For most reactions the total energy of products is different to the total energy of the reactants.
Therefore as the reaction proceeds heat energy is either released to or absorbed from the surroundings.
Thermal changes for chemical reactions are described using a quantity enthalpy (H) where the change in enthalpy (ΔH) equals qp, the heat transferred at constant pressure.
The enthalpy change is positive if the products have a higher energy than reactants due to heat (q) having been transferred from the surroundings to the reaction system (endothermic reaction).
The enthalpy change is negative if the products have a lower energy than reactants due to heat (q) having been transferred from the reaction system to the surroundings (exothermic reaction).
The unit for ΔH is the same as the unit for heat q (J or kJ). The sign for q is the same as for ΔH. This sign shows the direction in which heat flows.
If heat is transferred from the system q(system) is negative.
If heat is transferred to the system q(system) is positive.
ΔH depends on the amount of substance reacting as different amounts of reaction release or consume different amounts of heat.
Therefore as the reaction proceeds heat energy is either released to or absorbed from the surroundings.
Thermal changes for chemical reactions are described using a quantity enthalpy (H) where the change in enthalpy (ΔH) equals qp, the heat transferred at constant pressure.
Many reactions are carried out at constant pressure in containers open to the atmosphere. For these volume occupied and the external pressure are constant AND ΔH is also equal to the difference between the energy of the products and the reactants.
The enthalpy change is positive if the products have a higher energy than reactants due to heat (q) having been transferred from the surroundings to the reaction system (endothermic reaction).
N2(g) + O2(g) 
2NO(g)
ΔH = +180 kJ
2NO(g) ΔH = +180 kJ
2H2(g) + O2(g) 2H2O(g)
ΔH = -484 kJ
2H2O(g) ΔH = -484 kJ
The unit for ΔH is the same as the unit for heat q (J or kJ). The sign for q is the same as for ΔH. This sign shows the direction in which heat flows.
If heat is transferred from the system q(system) is negative.
If heat is transferred to the system q(system) is positive.