Mavs04
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Hi so I’ve got myself in a muddle about energy changes. An exothermic reaction transfers energy out of the system and heats up the surroundings, which would make the temperature of a thermometer increase. I’m a bit confused because if the system is transferring energy away, wouldn’t the particles be losing energy so it would cool down?
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m.aurelius
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(Original post by Mavs04)
Hi so I’ve got myself in a muddle about energy changes. An exothermic reaction transfers energy out of the system and heats up the surroundings, which would make the temperature of a thermometer increase. I’m a bit confused because if the system is transferring energy away, wouldn’t the particles be losing energy so it would cool down?
So I’m not sure I’m qualified to answer (I’m doing y13 chem) but when we talk about enthalpy change, it’s important to keep in mind that energy is always conserved - ie it cannot be created or destroyed. Therefore any energy gained must have been lost from somewhere. So enthalpy change can be broken down to the enthalpy change of the system (the reactants and products) and enthalpy change of the surroundings (eg the water, thermometer, flask).

In an exothermic reaction, energy is lost from the system to the surroundings and so the chemical system LOSES energy. This is why delta H is negative. The temperature can only measure the SURROUNDINGS, not the system (particles) so if we see that the temperature has increased, we know that that energy must have come from somewhere —it was lost from the particles. The system loses chemical energy but I don’t think it’s the same as the heat energy that is transferred to the surroundings so I don’t think you can say the particles themselves cool down exactly (but I’m not sure about this part) The universe is both system and surroundings so overall, no energy is lost or created.
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m.aurelius
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I think I went into a bit too much detail but hopefully that answers it?
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Mavs04
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(Original post by m.aurelius)
So I’m not sure I’m qualified to answer (I’m doing y13 chem) but when we talk about enthalpy change, it’s important to keep in mind that energy is always conserved - ie it cannot be created or destroyed. Therefore any energy gained must have been lost from somewhere. So enthalpy change can be broken down to the enthalpy change of the system (the reactants and products) and enthalpy change of the surroundings (eg the water, thermometer, flask).

In an exothermic reaction, energy is lost from the system to the surroundings and so the chemical system LOSES energy. This is why delta H is negative. The temperature can only measure the SURROUNDINGS, not the system (particles) so if we see that the temperature has increased, we know that that energy must have come from somewhere —it was lost from the particles. The system loses chemical energy but I don’t think it’s the same as the heat energy that is transferred to the surroundings so I don’t think you can say the particles themselves cool down exactly (but I’m not sure about this part) The universe is both system and surroundings so overall, no energy is lost or created.
Ahhh that’s rlly helpful I think I understand now tysm!!
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Pigster
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(Original post by Mavs04)
Ahhh that’s rlly helpful I think I understand now tysm!!
The thermometer is part of the surroundings.

Heat is being released by the reactions (i.e. exothermic), the system heats up and then heat flows from the system to the surroundings, heating the thermometer up.
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