and E as trans. Using either is fine.

2) -OH is treated as a substituent when present with an aldehyde or a ketone or an acid. Always start numbering the C at the functional group.

Chem_Mod

1) Is it possible to name ring structures as E, Z? I ran across a problem that asks to determine if a cyclic structure is E or Z. 2) How can we learn the priority of each functional group. That is, if we had a species with multiple functional groups, which ones should be named first? Which ones woul...

Chem_Mod

A perpetual motion machine is a hypothetical device that uses and produces the same amount of energy. One outcome would be that we could build perfect machines.

Chem_Mod

What is the concept of a "perpetual motion machine?" It doesn’t make sense having an isolated system, removing it from isolation (work on the surroundings), and then put it back in isolation, and the internal energy returning to its initial value. I know it hasn’t been proven but what if i...

Chem_Mod

The reason for this is that when we talk about standard energies of formation (either enthalpy or Gibb's free energy) it has units of kJ/mol, or rather, the amount of energy associated with forming 1 mole of your molecule. To say that Δ G_{reaction} = Δ G_{formation} is just saying that you're writi...

Chem_Mod

How are the Standard Reaction Gibbs energy and the Standard Gibbs energy of formation equal?

Chem_Mod

Yes, heat capacity always has a positive value. It is the amount of energy (heat) required to raise the temperature of the system. When talking about the specific heat capacity of solids, it has the units of \frac{J}{gram X degrees Celsius} . This says that the heat capacity of a solid is the amount...

Chem_Mod

Is heat capacity always positive because it is the amount of heat required to raise the temperature of a system?

Chem_Mod

Kinetic stability occurs when reactants react extremely slowly, or not at all. Kinetic stability can be measured by the time it takes for a reaction to occur. The slower a reaction occurs, the greater the kinetic stability. The rate at which the reaction occurs will tell you if the reactants are kin...

Chem_Mod

What does kinetically stable mean and how we can tell if the reactants are kinetically stable?

Chem_Mod

ΔE is the change in internal energy of a system. ΔE = q + w (1st law of thermodynamics). ΔE is also equal to $\frac{3}{2}RT$ .

Chem_Mod

What exactly is ΔE?

Chem_Mod

k' generally refers to the rate constant of the reverse reaction. k is the rate constant for the forward reaction.

Chem_Mod

When calculating activation energy, how do we know which value to consider: k' or k.

Chem_Mod

You are correct in that angle strain is related to angles being both smaller and larger than the ideal (109.5º for sp^{3} ). Also, the larger the C-C-C bond angle, the smaller the angle for the substituents on that central carbon, which would add strain. As we change those substituents, of course th...

Chem_Mod

What if a molecule included an angle being larger than the ideal bond angle for a tetrahedral molecule? For example, if the bond angle was 120º instead of 109.5º, does that create angle strain? In lecture you stated that bigger ring structures than cyclohexane are less stable. Does this have to do w...

Chem_Mod

The biggest difference is that neo- comes from a common naming system that is more or less not used any more while tert-butyl is a mix of common name and IUPAC that gets used for substituents. When you see neo-, it is a prefix to something pent- or larger. It means that the number of carbons from th...

Chem_Mod

What is the difference between neo- and tert- (because neo-butyl and tert-butyl have the same structural formula)?

Chem_Mod

Follow the book.

Chem_Mod

According to the Organic Chemistry book, "the prefixes di-, tri-, sec-, and tert- are IGNORED in alphabetizing, but iso-, neo-, and cyclo- are NOT ignored." Is this true? Do we use iso-, neo-, and cyclo- when we alphabetize?

Chem_Mod

Answer: The p-block elements form anions. So their neutral and anionic e- configurations are known. The s-block and d-block elements form cations. So their neutral and cationic e- configurations are known.

Chem_Mod

Question: When asked to write the electron configuration of the Vanadium ion V-, do you need to follow the chromium rule and switch the notation to [Ar] 3d5 4s1?

Chem_Mod

Answer: Starting at the 7th period with francium with [Rn]7s1 you go to Radium [Rn]7s2, Actinium [Rn]7s2 6d1 and finally Thorium [Rn]7s2 6d1 5f1. Rutherfordium, Rf, actually has an electron configuration of [Rn]7s2 6d2 5f14.

Chem_Mod

Question: How does the electron configuration work for elements in the 7th period?

Chem_Mod

Answer: According to Hund's rules, when determining electron configuration you place electrons with the same spin into different orbitals before filling up the orbitals with electron with opposite spin. That means that for say nitrogen's 2px 2py 2pz orbitals, you would place one electron into each ...

