For a chemical reaction dH denotes the change in the internal energy of the reactants and the internal energy of the products. When we look at exothermic reactions dH is negative as the sum of the internal energy of the products is lesser than the sum of the internal energy of the reactants. The opposite is true for endothermic reactions.
In the reaction 6C + 3H2 --> C6H6 , dH = 49.03 kJ/mole
This is an endothermic reaction and for one mole of carbon to combine with hydrogen to form C6H6, 49.03 kJ of energy is required.
The molar mass of carbon is 12.0107 g/mole
5.81 g of carbon is 0.4837 moles of carbon. The energy required for 5.81 g of carbon to form C6H6 is equal to 49.03*0.4837 = 23.71 kJ.
This energy is obtained as heat absorbed from the surrounding. Therefore when 5.81 g of carbon reacts with hydrogen to form C6H6, 23.71 kJ of heat is absorbed.