If we have an exothermic reaction ( i.e. ΔH = -ve) , using the equation ΔH = ΔU + P(external)ΔV, am I right in thinking that if ΔH is to be negative, ΔU will also be negative since the thermal energy of the molecules will decrease due to transfer of energy with the surroundings? If this is correct, what does this mean for the volume of the system in this case, does it increase or decrease, or stay the same?

Thanks in advance.