Energy can no be created or destroyed just changed into another form.
Enthalpy change = enthalpy of products - enthalpy of reactants
Exothermic reaction - heat is lost to the surroundings so the energy of the reaction is lower. Enthalpy change is negative
Endothermic reaction - heat is taken in so the energy of the reaction is higher. Enthalpy change is positive
Standard conditions for enthalpy - 298 K (25 degrees Centigrade), 1 atmosphere pressure (101 kPa).
Enthalpy change of formation - when a substance is formed
Enthalpy change of combustion - when a substance is burned
- taking the temperature for 3 minutes before adding the solid allows for any fluctuations and so the temperature of the acid stabilises