Chemistry exothermic and endothermic ENERGY CHANGED HELP

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Gus Fraser
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So I don’t understand this exothermic and endothermic question.
1) During a reaction between solutions of citric acid and sodium hydrogen carbonate, the temperature of the reaction mixture fell from 18c to 4c. Is this reaction exo or endothermic?

I thought it would be exothermic becuase temperature is lost so therefore energy (thermal) is being given out to the surroundings, which is the definition of exothermic. But no, it’s endothermic even though energy is taken in which implies it would get hotter.
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mercuryman
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(Original post by Gus Fraser)
So I don’t understand this exothermic and endothermic question.
1) During a reaction between solutions of citric acid and sodium hydrogen carbonate, the temperature of the reaction mixture fell from 18c to 4c. Is this reaction exo or endothermic?

I thought it would be exothermic becuase temperature is lost so therefore energy (thermal) is being given out to the surroundings, which is the definition of exothermic. But no, it’s endothermic even though energy is taken in which implies it would get hotter.
Not really a chem student anymore but from my understanding It has to be an endo reaction cos its mentioning the system's temperature being dropped by 14c. If the temp change of a reaction mixture is negative then that means its an endo reaction. There would be a positive increase in mixture temp if it was exo since the reaction generates more energy in terms of heat.

Ygm?
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Pigster
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(Original post by Gus Fraser)
So I don’t understand this exothermic and endothermic question.
1) During a reaction between solutions of citric acid and sodium hydrogen carbonate, the temperature of the reaction mixture fell from 18c to 4c. Is this reaction exo or endothermic?

I thought it would be exothermic becuase temperature is lost so therefore energy (thermal) is being given out to the surroundings, which is the definition of exothermic. But no, it’s endothermic even though energy is taken in which implies it would get hotter.
The issue is historical, in that energy changes involve heat flowing from one place to another, e.g. into a test tube full of reacting chemicals or out of it. Soooo, in order to do calculations, all me have to do is agree which direction of energy change (in or out) will have the +ve sign. It was decided to make 'in' get the + sign.

In your case, your chemicals are happily sat at 18 oC. When added together T drops to 4 oC. Now, heat will flow into the mixture (as heat always goes from where it is hot to cold), i.e. you are adding a +ve amount of heat and hence the energy change is +ve, i.e. endo.

Compare this with an exotheremic reaction, where heat flows outwords, i.e. you must add a -ve amount of energy.
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sotor
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(Original post by Gus Fraser)
So I don’t understand this exothermic and endothermic question.
1) During a reaction between solutions of citric acid and sodium hydrogen carbonate, the temperature of the reaction mixture fell from 18c to 4c. Is this reaction exo or endothermic?

I thought it would be exothermic becuase temperature is lost so therefore energy (thermal) is being given out to the surroundings, which is the definition of exothermic. But no, it’s endothermic even though energy is taken in which implies it would get hotter.
are you GCSE or A level?

temperature of the reaction mixture is a measure of the kinetic energy in the environment (surroundings).
the energy in the reaction system is a measure of the chemical energy stored in bonds.
from physics, you should know that energy is only transferred, never destroyed.

when a reaction is exothermic, energy is released from the bonds and given to the environment. this increases the kinetic energy of the environment and so the temperature goes up. therefore if the temperature of the mixture goes up, the reaction was exothermic.

when a reaction is endothermic, energy is taken from the environment and stored in the chemical bonds. this means there is less kinetic energy in the environment and so the temperature goes down. therefore if the temperature goes down, the reaction is endothermic.

does this make more sense? your mistake is thinking that the temperature is a measure of the energy in the system, not the surroundings.
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Gus Fraser
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Im GCSE, but planning to take chemistry into A level so thanks for your help!
(Original post by sotor)
are you GCSE or A level?

temperature of the reaction mixture is a measure of the kinetic energy in the environment (surroundings).
the energy in the reaction system is a measure of the chemical energy stored in bonds.
from physics, you should know that energy is only transferred, never destroyed.

when a reaction is exothermic, energy is released from the bonds and given to the environment. this increases the kinetic energy of the environment and so the temperature goes up. therefore if the temperature of the mixture goes up, the reaction was exothermic.

when a reaction is endothermic, energy is taken from the environment and stored in the chemical bonds. this means there is less kinetic energy in the environment and so the temperature goes down. therefore if the temperature goes down, the reaction is endothermic.

does this make more sense? your mistake is thinking that the temperature is a measure of the energy in the system, not the surroundings.
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TheTenthBox
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Temperature is NOT a measure of energy in this sense. If the temperature of the solution were to decrease, that means that the energy was actually used up in the reaction by the reactant molecules themselves, and so they attain a higher energy level. To attain more energy, it must be provided from the temperature.
Visit RevisePro - Chemistry for excellent chemistry resources
(Original post by Gus Fraser)
So I don’t understand this exothermic and endothermic question.
1) During a reaction between solutions of citric acid and sodium hydrogen carbonate, the temperature of the reaction mixture fell from 18c to 4c. Is this reaction exo or endothermic?

I thought it would be exothermic becuase temperature is lost so therefore energy (thermal) is being given out to the surroundings, which is the definition of exothermic. But no, it’s endothermic even though energy is taken in which implies it would get hotter.
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