CHEMISTRY COMMUNITY

Mika Sonnleitner 1A

Either way is fine; technically it is more accurate to write K as k/k', but you can leave the answer in either form.

Mika Sonnleitner 1A

The lecture notes for cell potential state that maximum work = -charge*E. From this, we get that max w=-nFE, and since max w=ΔG (not ΔG°, according to the notes), then ΔG=-nFE.

Mika Sonnleitner 1A

The state of the molecule matters more than the weight. So, when you are comparing entropies, you would first look at gas vs. liquid etc., and then if they are in the same phase, then you would calculate the molar mass. Whichever molecule is heavier has the higher entropy.

Mika Sonnleitner 1A

Pseudo rate laws are used essentially when the order is determined by increasing the concentrations of one or more reactants. For example, in the rate law Rate=k[A][B][C], if [B] and [C] are in large excess, then they remain essentially constant, so [A] determines the rate. Since only [A] matters in...

Mika Sonnleitner 1A

A nucleophile is any electron rich species that donates electrons to elecrophiles to form a chemical bond in a reaction. In substitution reactions, for example, nucleophiles can be electrostatically attracted to a positive central atom, and therefore donate electrons to bond with that central atom. ...

Mika Sonnleitner 1A

To identify the intermediates, you would choose the compound that appears as a product in one reaction, and then as a reactant in the next reaction. To find the overall reaction, you would remove all intermediates and simply write the reaction using the reactants and products that are left.

Mika Sonnleitner 1A

The purpose of taking the ln of the concentration in the graph for a first order reactant is to have a straight-line graph instead of an exponential curve, as a linear graph is more useful for calculations. Similarly, the graph for second order reactants plots 1/[A] vs. time, and the graph of a zero...

Mika Sonnleitner 1A

It is possible that problems will describe the graph of a reaction and expect you to determine the order. You could also determine the order of the reaction if the problem explicitly states the orders of each reactant, or if the problem gives the units of k.

Mika Sonnleitner 1A

The order of the reactants should be given in the problem in some form. The problem will either explicitly state "first order reactant" and "second order reactant", or give some other type of information that would help you determine the order. For example, the problem could stat...

Mika Sonnleitner 1A

To determine the states of the molecules in the cell diagram, you can also look at Appendix 2B for the cell potential values. All the half reactions will be listed there with the states of each substance.

Mika Sonnleitner 1A

A single line is used to separate substances in different states, so in this case you would use a single line, instead of a comma, which is used to separate substances in the same state. Furthermore, two double lines are used only to represent a salt bridge, and one single line is used to represent ...

Mika Sonnleitner 1A

Like density, cell potential is an intensive property. This means that cell potential does not depend on the amount of substance present, so even if you multiply your chemical reaction by a coefficient in order to balance the half reactions, you do not need to do anything to change the cell potentia...

Mika Sonnleitner 1A

Step 1: write the half-reactions Oxidation: Cl2(g) + 2e- --> 2 Cl-(aq) Reduction: 2Br-(aq) --> Br2(l) + 2e- Step 2: calculate the standard cell potential for each reaction by looking at Appendix 2B *you should reverse the anode reaction in the table to get the correct cell potential value for the ha...

Mika Sonnleitner 1A

This only occurs when the reaction is isothermal, because in the equation ΔU=(3/2)nRΔT, if there is no change in temperature, then there will be no change in internal energy.

Mika Sonnleitner 1A

Since the Nernst Equation is as follows: E=E°-(RT/nF) ln Q, we can see that the cell potential (E) depends on the value of Q, which is the concentration of the products over the concentration of the reactants.

Mika Sonnleitner 1A

For irreversible expansion, ΔS is the same as for reversible expansion, because entropy is a state property. So from part (a) calculations, ΔS=+3.84 J/K. To calculate this value, you would use the equation ΔS=nRln (V2/V1). In free expansion, w=0. This is because in free expansion, the gas expands fr...

Mika Sonnleitner 1A

For this problem, you need to find the temperature at which the reaction is spontaneous, so you first have to find ΔG. For part (a), you find ΔG by first finding ΔH and ΔS. Step 1: Find ΔG° by finding ΔH° and ΔS°, and using the equation ΔG°= ΔH°-TΔS°. You can look up the values in Appendix 2A, using...

Mika Sonnleitner 1A

The products and reactants are never supposed to be in the same solution. They are always kept separate with a salt bridge or porous disc in between. The anode and cathode are always separated. If the substances in the anode or cathode are both aqueous, then a conductor is required in order to trans...