Chem_Mod

Question: How can you predict the number of unpaired electrons an atom has by determining its electron configuration?

Chem_Mod

Answer: These diagrams only apply for elements in period two since they only deal with the interaction of 2s and 2p atomic orbitals. If one of the atoms had an extended octet(meaning it was in the 3rd period or lower), then there would be a much more complex diagram.

Chem_Mod

Question: For period 2, Groups 1-5 (pi 2p bonds come first after 2s*) and Groups 6-7 (sigma 2p comes first instead). Do these patterns apply for the valence electron configurations for the period 3 elements below them, or is there a complication because of the possibility of d-orbitals in an expand...

Chem_Mod

Answer: In general, the 3d orbitals are lower in energy than the 4s orbital, which is why the 4s orbital is always filled before the electrons are placed in the 3d orbitals (if you think about it in terms of the Aufbau, or "building up principle"). The exceptions are chromium and copper. ...

Chem_Mod

Question: What is the exception when the 3d-orbitals would be lower than the 4s-orbitals? Also, what results in half full d^5 subshell?

Chem_Mod

Answer: Copper is an exception to the normal manner of writing electron configurations: it is ([Ar]3d10 4s1). When it loses one electron (Cu+ has net positive charge of 1, resulting from the loss of 1 electron) it loses it from the 4s orbital, because these orbitals are furthest from the nucleus an...

Chem_Mod

Question: When writing electron configurations, how come sometimes there is a configuration like [Ar]3d10 instead of 3d8 4s2. Are there exceptions to the order of filling?

Chem_Mod

Answer: This is dependent of the atom you are considering. If for instance you are looking at the transition metals, then the outer most electrons are those in the s and d orbitals. In that case, the number of s and d electrons will be your valence electrons. If you were looking at p block elements...

Chem_Mod

Question: How do we figure out the number of valence electrons in metals? They seem to be variable.

Chem_Mod

Answer: The valence electron configuration would only require you to say the valence electrons (outermost shell). The ground state electron configuration would require all electrons which can either be written with all orbitals or the noble gas preceding the row in question then the valence electro...

Chem_Mod

Question: Is the valence electron configuration the same as a ground state electron configuration?

Chem_Mod

Answer: F2 the following: (sigma 2s)^2. (sigma 2s*)^2. (sigma 2px)^2. (pi 2py)^2. (pi 2pz)^2. (pi 2px*)^2. (pi 2py*)^2.

N2 the following: (sigma 2s)^2. (sigma 2s*)^2. (pi 2px)^2. (pi 2py)^2. (sigma 2pz)^2.

Chem_Mod

Question: What is the electron configuration of both N2 and F2, including the x, y, and z of the p-orbitals?

Chem_Mod

Answer: The electron configuration for Cr is [Ar]3d6 4s2 (note that 4s is higher in energy than 3d - that is important). Cr2+ means we remove 2 electrons, and we remove them from the highest energy level, which is 4s, so the electron configuration for Cr2+ is [Ar]3d6. The d orbital has 5 orbitals a...

Chem_Mod

Question: How does Cr2+ has 4 unpaired electrons? How do you tell in general?

Chem_Mod

Answer: The two 5s e- are removed. Just like Ca or Mg losing two s e- to form Ca2+ and Mg2+. They are all metals.

Chem_Mod

Question: I would think that Cadmium would not become an ion since its s and d orbitals are filled, so why is it possible for Cd2+ to exist?

Chem_Mod

Answer: Since the electron configuration of Cu is [Ar]3d10 4s1, then Cu2+ is [Ar]3d9.

Chem_Mod

Question: Since we know that the electron configuration of Cu is [Ar]3d10 4s1, what would the electron config. of Cu2+ be?

Chem_Mod

Answer: As ground state for Cu is [Ar]3d10 4s1, ground state for Cu+ is [Ar]3d10 because we remove the last e- to make the cation.

Chem_Mod

Question: Why is the ground state electron configuration of Cu+ equal to [Ar]3d^10 instead of [Ar]3d8 4s2? I thought that electrons were taken from the s orbital to fill a d orbital only if there are 4 or 9 electrons in the d orbital.

Chem_Mod

The valence electron configuration of Cr is 1s2, 2s2, 2p6, 3s2, 3p6, 3d5, 4s1 instead of 1s2, 2s2, 2p6, 3s2, 3p6, 3d4, 4s2 because one of the electron from the s orbital jumped to the d orbital. By distributing its electrons along the empty orbitals, it becomes more stable. Since the ground state fo...

