Thermodynamics. 1. Define temperature and state the units in which it is measured. 2. Define heat and state its units.

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2 Thermodynamics 1. Define temperature and state the units in which it is measured. 2. Define heat and state its units. 3. Perform specific heat problems.

3 Temperature Temperature is the measure of the average kinetic energy of the particles of a substance. Thermometers are used to measure temperatures. Units: C and K (and F of course)

4 Heat (q) Heat is the flow of energy from a substance of higher temperature to a substance of lower temperature, when they are placed in thermal contact with each other. HOT COLD Units of Heat: 1000 J = 1 kj Calorimeters: Measure heat flow and direction

5 Venn Diagram Temperature C, K, F Heat J, kj, cal

6 Venn Diagram Temperature C, K, F Heat J, kj, cal Energy

7 Other Energy Units 1 J = 1 kg m 2 / s J = 1 cal 1000 cal = 1 kcal = 1Cal (Nutritional Calorie)

8 q = m c p ΔT q = Heat ( units are J and kj) m = Mass (units are g or kg) ΔT = Change of Temperature (T f T i ) for ΔT: C is the same size as a K c p = Specific Heat (units are J / g C )

9 Specific Heat The amount of heat (energy) required to raise the temperature of 1 gram of a substance by 1 C. Water J / g C Aluminum J / g C Iron J / g C Silver J / g C

10 Calorimeter A device that uses a thermometer to measure the change of temperature of water caused by a physical change or a chemical change. Foam Cup Calorimeters (styrofoam) Bomb Calorimeters (steel)

11 Foam Cup Calorimeter These calorimeters are used for mixing aqueous solutions and then measuring the temperature change. Exothermic: If the temperature goes up. Endothermic: If the temperature goes down.

12 Bomb Calorimeter

13 Bomb Calorimeters Bomb calorimeters are used for determining the heat lost or gained by combustion reactions and others that release a lot of heat. Exothermic: If the temperature of the water increases (rises). Endothermic: If the temperature of the water decreases (drops).

14 Calculations q mass How much heat is required to heat 53.8 g of water from 20.0 C to 45.0 C? Temperature Waters C p = J / g C q = mc p T q = (53.8 g) (4.184 J /g C)( C) = J or 5.63 kj

15 q Specific Heat of Unknown Substance An unknown substance with a mass of kg required kj of heat to raise its temperature 8.0 C. T What is the unknown substances specific heat? Identify the substance kj kg mass q = mc p T 567 J = (506 g) (c p )(8.0 C)

16 Specific Heat of Unknown Substance An unknown substance with a mass of kg required kj of heat to raise its temperature 8.0 C. What is the substances specific heat? Identify the substance kj kg 567 J = (506 g) (8.0 C) (c p ) 567 J = c p = J / (g C ) (506 g) ( 8.0 C) Mercury

17 Heat Lost = Heat Gained When one substance loses heat during a chemical or physical change, another substance must gain that lost heat. When two aqueous solutions of chemicals are being mixed together, the reaction will either give off heat to the water or absorb heat from the water.

18 When the Water in a Calorimeter Changes Temperature Does the reaction release heat into the water or absorb heat from the water, when the temperature rises? ****If it releases heat exothermic Temperature rises! If it absorbs heat from the water - endothermic Temperature decreases!

19 Heat Lost = Heat Gained Diagram 25.0 g Metal at 95 C = t i Final temperature at 37 C = t f 50.0 g Water at 25 C = t i

20 Heat Lost = Heat Gained Diagram 25.0 g Metal at 95 C = t i Heat Lost Heat Gained Final temperature at 37 C = t f 50.0 g Water at 25 C = t i

21 The formula q=mc p T 25.0 g Metal at 95 C = T i Final temperature at 37 C = T f 50.0 g Water at 25 C = T i -(Heat (q) of Metal) = (Heat (q) of Water) -q (metal) = q water -m (metal) c p (metal) ΔT (metal) = m (water) c p (water) ΔT (water) c p (metal) = - m (water) c p (water) ΔT (water) m (metal) ΔT (metal) c p (metal) = - (50.0 g)(4.184 J/g 0 C)(37 0 C-25 0 C) (25.0 g)(37 0 C-95 0 C) c p = 1.73 J / g C

22 Changing a solid to a liquid Changing liquid into gas Heating gas Heating liquid Heating solid

23 Hess s Law and Physical Changes Heat Curve for water Be Sure that all the q s are in the same units! Joules (J) or kilojoules (kj) ΔH = q 2 = Mole( H fus ) ΔH =q 4 = Mole( H vap ) ΔH = q 5 = m c p ( of gas) ΔT ΔH = q 3 = m c p ( of liquid ) ΔT ΔH = q 1 = m c p ( of solid) ΔT q t = q 1 + q 2 + q 3 + q 4 + q 5

24 Example: A 32.5 g popsicle is in a freezer at o C. It is removed from the freezer and left on the counter. It melts and when mom comes home she finds a puddle of flavored water at 25.0 o C. How much energy did it take

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26 Example: A 32.5 g popsicle is in a freezer at o C. It is removed from the freezer and left on the counter. It melts and when mom comes home she finds a puddle of flavored water at 25.0 o C. How much energy did it take to do this? Steps: 1-Heat Solid -20 o C to 0 o C q 1 =mc p T = 32.5 g(2.06j/g o C)(20 o C) = 1339 J = 1.34 kj = q 1 2 Melt Solid q 2 = mole H fus = 32.5 g (6.01 KJ/mole) = 10.8 kj = q g/mole 3 Heat the liquid from 0 o C to 25 o C q 3 =mc p T = 32.5 g(4.184j/g o C)(25 o C) = J = 3.40 kj = q 3 + Then add q 1 + q 2 + q 3 = H = 15.5 kj

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