A system gives out $$\mathrm{x} \mathrm{~J}$$ of heat and does $$\mathrm{y} \mathrm{~J}$$ of work on it's surrounding. What is the internal energy change?

A system does 394 J of work on surrounding by absorbing 701 J heat. What is the change in internal energy of the system?

Formation of $$\mathrm{NO}_{2(\mathrm{g})}$$ from $$\mathrm{N}_{2(\mathrm{g})}$$ and $$\mathrm{O}_{2(\mathrm{g})}$$ is an endothermic process. Which of the following is true for this reaction?

What is enthalpy of formation of $$\mathrm{NH}_3$$ if bond enthalpies are as $$(\mathrm{N} \equiv \mathrm{N})=941 \mathrm{~kJ},(\mathrm{H}-\mathrm{H})=436 \mathrm{~kJ},(\mathrm{N}-\mathrm{H})=389 \mathrm{~kJ}$$ ?