4 what is the net amount of heat released when two moles of c2h6 (g) are formed from its elements at 101.3 kpa and 298 k?

Enthalpy of formation is defined as the heat change that is needed for formation of one mole of substance from its constituent elements in pure state.

In present case, Heat of formation of one mole of C2H6 at 101.3 kpa and 298 k is - 84 kJ/mol. Negative sign indicates that, heat is evolved from the system.

Thus, for two mole of ethane, heat of formation will be 2 X (-84) = - 168 kJ/mol.

The enthalpy of formation is the heat change that is required to form one mole of a substance from its constituent elements.

In this case; the enthalpy of formation of one mole of ethane (C2H6) is - 84 kJ/mol; Heat released is 84 kj/mol

Therefore for two moles of ethane the amount of heat released will be twice the amount released with one mole of ethane

Thus; net amount of heat released will be 84 * 2 = 168 kJ/mol