Chemistry Exam Revision Questions

[kenhung123]

why does decreasing pressure decrease boiling pt of hydrocarb?

[m@tty]

The molecules are spread out more, hence, as the distance is greater, and the collisions are fewer, the intermolecular bonding is reduced. Thus less force must be overcome to transform into the gaseous state.

[kenhung123]

Thanks.

What chromatographic and spectroscopic techniques do not destory a sample?
What effects are there on the state of hydrocarbon when functional groups OH and Br are added?

[Martoman]

HPLC, Paper/TLC.

Spec: UV, IR, NMR

The chemical properties change. Becoming more polar (and in the case of OH becoming soluble on a sufficiently short hydrocarbon) and more reactive. Again, on  a relatively short chain, bioling, melting and viscoscity would increase and volatility would decrease.

[kenhung123]

Nice stuff. Thanks.
Just to confirm in organic pathways, Br, HCl reactants are (aq) and caroxbylic acids, alcohol are liquid and esters are generally (g)?

[Mao]

Thanks guys

Problem: Why is it a problem to directly titrate weak acid with weak base when you can simply use a strong acid or strong base to titrate? I mean why do we need to use back titration in that case? I know it doesn't produce sharp end pt but just use a strong acid/base and problem solved?

This isn't a problem because you can use just that. This is just in the textbook to warn us of the folly of doing so and give us some intuition on the equivalence point...... i think. :laugh:

Not true. There is a very big problem, that being the amount of uncertainty [error] goes crazy. In a strong-strong titration curve, the curve is very steep around the end point, thus a colour change (over a pH range) corresponds to a single point (very small change in volume, maybe 0.1 mL). In a strong-weak titration curve, the curve is a lot less steep, and a change in colour (over a pH range) corresponds to a finite volume (usually a couple of mLs), so there is a fair amount of error. In a weak-weak titration curve, well, no one is stupid enough to do a weak-weak titration.

The principle is, if you want to determine the concentration of a weak acid/base, back titration gives accurate results. In other words, back titration is always used to avoid doing strong-weak titration, and instead perform an accurate strong-strong titration.

[Mao]

HPLC, Paper/TLC.

Spec: UV, IR, NMR

The chemical properties change. Becoming more polar (and in the case of OH becoming soluble on a sufficiently short hydrocarbon) and more reactive. Again, on  a relatively short chain, bioling, melting and viscoscity would increase and volatility would decrease.

Try to avoid statements like this. Methanol and ethanol are much less reactive than methane and ethane (in terms of combustion), but much more reactive in terms of substitution pathways.

[kenhung123]

Thanks guys

Problem: Why is it a problem to directly titrate weak acid with weak base when you can simply use a strong acid or strong base to titrate? I mean why do we need to use back titration in that case? I know it doesn't produce sharp end pt but just use a strong acid/base and problem solved?

This isn't a problem because you can use just that. This is just in the textbook to warn us of the folly of doing so and give us some intuition on the equivalence point...... i think. :laugh:

Not true. There is a very big problem, that being the amount of uncertainty [error] goes crazy. In a strong-strong titration curve, the curve is very steep around the end point, thus a colour change (over a pH range) corresponds to a single point (very small change in volume, maybe 0.1 mL). In a strong-weak titration curve, the curve is a lot less steep, and a change in colour (over a pH range) corresponds to a finite volume (usually a couple of mLs), so there is a fair amount of error. In a weak-weak titration curve, well, no one is stupid enough to do a weak-weak titration.

The principle is, if you want to determine the concentration of a weak acid/base, back titration gives accurate results. In other words, back titration is always used to avoid doing strong-weak titration, and instead perform an accurate strong-strong titration.

Thanks that clears things up!

[kenhung123]

Zn(s) +2HCl(aq)=>ZnCl2(aq)+H2(g)

What best describes this reaction:
acid base
precipitation
redox
gravimetric

[Martoman]

Thanks guys

Problem: Why is it a problem to directly titrate weak acid with weak base when you can simply use a strong acid or strong base to titrate? I mean why do we need to use back titration in that case? I know it doesn't produce sharp end pt but just use a strong acid/base and problem solved?

This isn't a problem because you can use just that. This is just in the textbook to warn us of the folly of doing so and give us some intuition on the equivalence point...... i think. :laugh:

hahaa I try  

Not true. There is a very big problem, that being the amount of uncertainty [error] goes crazy. In a strong-strong titration curve, the curve is very steep around the end point, thus a colour change (over a pH range) corresponds to a single point (very small change in volume, maybe 0.1 mL). In a strong-weak titration curve, the curve is a lot less steep, and a change in colour (over a pH range) corresponds to a finite volume (usually a couple of mLs), so there is a fair amount of error. In a weak-weak titration curve, well, no one is stupid enough to do a weak-weak titration.

The principle is, if you want to determine the concentration of a weak acid/base, back titration gives accurate results. In other words, back titration is always used to avoid doing strong-weak titration, and instead perform an accurate strong-strong titration.

Thanks that clears things up!

hahaa I try 

[Martoman]

Zn(s) +2HCl(aq)=>ZnCl2(aq)+H2(g)

What best describes this reaction:
acid base
precipitation
redox
gravimetric

Redox Zn changes from 0 to +2
         H changes from +1 to 0

[kenhung123]

Can you also say its acid base ? metal cloride+H gas?

[Mao]

Thanks guys

Problem: Why is it a problem to directly titrate weak acid with weak base when you can simply use a strong acid or strong base to titrate? I mean why do we need to use back titration in that case? I know it doesn't produce sharp end pt but just use a strong acid/base and problem solved?

This isn't a problem because you can use just that. This is just in the textbook to warn us of the folly of doing so and give us some intuition on the equivalence point...... i think. :laugh:

hahaa I try 

Not true. There is a very big problem, that being the amount of uncertainty [error] goes crazy. In a strong-strong titration curve, the curve is very steep around the end point, thus a colour change (over a pH range) corresponds to a single point (very small change in volume, maybe 0.1 mL). In a strong-weak titration curve, the curve is a lot less steep, and a change in colour (over a pH range) corresponds to a finite volume (usually a couple of mLs), so there is a fair amount of error. In a weak-weak titration curve, well, no one is stupid enough to do a weak-weak titration.

The principle is, if you want to determine the concentration of a weak acid/base, back titration gives accurate results. In other words, back titration is always used to avoid doing strong-weak titration, and instead perform an accurate strong-strong titration.

Thanks that clears things up!

Don't stress miss, you are doing excellent. You won't pick up these quirky stuff until you're well settled in uni [if you even do chemistry at uni, that is]. I didn't know any of these back in year 12.

[Mao]

Can you also say its acid base ? metal cloride+H gas?

For the sake of VCE (Bronsted-Lowry definition), no, you cannot say it's acid-base reaction. It is redox.

Eventually though, you'll learn acid-base is almost the same thing as redox reaction (Lewis and Arrhenius definition of acid/base).

[kenhung123]

1. But i thought if products where Acid+metal=Salt+H2 or Salt+H2O or salt+H2O+CO2 is an acid-base?
Perhaps it can be acid and base and redox? But why would redox be a more accurate one?
2. Is the OH in carboxy group considered a hydroxy group i.e. can be ionised?
3. Do I need to use words to describe an organic pathway e.g. oxidise ethanol to form ethanoic acid with the use of dichromate oxidising agent