In the previous section you learned how to calculate Q, which is the amount of thermal energy transferred in a chemical reaction. Recall from Lesson 1.1 that the enthalpy change of a chemical system is equal to the change in thermal energy of the surroundings .

Enthalpy of reaction (ΔrH) refers to the enthalpy change for an entire chemical system as reactants change to products. In other words, it is the enthalpy change for the reaction, as written.

\( \mathrm { \Delta_rH = Q } \)

Because Q = mcΔt, then

\( \mathrm { \Delta_rH = mc\Delta t } \)

This means that we can use calorimetric data to infer enthalpy change for a given chemical reaction.

When using calorimetry to infer enthalpy change, assume that the energy lost or gained by the chemical system is equal to Q (the thermal energy lost or gained by the calorimeter water).

Conservation of energy problems are solved using various methods. The method shown in your textbook uses absolute values. That means no positive or negative values should appear in the calculation. For that reason, you need to add a positive or negative sign to your final answer for Δ_rH

• If the chemical reaction absorbs energy, resulting in a temperature decrease in the surrounding water, the reaction is endothermic. In this case, add a positive sign to the Δ_rH.
  
  
• If the chemical reaction produces energy, resulting in a temperature increase in the surrounding water, the reaction is exothermic. In this case, add a negative sign to the Δ_rH.

---

 Read pages 487 to 489 in the textbook and work through Sample Problem 11.1

---