In the reaction, ${\mathrm{CO}}_2\left(g\right)+{\mathrm{H}}_2\left(g\right)⟶\mathrm{CO}\left(g\right)+{\mathrm{H}}_2\mathrm{O}\left(g\right)$ $\mathrm{\Delta }H=2.8\mathrm{kJ}$ $\mathrm{\Delta }H$ represents

Here, the heat of reaction, is the amount of heat which has to be added or taken out from a chemical reaction to keep all substances present at the same temperature. 

Here, change in enthalpy represent the heat of reaction.