Confusion in understanding temperature change in exothermic reactions

The question due to which my doubt arose $\ce{->}$



> 
> The following system is in equilibrium:
>  $\ce{SO2Cl2 + {Heat} -> SO2 +Cl2}$
> 
> 
> What will happen to the temperature of the system initially if some $\ce{Cl2}$ is added into it at constant volume ? 
> 
> 
> 


**My attempt at the question** : Since if we add more $\ce{Cl2}$, concentration of products will increase which will in turn shift the equilibrium in backward direction, which will in turn release more heat. And since the heat is being released to the surrounding, temperature of the system should decrease. 


Though according to my textbook the answer is **temperature of the system will increase**.


So please do explain where my understanding is wrong.




I think this comes down to what constitutes the system. If we think of the system as the reactants and products in a container, releasing heat will increase the temperature of the system. 


 1

This really a comment but too long to fit. The question does not specify if the container holding the gas is to be treated isothermally or adiabatically. However, by inference since the question asks about temperature changes then it cannot be isothermal, i.e. it must be that no heat is gained or lost to the 'world' outside the container otherwise the question does not make sense. It is also implicitly assumed that the added gas is at the same temperature as the gases currently there.


The fact that the volume is fixed means that there is no work done against any external pressure, so all the heat generated by the reaction moving towards reactants heats up the gases.  

( Note also that in thermodynamics we always move from one equilibrium state to another equilibrium state; time is never considered.)


 1