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The heat content of the products is more than that of the reactant in an ............. reaction.

In this question the answer given is "exothermic"; yet I thought the answer would be endothermic as the products have more heat content and energy cannot be created nor destroyed. Therefore, from the reactants' side, more energy should have been put in for the heat content of the products to be greater.

Also mathematically endothermic make sense as: $$\Delta_\mathrm{r}H = \sum \Delta_\mathrm{f}H_\mathrm{P}-\sum \Delta_\mathrm{f}H_\mathrm{R}$$

And as: $\sum \Delta_\mathrm{f}H_\mathrm{P} \gt \sum \Delta_\mathrm{f}H_\mathrm{R}$ ,$\Delta_\mathrm{r}H \gt 0$

Could someone please explain where am I wrong and why the answer is 'exothermic'?

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Don't blame yourself. This is a badly-worded question. Products and reactants don't have "heat content", they have thermal energy. I.e., heat is not a property of substances—they don't 'contain heat.' Instead, heat is a property of processes; specifically, it is the flow of thermal energy across a boundary. So, for instance, in an exothermic reaction in a system with diathermal walls (walls that allow free flow of heat across them), thermal energy flows out of the system, and thus the heat (q) for that reaction is negative.

Having said that, it seems what they mean by "heat content" is the thermal energy of the system after a reaction if the system is adiabatic (no heat flow in or out, i.e., perfectly insulating walls). Thus, if you have an exothermic reaction in an insulated container, the products you get will be hotter than the reactants you started with.

A much better way of asking the question would be: Suppose a reaction takes place at constant pressure in an adiabatic (insulated) container, and the temperature increases. Is the reaction exothermic or endothermic? Answer: exothermic.

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    $\begingroup$ " Suppose a reaction takes place at constant pressure in an adiabatic (insulated) container, and the temperature increases. Is the reaction exothermic or endothermic? Answer: exothermic." - what? That doesn't make sense. First, you're leaving open the possibility of energy entering or leaving the system in the form of work (such as the container walls expanding). Second, if you require no energy entering or leaving through either heat transfer or work, then you can't also hold the pressure constant. $\endgroup$ – user2357112 supports Monica Aug 7 at 15:49
  • $\begingroup$ The IUPAC Gold Book definition for exothermic is that the std. enthalpy change ($\Delta H^o$) ("$^o$" =1 bar) is negative (goldbook.iupac.org/terms/view/E02269). That means if you did the reaction at 1 bar in a diathermal container, q would be negative (heat flow out of the system). Now suppose you did the same reaction at const. P (1 bar), but adiabatically. The latter describes a constant-pressure calorimeter (look it up). The walls are insulated, but P is kept constant. In that case, the energy released by the rxn causes an increase in T (even if some of that energy also goes towards work). $\endgroup$ – theorist Aug 7 at 20:43

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