VCE Chemistry Question Thread

[anoushka_iyer]

Hey,
I've been confused about this for the whole year- do you need a negative sign for molar heat of combustion/heat of combustion in KJ/g? In general, which values in chemistry need a sign?

Thanks

[hums_student]

Hey,
I've been confused about this for the whole year- do you need a negative sign for molar heat of combustion/heat of combustion in KJ/g? In general, which values in chemistry need a sign?

Thanks

There's more discussion on this if you scroll down from this post but the molar heat of combustion is always positive. You do, however, need a sign for enthalpy.

[anoushka_iyer]

Hey guys,
Would a colour change in an indicator when performing an acid-base titration be quantitative or qualitative? I think it's qualitative but my teacher said it was a 'quantitative estimate' since your estimating the approximate pH of the solution based on the colour of the indicator.

thanks

[anoushka_iyer]

These are some questions from the jacaranda textbook (I don't have the solutions) that I am struggling to answer. It would be great if you could help me out.

1) List two procedures that could adversely impact on the internal validity of an experiment.
- if factors other than the independent variable are not controlled and influence the dependent variable (eg. in an experiment to determine the effect of the length of hydrocarbon chain on its retention time in HPLC, if the temperature/pressure/length of column is not controlled, results will be inconclusive)
- if you want to measure a temperature change of 0.1șC, but use a thermometer with an uncertainty of ±2șC, you will not be able to measure the dependent variable accurately

2) Is it essential that the results of an experiment can be replicated in order for the experiment to be considered
reliable? Explain your answer
- yes, because reliability is the ability of another researcher to repeat your experiment by following the same method and obtain the same results, so in order to be reliable, results must be replicable.
3) Explain, how an experiment can be reliable but not valid, and why an experiment that is valid must also be reliable.
- an experiment can yield the same results when it is repeated but the results might be consistently affected by an uncontrolled variable in the same way, so an exclusive relationship between the independent variable and dependent variable cannot be identified.
- if the results are not reliable, they must be influenced by random or systematic errors which affect their repeatability, and also reduce the accuracy of the experiment which reduces the validity.

4)  Give an example of a strength and a weakness of quantitative and qualitative data.
- quantitative:
     - strength: measurable, more objective than qualitative data
     - weakness: its accuracy and precision can be compromised by sources of error, can be difficult to obtain for some experiments

- Qualitative:
     - Strength: consists of descriptive observations of results, provides more depth and detail than quantitative results, open to interpretation
     - weakness: subjective, less reliable

Thank you so much.

[turtlebanana]

Hey guys, just a question about addition reactions

Let's say you react ethene (C₂H₄) with hydrogen bromide (HBr).

When the addition reaction occurs, do each of the atoms in the small molecule you are reacting (in this case HBr) always bond to a different carbon?
So in this case, H would bond to one C atom and Br would bond to the other C atom?

Or could they both bond to the same C atom?

[KiNSKi01]

Hey guys, just a question about addition reactions

Let's say you react ethene (C₂H₄) with hydrogen bromide (HBr).

When the addition reaction occurs, do each of the atoms in the small molecule you are reacting (in this case HBr) always bond to a different carbon?
So in this case, H would bond to one C atom and Br would bond to the other C atom?

Or could they both bond to the same C atom?

I don't think they will bond to the same C atom due to the nature of the double bond in ethene. Each carbon atom in ethene can form one other bond after the addition reaction because they share a double bond before. If both the H and Br bonded to the same C atom that would imply Carbon has 5 bonds because the addition reaction is just adding, its not like one of the existing hydrogen atoms bonded to this C atom would be kicked off onto the other C atom
Someone will probs explain this better than me 

[Erutepa]

Hey guys,
Would a colour change in an indicator when performing an acid-base titration be quantitative or qualitative? I think it's qualitative but my teacher said it was a 'quantitative estimate' since your estimating the approximate pH of the solution based on the colour of the indicator.

thanks

Im not sure about this, but I would say that the colour change itself is qualitative data (if you were to record that the colour changed from blue to red for example) but If you were recording the volume of titre at the colour change, then that would be quantitative data.
This is becuase qualitative data deals with decriptions/characteristics while quantitative data deals with numerical values

Hey guys, just a question about addition reactions

Let's say you react ethene (C₂H₄) with hydrogen bromide (HBr).

When the addition reaction occurs, do each of the atoms in the small molecule you are reacting (in this case HBr) always bond to a different carbon?
So in this case, H would bond to one C atom and Br would bond to the other C atom?

Or could they both bond to the same C atom?

They would bond to different Carbons. Carbon atoms will form 4 single bonds/1 double bond and 2 single bonds/1 triple bond and 1 single bond generally. If H and Br from hydrogen bromide were to both be added onto the same carbon from ethene, you would have a carbon with 3 single H-C bonds, 1 C-Br bond and a C-C bond (to the other carbon) making a total of 5 single bonds on that carbon, while you will also have another carbon with 3 single bonds (2 C-H bonds and a C-C bond). This just doesn't happen - it doesn't follow the rules of how carbon likes to bond.
Instead each atom from HBr will be added to another Carbon atom so that you have 2 carbons which each make 4 single bonds with other atoms.

