sir first law of thermodynamics mein delta U = q - w hoga na
video mein delta U = q + w likha h.....................
donot get confuse it is just a convention
in physics our priority is to get the work by the system to surrounding hence we take as positive and work done on the system is negative
but in chemistry our priority is to make change in system of reactants in order to make new system of chemical reactants at higher temperature hence here we take work done on the system as possitive and work done by the system as negative