Journal Entry:

 How do calculate the enthalpy of solution (kJ/mol) for dissolving a solid in water?

Cold packs are commonly used in first aid to “ice” injuries (to reduce inflammation). They do not get as cold as ice but they can significantly lower the temperature of the surface to which they are applied. Cold packs contain ammonium nitrate in one compartment and water in a separate, adjacent compartment. When the cold pack is needed, the barrier between compartments is broken and the contents mix. Upon mixing, the temperature significantly lowers.

(a)  Explain how the mixing of the contents of the two compartments lowers the temperature.

(b)  Calculate the mass of NH₄NO₃ (molar mass= 80 g mol^-1, ᐃH_sol=25.6 kJ mol^-1) needed to lower the temperature of 250 grams of water from 25 ^°C to 5 ^°C. Assume the specific heat of the solution is the same as that of water, 4.18 kJ ^°C^-1 kg^-1). You do not need to include the mass of the NH₄NO₃ in the mass of the water or solution.

Learning Intentions

●     We will learn about the energy transformations and particle changes involved in dissolving a variety of solutions.

●     We will learn how to follow and modify a procedure and interpret collected data in which calorimetry is used to create an ideal, cost effective hand warmer.

Closing Task:

You can follow and modify a procedure and analyze results for determine an ideal cost effective hand warmer using ionic solids and water.

Chemical system undergo three main processes that change their energy: heating/cooling, phase transitions, and chemical reactions. (E.K. 5.B.3.)

At the particulate scale, chemical processes can be distinguished from physical processes because chemical bonds can be distinguished from intermolecular interactions. (E.K. 5.D.2)

Calorimetry is an experimental technique that is used to determine the heat exchange/transferred in a chemical system. (EK 5.B.4)

 The net energy change during a reaction is the sum of the energy required to break bonds in the reactant molecules and the energy release in forming the bonds of the product molecules. The net change in energy may be positive for endothermic reactions where energy is required, or negative for exothermic reactions where energy is released. (E.K. 5.C.2)