Enthalpy Change

Enthalpy change is also known as heat of reaction, enthalpy of reaction, change in heat content.

Enthalpy = total energy of a system

It is impossible to measure the total kinetic and potential energy of a system, so instead we measure enthalpy change (using calorimeters).

Enthalpy Change –  is the energy absorbed from or released to the surroundings when a system changes from reactants to products.  It is given the symbol, DH.

DH = DH_products – DH_reactants

DH is measured at STP or SATP for allowance of comparisons between enthalpy changes.

This idea is consistent with the law of conservation of energy, therefore;

Enthalpy change of system  =  quantity of heat that flows from system to  surroundings or vice versa

In other words;

DH  = ± q (DH= Delta H- Enthalpy Change)

Sign of DH

Endothermic Reaction:

• System has absorbed heat energy from surroundings
• H_products >  H_reactants
• DH = H_products –  H_reactants

= positive value

• DH is positive

Exothermic Reaction:

• System releases heat energy into surroundings
• H_products < H_reactants
• DH = H_products –  H_reactants

= negative value

• DH is negative