VCE Chemistry Question Thread

[Cassidyhogi]

Hi, I found this practical report on the internet and there were no solutions with it, so I have a few questions I need help with please:

1. Given that the biodiesel produced is a fatty acid ester molecule, discuss the similarities and differences of the intermolecular forces, polarity and melting points between petrodiesel and biodiesel.

2. In reference to their structures, would biodiesel or petrodiesel be more hygroscopic and how would this affect the efficiency of the fuel when burned?

3. What contaminants might biodiesel contain? Discuss the impacts that any impurities would have on the efficiency of which fuel burns.

4. Compare the suitability of petrodiesel and biodiesel as transport fuels in different climates.

Thank you.

[asdfdsa]

Think of it as a circuit, if you have a circuit that isn't open, it doesn't work. However, if you have a closed circuit, the electricity (or this case, electrons) can flow through the "circuit" as it is completed.
Thus the salt bridge containing K2SO4 instead of SO4 2- anions and K+ cations allows the electrons to flow through, but SO4 2- and K+ on its own does not do anything, the two half cells remain as half cells.
(This may be a really haphazard answer, I'll fix it up when I get home, haha)

so the electrons flow through the salt bridge to complete the circuit?

[exit]

so the electrons flow through the salt bridge to complete the circuit?

The salt bridge is there to ensure the circuit continues and to constantly balance the charges of the cell, from what i understand. For example, If the electrode gains electrons (cathode) , then the cations are available from the other half cell travelling through the salt bridge to ensure that the electrode remains instead of accumlating charge, which may cause undesired reactions/effects

If there are anions and cations in separate containers, then those will run out. It's better to ensure that all the anions/cations stay within the circuit. Correct if wrong..

[cookiedream]

so the electrons flow through the salt bridge to complete the circuit?

No, the electrons flow through the external circuit (the wire that connect the anode to the power source/voltmeter and the wire that connects the cathode to the power source/voltmeter)
Remember that an electric current involves movement of charged particles, which are not only electrons but ions as well.

Therefore, anions and cations flow through the salt bridge/internal circuit (not electrons) to allow the current to flow and for the circuit to be complete. It also allows the charges to be balanced at either electrode so that the chemical reaction can continue

[anotherworld2b]

hi, i'm a bit confused about what to do for q2. I'm not really sure how its different from q1

Q1. Using the equation for the reaction, calculate the number of moles of sodium carbonate used in each titration

Na2CO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)
n(HCl) = 0.05
n( Na2CO3 ) = ½ n(HCl)
n(Na2CO3) = ½ x 0.05 = 0.025 moles

Q2. From the equation determine the number of moles of HCL that react with each mole of sodium carbonate. Use this to determine the number of moles of HCL used in the titration.

Na2CO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)

Q5. Is the equivalence point of a reaction acidic, basic or neutral? Explain.
It should be a bit acidic? but wouldn't it be neutral?

[pmmenotes]

Any one done a SAC on equilibrium next? any idea what the potenital experiment could be? have a SAC coming up in 2weeks and 100% doing a experiment the week before

[Quantum44]

Any one done a SAC on equilibrium next? any idea what the potenital experiment could be? have a SAC coming up in 2weeks and 100% doing a experiment the week before

We did a prac for our equilibrium SAC and it was on how changes to the system change the equilibrium position. We used two systems and tested the effects of adding/removing products/reactants, changing volume/pressure and changing temperature.

[zsteve]

hi, i'm a bit confused about what to do for q2. I'm not really sure how its different from q1

Q1. Using the equation for the reaction, calculate the number of moles of sodium carbonate used in each titration

Na2CO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)
n(HCl) = 0.05
n( Na2CO3 ) = ½ n(HCl)
n(Na2CO3) = ½ x 0.05 = 0.025 moles

Q2. From the equation determine the number of moles of HCL that react with each mole of sodium carbonate. Use this to determine the number of moles of HCL used in the titration.

Na2CO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)

Q5. Is the equivalence point of a reaction acidic, basic or neutral? Explain.
It should be a bit acidic? but wouldn't it be neutral?

Hmm, that doesn't look right. Are you sure about starting off with n(HCl) = 0.05? The way the questions are put suggests that you should first know , and use that to calculate n(HCl). Are you sure you read the questions correctly?

