Question

You find two bottles of white solid on a shelf in the stock room without labels. You then find two labels CaCl2 (s) and KCl (s0 lyring right beside. You want to determine which is which. The solids can be differentiated by dissolving each solid in water and measuring the change in temperature.

delta H(f) CaCl2 (s) = -796 kJ/mol     

delta H(f) KCl (s) = -437 kJ/mol         

You dissolve 0.100 moles of unknown A in enough water to make a 200g solution at 22 degress C. The final temperature of the system is 31.7 degress C. Which solid is unknown A?         

What would be the expected Final Temperature if you dissolved 0.100 moles of unknown B in enough water to make a 200g solution at 22 degrees C?                  

Draw an enthalpy diagram for the dissolution of unknown B, be sure to include reactants, products, enthalpy, delta H, reaction progress.

Answer 1

1. The process of dissolving a substance in water is called dissolution, and usually it's an exothermic process because energy is released when weak bonds of the substance are broken apart into it's respective ions (one positive charged and one negative charged). Water is a polar molecule, and has a higher lattice energy than CaCl₂, so water will break the weak bonds of CaCl₂ into producing Ca²⁺ and Cl^- ions. So it's an exothermic reaction because energy is released to the surrounding.
   
   But the simplest way to put it is, it is an exothermic reaction because heat is being released when calcium chloride is dissolved in water.

solid is unknown A CaCl₂

1. When KCl solid is mixed in water, it must breakdown into its individual ions, this is an endothermic reaction. KCl, the first endothermic step absorbs more heat than it is releasing in the second step.

Final Temperature if you dissolved 0.100 moles of unknown B in enough water to make a 200g solution at 22 ^oC will be less than 22 ^oC