Experiment results
a) The chemicals before mixing - note the colours
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| Br-water |
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| Cyclohexane |
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| Cyclohexene |
b) The chemicals immediately after mixing - are there any differences?
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| Cyclohexane and Br-water (immediate) |
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| Cyclohexene and Br-water (immediate) |
c) The mixtures after 10, 15 and 20 minutes
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| Cyclohexane (a) & Cyclohexene (e) 10 min after mixing with Br-water |
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| Cyclohexane (a) & Cyclohexene (e) 15 min after mixing with Br-water |
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| Cyclohexane (a) & Cyclohexene (e) 20 min after mixing with Br-water |
i) Which chemical reacted more rapidly with the Br-water and what evidence led you to that conclusion
ii) Why were two layers formed when the chemicals were mixed (ie why were they immiscible)?
iii) Which layer was on the bottom and why?
iv) The cyclohexane layer got browner over time suggesting that the bromine was dissolving into it - why is bromine soluble in cyclohexane?
v) For the same reason as (iv) bromine should also be soluble in cyclohexene - why didn't the cyclohexene turn brown?vi) For the chemical that reacted with bromine water - can you name and draw a structural diagram of the product?
Using a control
In the fume cupboard were three extra test tubes:
*One with just cyclohexane
*One with just cyclohexene and
*One with just Br-water
They just sat there and weren't mixed with anything. Throughout the whole time the Br-water stayed brown and the cyclohexene and cyclohexane stayed clear.
Thus, by comparing our results to the control, we can be sure that the change in colour was due to the reaction between the cyclohexene and the Br-water, ie not just due to some other environmental effect or because the Br-water naturally changes colour.
Reliability
We repeated the experiment with consistent results AND we can use other secondary sources to check the reliability of our results. See the clips below
PORTFOLIO #1 Feedback on Worksheets
MOST IMPORTANT TIP – DO NOT EVER leave any of these questions blank. They are a compulsory part of the coursework and are a prerequisite for getting your draft summary marked. IF you cannot do a question you need to:
- Look up the answer in your notes/text
- Ask a classmate
- Ask Dr Burg
Then you need to do the question
Q2)
When a question states structural formula and names you have to give structural formula AND names - amazing! Also – unless you have finished you HSC exam you are obligated to put ALL the hydrogens on ALL your carbon chains. Seeing students take shortcuts in these practise questions makes me want to use octane, nonane and decane for ALL hydrocarbon questions in your exams. You have been warned.
Q3) & Q5)
For combustions make sure you balance the carbon , then hydrogen, then oxygen (CHO)
Q4)
Remember that in substitution reactions (for alkanes) only ONE species is added to the chain in the place of ONE hydrogen (i.e. the hydrogen is substituted for something else)
Q6)
Alkenes undergo addition reactions. TWO species are added to the alkene AT THE SITE OF THE DOUBLE BOND. Thus, if the double bond is between the first and second carbon, the species added will be on the first and second carbon. In addition reactions (unlike substitution reactions) only one product is produced.
E.g. if chlorine gas is added to 1-pentene the product will be 1,2-dichloropentane.
NOTE: there is commas between numbers and hyphen between letters and numbers when naming
E.g. if water is added to hex-2-ene (with a sulfuric acid catalyst) we are adding a H and a OH group to a hexene chain with a double bond between the second and third carbon. Thus the OH will go on either the second or third carbon. So the product could be hexan-2-ol or hexan-3-ol.
NOTE: When adding asymmetrical molecules (water, HCl, HBr etc) to alkenes bigger than ethylene there are always two possible products
Q7)
*It is important to include any necessary catalysts for these reactions.
* Sometimes there will be more than one way to make the chemical.
* You need to make sure your equations are balanced. It is easier to do this if you draw and think about the structures.
E.g. If I want to make 1-bromobutane (CH2BrCH2CH2CH3) from but-1-ene (CH2CHCH2CH3) I need to add an extra hydrogen and a bromine atom. Thus I have to react the but-1-ene with HBr!
Q8)
Where the table says ‘Burns’ it means ‘Combusts’ – so most people still need to write the combustion equations for pentane, hexane and ethanol. Balancing combustion reactions is something we all need to practise (CHO!)
PORTFOLIO #1 – Feedback on Summary Sheets
Note – these summaries were very well done. The addition (lol, that’s very punny Dr Burg!) of skills to this experiment summary made it quite challenging and you all did very well. I had to increase difficulty of the marking scheme to distinguish between you.
NOTE – in this experiment decoloured (which means REMOVE colour) is a better term to use than discoloured (which just means CHANGE colour).
(A) There is something in Engineering (and Science) called the KIS principle (Keep It Simple). Don’t say more than you need to. Answer the question and that is it. You only had to explain why alkenes are more reactive than alkanes. Eg “Unlike single-bonded alkanes, alkenes have a carbon-carbon double bond that allows them to undergo addition reactions. Thus, alkenes are more reactive than alkanes.”
(B) The dependent variable is the factor that is measured in order to determine how it responds to changes made to the independent variable (i.e. the value of the dependent variable DEPENDS on the independent variable) . In this experiment the dependent variable is the reactivity of the molecules (which we measured by the rate at which they decoloured bromine water).
