1)1200kJ of heat is evolved when2 moles of magnesium react completely with 1 mole of oxygen. How much energy is released if0.600g of Mg isburnt?

2Mg(s) + O2(g) à 2MgO(s)  

So DHc(Mg) = -1200kJmol-1/2 = -600 kJmol-1

n(Mg) = m/M = 0.600g/24.3g/mol= 0.0247mol

DH = q/n

so q = DH x n =-600 kJmol-1 x 0.0247mol = 14.8 kJ    

enthalphy change 후에 q 값을 찾아야 합니다.

​2)  20.0cm3 of 2 mol dm-3 NaOH is added to 30.0cm3 HCl of the same concentration. The temperature increases by12.0oC. Determine the heat released.

Total volume is 50.0cm3, water density is 1.00 g cm-3 so mass of aqueous solution is50.0g.

Amount of heat required to heat thewater, Q

= mcDT = 50.0 g x 4.18Jg-1ᵒC-1 x 12.0ᵒC= 2508 J or 2.51 kJ

So, 2.51 kJ is the amount of heatenergy evolved in the reaction

HCl(aq)  + NaOH(aq)à NaCl(aq)  + H2O(l)

So sodium hydroxide is the limitingreagent.

n(NaOH) = CV = 2M x 0.02dm3 = 0.0400 moles

(so the enthalpy PER MOLE can bedetermined)

DH(NaOH)= Q/n = 2.51kJ/0.04moles = 62.7 kJ/mol

The energy is evolved, exothermic, so

DH(NaOH)= -62.7 kJ/mol

​1번과 2번의 차이는 1번은 Mg의 energy enthaphy 의 값으로 0.6 g Mg 의 energy release 값을 구하는 것이고 2번은 heat energy  (must supplied to raise the temperture of mass m)값으로 amount of heat energy taken in/out in a chemical reaction (=enthapy change)의 값을 찾는것입니다 .