Demo: freezing of stearic acid (QOTD coming up….) Watch the how the temperature of the chemical changes over time as it cools down.
QOTD 4/1/14 Q = heat (cal) m = mass (g) Cp = heat capacity (cal/gᵒC) DT = change in temperature (ᵒC) Use dimensional analysis (look at the units!) to determine which of these equations are valid rearrangements of the “heat equation”: DT = Q m = Q Cp = Q m x CpDT x CpDT x m
QOTD Answer Q = heat (cal) m = mass (g) Cp = heat capacity (cal/gᵒC) DT = change in temperature (ᵒC) Use dimensional analysis (look at the units!) to determine which of these equations are valid rearrangements of the “heat equation”: DT = Q m = Q Cp = Q m x CpDT x CpDT x m All of these! Because all of the units work out!
Homework Answers 1. The ocean feels hot to the iceberg. What is meant by this? The ocean has a higher temperature than the iceberg, so heat leaves the ocean and enters the iceberg, causing it to melt. Heat flows until THERMAL EQUILIBRIUM is reached. 2. Does your body come to thermal equilibrium with the classroom? Why or why not? No! You maintain your body temperature by respiration (slow burning of sugar). This keeps you warm even though you continually lose heat to the surroundings. If you did come to thermal equilibrium, you would suffer hypothermia and die. There’s an old saying: old chemists don’t die; they just reach equilibrium!
Homework Answers • What is the difference between heat and temperature Heat is energy that is transferred between objects Temperature is a measure of how fast molecules are moving • Imagine a thermometer is placed in a beaker of water and the temperature is noted. An ice cube is dropped in the water and after ten minutes the temperature is noted again. • Define the system and the surroundings • System = ice cube. Surroundings = everything else • What is the direction of heat transfer? Explain. • From the surroundings into the system (endothermic) • How does the energy of the system change? • It increases
Learning Target: • You should be able to identify what phases are present in various parts of a heating/cooling curve as well as the melting/freezing and boiling/condensation points. • You should also be able to describe what is happening at the particle level.
Back to the demo… • Let’s check and see how the stearic acid is cooling…
Notes Time Please send a group member to get notes sheets for your group. Answer Questions 1 and 2 on the notes sheet before we begin taking notes.
Similarities Both lines should have 3 regions: Initial temp change (while still a liquid) Plateau where temp stays the same (phase change- freezing point) Final temp change (while a solid) Differences Plateaus at a different temperature (substances have different freezing point temps) Plateaus are different length (may be short or long) Steepnessof graph during temperature change may differ Lesson 2: shape of the graph
3. What is the freezing point for stearic acid? Lauric acid?How can you tell? Freezing point = 68˚C Stearic Acid Lauric Acid Freezing point = 44˚C
4. Phase Change Vocab (notes) • Freezing point(fusion) & melting point • temperature where solid↔ liquidphase change • water freezes at 0 ˚C, ice melts at 0 ˚C • Boiling point(vaporization) & condensationpoint • temperature where liquid↔ gasphase change • water boils (becomes gas) at 100 ˚C • water vapor (gas) also condenses to become liquid at 100 ˚C
5. Cooling Curve • On the graph below, label the areas which represent… • A gas cooling • A liquid cooling • A solid cooling • Condensation point (phase change from gas to liquid) • Freezing point (phase change from liquid to solid) • A time where both solid and liquid phases are present Temperature (˚C) Time Unit 3 • Investigation I
Check your work! • On the graph below, label the areas which represent… • A gas cooling • A liquid cooling • A solid cooling • Condensation point (phase change from gas to liquid) • Freezing point (phase change from liquid to solid) • A time where both solid and liquid phases are present Temperature (˚C) Time Unit 3 • Investigation I
