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...

UltimaOnline SG

Page 248 K.S. Chan planning book

Test for phenol

Add aq NaOH . Observation : phenol dissolves, two layers become one

What does this mean?

Say we want to distinguish two organic liquids, both of them are colorless, in two different test tubes

When we add aq NaOH to the one containing phenol,

Is it that aq NaOH and phenol forms two distinct layers momentarily, seen from the difference in translucency, and then when phenol dissolves it becomes a homogeneous translucent soln?

Same for RCOOH pg 249

Not even momentarily. If NaOH(aq) is added drop by drop, the phenol layer dissolves bit by bit into the aqueous layer, as phenoxide is more soluble in water than ether. If excess NaOH(aq) is added immediately, the phenol layer completely dissolves into the aqueous layer immediately.

Flying grenade SG

OOOHH !!! THANKS ULTIMA !

Flying grenade SG

Difference between acidity and strength of acid?

Acidity of soln (molarity of H+) and the tendency for a acid to deprotonate is it?

Acids do not necessarily have COOH functional group

https://www.dropbox.com/s/99fuo0hj966t1c3/20160609_223822.jpg?dl=0

COOH functional group for carboxylic acid

Flying grenade SG

Water is an acid

UltimaOnline SG

Difference between acidity and strength of acid?

Acidity of soln (molarity of H+) and the tendency for a acid to deprotonate is it?

Acids do not necessarily have COOH functional group

https://www.dropbox.com/s/99fuo0hj966t1c3/20160609_223822.jpg?dl=0

COOH functional group for carboxylic acid

Measure of acidity (ie. pH) is referring to for solutions.

Strength of an acid is measured by its Ka or pKa value.

Proticity refers to how many moles of H+ one mole of the acid can donate, or how many moles of H+ one mole of base can accept.

Obviously only those acids with the COOH group are called carboxylic acids. Obviously not all acids have that group. Why are you stating the obvious?

Option D is the answer, because its conjugate base is most effectively stabilized (among the 4 options), by delocalizing the uninegative formal charge over 2 electronegative, electron-withdrawing O atoms (ie. similar acidic strength to carboxylic acids). You *have* to draw the mechanism for the resonance contributors to fully understand this.

Flying grenade SG

thanks oTL

Flying grenade SG

page 265 planning book

want to isolate the filtrate

what does it mean by retain the filtrate and carry out the extraction process on the filtrate?

isn't the filtrate alr seperated from the residue alr (in the conical flask/buchner flask alr)?

what does it mean evaporate the filtrate to dryness using water bath, when it's the filtrate that we want???

have something attached to the top to collect the filtrate gas?? or fractional distillation?

UltimaOnline SG

page 265 planning book

want to isolate the filtrate

what does it mean by retain the filtrate and carry out the extraction process on the filtrate?

isn't the filtrate alr seperated from the residue alr (in the conical flask/buchner flask alr)?

what does it mean evaporate the filtrate to dryness using water bath, when it's the filtrate that we want???

have something attached to the top to collect the filtrate gas?? or fractional distillation?

This is the sort of qn that u probably still wun understand after I type out a reply. So if u still dun get it after this, go ask ur school teacher or private tutor. Dun flog the dead horse.

U misread the book : it says, if you don't want the species in the filtrate, then evaporate the filtrate to dryness, then take the residue. This will work if the filtrate is simple covalent rather than soluble ionic. If the filtrate is wanted, then retain (ie. don't throw away) the filtrate, and carry out various extraction processes on the filtrate (the exact extraction process depends on whether the species in the filtrate is soluble ionic or covalent molecular).

Flying grenade SG

OOOOOH THANKS ULTIMA !!!

Flying grenade SG

yeah lor, provided the solid residue is insoluble in the filtrate, just boil the filtrate off and collect the residue

wonder why ks chan wrote this in this manner, making it seem like boiling of the filtrate is a secondary technique

shld be at the same primary level - the two techniques expounded in pg 264 and 265

ArJoe SG

Hi, for a level 2012 paper 2 question 2iii which asks why so2 and not so3 is formed, can i say it is because when heated, temperature increase, hence by lcp system attempts to decrease temperature by favouring reverse reaction?

UltimaOnline SG

Not acceptable. Nice try though. The question specifies it is about S to SO2 versus S to SO3. This isn't about the equilibrium between SO2 and SO3. So the only acceptable answer is the most directly relevant one : ie. this is primarily a matter of kinetics, not equilibria.

ArJoe SG

Hi ultimaonline, thanks for your detailed response.

For 2011 paper 2 question bii, i wrote atomic radius increases from c to si to ge, hence extent of orbital overlap decreases from cc bond to sisi bond to ge-ge bond, resulting in decreasing covalent bond strength hence less energy required to overcome. Is it acceptable?

for 2011 paper 2 question 5, for the further relevant piece of information, for geometrical isomers i said that geometrical isomers have c c double bond to restrict free rotation of the molecule. There must also be 2 different atoms or group of atoms directly bonded to each c atom of the double bond.
for optical isomerism, i said that if both enantiomers are not present in equal amounts, the compounds can rotate plane polarised light. For optical isomerism to arise, compound must have a chiral carbon atom, carbon bonded to 4 different atoms.
Are my answers acceptable?

UltimaOnline SG

Hi ultimaonline, thanks for your detailed response.

Yes ArJoe, your answers are acceptable.

Flying grenade SG

when the Eθcell of an overall cell reaction is +ve , do we say the reaction is thermodynamically/energetically favourable/feasible?

UltimaOnline SG

when the Eθcell of an overall cell reaction is +ve , do we say the reaction is thermodynamically/energetically favourable/feasible?

Best to use the phrase "thermodynamically feasible". Because a positive cell potential = a negative Gibbs free energy change, by the formula (tested in the new H2 Chem syllabus) : delta G = - n F cell potential

Flying grenade SG

THANKKSSSS ULTIMAAA !!!!

Flying grenade 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 ?

Flying grenade 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?

MightyBiscuits 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/

« Previous Page Next Page »