HSC Chemistry Question Thread

[RuiAce]

I have a question about a 'control experiment'. In trying to find an answer on the web, all that came up were things about controlled variables. To elaborate, the control experiment in which I'm not completely certain of its purpose is always seen in the discussion of a first hand experiment, addressing improvements in validity. E.g. in the acidic topic the experiment of decarbonating a carbonated drink through heating on a hotplate/boiling water bath is not completely valid. To improve validity a control of water is used.

Could you please explain to me what a control experiment is and its purpose. Thank you

Having a control is basically having a basis. The same experiment with a carbonated drink should not yield the same results as water.

In the experiment, you could say that the variable is what your drink is. If it's (deionized) water, which is "probably" liquid in its most purest form, you should not expect results. I think of it like a "ground state" - we do nothing to it whatsoever.

Whereas a carbonated drink is full of all sorts of things to give it flavour. These "things" should be what's producing the CO2

(Also, note that if the WATER yielded positive results, then supposedly water would be a carbonated drink. If that's the case, in your coke can what if it was the water producing the CO2 and not something else?)

Sorry - Much like the above answer, all over the place

[Butterflygirl]

Also another question...

2H2 + O2 --> 2H20 (g), enthalpy change= -484kJ/mol

2H2 + O2 --> 2H20 (l)

Would you expect the heat change for the second reaction to be greater than, less than or the same as the heat change in the first reaction? Explain your answer.



Thanks!!

[RuiAce]

Also another question...

2H2 + O2 --> 2H20 (g), enthalpy change= -484kJ/mol

2H2 + O2 --> 2H20 (l)

Would you expect the heat change for the second reaction to be greater than, less than or the same as the heat change in the first reaction? Explain your answer.



Thanks!!

Less.

We are told that the reaction between hydrogen and oxygen to produce gaseous water is an exothermic reaction. But then we switch out the product with water in its liquid state. So the only thing to analyse is the product.

So consider this: How does water go from a liquid state into a gaseous state? The answer is that it simply gets heated. The water absorbs the heat energy around it to go from the liquid state upwards.

Essentially, if gaseous water is immediately produced, then this has already happened. The water has already absorbed the heat in its surroundings (which INCLUDES the heat released from the reaction) and changed states. But if liquid water is produced, then this did not happen. It may or may not have absorbed some of the heat that was already in the surroundings, but it did NOT absorb the heat produced from the reaction.

If the system (i.e. the water product) does not absorb this heat, then since the reactants are the same, we can only assume that the heat was simply released into the surroundings. This "unused" heat released to the surroundings makes the reaction more exothermic, thus lowering the enthalpy change value.

[legorgo18]

Hey um got a question

A studemt was told to dissolve 2g of NaOH in 2L of water. Calculate the concentration and pH of this solution

I searched on net and the eqn seems to be: NaOH + H2O > Na + OH + H2O

If this eqn is legit, then what is the solution? The na? or what is the actual equation?

[Kle123]

Hey um got a question

A studemt was told to dissolve 2g of NaOH in 2L of water. Calculate the concentration and pH of this solution

I searched on net and the eqn seems to be: NaOH + H2O > Na + OH + H2O

If this eqn is legit, then what is the solution? The na? or what is the actual equation?

I had to reduce the resolution to post. The working out doesn't show all steps. Hope you still understand.

[Butterflygirl]

Thanks for the help guys I have another question also...

For the question:
A cold pack contains 32.0g of ammonium nitrate that dissolves in 120g of water at 20 degrees celsius. The energy absorbed when the salt dissolves in the water is 10.28kJ. Calculate the minimum temperature that could be reached by this solution (the specific heat capacity of the solution is 4.18 J/g/degrees celsius).

I can calculate up to finding delta T but I'm not sure how to find the minimum temperature. I thought the minimum temperature was 20 because the solution absorbs energy so it should increase when the ammonium nitrate is dissolved in the water?

Help please, I'm finding hard to visualise what's actually going on
Thankyou so much

[sweetcheeks]

Thanks for the help guys I have another question also...

For the question:
A cold pack contains 32.0g of ammonium nitrate that dissolves in 120g of water at 20 degrees celsius. The energy absorbed when the salt dissolves in the water is 10.28kJ. Calculate the minimum temperature that could be reached by this solution (the specific heat capacity of the solution is 4.18 J/g/degrees celsius).