Q5. It depends on what your acid/base strengths are. For instance, strong acid + weak base => equivalence point should be acidic. On the other hand, strong base + weak acid => equivalence point should be basic.

Hopefully that helps - maybe you could clarify the first two questions a bit?

[pmmenotes]

We did a prac for our equilibrium SAC and it was on how changes to the system change the equilibrium position. We used two systems and tested the effects of adding/removing products/reactants, changing volume/pressure and changing temperature.

Thank You !

[-273.15]

Hello
I have a couple questions about electrochemistry

1. When the products of electrolysis come into contact, a spontaneous reaction will occur. However if they dont come into contact, this wont occur. But in galvanic cells, the reactants arent in contact yet a reaction still occurs. So why must the products of electrolysis be in contact in order for the spontaneous reaction to occur?

2. In electrolysis, if e.g. via oxidation, a product is formed that is a stronger oxidant than the original strongest oxidant, apparently this would react in a reduction reaction instead of the original reduction reaction but I dont get how?

3. When does electrolysis end? It is when either the anode is depleted (if non a inert electrode) or also when the electrolyte has been depleted?

4. WHy isn't a salt bridge needed in electrolysis?

Thanks

[peanut]

Are we expected to know how to balance half-equations in basic conditions? If so, how do you do this?

[Sine]

Are we expected to know how to balance half-equations in basic conditions? If so, how do you do this?

Balance them as usual then ad OH- ions to both sides  - this will turn the H+ (acidic) to water  and you will be left with some OH- ions on the other side this will be a half equation in basic conditions. (Basically make sure there aren't any H+ ions present)

[-273.15]

Cotton wool may be used to seperate the products in electrolysis so a spontaneous reaction doesnt occur but does this only stop solids from moving through or does it also block ions?

thanks

[Syndicate]

Hello
I have a couple questions about electrochemistry

1. When the products of electrolysis come into contact, a spontaneous reaction will occur. However if they dont come into contact, this wont occur. But in galvanic cells, the reactants arent in contact yet a reaction still occurs. So why must the products of electrolysis be in contact in order for the spontaneous reaction to occur?

2. In electrolysis, if e.g. via oxidation, a product is formed that is a stronger oxidant than the original strongest oxidant, apparently this would react in a reduction reaction instead of the original reduction reaction but I dont get how?

3. When does electrolysis end? It is when either the anode is depleted (if non a inert electrode) or also when the electrolyte has been depleted?

4. WHy isn't a salt bridge needed in electrolysis?

Thanks

1. Because chemical energy is being converted into electricity in galvanic cells. To convert electrical energy into chemical energy, you would require a source (ie. battery). Because it forms a complete circuit, the electrons will just flow across each electrode, cause a spontaneous reaction as the cation would be attracted towards both the electrodes (not sure how correct this is).

2. Because there is a stronger oxidant present it will reduce first, only if concentration(stronger oxidant product) > concentration(original oxidant)

3. If the anode provides the electrolyte with the cations, then the electrolytic cell stops are the anode is consumed. Otherwise, it stops when there is no oxidant present.

4. Because the electrodes are placed in one cell. You don't need ions to travel to prevent polarisation at one electrode in electrolysis. Polarisation is automatically prevented due to cations moving towards the cathode and the anions moving towards the anode from the electrolyte.

Cotton wool may be used to seperate the products in electrolysis so a spontaneous reaction doesnt occur but does this only stop solids from moving through or does it also block ions?

thanks

Cotton wool is semi-permeable. Look at Membrane cells for example.

[-273.15]

1. Because chemical energy is being converted into electricity in galvanic cells. To convert electrical energy into chemical energy, you would require a source (ie. battery). Because it forms a complete circuit, the electrons will just flow across each electrode, cause a spontaneous reaction as the cation would be attracted towards both the electrodes (not sure how correct this is).

2. Because there is a stronger oxidant present it will reduce first, only if concentration(stronger oxidant product) > concentration(original oxidant)

3. If the anode provides the electrolyte with the cations, then the electrolytic cell stops are the anode is consumed. Otherwise, it stops when there is no oxidant present.

4. Because the electrodes are placed in one cell. You don't need ions to travel to prevent polarisation at one electrode in electrolysis. Polarisation is automatically prevented due to cations moving towards the cathode and the anions moving towards the anode from the electrolyte.

Cotton wool is semi-permeable. Look at Membrane cells for example.

Thank you so much! this is amazing