(C) We need to control all variables other than the DV and IV to make the experiment valid – ie so that measured changes to the DV are ONLY due to the changes made to the IV. Some examples from this experiment include temperature, time of measurement, light intensity, and the fact that we used corresponding molecules (same # of carbons with the same shape).
(D) A control is a sample that is left unchanged so that all other results can be compared to it (ie to determine if the IV really had any effect on the DV). In this case we left samples of bromine water, cyclohexene and cyclohexane unmixed over the same time period as the experiment. The fact that they did not change colour indicates that the observed colour changes in the experiment were due to the reaction between cyclohexene and bromine water.
(E) Make sure you name the reactants and products. Remember that when naming hydrocarbons there are commas between numbers and hyphens between letters and numbers. Eg CCl3-CClF2 is called 1,1,1,2-tetrachloro-2,2-difluoroethane. Also - bromine (the element) is a liquid but bromine water is a aqueous solution of bromine – ie Br2 (aq)
(F) Even though cyclohexane was the only chemical identified as being dangerous for the environment, we should never tip harmful chemicals down the sink if we can avoid it. All three were stored for proper disposal.
(G) Reliability is a measure of whether the results are reproducible. This requires the experiment to be repeated with consistent results. Validity is a measure of whether the investigation was able to measure what was intended. The most important way to ensure a valid experiment is to ensure that all variables are controlled so that the only thing affecting the DV is the chnages made to the IV (but it also requires accurate measurement and reliability).
(H) Don’t forget that we did the experiment in a fume cupboard!
(I) A cause is a REASON for something happening. An effect is what happens as a RESULT of the cause. A great ay to explain a cause-and-effect situation is this awesome word: ‘because’ (it introduces a cause). Eg “Because alkenes have a reactive double bond (the cause), they undergo an addition reaction with bromine water (the effect)”.
(J) To explain reliability we need to consider whether we repeated the experiment with consistent results. Even though no individual in the class repeated the experiment, each group did the same experiment and all groups obtained the same result.
(K) You have been asked to write four different conclusion sentences (from slightly different perspectives). You need to treat each as if it was a stand-alone conclusion. So each needs to say (somehow) that the cyclohexene reacted faster than cyclohexane with bromine water and thus that alkenes are more reactive than alkanes.
(L) Quantitative results involve measured numerical values (quantities, eg mass, time, volume). Qualitative results involve descriptive measurements of observations (eg colour, shape, size, identity).
(M) You need to include labels on your diagrams. If your diagrams are not coloured you need to label the colours. You should label that the bottom layer was always the aqueous (bromine water) layer. It is important to note that the hydrocarbon layer NEVER mixed with the aqueous layer (remember ‘like dissolves like’). Even when the bromine water was completely decoloured there were still two layers: a clear cyclohexene mixture on top and a clear aqueous mixture below.
(N) It is a good idea to make your safety section concise by combining all the similar risks together but be careful not to make erroneous statements. Eg you cannot say that the bromine water risks and precautions are identical to cyclohexene, as cyclohexene is flammable and bromine water is not.
(O) All hydrocarbon reaction equations (apart from combustion, photosynthesis and fermentation should be structural equations - with states.
(P) You need to make your statements generalised - ie we studied this one simple system of cyclohexene and cyclohexane (ignore the hexane) so we know that the only thing affecting the rate of reaction is the double bond. Thus, we can conclude more than just cyclohexene reacts faster than cyclohexane - we can make the generalisation that alkenes are more reactive than...
(Q) Specify what your controlled variables were
PORTFOLIO #1 – Feedback on Summary Sheets
Note – these summaries were very well done. The addition (lol, that’s very punny Dr Burg!) of skills to this experiment summary made it quite challenging and you all did very well. I had to increase difficulty of the marking scheme to distinguish between you.
NOTE – in this experiment decoloured (which means REMOVE colour) is a better term to use than discoloured (which just means CHANGE colour).
(A) There is something in Engineering (and Science) called the KIS principle (Keep It Simple). Don’t say more than you need to. Answer the question and that is it. You only had to explain why alkenes are more reactive than alkanes. Eg “Unlike single-bonded alkanes, alkenes have a carbon-carbon double bond that allows them to undergo addition reactions. Thus, alkenes are more reactive than alkanes.”
(B) The dependent variable is the factor that is measured in order to determine how it responds to changes made to the independent variable (i.e. the value of the dependent variable DEPENDS on the independent variable) . In this experiment the dependent variable is the reactivity of the molecules (which we measured by the rate at which they decoloured bromine water).
(C) We need to control all variables other than the DV and IV to make the experiment valid – ie so that measured changes to the DV are ONLY due to the changes made to the IV. Some examples from this experiment include temperature, time of measurement, light intensity, and the fact that we used corresponding molecules (same # of carbons with the same shape).
(D) A control is a sample that is left unchanged so that all other results can be compared to it (ie to determine if the IV really had any effect on the DV). In this case we left samples of bromine water, cyclohexene and cyclohexane unmixed over the same time period as the experiment. The fact that they did not change colour indicates that the observed colour changes in the experiment were due to the reaction between cyclohexene and bromine water.
