Heat in Reactions

The study of heat in reactions is: Thermochemistry, a “branch” of Thermodynamics

Reactions involve energy because: chemical bonds are BROKEN (needs energy) and FORMED (releases energy)

Heat is: the energy transferred between objects due to a difference in temperature. It flows from the higher temperature object to the lower temperature object.

Some reactions release energy: EXOTHERMIC C3H8 + 5O2 → 3CO2 + 4H2O + energy C3H8 + 5O2 → 3CO2 + 4H2O + 2043 kJ

Some reactions absorb energy: ENDOTHERMIC C + H2O + energy → CO + H2 C + H2O + 113 kJ → CO + H2

To study energy in reactions, scientists use a concept called ENTHALPY symbolized H and sometimes you will see it as ΔH

ENTHALPY is the part of the energy of a substance due to the motion of its particles, or heat content. H = E + PV

We cannot determine absolute H, rather we can determine change in H (DH). DH = Hproducts - Hreactants

(This works because pressure is essentially constant.) For our purposes, the heat absorbed or released during a chemical reaction is equal to the enthalpy change for the reaction (This works because pressure is essentially constant.)

standard enthalpy change (DHo) For easy comparisons, define: standard enthalpy change (DHo) when reactants and products are in their standard states (at 25 oC and 1 atmosphere)

The sign of DH is informative. When DH > 0, (positive) energy is absorbed, Endothermic reaction When DH < 0, (negative) energy is released, Exothermic reaction