Consider the reaction at equilibrium:
2NO2(g) <---> N2O2(g) + O2(g)
for which, H = −20.4 kJ · mol−1. If you
decrease the temperature of the system, what will occur?
Rewrite the equations so the heat shows.
2NO2(g) <---> N2O2(g) + O2(g) + heat
Now, decreasing T means the reaction will try to heat the reaction. so it shifts to the ......
how do I know which side heat is on?
In this case I wrote it on the correct side which is the right side. In any case however, remember that -delta H means the reaction is exothermic and + delta H means the reaction is endothermic. Look at this reaction; the problem tells you that delta H is -20 kJ/mol which means it is exothermic so you write heat as a product. If delta H is a + number it means the reaction is endothermic so you write heat as a reactant.
does the decrease mean the reaction shifts to the right?
To determine the effect of decreasing the temperature on this reaction, you need to consider the Le Chatelier's principle. According to Le Chatelier's principle, when a system at equilibrium is subjected to a change in temperature, it tends to shift in a way that counteracts the change.
In this case, the reaction is exothermic because H is negative (-20.4 kJ/mol). This means that heat is released during the forward reaction, and an increase in temperature will favor the reverse reaction (the reaction that absorbs heat). Conversely, decreasing the temperature will favor the forward reaction (the reaction that releases heat).
To understand this concept, you can think of temperature as a reactant in the equilibrium equation. By decreasing the temperature, you effectively decrease the concentration of the "reactant" side of the equilibrium equation. According to Le Chatelier's principle, a decrease in concentration of reactants will cause the reaction to shift in the direction that produces more reactants. Therefore, in this case, decreasing the temperature will favor the forward reaction, resulting in an increase in the concentration of NO2(g).
In summary, when you decrease the temperature of the system, the equilibrium will shift toward the forward direction, favoring the formation of more NO2(g).