Chemistry 3/4 2013 Thread

[barydos]

Oh haha, my bad.

An amino acid residue is "whatever's left" of the amino acid after it undergoes condensation to form the polypeptide.

For example, if three NH₂-CHR-COOH molecules bond to form a polypeptide, each one will have a different residue (assume I've bonded them end to end with amide to the left and carboxyl to the right):

• Left molecule residue: NH₂-CHR-CO-
• Middle molecule residue: -NH-CHR-CO-
• Right molecule residue: -NH-CHR-COOH

Hope that makes sense!

EDIT: If that doesn't make sense, here's a definition straight from the horse's mouth (IUPAC): http://goldbook.iupac.org/A00279.html (may be more confusing than me!).

Idk if it matters that much, but in the solutions for the sample exam question 5 a)
http://www.cea.asn.au/sites/default/files/sample_paper_answers_2013.pdf
"Circle the methionine residue on the diagram above" the circled part doesn't include the COOH part which didn't react..
so maybe im just being picky here, but just need some clarification once more

[Jaswinder]

1 - What exactly are enzymes? (In simple terms ) What role do they play?

2 - Consider the following statements regarding the effect of temperature on the particles in a reaction mixture. A) At higher temperature, particles move faster and the reactant particles collide more frequently. B) At a higher temperature, more particles have energy greater that the activation energy. Which of the statements provides an explanation as to why the observed rate is greater at higher temperature.

Just wanted to know if I have the correct reasoning. The answer if A and B, but B to a greater extent that A, this is because the heat energy transforms into kinetic energy required to overcome the activation energy, and A although is correct doesn't necessarily imply that there are more 'fruitful' collision?

3 - At 25 C the pH of 0.0050M Ba(OH)2 is? Everytime I do this I get it wrong. What I did was which gives me the wrong answer

4 - attached Image
5 - attached Image

[lzxnl]

1 - What exactly are enzymes? (In simple terms ) What role do they play?

2 - Consider the following statements regarding the effect of temperature on the particles in a reaction mixture. A) At higher temperature, particles move faster and the reactant particles collide more frequently. B) At a higher temperature, more particles have energy greater that the activation energy. Which of the statements provides an explanation as to why the observed rate is greater at higher temperature.

Just wanted to know if I have the correct reasoning. The answer if A and B, but B to a greater extent that A, this is because the heat energy transforms into kinetic energy required to overcome the activation energy, and A although is correct doesn't necessarily imply that there are more 'fruitful' collision?

3 - At 25 C the pH of 0.0050M Ba(OH)2 is? Everytime I do this I get it wrong. What I did was which gives me the wrong answer

4 - attached Image
5 - attached Image

1. Enzymes speed up reactions. By a LARGE factor of several million at least. They're effectively required for biological reactions.
2. Heat energy. Hmph. Heat isn't "energy"; it's a transfer for energy
Strictly speaking, higher temperatures only change the average kinetic energy of the particles, which increases the average energy for those particles, meaning a larger proportion of them have more energy than the activation energy. Colliding more frequently is important as well; indeed, if you have really, really high temperatures, when nearly all particles are above the activation energy, the collision frequency would be predicted to be the main determinant of rate.

3. 0.0050M Ba(OH)2? OK. Every one Ba(OH)2 group has 2 OH- ions => 0.01 M OH-. You can go from here. You're not meant to divide here

4. So...one mole of methane=>900 kJ of energy. Divide by 44 kJ/mol to see how many moles of the evaporation reaction you can have at most=>multiply this number by molar mass of water to get mass of water.

5. Chromium has a +3 charge. Needs three times as many electrons as the silver(I), and twice as much as the copper(II). You have 0.03 F=>0.03 mol electrons, and this reacts in a 1:1 ratio with silver=>0.030 mol silver, 0.015 copper, 0.010 chromium. B?

[barydos]

I've got quite a lot of questions here sorry!

1. The pH of 0.10 M CH3COOH(aq) can be determined using its Ka value and, from the simplified equilibrium law, Ka= [H3O+]2/0.10. State two assumptions made in establishing this simplified relationship and explain why the [H2O] is not included.
- ?

2. A catalyst increases the proportion of‘‘successful’’ collisions by lowering the activation energy, but how does it lower the activation energy?
- is it just providing an alternative reaction pathway? (doesn't sound like it's enough)

3. CH3CH2OH is oxidised to CH3COOH by acidified Cr2O7^2-(aq).Write balanced half-equations for both the oxidation and the reduction?
- With the oxidation reaction CH3CH2OH + H2O(l) --> CH3COOH + 4H⁺(aq) + 4e^-; are the states for ethanol and ethanoic acid both aqueous?
- And what is the reduction reaction?

