CHEMISTRY 122 HEAT OF SOLUTION

HEAT OF SOLUTION (∆H soln )  During the formation of a solution, heat is either released or absorbed  The enthalpy change by dissolution of one mole of substance is the molar heat of solution  Remember, a reaction can be either exothermic or endothermic.

Exothermic Reactions  The heat of solution is written as a negative number to indicate that heat is being released from the system.  In the case of examples given in the textbook, both reactions are the result of heat being released from a heat pack once the substance comes into contact with water.

Endothermic Reactions  The heat of reaction is positive. This indicates heat being absorbed by the system from the surroundings.  In the example given in the textbook, a cold pack, when activated by a chemical reaction with water, becomes extremely cold. Heat is moving from the surroundings to the system.

Figure  Is the process that occurs in the cold pack endothermic or exothermic? How do you know?  In which direction is the heat flowing?  Is the process in a hot pack endothermic or exothermic? How do you know?  In which direction is heat flowing?

For tomorrow…  Answer the following questions:  How many kilojoules of heat are released when 25.0g of NaOH are dissolved in water?  Sketch a heating curve for 1 mole of ice being heated from -10°C to 110°C. Write a brief explanation of the graph. Calculate how much energy would be required to change 54.0g of ice at -10°C to water vapour at 110°C.  Questions 27 – 31, page 526