BedokFunland JC's A Level H2 Chemistry Qns (Part 2)

last SG · 2010-07-26T23:40:35+00:00

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Generally speaking increasing the concentration of a reactant incraesing the yield of a product. But if the yield of the product increases will the value of the Kc increases because the reaction moves to formward direction?? If not why?? What can change the value of Kc??

Only changing temperature will change the Kc value. Changing any other variable (eg. moles or molarity or partial pressure of a reactant or product) will change the Qc value.

So even if we say position of equilibrium has shifted, it doesn't necessarily mean the Kc value changed. It could be that Kc value has changed (if we changed temperature), or it could be that Qc value has changed (if we changed the moles or molarity or partial pressure of a reactant or a product).

Either way, when changing either the Qc value or the Kc value, we say the position of equilibrium (ie. Kc) will have shifted, relative to Qc.

If Qc is now less than Kc, the position of equilibrium now lies to the right, and the rate of the forward reaction will exceed the rate of the backward reaction, until Qc = Kc and equilibrium is re-estabilished.

If Qc is now more than Kc, the position of equilibrium now lies to the left, and the rate of the backward reaction will exceed the the rate of the forward reaction, until Qc = Kc and equilibrium is re-estabilished.

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Jasmine ngjiamin asked :

Why does diamond have a higher melting point than silicon?

Answer :

Both species in their standard sates have giant covalent lattice structures.

Hence, melting involves breaking the multitude of covalent bonds in their lattice structures.

In diamond (carbon), the C-C sigma bonds are formed by the more effective overlap using the less diffuse 2nd electron shell's sp3 hybridized orbitals, as opposed to the case of silicon in which the Si-Si sigma bonds are formed by the less effective overlap using the more diffuse 3rd electron shell's sp3 hybridized orbitals.

Consequently, the bond dissociation enthalpy/energy and thus the activation enthalpy/energy involved in melting diamond, is significantly more endothermic compared to that involved in melting silicon.

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If steam and water has the same volume at 100 degree celsius and atmospheric pressure, will they have equal number of molecules. Are the molecules able to move from water into the steam and from steam into water? It's in a closed setup.

At boiling point (for a given pressure) of a species, Gibbs free energy for the reaction in either direction is zero, and the system is at equilibrium between the liquid and gaseous states. Hence, the rate of the forward reaction (eg. vaporization of water) is equal to the rate of the backward reaction (eg. condensation of water).

Hence, the final answer to your final question is, yes of course.

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Standard electrode potential is the emf measured between a test electrode and S.H.E.

under S.T.P. Standard redox potential is same as standard electrode potential??

How will we define standard cell potential in words??

There is no standard (pun intended) word definition for cell potential.

For A level purposes, Cambridge only requires students to define electrode potential in words (as you've done), and cell potential by formula, ie.

Cell Potential = Reduction Potential @ Cathode + Oxidation Potential @ Anode.
or
Cell Potential = Reduction Potential @ Cathode - Reduction Potential @ Anode.

Regarding RTP vs STP, the Data Booklet values are for standard conditions (of 1 bar for all gases, and 1 mol/dm3 for all solutions), and for temperature at 25 deg C.

UltimaOnline SG

Both Standard electrode potential and standard cell potential are same thing since the equation that you gave above regarding Cell potential will still give the standard electrode potential of any cell in consideration. Is that so?

Not quite.

Technically, the term Standard Electrode Potential refers to either the Oxidation Potential or (more usually, by convention) the Reduction Potential, for a particular Electrode (which could be either the Cathode or the Anode, relative to the SHE), under Standard conditions.

In contrast, the term Standard Cell Potential compulsorily takes into consideration both Redox Potentials, again under Standard conditions, of both Electrodes (ie. both the Cathode and the Anode) that constitute the ElectroChemical (usually Galvanic aka Voltaic) Cell.

Perhaps the source of many students' (and some school teachers' as well) confusion, is that the Standard Electrode Potential = Standard Cell Potential, if and only if, one of the Electrodes in the Cell, happens to be (by deliberate choice) the Standard Hydrogen Electrode (SHE).