Chem_Mod

The full electron configuration for Cr+. The answer was 1s2, 2s2, 2p6, 3s2, 3p6, 3d5. Shouldn't it be 1s2, 2s2, 2p6, 3s2, 3p6, 3d3, 4s2 instead?

Chem_Mod

Answer: Since the electrons in 5p have higher energy than the electrons in 5s, the electrons in 5p is removed first. It is no different from why we would remove electrons from the 3p before the 3s orbital.

Chem_Mod

Question: Why do you take out an e- from the 5p before the 5s when you did the electron configuration of In+?

Chem_Mod

Answer: First write the e- configuration for the neutral atom. Then remove the last e- that you wrote to get the appropriate e- configuration for the cation.

Chem_Mod

Question: How do you write the electron configuration of a metal cation?

Chem_Mod

The 5s should come after the 4d. Once the d orbitals gain electrons, the d orbitals become lower in energy than the 5s, so we list the d orbitals first (since we list the orbitals from lowest to highest energy).

Chem_Mod

Question: Why is the e- configuration for Ag [Kr]4d10 5s1 not [Kr]5s1 4d10? Which is written first?

Chem_Mod

Answer: Count the # of e- in the s- and d- orbitals for the outer shell. Chromium would have 6 valence electrons.

Chem_Mod

Question: How would we determine the number of valence electrons for an element in the d-block (transition elements)? For instance, how many valence are there for Chromium?

Chem_Mod

Answer: Valence electrons are the electrons in the outermost shell of the atom, that is the electrons with the highest principle quantum number. For instance Carbon has electrons in the n=2 shell. The electrons in this shell are the valence electrons. Specifically, it has two electrons in the 2s su...

Chem_Mod

Question: How do you find valence electrons?

Chem_Mod

Answer: Yes, you should consider the (n-1) d-electrons of d-block elements as valence electrons. Cr has 6 valence e-. Count the e- from the previous noble gas.

Chem_Mod

Question: Do you consider the (n-1) d-electrons of d-block elements as valence electrons? How many valence electrons does Cr have?

Chem_Mod

Answer: f-orbitals are used in periods 6 & 7. We covered the filling of f-orbitals (Lanthanides and Actinides).

Chem_Mod

Question: For a ground state configuration, when is a f-orbital used?

Chem_Mod

Answer: Electron configurations are only written without the s orbitals if you are talking about a transition metal ion which has already lost the electrons in the s orbital. There are special cases (see the column with Cr and Cu) where a single s electron can be promoted to the d orbital.

Chem_Mod

Question: Could you explain why electron configurations are written with the d orbital, excluding the s orbital (whose electrons are added to the preceding d orbital). Do you always transfer the electrons from the s orbital into the d orbital?

Chem_Mod

Answer: Count only the electrons in the same shell, that is, the outermost shell. For Xe and Br the s and p electrons are in the same shell, but d electrons are not in the outermost shell.

Chem_Mod

Question: Is it just a general rule that you don't count the d-orbital electrons as valence electrons for elements in the p-orbitals? For example, Xe has 8 valence e- rather than 18. Would the element Br only have 7 valence electrons, rather than 17? If this is so, what is the reason?

Chem_Mod

Answer: The 4s is lower in energy than the 3d due to its shape and it has e- density closer to the nucleus.

Chem_Mod

Question: For electron configurations, why is the 4S orbital filled first than the 3d orbital? Is this connected to orbital's shapes or electron repulsion?

Chem_Mod

Answer: The two are technically the same, in that they both state that there are 2 electrons in the 7s subshell and the 6d2 subshell. There is a difference in nomenclature as to how to order the orbitals, whether they should be in increasing principle quantum number ([Rn]6d2 7s2) or the way in whic...

Chem_Mod

Question: Does it matter whether an electronic configuration is written as [Rn]7s2 6d2 or [Rn]6d2?

Chem_Mod

Answer: For n=5 we can have l=4, 3, 2, 1, and 0. For each l, we have ml ranging from -l to l. The total number of ml will tell us the number of orbitals. l=4, ml=-4,-3,-2,-1,0,1,2,3,4 for a total of 9 / l=3, ml=-3,-2,-1,0,1,2,3 for a total of 7 / l=2, ml=-2,-1,0,1,2 for a total of 5 / l=1, ml=-1,0,...

Chem_Mod

Question: How many orbitals are there for n=5?

Chem_Mod

Answer: The orbital is determined by the ml value while the subshell is determined by the l value. So if a question asks you how many orbitals there are, it's basically asking how many ml values you can have.

Chem_Mod

Question: What is the difference between an electron orbital and an electron subshell?

