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So I got a problem recently which asked to calculate the amount of energy needed to increase the temperature of 200 g of water from 26 to 51 degrees Celsius. The value I got was 20900 J.

So far I feel like it is endothermic because heat had to be added to the water in order for its temperature to go up, so the water essentially absorbed heat, but I'm really not sure.

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    $\begingroup$ Just heating water is not a chemical reaction. $\endgroup$ – MaxW Apr 11 at 23:31
  • $\begingroup$ Oh well I got this on a test...weird. The wording was just to calculate the amount of thermal energy needed to increase the temperature, and then to say if that was endothermic or exothermic. You're saying it's neither then ? $\endgroup$ – Sajjad Abbas Apr 12 at 0:02
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    $\begingroup$ Heating is an endothermic process, but not a chemical reaction. $\endgroup$ – A.K. Apr 12 at 0:59
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So far I feel like it is endothermic because heat had to be added to the water in order for its temperature to go up, so the water essentially absorbed heat, but I'm really not sure.

This is mostly semantics. Such exam questions are annoying when they make you argue about words. The term endothermic and exothermic are reserved for reactions by IUPAC. An endothermic reaction is one where the overall enthalpy change is positive. However one can extend it to endothermic process, which is any process that acquires energy (from external surroundings). Atkin's physical chemistry clarifies it nicely "it must never be forgotten that heat is a process (transfer of energy as a result of temperature difference), not an entity"

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  • $\begingroup$ Really playing with names. It is what most of the high school books do in half their size. Think about how much time is devoted to nomenclature of salts and balancing reaction. ...too. Instead of giving solid bases, they reduce to sudoku activity :( $\endgroup$ – Alchimista Apr 12 at 9:47
  • $\begingroup$ I cannot agree more. Excellent point. There is a beautiful article by Gillespie (VSEPR guy) from1970s. He said chemistry gets the lowest ranks among other sciences and engineering. He meant that chemistry is not choice of the brightest. He was lamenting Canadian system, which is almost universally true. $\endgroup$ – M. Farooq Apr 12 at 20:29
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Exothermic means the enthalpy of a system decreases ($\Delta H < 0$) and endothermic means it increases ($\Delta H > 0$). If you choose your system as just the water, the process is endothermic. This makes sense because you are increasing the thermal energy of the water, and if nothing else changes in the system, the energy has to come from the surrounding.

The question does not specify how you increase the temperature. You could spin a disk on the bottom of your containiner, using friction to turn mechanical work into thermal energy. You could increase the temperature of the water using a microwave oven. Probably, however, you would just place the container with water on a hot surface, and increase the temperature by heat transfer.

If you choose your system to be elemental sodium and your water mixed together, the enthalpy of the system would be constant, and the temperature of the water would increase from the energy released by the chemical reaction. If you choose your system to be just the sodium and the water it reacts with, it is an exothermic process that heats up the surrounding (the water that does not react increases in temperature as a consequence of the exothermic reaction).

Is heating water endothermic?

It depends what you choose as the system and as the surrounding. As an aside, we say "heating up water" to signify we are somehow increasing its temperature, but as the examples above show, this increase in temperature does not necessarily require the transfer of heat.

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