4. The production of large biomolecules such as lipids, carbohydrates, proteins and DNA involve condensation reactions, i.e. water is a product. Describe the functional group changes that occur when lipids, carbohydrates and proteins are formed.
- carbohydrates: OH + OH -> ether/glycosidic links
- proteins: COOH + NH2 -> amide/peptide links
- lipids though?

5. Enzyme activity depends on the proteins tertiary structure. Changes in pH can disrupt this tertiary structure and denature the protein. Using aspartic acid as an example, explain how a change in pH can alter the charge on a side group in a protein chain.
- is it in low pH, the NH2 becomes NH3+
- in high pH the COOH becomes COO- 
- are those sufficient?

6. Aspirin can be produced from reaction between salicylic acid and ethanoic acid. However the yield is significantly better if ethanoic anhydride is used and H2O is not a product. The theoretical yield is determined stoichiometrically from the amounts of reactants used. The actual yield is lower than the theoretical yield since the reaction reaches equilibrium. Explain, in terms of concentration changes and rates, how a reacting mixture of salicylic acid and ethanoic anhydride ‘‘reaches equilibrium’’.
- yeah im not sure here..

Ahh, any help at all in any of these would be greatly appreciated!

[lzxnl]

I've got quite a lot of questions here sorry!

1. The pH of 0.10 M CH3COOH(aq) can be determined using its Ka value and, from the simplified equilibrium law, Ka= [H3O+]2/0.10. State two assumptions made in establishing this simplified relationship and explain why the [H2O] is not included.
- ?

2. A catalyst increases the proportion of‘‘successful’’ collisions by lowering the activation energy, but how does it lower the activation energy?
- is it just providing an alternative reaction pathway? (doesn't sound like it's enough)

3. CH3CH2OH is oxidised to CH3COOH by acidified Cr2O7^2-(aq).Write balanced half-equations for both the oxidation and the reduction?
- With the oxidation reaction CH3CH2OH + H2O(l) --> CH3COOH + 4H⁺(aq) + 4e^-; are the states for ethanol and ethanoic acid both aqueous?
- And what is the reduction reaction?

4. The production of large biomolecules such as lipids, carbohydrates, proteins and DNA involve condensation reactions, i.e. water is a product. Describe the functional group changes that occur when lipids, carbohydrates and proteins are formed.
- carbohydrates: OH + OH -> ether/glycosidic links
- proteins: COOH + NH2 -> amide/peptide links
- lipids though?

5. Enzyme activity depends on the proteins tertiary structure. Changes in pH can disrupt this tertiary structure and denature the protein. Using aspartic acid as an example, explain how a change in pH can alter the charge on a side group in a protein chain.
- is it in low pH, the NH2 becomes NH3+
- in high pH the COOH becomes COO- 
- are those sufficient?

6. Aspirin can be produced from reaction between salicylic acid and ethanoic acid. However the yield is significantly better if ethanoic anhydride is used and H2O is not a product. The theoretical yield is determined stoichiometrically from the amounts of reactants used. The actual yield is lower than the theoretical yield since the reaction reaches equilibrium. Explain, in terms of concentration changes and rates, how a reacting mixture of salicylic acid and ethanoic anhydride ‘‘reaches equilibrium’’.
- yeah im not sure here..

Ahh, any help at all in any of these would be greatly appreciated!

1. Water is the solvent and is the dominating species here; its concentration doesn't appreciably change at all and remains constant at around 55 M
An assumption is that the concentration of ethanoic acid stays constant; the actual equilibrium law would be [H3O+][CH3COO-]/([CH3COOH]-[CH3COO-]) to account for the change in concentration of the ethanoic acid.

2. It forms bonds to the reactants to lower the activation energy. Alternative reaction pathway? Not specific enough, although VCE chemistry doesn't really say so

3. Yes, states for ethanol and ethanoic acid are aqueous as they dissolve well in water.
Reduction of dichromate? Cr2O7 2- + 14H+ + 6e- => 7H2O + 2Cr 3+ I think

4. Lipids have esters?

5. Not quite; if you change the charges, then you form new ionic bonds/remove old ionic bonds depending on what charge change there  is. For instance, if you go from R-NH3+ to R-NH2, you remove potential bonds between the NH3+ and a R-COO- group, for instance. Thus, the bonding in the tertiary structure is affected.

6. At equilibrium, the forwards and backwards reaction rates become equal and concentrations no longer change. Initially, the salicylic acid and the ethanoic anhydride are the only species present, so there is no back reaction. However, as the reaction progresses, more acetylsalicylic acid is formed, which increases the backwards reaction rate until the forwards and backwards rates are equal.

[barydos]

1. Water is the solvent and is the dominating species here; its concentration doesn't appreciably change at all and remains constant at around 55 M
An assumption is that the concentration of ethanoic acid stays constant; the actual equilibrium law would be [H3O+][CH3COO-]/([CH3COOH]-[CH3COO-]) to account for the change in concentration of the ethanoic acid.