I can calculate up to finding delta T but I'm not sure how to find the minimum temperature. I thought the minimum temperature was 20 because the solution absorbs energy so it should increase when the ammonium nitrate is dissolved in the water?

Help please, I'm finding hard to visualise what's actually going on
Thankyou so much

When ammonium nitrate dissolves it is an endothermic reaction, resulting in the solution cooling down. The minimum temperature will be 20-deltaT. In general, if there is an absorption of energy there will be a decrease in temperature. If there is energy released the temperature will increase.

What is happening in an endothermic reaction is that the bonds of the products has a higher energy then the bonds of the reactants. This energy needs to come from somewhere, it is taken from the environment.

[Butterflygirl]

When ammonium nitrate dissolves it is an endothermic reaction, resulting in the solution cooling down. The minimum temperature will be 20-deltaT. In general, if there is an absorption of energy there will be a decrease in temperature. If there is energy released the temperature will increase.

What is happening in an endothermic reaction is that the bonds of the products has a higher energy then the bonds of the reactants. This energy needs to come from somewhere, it is taken from the environment.

Makes so much sense now, thankyou!!

[parthie]

If anyone has any of these experiments written up and would like to lend to me that would be a huge help I have a practical exam coming up and its worth 25% and I am really bad at writing up experiments

Bromine Water Decolourisation
Heat of Combustion of Alcohols
Galvanic Cells - measure voltage
Using flower petals as an indicator

Thankyou

Also if anyone has any tips about how to properly write up experiments because I lose all my marks there and practicals are worth 60% of my school mark

[anotherworld2b]

I was wondering would anyone know a good site/notes for learning the haber process and sulfuric acid the contact process?

[RuiAce]

I was wondering would anyone know a good site/notes for learning the haber process and sulfuric acid the contact process?

When I was learning the contact process I just used my textbook with "The Student's Guide to HSC Chemistry"

You could try "easychem" for the Haber process.

[kiwiberry]

If anyone has any of these experiments written up and would like to lend to me that would be a huge help I have a practical exam coming up and its worth 25% and I am really bad at writing up experiments

Bromine Water Decolourisation
Heat of Combustion of Alcohols
Galvanic Cells - measure voltage
Using flower petals as an indicator

Thankyou

Also if anyone has any tips about how to properly write up experiments because I lose all my marks there and practicals are worth 60% of my school mark

Hey! So the general things you should be including in your prac reports are:
- Aim
- Hypothesis
- Diagram of set-up - labelled!
- Method - not sure if it's just my school but this should be written in past tense
Eg. Change "Measure 100mL of water and place it into an aluminium can" to "100mL of water was measured and placed into an aluminium can"
- Risk assessment - state the risk, and ways to minimise this risk
- Results - could include observations, tables/graphs, calculations
For graphs, always remember to include a title, label your axes with units, and make sure your scale is appropriate (increments of 1, 2, 5, 10 etc.)
For tables, also remember to write a title, and make sure you draw in the borders - I know this might sound stupid but the teachers at my school are really picky and they took marks off people who didn't draw the outside borders on their tables :/
- Discussion - here you have to talk about accuracy, reliability and validity, as well as any improvements you could make to your experiment.
Accuracy refers to your measuring instruments - How sensitive were the instruments that you used? Could you have used something more accurate (scales with more decimal places or volumetric flask vs. measuring cylinder)? For things like the heat of combustion prac, how close were your results to the actual value?
Reliability refers to consistency of results - How many times did you repeat it? Did you obtain similar results everytime? Important to note that your results aren't reliable if you repeat 100 times but obtain completely different results
Validity refers to whether your method actually tests your aim - Did you control all your variables? Did you pick the most appropriate equipment?
- and finally, Conclusion - just a sentence or two about your results with reference to your aim/hypothesis

Hope this helps

[RuiAce]

- Method - not sure if it's just my school but this should be written in past tense

I think at my school, we used third person language and avoided the use of tense. Our steps were more like instructions.

[anotherworld2b]

okay
thank you

When I was learning the contact process I just used my textbook with "The Student's Guide to HSC Chemistry"

You could try "easychem" for the Haber process.

[anotherworld2b]

Would anyone have  advice on how to do these questions?