Bellwork 3/6/18 What is energy? What units can energy be measured in? 2. What is it called when a reaction releases energy?___________How does it feel?___________ 3. What does it mean to “heat something up”?

Answers 1. The ability to do work or to produce heat. Measured in Joules or Calories 2. exothermic / hot 3. transfer energy from a warm surrounding to a colder surrounding.

Discussion If you want to make water boil, what do you have to do? What do you think is happening at the molecular level? When we get out of the shower we feel cold. Why is this? IT TAKES ENERGY TO VAPORIZE!

States of matter review… The phase of a material depends on the energy of its particles

Phase changes

Phase Change Diagram How phases of matter change with the addition of heat energy Note vaporization takes a lot more energy than melting.

From a solid to liquid (melting): From a liquid to gas (vaporization): How does energy play a role in phase changes? Label each process as exothermic or endothermic From a solid to liquid (melting): From a liquid to gas (vaporization):

From a gas to a liquid(condensation): From a liquid to a solid(freezing/fusion):

Checking for understanding: answer after changes of state movie In terms of heat energy, what happens when a solid changes to a liquid and then to a gas? 2. Would this be considered exothermic or endothermic? Why? 3. When does temperature increase? Why? 4. When does temperature remain constant? Why?

Classwork

Enthalpy (H) - Is the heat content of a system at constant pressure You can measure the enthalpy of reaction (Hrxn) This is negative if heat is released (exo) This is positive if heat is absorbed (endo)

Fusion: (melting)/freezing 00C Latent heat of fusion= (Hfus): There is a tool to calculate the amount of energy gained or released during phase changes: Fusion: (melting)/freezing 00C Latent heat of fusion= (Hfus): Energy required to melt a substance, or energy given off when a substance freezes Equation… Q=mHfus m=mass Hfus (H2O)= 334 j/g = 80 cal/g

Example 1: how much energy is released (in joules) when 50 grams of water freezes? Example 2: How many grams of water can you melt with 3,555 calories of energy?

vaporization: (boiling)/condensing 1000C Latent heat of vaporization= (Hvap) Energy required to boil a substance, energy released when a substance condenses Equation… Q=mHvap m=mass Hvap (H2O)= 2260 j/g =540 cal/g

Example 1: how much energy is needed (in calories) to boil 250 grams of water? Example 2: How much energy is given off when 5.0 moles of water (H2O) condenses? (hint, first convert mol  g)

Group Talk… If you want to make water boil, what do you have to do? What do you think is happening at the molecular level? When we get out of the shower we feel cold. Why is this? IT TAKES ENERGY TO VAPORIZE!