2. It forms bonds to the reactants to lower the activation energy. Alternative reaction pathway? Not specific enough, although VCE chemistry doesn't really say so

3. Yes, states for ethanol and ethanoic acid are aqueous as they dissolve well in water.
Reduction of dichromate? Cr2O7 2- + 14H+ + 6e- => 7H2O + 2Cr 3+ I think

4. Lipids have esters?

5. Not quite; if you change the charges, then you form new ionic bonds/remove old ionic bonds depending on what charge change there  is. For instance, if you go from R-NH3+ to R-NH2, you remove potential bonds between the NH3+ and a R-COO- group, for instance. Thus, the bonding in the tertiary structure is affected.

6. At equilibrium, the forwards and backwards reaction rates become equal and concentrations no longer change. Initially, the salicylic acid and the ethanoic anhydride are the only species present, so there is no back reaction. However, as the reaction progresses, more acetylsalicylic acid is formed, which increases the backwards reaction rate until the forwards and backwards rates are equal.

Time and time again, you've saved me. Have all my upvotes.
Thanks a lot Mr 50raw!

@Dismounted: Alright, I understand. Thanks for the help

[Dismounted]

Idk if it matters that much, but in the solutions for the sample exam question 5 a)
http://www.cea.asn.au/sites/default/files/sample_paper_answers_2013.pdf
"Circle the methionine residue on the diagram above" the circled part doesn't include the COOH part which didn't react..
so maybe im just being picky here, but just need some clarification once more

I would consider that answer incorrect. Should have circled -NH-CH(CH₂-CH₂-S-CH₃)-COOH (so including the -NH- and -COOH on either end).

A residue is "what's left after it's reacted". So take methionine from the data book, and circle whatever is left of it on the diagram. (For a more technical explanation, click on the IUPAC link I had in my previous post.)

[Edward21]

I don't want to be hated for saying this, but I'm sorting of hoping for a hard exam like the VCAA 2009s on tuesday. It's just when they're easier like the 2012s the 1 mark factor up the top end becomes extremely cut throat and competitive. Like I know it's a competition, it's just scary how high the A+ cutoffs were for last year's exam! I'd prefer that 1 mark from a stupid mistake not drop me from a potential 45 to 43 just because of the tiny A+ range at the top. Does anyone else know what I mean?

[psyxwar]

I don't want to be hated for saying this, but I'm sorting of hoping for a hard exam like the VCAA 2009s on tuesday. It's just when they're easier like the 2012s the 1 mark factor up the top end becomes extremely cut throat and competitive. Like I know it's a competition, it's just scary how high the A+ cutoffs were for last year's exam! I'd prefer that 1 mark from a stupid mistake not drop me from a potential 45 to 43 just because of the tiny A+ range at the top. Does anyone else know what I mean?

Yup, and that's why I was annoyed at this year's biology exam.

[psyxwar]

1 - What exactly are enzymes? (In simple terms ) What role do they play?

Enzymes are biological (protein) catalysts that increase the rate of reaction by lowering the activation energy necessary to initiate reactions. Like all catalysts, they don't have any affect on equilibrium/ yield.

Unlike inorganic catalysts (eg, manganese dioxide) enzymes are highly specific for one specific reaction (something that has to do with the specificity of their shape to their substrate (ie. reactant)). They play a vital role in biological systems in ensuring that metabolism occurs at a rate fast enough to sustain life (without them some reactions might take millions of years, which is a bit slow...)

[Jeggz]

Yup, and that's why I was annoyed at this year's biology exam.

I have a strong feeling that this year's Chem exam will be a similar style to the Bio exam though 

[psyxwar]

I have a strong feeling that this year's Chem exam will be a similar style to the Bio exam though 

Yeah I think so too, which means you'll all have to be really pedantic.

I guess it's better than bio in that for chemistry you can go "oh yeah I definitely got that right" for a lot of the questions, whereas for biology they might just decide to be really dodgy and require you to write super specific terms for full marks.

[neonperson]

How can you determine whether a system has reached equilibrium using titration?

[Yacoubb]

1 - What exactly are enzymes? (In simple terms ) What role do they play?

An enzyme is an organic compound (a protein) biological catalyst that increases the rate of a chemical reaction by lowering its activation energy. Something important to consider that is unique about enzymes is that they are neither consumed nor used up by the reactions they catalyse. Plus, enzymes are highly specific. Because of the specific tertiary configuration of the active site of an enzyme, an enzyme molecule can only combine with a specific substrate molecule. Enzymes catalyse anabolic and catabolic reactions.

[Jaswinder]

thanks boys.
this one too