Because the SHE's Electrode Potential (either/both Reduction and/or Oxidation Potentials) is designated by humans to be taken as the default zero Volts (ie. 0 V), hence the Standard Cell Potential for such a setup (and only such a setup) = the Electrode Potential for the Electrode being investigated.

See Wikipedia :
http://en.wikipedia.org/wiki/Electrode_potential

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Hii, I have a few qns here.

Miscellanous qn: Do we still need to know what’s a mass spectrometer, NMR spectrum for H2 Chem?

Stoichiometry: A giant molecule contains a large amount of carbon, mainly of isotopes 12C and 13C. It was found that the relative atomic mass of carbon in the molecule is 12.20. What is the ratio of 12C to 13C ?
Ans: 4:1

It happens that this is a MCQ qn. So the approach is just (12×0.8) + (13×0.2) = 12.2.
I was wondering, what if this is not a MCQ qn? I know we can let them be x and y.
So it becomes 12x + 13y = 12.2 . However there is only one eqn.

Gasesous State: Under extereme conditions, gallium chloride Ga2Cl6 can also be formed via the endothermic dimerisation of GaCl3. The two species can exist in dynamic equilibrium under suitable conditions.
2GaCl3(g) two way arrow Ga2Cl6(g)

(1) In an experiment, a total pressure of 7.75×10^{-3 N
m}-2 was measured when a 0.661g sample of GaCl3 was allowed to dimerise in a 1.80dm3 vessel at 200 degrees.

Calculate x, the fraction of GaCl3 that dimerised at 200 degrees, assuming ideal gas behaviour.

I apply pV=nRT, where turns n is 0.00354904

Initial mass is 0.661g of GaCl3, n = 0.00375141

Fraction that dimerised, x = (0.00375141 – 0.00354904) / 0.00375141 = 0.053945

But the answer is 0.108! (No working provided)

Thanks for answering!!!

For the 1st qn, let X be the % of C12. Then (X/100)(12)+((100-X)/100)(13) = 12.2 and solve for X.

For the 2nd qn, the number of moles when using PV=nRT is the total number of moles of both gaseous species (ie. both the monomer and the dimer).

2GaCl3 <---> Ga2Cl6

Initial : 0.00375141 and 0

Change : -2X and +X

Equilibrium : (0.00375141 - 2X) and X

Total moles of gaseous species = (0.00375141 - 2X) + X = 0.00354904 (from PV=nRT)

Solve for X.

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Hi, would appreciate if someone could answer the following question :)

When we calculate Kc of reactions that place in aqueous medium, do we take into account the concentration of water, if it is a reactant or product?

ie. CH3CO2H (l) + C2H5OH (l) <=> CH3CO2C2H5 (l) + H20 (l)

Is the Kc = [CH3CO2C2H5][H20] / [C2H5OH][CH3CO2H] ?

Thanks ! :)

It depends on whether water was already initial present as a solvent (ie. in large excess), or if it functions solely as either a reactant or product.

Notice that in your particular question, it is implied that no water was initially present. Hence water here is a product, and must therefore be included in your Kc expression and formula.

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Hi, it would be great if I could have some help with the following questions :)

1. Given the reasoning that the standard enthalpy change of neutralisation between weak acids/bases and strong acids/bases is lower as compared to those between strong acids and bases because energy is absorbed to ionize the undissociated weak acid/base, why is the standard enthalpy change of the reaction:

CH3COOH + NH3 -> H2O + CH3COONH4

greater than that of neutralisation between weak acids/bases and strong acids/bases since there should be more energy absorbed to ionize the greater number of undissociated acid/base?

2. Since the units of enthalpy change of reaction is kJ/mol, why does changing the stochiometry coefficients still change the enthalpy change of a reaction? Should this not be analogous to the concept of heat capacity and specific heat capacity, where the value of the specific heat capacity is constant for a particular material, regardless of mass, since the units of specific heat capacity already “accounts” for the mass?

