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Heat in chemical reactions

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Title: Heat and Calorimeters Author: Peter Last modified by: SASD Created Date: 10/20/2002 3:57:25 PM Document presentation format: On-screen Show (4:3) – PowerPoint PPT presentation

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Title: Heat in chemical reactions


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 BTUs
  • joules in science (metric unit)
  • BTUs for air conditioning, heaters etc
  • Calories in some science like food
  • 1 cal 4.184 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 4.184 J/(g oC)
  • Copper has a specific heat of 0.387 J/(g oC)

7
Specific heat problems
  • 1. Calculate the Specific heat of Fe if
  • heat 145 J
  • mass 89.3 g
  • ? temperature 13.2 oC
  • Calculate the heat of water if
  • Specific heat 4.18 J/g C
  • mass 43.6 g
  • ? 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
  • Cant 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

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 23.8g which is heated to 100.0C
and dropped into a calorimeter containing 50.0g
of water at 24.0C. The final temperature is
32.5C.
H2O mass 50.0g T1 24.0C, T2 32.5C Sp
Heat(S) 4.184.J/gC 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?
q 100.0g x 4.184 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

20
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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 4.184 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
  • Is this process endo or exothermic?

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 2.07j 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 4.18j x
    100oC
  • goC
  • 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 19.5C. A 0.50mole sample
of solid NaOH is added and the temperature
increases to 26.7C. 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 23.8g
24.0C 100.0C 32.5C
32.5C 8.5C
67.5C 4.184J/gC x massH2O
?TH2O Sp Heat (Cp)H2O massmetal?Tmetal Sp Heat(
Cp)metal (50.0g)(8.5C)(4.184J/gC)
(23.8g)(67.5C)x
x 1.1 J/gC
31
Make chart put data in chart H2O NaOH m
75.0g (0.050 mol) T1 19.8
C T2 26.7 C ?T 6.9 C Cp 4.184 J/g C
Heat(q)H2O mass ?T Sp Heat(Cp)
(75.0g)(6.9C)(4.184J/gC) 2200J
Heat(q)H2O Heat(q)NaOH therefore Heat(q)Na
OH 2200J 2200J x x
44000J 0.050 mol 1 mol ?H
-44000J/mol NaOH
32

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39
Calculation of enthalpy changes
  • 2H2O2 ? 2H2O O2 ? 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!


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 ( 0.879 j/g C) x (100. 22)
  • q 340 or 3.4 x 102 j are required
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