6. Heating Curve • On the graph below, label the areas which represent… • A gas increasing in temperature • A liquid increasing in temperature • A solid increasing in temperature • Melting point (phase change from solid to liquid) • Vaporization point (phase change from liquid to gas) • A time where both liquid and gas phases are present Temperature (˚C) Time Unit 3 • Investigation I
Check your work! • On the graph below, label the areas which represent… • A gas increasing in temperature • A liquid increasing in temperature • A solid increasing in temperature • Melting point (phase change from solid to liquid) • Vaporization point (phase change from liquid to gas) • A time where both liquid and gas phases are present Temperature (˚C) Time Unit 3 • Investigation I
7. Cooling Curve Kinetic energy changes on the slanted portions of the graph while potential energy changes on the plateaus. Potential energy decreasing (molecules moving closer together) Temperature (˚C) Kinetic energy decreasing (molecules slowing down) Time
Heating Curve Potential energy increasing (molecules moving further apart) Temperature (˚C) Kinetic energy increasing (molecules speeding up) Time
8. Why does the temperature rise, fall or level off? • Sections where temperature changed… (rose or fell) Q: What happens to the molecules as they change temperature? A: During these times, the kinetic energy (KE) of the molecules changed. • They speed up or slow down when temp changes • Sections where no temperature change… (plateau) • kinetic energy stays the same (no temp change) • phase change is occurring • BUT, acid sample is still losing heat energy • What type of energy is lost? potential energy (PE)
Kinetic Energy changes as the temperature changes Lower temp Higher temp Kinetic Energy = energy of motion Higher temp = higher kinetic energy = moving faster
9. Potential Energy Change during phase change… • You can think of potential energy as the distancebetween the molecules • The molecules in a liquid are more spread out than those in a solid • so, during “melting”, the molecules spread out, potential energy increases
Check In - water heating curve Kinetic energy stays the same Kinetic energy increasing
Units for Energy: calorie: the amount of energy needed to heat one gram of water one degree (Celsius). Kilocalorie: unit we use to measure the energy in food(Calories) = 1000 calories (1 kcal) J = Joules= a measure of energy kJ = Kilojoules = 1,000 J Conversion: 1 calorie = 4.184 Joules
Specific Heat Capacity:the heat energy required to increase the temperature of one gram of substance by one degree (Celsius or Kelvin) • Represented by symbol “Cp” • Units are Joules/gram*Kelvin or Joules/gram*Celsius
Formula to find Energy needed to change temperature: (the “heat equation”) Q = mCpΔT Q = energy (J) m = mass (g) Cp = specific heat capacity (at const. P) ΔT = change in temperature
energy of temperature change Use Q = mCDT Q = energy (J) m = mass (g) Cp= specific heat (J/g˚C) DT = change in temp (˚C) Temperature (˚C) Note: the substance will have a different value for C (specific heat) depending on its phase (liquid, gas, etc…) Time
Sample Problem: A 622 g piece of copper metal is heated from 20.5°C to 324.3°C. Given that the specific heat of Cu is 0.385 J/g°C, calculate the heat absorbed (in J) by the metal. ΔT = 324.3°C – 20.5°C = 303.8°C Q = mCΔT = (622g)(0.385 J/g°C)(303.8°C) Heat absorbed = 72,750 J = 72.8 kJ
Practice: The specific heat of water is 4.18 J/g°C. How much energy is required to heat 45.0 grams of water from 15.5°C to 26.5°C? ΔT = 26.5 – 15.5 = 11°C Q = mCΔT = (45 g)(4.18 J/g°C)(11°C) = 2069.1 J = 2.07 kJ of energy
Energy for phase changes DHv (change in heat for vaporization, liquid ↔ gas) How much energy is transferred during phase changes? (the plateaus on the graph) DHf (change in heat for fusion, liquid ↔ solid) Temperature (˚C) Time
Energy for Phase Changes ΔHf = Heat of fusion ΔHv = Heat of vaporization
Energy needed for Freezing/Melting (solid liquid): Qfusion = ΔHfx mol Energy needed for Vaporization/Condensing (liquid gas): Qvaporization = ΔHv x mol
Practice: The molar heat of fusion for water is 6,100 J/mol. Calculate how much energy is required to melt 3.00 moles of ice at 0°C. Qfusion = 6,100 J/mol x 3.00 mol = 18,300 J