1. Heat in chemical reactions
How Do We Measure Heat?

2. What is Heat? What will cause more discomfort?
bucket of boiling water or
drop of boiling water
They are at the same temperature but the amount of stored heat energy is different

3. Temperature Hotness or coldness Proportional to average kinetic energy
Intensive property (does not depend on amount)
Put a thermometer in a hot oven
The fast oven particles bounce off the thermometer
The thermometers particles speed up
The oven particles slow down
Heat is transferred!

4. Energy The capacity to do work or produce heat
Energy transfer is measured
Extensive property – depends on amount
units are joules, calories or BTU’s
joules in science (metric unit)
BTU’s for air conditioning, heaters etc
Calories in some science like food
1 cal = j
1 Calorie = 1 Kcal

5. Heat transfer (flow) Depends on
The capacity of substance to absorb heat
The mass
The change in temperature
Why are pots made of metal and handles made of wood or plastic?

6. Specific heat
amount of energy required to raise the temperature of 1 gram of that substance 1 oC
Specific heat = heat (j or cal)
mass (g) x change in temp.(C)
Water has a specific heat of J/(g oC)
Copper has a specific heat of J/(g oC)

7. Specific heat problems
1. Calculate the Specific heat of Fe if
heat= 145 J
mass= 89.3 g
Δ temperature= oC
Calculate the heat of water if
Specific heat= 4.18 J/g C
mass= 43.6 g
Δ temperature= 8.3 o C

8. Lab: Specific heat
Purpose: To determine the specific heat of Copper and an unknown metal

9. Measuring heat transfer
Calorimeter – well insulated container that minimizes the amount of heat transferred to the surroundings

10. Their Calorimeter

11. Our calorimeter

12. Heat transfer Use water in calorimeter to absorb or release heat .
We can
measure mass of water in calorimeter
measure the change in temperature of the water in the calorimeter.
know the specific heat of water.
Heat(q)H2O = massH2O∆TH2OSpHeat(S)H2O
qsur = -qsystem
qH2O = -qreaction

13. Can’t measure heat directly
But can see measure the effects
of energy being released or
absorbed by the reaction.
Water surrounding the rxn
chamber will increase or decrease
in temperature. Can measure this
temperature change.

14. Coffee Cup Calorimeter
M = mass of water in cup
SWater =4.184 j/ g C
TAfter – Tbefore = Tchange
Tf - Ti = T (for the water)
Heat water= mw x Sw x Tw
calculate heat collected by calorimeter
Thermo

15. Calorimeter Basics Heat absorbed = - Heat released
Heat water = heat metal
mwater x Swater x Twate = -[mmetal x Smetal x Tmetal]
Final temp. of metal= final temp. of the water

16. Determine the specific heat of an unknown metal with a mass of 23
Determine the specific heat of an unknown metal with a mass of 23.8g which is heated to 100.0°C and dropped into a calorimeter containing 50.0g of water at 24.0°C. The final temperature is 32.5°C.
H2O: mass = 50.0g
T1= 24.0°C, T2= 32.5°C
Sp Heat(S)= J/g°C
Metal: mass= 23.8g
T1= 100.0C, T2= 32.5C
Sp Heat (S) = ?

17. Calorimeter hot metal is added to 100.0 g of 25.6 oC water. The
Final temp is 35.9 oC, How much heat is transferred?
Thermo
q = 100.0g x J x ( 35.9 – 25.6 C)
g C
q = 4300 J or 4.30 x 103 J or 4.30 kJ

18. Specific Heat Lab Find the Specific Heat of a metal
Mass metal
Heat metal to 100 °C
Place water in calorimeter
Know mass water
Know Initial temp
Put hot metal in cold water
What happens to temp of water?
What happens to temp of metal?
When does the temp stop changing?

19. Specific Heat Lab Find the Specific Heat of a metal
Heat into water = -heat out of metal
qwater = -qmetal
mwater x Swater x Twater = -[mmetal x Smetal x Tmetal]

21. Temperature
What happens to the particles of water when I put them into the freezer?
The slow moving air particles hit the water
The fast water molecules move slower
The slow air particles move faster
Heat is transferred!

22. Energy Change Exothermic – energy (heat) given off in a reaction
Condensing, freezing
Endothermic – energy (heat) absorbed in a reaction
Vaporizing, melting

23. Changes of State
How much energy does it take to heat 10.0 g ice from –20.0 to 100.0o C steam
Specific Heat ice 2.07 j/gC
Heat of fusion 334 j/g
Heat of evaporation 2260 j/g
Specific heat water j/gC

24. Changes of State It takes energy to change from one state to another
Heat of fusion
energy to change a solid into a liquid
Is this process endo or exothermic?
Heat of vaporization
energy to change a liquid to a gas

25. Cooling Curve (p.481) Gas Heat of vaporization Liquid Heat of fusion
Solid

26. Heat or Energy Problems
heat = mass x specific heat x temp change
q (∆H) = m x S x  T
joules = g x J x C°
g C°

27. Energy from -20 to 0
q = m x S x T
= 10.0g x j x 20oC
goC
Energy from solid to liquid
q = 10.0g x 6.7 j/g
Energy from 0 – 100
q = m x S x T
= 10.0g x j x 100oC
Energy from liquid to steam
q = 10.0 x 41.0 j/g

28. Heat of reaction
A calorimeter is filled with 75.0g of water at an initial temperature of 19.5°C. A 0.50mole sample of solid NaOH is added and the temperature increases to 26.7°C. What is the enthalpy change per mole of NaOH for this solution process.
NaOH(s) Na+(aq) + OH-(aq)

29. We can write expression for heat for both the metal and water Heat(q) H2O = massH2O ∆T H2O Sp Heat(Cp) H2O Heat( q )metal = mass metal ∆T metal Sp Heat( Cp )metal Heat( q) H2O =Heat( q)metal massH2O ∆T H2O Sp Heat(Cp) H2O = massmetal ∆Tmetal Sp HeatCpmetal

30. Make chart with data H2O Metal m T1 T2 ∆T Sp Heat (Cp)
50.0g g
24.0°C °C
32.5°C °C
8.5°C °C
4.184J/g°C x
massH2O ∆TH2O Sp Heat (Cp)H2O = massmetal∆Tmetal Sp Heat( Cp)metal
(50.0g)(8.5°C)(4.184J/g°C)= (23.8g)(67.5°C)x
x= 1.1 J/g°C

31. Make chart & put data in chart H2O. NaOH m 75. 0g (0. 050 mol) T1 19
Make chart & put data in chart H2O NaOH m g (0.050 mol) T °C T °C ∆T 6.9 °C Cp J/g °C
Heat(q)H2O = mass ∆T Sp Heat(Cp) = (75.0g)(6.9°C)(4.184J/g°C) = 2200J Heat(q)H2O = Heat(q)NaOH therefore
Heat(q)NaOH= 2200J 2200J = x x= J mol mol ∆H= J/mol NaOH

39. Calculation of enthalpy changes
2H2O2  2H2O + O2 Δ H=-190 kJ What is the heat released if 2.3 g H2O2 react?

40. Measuring Heat Transfer
“All” heat went into water
Know heat into water
All came from your experiment.
Heat water = -heat experiment
Minus sign important!
Thermo

41. Heat Problem
How much does it take to heat 5.0 g of aluminum (Cp =0.879 j/g C) from 22 C to 100. C?
q = m x Cp x T
q = 5.0g x ( j/g C) x (100. – 22)
q = 340 or 3.4 x 102 j are required