PLEASE EXPLAIN THE TERM

Thermochemistry

AND WHAT IS Standard enthalpy changes

Thermochemistry
The study of the energy transferred as heat during the course of chemical reactions is
called thermochemistry. Thermochemistry is a branch of thermodynamics because
a reaction vessel and its contents form a system, and chemical reactions result in the
exchange of energy between the system and the surroundings. Thus we can use
calorimetry to measure the energy supplied or discarded as heat by a reaction, and can
identify q with a change in internal energy (if the reaction occurs at constant volume)
or a change in enthalpy (if the reaction occurs at constant pressure). Conversely, if
we know ΔU or ΔH for a reaction, we can predict the energy (transferred as heat) the
reaction can produce.
We have already remarked that a process that releases energy by heating the surroundings
is classified as exothermic and one that absorbs energy by cooling the surroundings
is classified as endothermic. Because the release of energy by heating the
surroundings signifies a decrease in the enthalpy of a system (at constant pressure), we
can now see that an exothermic process at constant pressure is one for which ΔH
Conversely, because the absorption of energy by cooling the surroundings results in
an increase in enthalpy, an endothermic process at constant pressure has ΔH > 0.

Standard enthalpy changes