Heating/cooling of phosphoric acid (with methyl violet indicator)

[HarveyD]

Recently did a prac involving an investigation into the effect temperature had on reactions in equilibrium
this particular test (phosphoric acid) brought many different answers, so i was wondering if anyone would be able to confirm for me whether or not is it an exothermic or endothermic reaction
We got
hotter - darker
colder - lighter

so would that be right?

[Greatness]

Yeah we did this prac too!
Heated: Blue
Cooled: Light blue/green

[HarveyD]

:O wow lol, maybe we did the exact same prac O_O
did you do 3 reactions overall (with changing temperature)?
cause we did this one, another with NO2 going to N2O4 and something else
and I've been trying to remember what the third one was for ages lol

How do you think they'll ask us to determine whether its endo/exo for this one?
Something to do with the indicator/pH or you think they'll just give us the colours?
hopefully its simple...

[thatricksta]

How did heating your phosphoric acid make it go darker???
It should have gotten lighter if anything, from green to blue/violet or something... surely not 'darker'

The reaction is exothermic, and the increase in temperature drives the reaction backwards*, hence the 'increase' in pH/ decrease in acidity as the H+ ions and "H3PO2" ions are converted to H3PO4 (conjugate acids are also converted) and the indicator should go from Green to Blue.

Although the backwards reaction would in theory turn lighter, I am unsure if its actually possible in the lab to observe significant results...

on a side note, this 'third part' you are thinking of is the equilibrium of Fe(SCN)2+

[24study]

You mean the exothermic reaction H3PO4 ----> H2PO4- + H+?.
In the hot water, the endothermic back reaction would be favored and so there would be a decrease in concentration of H+. This results in decreased acidity of the solution causing the methyl violet indicator to be closer to a violet colour (it started off as a green colour when I did the prac).
The opposite happens in the cold water as the forward exothermic reaction is favored which causes heat to be released, and so minimizing the temperature drop from the cold water. Due to this, the conc. Of H+ increases, which increases the acidity of the solution resulting in a green colour (or if you start off with a green colour, the colour will be a lighter shade of green)

So:
Hot water = blue colour
Cold water = green colour (or lightish green)

So in essence, the hotter temperatures result in the darker colours and the cooler temperatures result in the lighter colours

[thatricksta]

I don't agree that 'hotter temperatures result in darker colours'

this is a pretty ambiguous statement as at pH=1, methyl violet is a very strong green; versus the lighter coloured blue/violet at pH=2.

[24study]

I don't agree that 'hotter temperatures result in darker colours'

this is a pretty ambiguous statement as at pH=1, methyl violet is a very strong green; versus the lighter coloured blue/violet at pH=2.

Really? The green that I observed in the prac actually seemed lighter than the blue colour I observed. But I would agree that it is a bit ambiguous as I don't think it's really the darkness that matters so much but rather the colour change in general observed by the methyl violet indicator due to the change in acidity

[Greatness]

You mean the exothermic reaction H3PO4 ----> H2PO4- + H+?.
In the hot water, the endothermic back reaction would be favored and so there would be a decrease in concentration of H+. This results in decreased acidity of the solution causing the methyl violet indicator to be closer to a violet colour (it started off as a green colour when I did the prac).
The opposite happens in the cold water as the forward exothermic reaction is favored which causes heat to be released, and so minimizing the temperature drop from the cold water. Due to this, the conc. Of H+ increases, which increases the acidity of the solution resulting in a green colour (or if you start off with a green colour, the colour will be a lighter shade of green)

So:
Hot water = blue colour
Cold water = green colour (or lightish green)

So in essence, the hotter temperatures result in the darker colours and the cooler temperatures result in the lighter colours

Yep this is correct.

@ thatricksta: when an exothermic reaction gets heated it, it undergoes a back reaction? and when cooled it undergoes a forward reaction?

[thatricksta]

I don't agree that 'hotter temperatures result in darker colours'

this is a pretty ambiguous statement as at pH=1, methyl violet is a very strong green; versus the lighter coloured blue/violet at pH=2.

Really? The green that I observed in the prac actually seemed lighter than the blue colour I observed. But I would agree that it is a bit ambiguous as I don't think it's really the darkness that matters so much but rather the colour change in general observed by the methyl violet indicator due to the change in acidity

This is much better with indicators it is always the change in colour you are observing

@ thatricksta: when an exothermic reaction gets heated it, it undergoes a back reaction? and when cooled it undergoes a forward reaction?

Yes, the increase in temperature drives the backwards reaction as the backwards reaction is 'endothermic', which requires the heat to react. If you think about it logically you can conclude that with an addition of temperature will supply the energy the endothermic reaction requires. (I don't know how true that statement is but I find it sometimes help if confused).

The decrease in temperature however, drives the exothermic reaction. Basically for the opposite reasons for above.

I don't know if this helps but think of the room as a 'neutral' heat and when you increase the temperature or decrease the temperature, the reaction will 'oppose this change' (sound familiar?). Then remember that "endo" means requires energy, so imagine if you increase the temperature, the ENDOthermic reaction will occur and the temperature is 'absorbed' so that this room returns to 'neutral' heat. Vise versa in the scenario of decreasing the temperature as EXOthermic reactions release heat.

I have no idea if any of this will help you, but it sorta helps me... for the weirdest reasons Hopefully it does though!

[HarveyD]

thanks heaps guys, really helped!
Sorry about the colour confusion in the first post D:

btw how does this explanation sound for an exothermic reaction (when increasing temperature)
According to Le Chatelier's principle the system will oppose the change caused by in increase in temperature/energy by removing energy. As it can be seen by the colour of the solution, the system has undergone a net back reaction in order to do so/re-establish equilibrium, thus the reaction must be exothermic.

Is that enough detail?

[Greatness]

thanks heaps guys, really helped!
Sorry about the colour confusion in the first post D:

btw how does this explanation sound for an exothermic reaction (when increasing temperature)
According to Le Chatelier's principle the system will oppose the change caused by in increase in temperature/energy by removing energy. As it can be seen by the colour of the solution, the system has undergone a net back reaction in order to do so/re-establish equilibrium, thus the reaction must be exothermic.

Is that enough detail?

Maybe mention what the reactants and products are and how their concetrations change.

[HarveyD]

According to Le Chatelier's principle the system will oppose the change caused by in increase in temperature/energy by removing energy. As it can be seen by the colour of the solution, the system has undergone a net back reactionincreasing the concentration of the reactants (name would be there instead) in order to do so/re-establish equilibrium, thus the reaction must be exothermic.

Like that? or do i need to include the products for exothermic reactions as well?

[Greatness]

According to Le Chatelier's principle the system will oppose the change caused by in increase in temperature/energy by removing energy. As it can be seen by the colour of the solution, the system has undergone a net back reactionincreasing the concentration of the reactants (name would be there instead) in order to do so/re-establish equilibrium, thus the reaction must be exothermic.

Like that? or do i need to include the products for exothermic reactions as well?

Explaining one should be ok imo, if youre teacher marks harshly then put both in lol

[HarveyD]

thanks for all the help again guys
hope you did well on your sac swarley (and tricksta when you do yours )
^_^

[thatricksta]

thanks for all the help again guys
hope you did well on your sac swarley (and tricksta when you do yours )
^_^

haha i did a lot of it today, it was painful... our teacher has given us a lot to write about so decided we can have another period... our chemistry SAC's this year have been really poorly organised so I can't say I'm a fan... im heavily dependent on the exam to say the least