Mika Sonnleitner 1A

To find if a reaction is favorable/spontaneous, you need to calculate ΔG. If ΔG is negative, then the reaction is favorable. There are 2 methods to calculate Gibbs free energy: Method 1: use standard enthalpy of formation and standard entropy values of reactants and products. ΔG°=ΔH°-TΔS° To find ΔH...

Mika Sonnleitner 1A

Larger molecules have a larger entropy because they are more complex and contain more molecules, which leads to more vibrational and rotational motions. Keeping this in mind, we can conclude that: (a) HBr has the higher entropy, because Br is larger than F, and therefore is more complex. (b) NH3 has...

Mika Sonnleitner 1A

From ΔH - TΔS = -RTln(K), you simply divide by -RT to get ln K = -(ΔH/RT) + (ΔS/R). However, this form of the Van't Hoff equation is not that commonly used. To derive the more widespread version of the Van't Hoff equation, you add two Van't Hoff equations in order to calculate the equilibrium consta...

Mika Sonnleitner 1A

Pure iron metal contains more energy than rusted iron. Therefore, energy would have to be released in order for iron to rust, creating an exothermic reaction. This same principle applies to other oxidation reactions. Since oxygen generally bonds with chemicals that have low-energy bonds, the oxygen ...

Mika Sonnleitner 1A

This formula for work is also used only for reversible processes, because temperature is constant, and as volume increases, pressure increases. However, the work formula for an irreversible process is w=-PΔV, because temperature is not constant. Less work is done in an irreversible process compared ...

Mika Sonnleitner 1A

Additionally, reversible systems involved, for example, a bicycle pump with equal pressure inside and outside of the pump. An irreversible system, however, would have differing pressure inside and outside of the bicycle pump example. However, all real/biological processes are highly irreversible, in...

Mika Sonnleitner 1A

If you use the equation w = -P∆V, the units are kg/m⋅s² (m³) ,which is equal to kg⋅m²/s², which is the equivalent of Joules. Alternatively, you could multiply by 101.325 J/L⋅atm, which would result in work with units of Joules.

Mika Sonnleitner 1A

q and ∆H are only equal to each other when pressure is constant. This gives rise to the equation for change in internal energy of a reaction that happens in a sealed container. If you substitute ∆H for q, the change in internal energy equation becomes ∆U = ∆H - w, or ∆U = ∆H -P∆V.

Mika Sonnleitner 1A

Question 8.25 in the book states: "A constant-volume calorimeter was calibrated by carrying out a reaction known to release 3.50 kJ of heat in 0.200 L of solution in the calorimeter (q= -3.50 kJ), resulting in a temperature rise of 7.32 C. In a subsequent experiment, 100.0 mL of 0.200 m HBr(aq)...

Mika Sonnleitner 1A

Heat and work both depend on the transfer of energy, regardless of the substance. This means that heat and work are not specific to a particular substance, i.e. they are not intrinsic properties so they cannot be state functions. Only properties like volume and pressure that are specific to the subs...

Mika Sonnleitner 1A

A bomb calorimeter is a closed system because it allows heat to be exchanged. While this system is insulated, an "insulated system" is not one of the main three types of systems: closed, open, and isolated. An isolated system would not allow for any transfer of matter or energy. In contras...

Mika Sonnleitner 1A

Question 8.67 asks to find the enthalpy of formation for benzene with and without resonance; how would you differentiate between the two? The problem only gives the chemical compound, so how would we know which chemical equation to use? For example, part (b) of this question asks to estimate the ent...

Mika Sonnleitner 1A

For part (d), it is helpful to write the net ionic equation: NH4I (am) + KNH2 (am) --> KI (am) + 2 NH3 (l) NH4+ + I- + K+ + NH2- --> K+ + I- + 2 NH3 This simplifies to: NH4+ + NH2- --> 2 NH3 In this case, NH3 is the conjugate acid of NH2-, because when NH2- accepts a proton, it becomes NH3 (Bronsted...

Mika Sonnleitner 1A

This problem gives us the % deprotonation of benzoic acid, and asks to find the Ka of benzoic acid. To start this problem, we first need to find [H3O+]. The problem gives us the information that the benzoic acid is in a 0.110 M solution. 1) Find [H3O+]: Since the % protonation is 2.4%, the concentra...

Mika Sonnleitner 1A

Dipole moments occur between two bonded atoms of different electronegativities. For example, H20 has a dipole moment between the O-H bond, since the two elements have different electronegativies. And since Oxygen has a greater electronegativity than Hydrogen, the dipole moment would point towards Ox...