These are some questions from the jacaranda textbook (I don't have the solutions) that I am struggling to answer. It would be great if you could help me out.

1) List two procedures that could adversely impact on the internal validity of an experiment.
- if factors other than the independent variable are not controlled and influence the dependent variable (eg. in an experiment to determine the effect of the length of hydrocarbon chain on its retention time in HPLC, if the temperature/pressure/length of column is not controlled, results will be inconclusive)
- if you want to measure a temperature change of 0.1șC, but use a thermometer with an uncertainty of ±2șC, you will not be able to measure the dependent variable accurately

2) Is it essential that the results of an experiment can be replicated in order for the experiment to be considered
reliable? Explain your answer
- yes, because reliability is the ability of another researcher to repeat your experiment by following the same method and obtain the same results, so in order to be reliable, results must be replicable.
3) Explain, how an experiment can be reliable but not valid, and why an experiment that is valid must also be reliable.
- an experiment can yield the same results when it is repeated but the results might be consistently affected by an uncontrolled variable in the same way, so an exclusive relationship between the independent variable and dependent variable cannot be identified.
- if the results are not reliable, they must be influenced by random or systematic errors which affect their repeatability, and also reduce the accuracy of the experiment which reduces the validity.

4)  Give an example of a strength and a weakness of quantitative and qualitative data.
- quantitative:
     - strength: measurable, more objective than qualitative data
     - weakness: its accuracy and precision can be compromised by sources of error, can be difficult to obtain for some experiments

- Qualitative:
     - Strength: consists of descriptive observations of results, provides more depth and detail than quantitative results, open to interpretation
     - weakness: subjective, less reliable

Thank you so much.

1. Internal Validity refers to the ability of the experimental design to accurately identify the effect of the IV on the DV
Both of your examples here do talk about potential aspects of an experimental design that does reduce its ability to accurately identify the effect of the IV on the DV, as such I think they are good responses.

2. Reliability isn't neccasarily the ability of another researcher to obtain the same data (this is an aspect of it though). Reliability refers to the ability of an experimental design to produce consistant data - this might be in the context of the same experimenter conducting the experiment multiple times, or by other experimenters conducting the experiment. Reliable experiments will produce precise (tightly grouped) data.
I think you would get the marks for what you have written though - its just that its probably best to refine your definition of reliability since VCAA does talk about it also in the context of repeating the same experiment/using large sample sizes (not reccasarily just others doing the experiment)

3. Rather than just saying that the experiment may be reliable but not valid due to uncontrolled varaiable, its probably best to identify systematic errors as the specific source of this inaccuracy of the experiment (systematic errors are things like incorrectly calibrated scales which will consistanly provide inaccurate results across all trials). Also rather than saying 'exclusive', I would personally write' accurate'.
Note that in your second point, systematic errors do not effect reliability - they still return reliable results as they have the same impact across all trials. Rather, if an experiment is unreliable, it indicares the presence of random errors which (like you said) reduced the accuracy of the relationship between the IV and the DV, thus reducing the validty of the experiment

4. I think these are mostly good responses. However, for the strength of qualitative data, your first point is not a strength but more of a description of what it is, and your last point is actually a weakness, not a strength - the strengths of qualitative data is that it can (like you said) provide additional depth to results, and may allow for data collection where quantitative aproaches are impractical (i.e. noting the nature of a color change during a reaction). For the weaknesses of qualitative data, its lower reliability is due to its subjectivitiy (something that you might want to explain) since it might rely on the experimenters own interpretation of an event which may change from experimenter to experimenter and may not actually be accurate - I wouldn't put reliability and subjectivity as seperate weaknesses.

Hope this helps and good luck with chem!

[turtlebanana]

Do we have to know how to draw monosaccharides?

I know how to draw alpha and beta glucose monomers, but do we also need to know how to draw galactose and fructose molecules?

EDIT: Nevermind, for anyone else wondering - they're all in the data booklet! (luckily)

[Erutepa]

Do we have to know how to draw monosaccharides?

I know how to draw alpha and beta glucose monomers, but do we also need to know how to draw galactose and fructose molecules?

The study design states:
"glucose, fructose, sucrose and the artificial sweetener aspartame with reference to their structures and energy content"
so I would take that to mean that they can ask you to draw any of those molecules, but do note that these are in your data booklet. However, only alpha glucose and beta fructose are in the booklet --> you draw beta glucose by flipping the hydroxyl group on the rightmost carbon (as its written in the data booklet) and alpha fructose by flipping the CH2OH group on the rightmost carbon (as its written in the data booklet).
That being said, I don't recall ever seing a question that asks you to draw fructose and certainly not galactose.

