For a chemical reaction,when does $\Delta H = \Delta U$ occur?

Fill in the blanks:
$(i)$ The conversion of graphite into diamond is a ........ process.
$(ii)$ $1 \text{ calorie} = ........ \text{ Joules}$.
$(iii)$ Work done by the system can be calculated by the equation ........ .
$(iv)$ The change in heat energy during a chemical reaction at constant volume is called ........ .

$A$ gas is allowed to expand against a constant external pressure of $2.5 \ bar$ from an initial volume '$x$' $L$ to a final volume of $4.5 \ L$. If the amount of work done is $5 \ dm^3 \ bar$,what is the value of '$x$' (in $L$)?

Calculate the work done in joule if $1 \ mole$ of an ideal gas is compressed from volume $24 \ dm^3$ to $13 \ dm^3$ at a constant external pressure of $3 \ bar$. (in $J$)

It is a general principle that the less energy a system contains,it is:

What is the enthalpy change (in $J$) for converting $9 \ g$ of $H_2O(l)$ at $+10^{\circ}C$ to $H_2O(l)$ at $+20^{\circ}C$? $(C_p(H_2O(l)) = 75 \ J \ mol^{-1} \ K^{-1})$