Question

7. Enthalpy of chemical changes: a.For each of the following reaction, the ∆H is the enthalpy change when the number of moles indicated by the stoichiometric coefficients react. Calculate the ∆H when 1.00 g of the italicized substance is consumed or produced:

4Na (s) + O2(g)---------2Na2O (s) ∆H = -828 KJ

CaMg(CO3)2(s)---------------CaO (s) +MgO (s) +CO2(g) ∆H = +302 KJ

H2(g) + 2CO (g)--------------H2O2(l) + 2C (s) ∆H = +33.3 KJ

Answer 1

In the question you are not mension the italicized substance.

4Na (s) + O2(g) -------> 2Na2O (s) :∆H = -828 KJ

Molar mass(g/mol) 23

4 moles = 4x23 g of Na releases 828 kJ of heat

1g of na releases (828/(4x23) n= 9 kJ of heat

So ∆H = -9 kJ

CaMg(CO3)2(s) ------> CaO (s) +MgO (s) + 2CO2(g) :∆H = +302 KJ

Molar mass (g/mol) 56

For the formation of 1 mole= 56 g of CaO the heat required is 302 kJ

For the formation of 1 g of CaO the hear required is 302/56 = 5.39 kJ

So ∆H = +5.39 kJ

H2(g) + 2CO (g) ----> H2O2(l) + 2C (s) :∆H = +33.3 KJ

Molar mass (g/mol) 12

For the formation of 2 moles = 2x12 = 24 g of C the heat required is 33.3 kJ

For the formation of 1 g of C the heat required is 33.3/24 = 1.39 kJ