[anoushka_iyer]

Im not sure about this, but I would say that the colour change itself is qualitative data (if you were to record that the colour changed from blue to red for example) but If you were recording the volume of titre at the colour change, then that would be quantitative data.
This is becuase qualitative data deals with decriptions/characteristics while quantitative data deals with numerical values
They would bond to different Carbons. Carbon atoms will form 4 single bonds/1 double bond and 2 single bonds/1 triple bond and 1 single bond generally. If H and Br from hydrogen bromide were to both be added onto the same carbon from ethene, you would have a carbon with 3 single H-C bonds, 1 C-Br bond and a C-C bond (to the other carbon) making a total of 5 single bonds on that carbon, while you will also have another carbon with 3 single bonds (2 C-H bonds and a C-C bond). This just doesn't happen - it doesn't follow the rules of how carbon likes to bond.
Instead each atom from HBr will be added to another Carbon atom so that you have 2 carbons which each make 4 single bonds with other atoms.1. Internal Validity refers to the ability of the experimental design to accurately identify the effect of the IV on the DV
Both of your examples here do talk about potential aspects of an experimental design that does reduce its ability to accurately identify the effect of the IV on the DV, as such I think they are good responses.

2. Reliability isn't neccasarily the ability of another researcher to obtain the same data (this is an aspect of it though). Reliability refers to the ability of an experimental design to produce consistant data - this might be in the context of the same experimenter conducting the experiment multiple times, or by other experimenters conducting the experiment. Reliable experiments will produce precise (tightly grouped) data.
I think you would get the marks for what you have written though - its just that its probably best to refine your definition of reliability since VCAA does talk about it also in the context of repeating the same experiment/using large sample sizes (not reccasarily just others doing the experiment)

3. Rather than just saying that the experiment may be reliable but not valid due to uncontrolled varaiable, its probably best to identify systematic errors as the specific source of this inaccuracy of the experiment (systematic errors are things like incorrectly calibrated scales which will consistanly provide inaccurate results across all trials). Also rather than saying 'exclusive', I would personally write' accurate'.
Note that in your second point, systematic errors do not effect reliability - they still return reliable results as they have the same impact across all trials. Rather, if an experiment is unreliable, it indicares the presence of random errors which (like you said) reduced the accuracy of the relationship between the IV and the DV, thus reducing the validty of the experiment

4. I think these are mostly good responses. However, for the strength of qualitative data, your first point is not a strength but more of a description of what it is, and your last point is actually a weakness, not a strength - the strengths of qualitative data is that it can (like you said) provide additional depth to results, and may allow for data collection where quantitative aproaches are impractical (i.e. noting the nature of a color change during a reaction). For the weaknesses of qualitative data, its lower reliability is due to its subjectivitiy (something that you might want to explain) since it might rely on the experimenters own interpretation of an event which may change from experimenter to experimenter and may not actually be accurate - I wouldn't put reliability and subjectivity as seperate weaknesses.

Hope this helps and good luck with chem!

Thank you so much for your help!!! this really cleared things up

[anoushka_iyer]

what is the difference between uncertainty and errors?

[turtlebanana]

what is the difference between uncertainty and errors?

I feel like uncertainties are somewhat related to systematic errors.

Uncertainties are defined (in the Heinemann textbook) as ''errors in measurements made during an experiment'' and it references to uncertainties associated with volumetric equipment (e.g. a 20 mL pipette that may give an actual measurement of 0.03 mL below or above the reading).

Systematic errors are similar in the sense that they produce a ''constant bias in a measurement'' (e.g. using a 20 mL pippete that delivers 20.2 mL)

Could be completely wrong, but those are my thoughts

[turtlebanana]

In regards to cellulose, can anyone explain why it is insoluble?

Is it similar to amylose (alpha-glucose), wherein the molecule can form spiral-like helices, with many of the -OH groups inside the helices and away from contact with water?

Cellulose has an alternating structure of the CH₂OH and OH group and seems like it can easily form hydrogen bonds? So why is it insoluble?

[Erutepa]

In regards to cellulose, can anyone explain why it is insoluble?

Is it similar to amylose (alpha-glucose), wherein the molecule can form spiral-like helices, with many of the -OH groups inside the helices and away from contact with water?

Cellulose has an alternating structure of the CH₂OH and OH group and seems like it can easily form hydrogen bonds? So why is it insoluble?

This image from bioninja does help understand the differnces in properties of the cellulose, amylose and amylopectin quite well:

Spoiler

Here we see that cellulose forms long unbranched molecules who's hydroxyl groups form strong hydrogen bonds with neighbouring chains - stronger than the attraction between water and individual cellulose chains - thus resulting in the production of long fibres of many individual cellulose chains which are insoluable in water.

So it does form hydrogen bonds, but with itself - not water

[turtlebanana]

This image from bioninja does help understand the differnces in properties of the cellulose, amylose and amylopectin quite well:

Spoiler

Here we see that cellulose forms long unbranched molecules who's hydroxyl groups form strong hydrogen bonds with neighbouring chains - stronger than the attraction between water and individual cellulose chains - thus resulting in the production of long fibres of many individual cellulose chains which are insoluable in water.

So it does form hydrogen bonds, but with itself - not water

AMAAAAAAAZING thank you