If at 298 K the bond energies of C - H, C - C, C = C and H - H bonds are respectively 414, 347, 615 and 435 kJ/mol, the value of enthalpy change for the reaction
H₂C = CH₂(g) + H₂(g) $$\to$$ H₃C - CH₃(g) at 298 K will be :

For the reactions
2C + O₂ $$\to$$ 2CO₂; $$\Delta H$$ = -393 J
2Zn + O₂ $$\to$$ 2ZnO; $$\Delta H$$ = -412 J

The heat required to raise the temperature of body by 1 K is called :

A heat engine absorbs heat Q₁ at temperature T₁ and heat Q₂ at temperature T₂. Work done by the engine is J (Q₁ + Q₂). This data :