Q1. There is a fundamental misconception with your question. Neutralization enthalpy is defined as per mole of water generated. No water is generated in the reaction between weak acids and weak bases.

Q2. kJ/mol is not per mole of any particular reactant or product, but per mole of the equation.

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Question :

Convert 3-methylhex-2-ene-1-ol to 2-hydroxy-4-methyl-hepta-1-amine in only and exactly 3 steps.

Answer (mouseover to highlight) :

1. Oxidize to aldehyde using pyridinium chlorochromate (PCC).

2. Nucleophilic addition of CN- (unto the electrophilic acyl carbon), followed by protonation. Reagent required is HCN (in the cold, 10-20 deg C to minimize vaporization of toxic HCN) with trace amount of NaCN or KCN or NaOH or KOH).

3. Simultaneous hydrogenation of both the alkene and nitrile functional groups, using gaseous hydrogen and transition metal catalyst (eg. platinum, palladium or nickel).

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Another BedokFunland JC original H2 Chemistry question.

Qn1. Explain why, despite cis-alkenedioic acids (eg. cis-butenedioic acid) being more polar compared to its trans isomer, the cis isomer may in some cases have a lower melting and/or boiling point.

Ans1 :
In the cis-alkenedioic acid, some of the molecule's capacity for hydrogen bonding has been used up to form intra-molecular hydrogen bonds, and consequently the cis isomer has less extensive inter-molecular hydrogen bonds, which accordingly require a lower amount of energy to overcome, and hence the cis isomer (despite being more polar) may have a lower melting and/or boiling point compared to the trans isomer. For the same reason, the cis-isomer may have lower solubility (ie. less extensive hydrogen bonding with water) compared to the trans isomer.

Qn2. Explain why, despite cis-alkenes being more polar compared to its trans isomer, and hence understandably having the higher boiling point, may in some cases have the lower melting point.

Ans2 :
As a trans-alkene is more symmetrical compared to its cis isomer, the trans isomer packs more efficiently and closely into its crystal lattice solid structure, which consequenty (ie. as a result of being able to achieve greater intermolecular proximities) allows for more extensive and stronger intermolecular van der Waals attractions. Hence, a trans-alkene (being more symmetrical) may have the higher melting point compared to its cis isomer, even as the cis-alkene (being more polar) may have the higher boiling point compared to its trans isomer.

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Energy is the capacity of doing work. Enthalpy is the total heat content of a system. What is the difference between energy and enthalpy??

For 'A' level Chemistry purposes, there is no difference, and Cambridge will accept either term.

Eg.

"bond dissociation enthalpy" = "bond dissociation energy",

"bond formation enthalpy" = "bond formation energy",

"lattice dissociation enthalpy" = "lattice dissociation energy",

"lattice formation enthalpy" = "lattice formation energy",

etc.

FYI, at higher levels, although there are several different ways to define the technical difference between enthalpy and energy, but for 'A' level Chemistry purposes, it'll more than suffice for the interested student to take it that :

Energy change is heat change under constant volume.

Enthalpy change is heat change under constant pressure.

For most chemical reactions, to 3 significant figures, there is negligible difference between the enthalpy change and energy change values, and thus Cambridge accepts either term used interchangeably.

Ideally when applying Hess Law, it's still good practice (though not compulsory) to standardize all processes in the cycle, in either "enthalpy" or "energy" values, rather than a mixture of both.

(Similarly, "Oxidation Number" versus "Oxidation State"; there is a technical difference at higher levels, but not for 'A' level Chemistry purposes).

But what do you eman by the statement that "enathalpy is the total heat content of a system."Does this mean all type of energies contained by a system viz. Potentilalenergy, Kinetic energy, vibrational energy, rotational energy?

All the energies you've mentioned, are included in both "enthalpy" and "energy", but "enthalpy" also measures the system's volume and pressure.

If you need to discuss this further, please visit a dedicated physics or chemistry forum, because anything beyond what's required for A level Chemistry (specifically the Singapore H2 syllabus) is not within my personal interest to discuss.

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