Chem_Mod

Answer: s orbital means zero angular momentum, p orbital means angular momentum 1, d orbital means angular momentum 2, f orbital means angular momentum 3, and so on

Chem_Mod

Question: What is the pattern for angular momentum quantum numbers when the orbital is given?

Chem_Mod

Answer: There are 4 subshells: 4s, 4p, 4d, and 4f. When the question asks you about how many subshells there are in a certain shell, what it basically is asking is how many different values of l there are. So if n=4, l can only be 0,1,2,3, which corresponds to s,p,d,f.

Chem_Mod

Question: How many subshells there are in the shell n=4?

Chem_Mod

Answer: It does not find anything. It simply labels the individual orbitals in a given subshell. All four quantum numbers are meant to designate the identity of an electron in an atom.

Chem_Mod

Question: What does the magnetic quantum number find?

Chem_Mod

Answer: The one dimensional box means that the particle can only move left or right (in 1-D, in other words, just a line). The distance in which it can move left or right before it hits the walls is L. In other words, the particle in confined within these two walls with a distance of L that it can ...

Chem_Mod

Question: What is a one dimensional box in regards to a particle in a box?

Chem_Mod

Answer: We did an example of using a wave function in the Schrodinger equation to see what result was obtained for the energy of an electron in an atom. In this example the Schrodinger equation gives us an expression for the energy of an electron. Second derivative is part of the Schrodinger equati...

Chem_Mod

Question: Does the Schrodinger equation manipulation show that the wave and particle models match up? What does the Schrodinger equation tell us? Why do we take the second derivation of the wave function?

Chem_Mod

Answer: Use the equal sign to represent the lower limit.

Chem_Mod

Question: Sometimes the uncertainty equation has an equal sign instead of of a greater than or equal sign. Is this a typo or is there a reason behind this?

Chem_Mod

Answer: We are interested in the spread when we calculate delta(x) or delta(p). The plus/minus 1 mm/s tells us that if are speed was 15 mm/s, then the possible values would be between 15-1 and 15+1, or 14-16 mm/s. That means there is a 2 mm/s uncertainty in our speed. If it was plus/minus 3, then y...

Chem_Mod

Question: The uncertainty principle and what it means when it says plus or minus 1.0 mm per second for the speed. Why do they multiply this by 2? If it were plus or minus 3.0 mm per second, would it be multiply it by 6 (because it's the spread)?

Chem_Mod

Answer: The Heisenberg indeterminacy principle uses calculations based on indeterminacy, which is twice the uncertainty, or in other words the spread of the uncertainty. For instance, say we had a velocity measurement of 2.00 +/- 0.01 m/s. The spread of possible values for velocity range from 1.99 ...

Chem_Mod

Question: For the equation of Heisenberg's uncertainty principle: delta(p) x delta(v)=h/(4xpi), why do we multiply the delta(v) by two to get the correct answer?

Chem_Mod

Answer: The textbook uses h-bar, which is h with a little line across the top of it.
h-bar just means h/2pi, so h/4pi is the same as (h-bar)/2.

Chem_Mod

Question: In the Heisenberg's indeterminacy equation the book uses h-bar/2 instead of h/4pi, which you stated in the course reader. Could you please clarify which is the correct one?

Chem_Mod

Answer: E=(1/2)mv^2 applies to particles with static mass, for example an electron or a base ball. So it cannot be used on photons. E=h(frequency) is valid only for photons. To calculate frequency of electrons or particles other than photons, you must use de Broglie relationship. c=(wavelength x fr...

Chem_Mod

Question: Would it be possible to use the E=(1/2)mv^2 equation to find the wavelength by substituting E=h(frequency) and c= (wavelength x frequency) instead of using the de Broglie relationship? If not, why is this not possible?

Chem_Mod

Answer: Actually, electrons do have mass. Its mass is 9.11*10^-31 kg. All waves have particle-like properties, but some not in the sense that you would normally think of for a particle. Like for example, photons (which are light waves) have no "rest mass", which means they have no mass th...

Chem_Mod

Question: From the de Broglie relation, all particles have wavelike properties, but all waves don't have particle-like properties, right? (i.e. electron lacks mass).

Chem_Mod

p=mv and lambda=h/p. The conversion from the work function and Avogadro's number is important, so it should be committed to memory.

Chem_Mod

Question: Where is the equation v = h/(m*wavelength) derived from? Also, in the same question, are we supposed to know how to convert the work function to energy per particle and the constant 6.022 x 10^23?

Chem_Mod

Answer: One result is negative of the other. What value you use for n1 or n2 depends on the question.

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