Lurkers, register with SgForums and ask ur H2 Chem qns here!

last SG · 2015-08-19T22:54:00+00:00

Topic : atomic radius Hi! This is from singapore cambridge nov a level exam year 2010,Paper 1 qn 1Qn: which element has the largest atomic radius?BrKKrScSo...

Flying grenade SG

george chong org chem page 7

how does valence bond theory and hybridisation theory help us to determine if an organic compound is e-rich or e- deficient?

UltimaOnline SG

2013/P1/11

for the equation of the auto ionisation of water, why is it that the equilibrium lies(when no change is made to the eqm) to the left? is it becos L.H.S of the eqn has 1 mol, R.H.S has 2 mol? (but a system always tends towards max entropy,one factor is increased no. of particles. so why would eqm lies to the left?)

is it correct that H2O《》H+ + OH- ΔH=+58kj/mol , is in a state of dynamic eqm but the equilibrium position lies to the left?

i thought that , under the right conditions, and also in a closed system, a reversible reaction will reach a state of dynamic equilibrium.

is it right that given enough time and in a closed system, All reversible reactions will reach dynamic equilibrium, and All has a equilibrium position either lying left,or right side of the equilibrium ah ?

Yes, correct. The position of equilibrium lies to the LHS (ie. Kc value less than 1), because H2O is more thermodynamically stable compared to H+ and OH-.

UltimaOnline SG

george chong organic chem book page 5, for the resonance structure of CO32- , the partial double bond is preferably drawn parallel to the single sigma bond, rather than like an arc right?

Correct. Other than benzene ring (in which a circle is used to denote partial pi bonds throughout the ring), all other resonance hybrids should use a dotted line parallel to the sigma bond.

UltimaOnline SG

george chong org chem page 7

how does valence bond theory and hybridisation theory help us to determine if an organic compound is e-rich or e- deficient?

From electron geometry, you can work out orbital hybridization. The greater the % s orbital character, eg. sp > sp2 > sp3, the more electron rich that atom is, because the unhybridized p orbitals (not used for hybridization to form sigma bonds) can overlap sideways to form pi bonds. Eg. Triply-bonded nitrile sp > doubly-bonded imine sp2 > singly-bonded amine sp3.

UltimaOnline SG

May I seek help for this question UltimaOnline? Here is the picture for the qns. Answer for this mcq question is B, I don't have the solution though.

https://www.instagram.com/p/BG6O0ZbDnn7/

Find OS of Fe in the reactant in terms of n. Find moles of electrons transferred per mole of anion. Hence find new OS of Fe in terms of n, in the product. Equate to actual OS of Fe in product. Hence solve for n.

Flying grenade SG

why cannot generate amide from amine and carboxylic acid

UltimaOnline SG

why cannot generate amide from amine and carboxylic acid

Flying grenade SG

N13/P1/11

as temp increase,(as predicted by LCP ) molarity of H+ and OH- increase by the same amount, is that right?

pH decreases and pOH increases is it correct?

as temp increase, will the soln be more acidic , neutral or more alkaline?

bewildering

correction : as pH=-lg [H+] , pOH=-lg [OH-] , as M of H+ or OH- increase, pH and pOH decrease.

UltimaOnline SG

N13/P1/11

as temp increase,(as predicted by LCP ) molarity of H+ and OH- increase by the same amount, is that right?

pH decreases and pOH increases is it correct?

as temp increase, will the soln be more acidic , neutral or more alkaline?

bewildering

correction : as pH=-lg [H+] , pOH=-lg [OH-] , as M of H+ or OH- increase, pH and pOH decrease.

Yes, as temperature increases, both pH and pOH decreases, since molarities of H+ and OH- both increase.

Flying grenade SG

page 157 cstoh advanced guide , under

WA-SB titration

why would H+ from water ,preferably bond with CH3CO2- (and consequently OH- is librerated) ,instead of Na+ , at the equivalence point, when what is left is salt and water?

is it becos Na+ like to be alone, want that unipositive formal charge, instead of dispersing it when it is bonded with CH3CO2-?

Flying grenade SG

Yes, as temperature increases, both pH and pOH decreases, since molarities of H+ and OH- both increase.

but is the soln more acidic, or more alkaline? or neutral?

both [H+] and [OH-] increases leh T-T

UltimaOnline SG

but is the soln more acidic, or more alkaline? or neutral?

both [H+] and [OH-] increases leh T-T

UltimaOnline SG

page 157 cstoh advanced guide , under

WA-SB titration

why would H+ from water ,preferably bond with CH3CO2- (and consequently OH- is librerated) ,instead of Na+ , at the equivalence point, when what is left is salt and water?

is it becos Na+ like to be alone, want that unipositive formal charge, instead of dispersing it when it is bonded with CH3CO2-?

Because H+ has higher charge density than Na+, which having a low charge density thermodynamically prefers to form ion - permanent dipole interactions (in the absence of ionic bonds) rather than covalent bonds, hence carboxylate salts are Bronsted-Lowry basic and hydrolyze (ie. abstract H+ from solvent water) to generate OH- at equivalence point.

Flying grenade SG

NH4+Cl- +H2O 《》NH3 + H3O+ + Cl-

is there a diff between the hydrolysis of NH4+Cl- (aq),

and the solvation of NH4Cl(s) (an endothermic rxn )?

Flying grenade SG

huh, idk sia, that one bewildering

i think the soln is neutral, but both pH and pOH decreases

izzit?

Flying grenade SG

OHH so is it that. the pH of a solution, which gives an indication of the acidity of a soln, is only a fair indication/measure for solutions at same temperature.

so for example , if a soln at 35°C, has a lower pH than a soln at 25°C , doesn't mean the soln with lower pH is more acidic . becos its not a fair comparison

DID I GET IT RIGHT

Flying grenade SG

Thank u so much Ultima !!!!

what abt NH4+Cl- +H2O 《》NH3 + H3O+ +Cl-

why H2O will deprotonate NH4+ ? is NH3 more stable than NH4+, becos there is a unipositive formal charge on N atom in NH4+ ?

Flying grenade SG

yes thank you for the detailed explanation. i was thinking why, then, H+ should just stick to OH- .

but becos Na+ prefers to form ion - permanent dipole interactions

thank u for enlightening

UltimaOnline SG

OHH so is it that. the pH of a solution, which gives an indication of the acidity of a soln, is only a fair indication/measure for solutions at same temperature.

so for example , if a soln at 35°C, has a lower pH than a soln at 25°C , doesn't mean the soln with lower pH is more acidic . becos its not a fair comparison

DID I GET IT RIGHT

Yes, that's right.

UltimaOnline SG

Thank u so much Ultima !!!!

what abt NH4+Cl- +H2O 《》NH3 + H3O+ +Cl-

why H2O will deprotonate NH4+ ? is NH3 more stable than NH4+, becos there is a unipositive formal charge on N atom in NH4+ ?

Because NH4+ is Bronsted-Lowry acidic, while H2O is amphiprotic.

And NH3 and NH4+ are both relatively stable (since the N atoms in both conjugate acid and base have a stable octet), and the positive formal charge on the N atom 'redeems' itself (ie. isn't too destabilizing) thermodynamically by forming ion - permanent dipole interactions and/or strong hydrogen bonds with water, hence both conjugate acid and base exist in equilibrium with each other, with fairly significant values for both Ka (of NH4+) and Kb (of NH3).

« Previous Page Next Page »