(E) Make sure you name the reactants and products. Remember that when naming hydrocarbons there are commas between numbers and hyphens between letters and numbers. Eg CCl3-CClF2 is called 1,1,1,2-tetrachloro-2,2-difluoroethane. Also - bromine (the element) is a liquid but bromine water is a aqueous solution of bromine – ie Br2 (aq)
(F) Even though cyclohexane was the only chemical identified as being dangerous for the environment, we should never tip harmful chemicals down the sink if we can avoid it. All three were stored for proper disposal.
(G) Reliability is a measure of whether the results are reproducible. This requires the experiment to be repeated with consistent results. Validity is a measure of whether the investigation was able to measure what was intended. The most important way to ensure a valid experiment is to ensure that all variables are controlled so that the only thing affecting the DV is the chnages made to the IV (but it also requires accurate measurement and reliability).
(H) Don’t forget that we did the experiment in a fume cupboard!
(I) A cause is a REASON for something happening. An effect is what happens as a RESULT of the cause. A great ay to explain a cause-and-effect situation is this awesome word: ‘because’ (it introduces a cause). Eg “Because alkenes have a reactive double bond (the cause), they undergo an addition reaction with bromine water (the effect)”.
(J) To explain reliability we need to consider whether we repeated the experiment with consistent results. Even though no individual in the class repeated the experiment, each group did the same experiment and all groups obtained the same result.
(K) You have been asked to write four different conclusion sentences (from slightly different perspectives). You need to treat each as if it was a stand-alone conclusion. So each needs to say (somehow) that the cyclohexene reacted faster than cyclohexane with bromine water and thus that alkenes are more reactive than alkanes.
(L) Quantitative results involve measured numerical values (quantities, eg mass, time, volume). Qualitative results involve descriptive measurements of observations (eg colour, shape, size, identity).
(M) You need to include labels on your diagrams. If your diagrams are not coloured you need to label the colours. You should label that the bottom layer was always the aqueous (bromine water) layer. It is important to note that the hydrocarbon layer NEVER mixed with the aqueous layer (remember ‘like dissolves like’). Even when the bromine water was completely decoloured there were still two layers: a clear cyclohexene mixture on top and a clear aqueous mixture below.
(N) It is a good idea to make your safety section concise by combining all the similar risks together but be careful not to make erroneous statements. Eg you cannot say that the bromine water risks and precautions are identical to cyclohexene, as cyclohexene is flammable and bromine water is not.
(O) All hydrocarbon reaction equations (apart from combustion, photosynthesis and fermentation should be structural equations - with states.
(P) You need to make your statements generalised - ie we studied this one simple system of cyclohexene and cyclohexane (ignore the hexane) so we know that the only thing affecting the rate of reaction is the double bond. Thus, we can conclude more than just cyclohexene reacts faster than cyclohexane - we can make the generalisation that alkenes are more reactive than...
(Q) Specify what your controlled variables were
i feel the following 'things to ponder' questions were asked sometimes in hsc exam papers.
ReplyDeleteii) Why were two layers formed when the chemicals were mixed (ie why were they immiscible)?
i want to know this one. is it because of density? and Br2 has a heavier mol. weight?
iii) Which layer was on the bottom and why?
this would be the denser layer (im guessing the bromine part, which separated from H2O as the latter bonded with cyclohexane/ene through addition/sub reactions and hence they turned clear? but through equations, its obvious this is not the case as cyclohexane bonds with a Br on the hydrocarbon ring plus an additional HBr, and the cyclohexene breaks its double bond for 2 Br. please explain on this thanks
iv) The cyclohexane layer got browner over time suggesting that the bromine was dissolving into it - why is bromine soluble in cyclohexane?
v) For the same reason as (iv) bromine should also be soluble in cyclohexene - why didn't the cyclohexene turn brown? this ive not been able to find anywhere; as why exactly does a solution decolour? does ions at play change the colour itself?
please send a copy of your reply to :redplayer0@hotmail.com thanks :)
I think:
ReplyDeleteii) the Br-water layer is predominantly water (polar) and cyclohexene and cyclohexane are non-polar, thus immiscible
iii) The Br-water layer is denser thus sits at the bottom - see photos (b) - the brown Br-water layer is at the bottom
iv) The Br2 is non-polar thus is more soluble in non-polar solvents - thus over time there is a net diffusion of Br2 fro the water layer to the cyclohexene and cyclohexane layers - this can be seen in photos (b) as well.
v) When the Br2 dissolves in the cyclohexane layers 'no' reaction occurs (there would be a slow substitution reaction but experiment was kept in the dark so there was no UV to facilitate the reaction) thus you can see the brown Br2 in the cyclohexane layer. But when the Br2 diffuses into the cyclohexene layer it reacts in an addition reaction with the cyclohexene to form a colourless product - thus the brown cannot be seen. What that product is depends on which source you read: 1,2-dibromocyclohexane, 2-bromocyclohexanol...
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