Heat of Solution

1
Heat of Solution

• Overall Energy Change in Dissolving

2
What would happen if I measured the temperature
while I was making solutions?
Sometimes, the water gets colder. Sometimes, the
water gets warmer. Rem Water is the
environment!
3
Heat flow
System to environment exothermic. Temp of
water ?.
Environment to system endothermic. Temp of
water ?
4
How would I calculate the energy change is this
experiment?

• Use Q mC?T
• Q energy in joules
• m mass of water in cup
• C specific heat of water table B
• ?T temperature change of water.

5
Why is there an energy change?
6
Energy Considerations Solid in Liquid

1. Solvent-solvent attraction has to be broken up
   energy absorbed.
2. Solute-solute attraction has to be broken up
   energy absorbed.
3. Solute-solvent interaction formed energy
   released.

7
Recall

• Breaking a bond absorbs energy.
• Making a bond releases energy.

Memorize!
8
Pulling solute particles apart absorbs energy
Solute-solvent attraction releases energy.
Also have to pull solvent particles apart.
9
Energy in
Pulling solvent molecules apart.
Pulling solute molecules apart.

Energy out
Attraction between solute solvent molecules.
10
Heat of Solution

• If exothermic component wins out, solution warms
  up.
• If endothermic component wins out, solution cools
  down.

11
Heat of Solution

• Has to be determined experimentally.
• Solubility traces with a ve slope
• Solute Solvent Heat ? Solution
• Dissolving is endothermic.
• Solubility traces with a ve slope
• Solute Solvent ? Heat Solution.
• Dissolving is exothermic.

12
Measuring Heat of Solution
Water is the environment!
source
13
Measuring Heat of Solution

1. Measure mass of water in cup.
2. Dissolve salt in water.
3. Observe temperature change of water.
4. Use Q mC?T to calculate energy change.

14
Energy Changes on Dissolving

• Dissolving may be either endothermic or
  exothermic. Cannot predict ahead of time. Which
  is which?
• Two things going on its a balancing act!

15
Energy Changes on Dissolving

• Things pulling apart from each other. This costs
  you energy breaking bonds or interactions is
  endothermic
• Forming interactions between solute solvent
  molecules. This gives you energy making bonds
  or interactions is exothermic.

16
Energy Considerations Gas in Liquid

• Solvent-solvent attraction has to be broken up
  energy absorbed.
• Solute-solute attraction has to be broken up
  energy absorbed.
• Solute-solvent interaction formed energy
  released.

Solute is a gas. There is no solute-solute
attraction.
3 gt 1 or gas wont dissolve. Always exothermic.