Mika Sonnleitner 1A

A Lewis acid is a compound that accepts an electron pair. So, the H+ ion could accept an electron pair from another compound (base) to form a bond. A Bronsted Acid is a compound that donates a proton. The H+ ion would be accepted by another compound (base) to form a bond. Essentially, H+ can be a Le...

Mika Sonnleitner 1A

"K" is used to denote Kc or Kp, which you can differentiate by determining if the elements are in a gas phase or an aqueous phase. If the elements are in a gas phase, then you would use the partial pressures to calculate K, which you could also write as Kp. If the elements are in an aqueou...

Mika Sonnleitner 1A

"In phase" refers to the physical states of the compounds in chemical equilibrium equations. However, it's important to keep in mind that the equilibrium constant does not include elements in the solid or liquid phase, only aqueous or gas phases. When an element is in the aqueous phase, th...

Mika Sonnleitner 1A

4.33 and 4.35 are asking the same thing; the wording is slightly different, but "state the hybridization" and "identify the hybrid orbitals" mean the same thing. The solutions manual answers are in the same format for both questions. You would just answer with the hybridization o...

Mika Sonnleitner 1A

If we look at C2H4 as an example, there is a pi bond between the two carbons. However, assuming you are looking at the Lewis structure of C2H2, you must think of the pi bonds as having one lobe pointing out of the page towards you, and the second lobe pointing back behind the page. This picture migh...

Mika Sonnleitner 1A

You would use the equation E=hv when asked to find the energy of the photon. However, if you were asked to find anything related to the emitted electron, you would use the overall photoelectric effect equation, E(photon) - work function = Ek. For example, if you were asked to find the velocity of th...

Mika Sonnleitner 1A

Some resonance structures are more favorable than others; this depends on the formal charge of each atom in the compound. The resonance structure that contains the fewest amount of formal charges will be the most favorable, because it is the most stable. Similarly, the resonance structure with the m...

Mika Sonnleitner 1A

Additionally, ionic bonds are salts that dissociate in water; this is because their electrons and transferred instead of shared (as is the case with covalent bonds), so they compound is much easier to break up/dissolve. In contrast, covalent bonds are molecules that do not easily dissolve in water; ...

Mika Sonnleitner 1A

Elements in period 3 and below contain an empty d-subshell (since the d-block starts at n=3), which creates room for extra electrons to exceed the octet rule. Elements in periods 1 and 2, however, only have s- and p-orbitals, therefore they cannot have more than 8 valence electrons.

Mika Sonnleitner 1A

The two particular exceptions we need to know are Copper and Chromium. Copper would have an electron configuration of [Ar]3d9 4s2, but notice that the d subshell is one electron away from being completely full. In order for the atom to become more stable, an electron would be needed to complete the ...

Mika Sonnleitner 1A

Yes, the exceptions would apply to all elements in the same groups as Chromium and Copper. Elements in groups 6 and 11 would have an electron configuration that includes nd4 or nd9, therefore it would be more stable if the d subshell was half full or completely full. In order to achieve this, one el...

Mika Sonnleitner 1A

n is the principle quantum number, and describes the energy level and shell size of the electron. l, the angular quantum number, describes the shape of the orbital. For reference: when l=0, the orbital is an s-orbital when l=1, the orbital is a p-orbital when l=2, the orbital is a d-orbital when l=3...

Mika Sonnleitner 1A

You would write the subshell notation by first writing the energy level n as the coefficient. Next, you would determine the type of orbital based on the angular quantum number, l. How to determine the type of orbital: When l=0, the orbital is an s-orbital When l=1, the orbital is a p-orbital When l=...

Mika Sonnleitner 1A

When a light source is shined on a hydrogen sample, it has the potential to excite the electron from its ground state to an excited state. Or, the incoming energy from the photon can bring an electron down from an excited state to a less excited state (ex. n=4 energy level to n=2 energy level). An e...

Mika Sonnleitner 1A

The equation E=-hR/n^2 represents the change of energy level that an electron goes through when there is a light source with enough energy to move the electron. An important thing to note: as the energy level approaches infinity, E approaches 0 (plug in infinity for the "n" in the equation...

Mika Sonnleitner 1A

To find the limiting reactant, you would first convert the given masses into moles using the molar masses for each reactant. Then, using the balanced equation, you can see that 1 mole of Al2O3 requires 6 moles of Na. When you calculate the moles of Al2O3 and multiply by 6 (to find the moles of Al2O3...

Mika Sonnleitner 1A

You must use 454g for part (b), because in order to find the mass of oxygen necessary to oxidize the fat, you must relate the moles of C57H110O6 to moles of O2 using their molar ratios from the balanced equation. In order to compare the moles of fat and the moles of oxygen, you must first use the ma...

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