# Help on a science chemistry question.

#1
During a reaction between solutions of citric acid and sodium hydrogen carbonate, the temperature fell from 18 degrees to 4 degrees.
Is this reaction exothermic or endothermic?
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2 years ago
#2
Endothermic - the temperature went down, so energy must have been absorbed from the surroundings
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#3
(Original post by Amiibo_Dealer)
Endothermic - the temperature went down, so energy must have been absorbed from the surroundings
The question also mentioned that the temperature of the mixture fell from 18 degrees to 14 degrees so would it still be endothermic?
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2 years ago
#4
(Original post by helpme2005)
The question also mentioned that the temperature of the mixture fell from 18 degrees to 14 degrees so would it still be endothermic?
Sorry, did you mean the temperature of the solution or the temperature of the surroundings?

If the temperature of the solution went from 18 degrees to 4 degrees, then the reaction is exothermic because heat is lost from the solution.
If the temperature of the surroundings went from 18 degrees to 4 degrees, then the reaction is endothermic because the solution gains heat from the surroundings (so surroundings get cooler and mixture gets hotter in an endothermic reaction).

So if the temperature of the mixture fell from 18 degrees to 14 degrees, it would be an exothermic reaction because the mixture loses heat
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2 years ago
#5
(Original post by helpme2005)
The question also mentioned that the temperature of the mixture fell from 18 degrees to 14 degrees so would it still be endothermic?
18 to 14 is a drop in T, so would be endo.
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2 years ago
#6
that is incorrect what you told him was bs pm me and i will tell you
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2 years ago
#7
(Original post by Amiibo_Dealer)
So if the temperature of the mixture fell from 18 degrees to 14 degrees, it would be an exothermic reaction because the mixture loses heat
Enthalpy changes are defined in terms of heat being added to a system.

If T decreases, energy will flow into the system from the surroundings. A positive amount of energy added to the system and hence is endothermic
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#8
(Original post by Amiibo_Dealer)
Sorry, did you mean the temperature of the solution or the temperature of the surroundings?

If the temperature of the solution went from 18 degrees to 4 degrees, then the reaction is exothermic because heat is lost from the solution.
If the temperature of the surroundings went from 18 degrees to 4 degrees, then the reaction is endothermic because the solution gains heat from the surroundings (so surroundings get cooler and mixture gets hotter in an endothermic reaction).

So if the temperature of the mixture fell from 18 degrees to 14 degrees, it would be an exothermic reaction because the mixture loses heat
I agree with what your saying, the question said that the temperature of the mixture fell from 18 degrees to 4 degrees. I wrote exothermic but the AQA mark scheme said endothermic but it doesn't make sense.
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#9
that is incorrect what you told him was bs pm me and i will tell you
(Original post by Pigster)
Enthalpy changes are defined in terms of heat being added to a system.

If T decreases, energy will flow into the system from the surroundings. A positive amount of energy added to the system and hence is endothermic
I have no idea what you are talking about Pigster?
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2 years ago
#10
(Original post by helpme2005)
I have no idea what you are talking about Pigster?
Put simply, if a reaction causes the temperature of your chemicals to increase it is an exothermic reaction.

If the temperature drops, it is an endothermic reaction.
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#11
(Original post by Pigster)
Put simply, if a reaction causes the temperature of your chemicals to increase it is an exothermic reaction.

If the temperature drops, it is an endothermic reaction.
Not really.
I think you meant if the temperature of the surroundings decreases the reaction is endothermic- it took in heat. If it is exothermic the temperature of the surrounding would increase- it gives out heat energy.
But if the temperature of the chemicals increases it is an endothermic reaction because it took in heat energy.
And if the temperature of the chemicals decreased it is an exothermic reaction- it gave out heat
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2 years ago
#12
(Original post by helpme2005)
I think you meant...
I know what I mean.

Let's put it another way:

The system (your chemicals) and the surrounding (everything else, simplistically: the room) are in thermal equilibrium (they are at the same temperature).

A reaction takes place in the system. Bonds break and new bonds form. There is less energy in the new bonds than the old bonds and therefore energy is released as thermal energy. The system heats up.

The system and surroundings are no longer in thermal equilibrium (the system is hotter), so heat flows from where it is hot to where it is cold, i.e. from the system to the surroundings.

A negative amount of energy is being added to the system, which we define as exothermic.

You might argue that your statement "if the temperature of the surroundings decreases the reaction is endothermic" is equivalent to what I have said, but your argument sounds to me like cart before horse. 1st the system heats up, 2nd this heats the surroundings up.

You followed with "it took in heat", but you did not clearly state what you meant by "it" - were you referring to the surroundings? It would seem so, since the previous part of the sentence had "surroundings" as the subject. If so, that you are wrong as taking in heat usually involves temperatures increasing, rather than decreasing.

vice versa for endothermic, but when the new bonds form, thermal energy is stolen from system by the reactions taking place. There is therefore less thermal energy in the system and therefore heat flows inwards from the surroundings.

Your last two sentences including "But if the temperature of the chemicals increases it is an endothermic reaction because it took in heat energy" are just plain wrong. Endothermic reactions involve the temperature of the chemicals decreasing. The temperature will increase once the reaction has finished, but this is just the system returning to thermal equilibrium with the surroundings and not a feature of the reaction.
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#13
(Original post by Pigster)
I know what I mean.

Let's put it another way:

The system (your chemicals) and the surrounding (everything else, simplistically: the room) are in thermal equilibrium (they are at the same temperature).

A reaction takes place in the system. Bonds break and new bonds form. There is less energy in the new bonds than the old bonds and therefore energy is released as thermal energy. The system heats up.

The system and surroundings are no longer in thermal equilibrium (the system is hotter), so heat flows from where it is hot to where it is cold, i.e. from the system to the surroundings.

A negative amount of energy is being added to the system, which we define as exothermic.

You might argue that your statement "if the temperature of the surroundings decreases the reaction is endothermic" is equivalent to what I have said, but your argument sounds to me like cart before horse. 1st the system heats up, 2nd this heats the surroundings up.

You followed with "it took in heat", but you did not clearly state what you meant by "it" - were you referring to the surroundings? It would seem so, since the previous part of the sentence had "surroundings" as the subject. If so, that you are wrong as taking in heat usually involves temperatures increasing, rather than decreasing.

vice versa for endothermic, but when the new bonds form, thermal energy is stolen from system by the reactions taking place. There is therefore less thermal energy in the system and therefore heat flows inwards from the surroundings.

Your last two sentences including "But if the temperature of the chemicals increases it is an endothermic reaction because it took in heat energy" are just plain wrong. Endothermic reactions involve the temperature of the chemicals decreasing. The temperature will increase once the reaction has finished, but this is just the system returning to thermal equilibrium with the surroundings and not a feature of the reaction.
I'm doing GCSE's and am not taught it in this much detail but thanks...
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