Heat & Phase Change (Latent Heat) Latent Heat of Fusion Energy required to melt a substance (add heat = melting, remove = freezing)

Heat & Phase Change (Latent Heat) Latent Heat of Vaporization Energy required to vaporize a substance (add = vaporizing, remove = condensing)

Heat & Phase Change (Latent Heat) Heat Energy goes to overcoming the intermolecular forces instead of changing temperature. Latent Heat of Fusion When enough Heat Energy is removed the intermolecular forces are allowed to lock into a crystal structure. Latent Heat of Vaporization Heat Energy is added to break the intermolecular forces and separate the molecules even further.

Heat & Phase Change Why does a liquid cool when some of it evaporates?

Why does a liquid cool when some of it evaporates?

Heat & Phase Change What are two things that effect evaporation rate?

Latent Heat Latent Heat of Fusion (freezing / melting) Lfus (water) = 334 KJ/Kg Latent Heat of Vaporization (boiling / condensing) Lvap (water) = 2260 KJ/Kg Example Problem – What Heat is required to raise 10kg of Pure Water from -10C to 110C?

Practice How much energy is needed to convert 50 grams of ice at 0° C to steam at 100° C? How much energy is needed to convert 10 kg of ice at -20° C to steam at 150° C? How much energy must be removed to change 500 grams of steam at 110° C to ice at -170° C?

Freezing Point Depression What keeps the oceans from freezing? What can keep the roads from freezing? What keeps your car from overheating or freezing?

What happens when something boils? Boiling What happens when something boils? How does a solute raise the boiling point?

What happens when something freezes? Freezing What happens when something freezes? How does a solute lower the freezing point?

Freezing Point Depression / Boiling Point Elevation DRAW

Boiling Point Elevation A substance is dissolved in a solvent, like salt in water. B.P. increases. There are less liquid molecules at the surface that can turn into a gas. The dissolved solid ions get in the way of the solvent molecules at the surface. It takes a higher temperature (more energy) to get the liquid molecules to the surface and boil the solution.

Freezing Point Depression A substance is dissolved in a solvent, like water. F.P. decreases. The solvent molecules begin to clump together (freeze) as energy is removed. The dissolved solid ions get in the way of the solvent molecule intermolecular forces. It takes more energy to be removed (lower temperature).

Freezing Point Depression / Boiling Point Elevation ΔTf = (Kf) (m) ΔTb = (Kb) (m) ΔTf = Tf (pure solvent) – Tf (solution) m = molality = moles of solute / kg of solvent Kf = Freezing point depression constant (solvent)

Freezing Point Depression / Boiling Point Elevation ΔTf = (Kf) (m) ΔTb = (Kb) (m) ΔT = C m = mol / Kg K = Kg C / mol

WATER Kf = 1.86 Kg C / mol Kb = 0.512 Kg C / mol

Enthalpy & Enthalpy Change (ΔH) H = Q + PV ΔH = Q + ΔPV ΔH = Q (for constant pressure system) Most reactions take place at constant pressure (atmospheric pressure).

Enthalpy & Enthalpy Change

Enthalpy (ΔH) Lower energy level is more stable. Positive ΔH = Endothermic = Reactants are more stable Negative ΔH = Exothermic = Products are more stable

Chemical Potential Chemical Potential Energy Energy stored in the bonds of molecules Lower chemical potential is more stable Relate to Gravitational Potential Energy Cliff Diver

Max Chemical Potential = top of the cliff Unreacted Chemicals Min Chemical Potential = bottom of the cliff Reaction has occurred Exothermic

Enthalpy (ΔH) C(s) + 2H2 (g) = CH4 + 74 kJ 74 kJ is released = Exothermic ΔH is Negative = -74 kJ Chemical Potential of reactants is higher than product, CH4. So (CH4) Product is more stable with lower potential energy.

Enthalpy (ΔH) O2 + 249 J = 2O Endothermic Requires heat to make O The heat breaks the bond between the two Oxygen atoms Reactants more stable; O2 has lower Potential than O This is why pure Oxygen is found as O2 in nature (Standard State)