Download Heat of Reaction

Unit 12 (Chapter 11)
Thermochemistry
The Flow of Energy--Heat

Thermochemistry—heat changes that occur
during chemical reactions

Energy—ability to do work or supply heat

Chemical potential energy—energy stored within
structural units of a chemical substance

Heat (q)—energy that flows from warmer objects
to cooler objects
– Heat cannot be detected, only changes caused by heat

All reactions and changes in state involve the release or absorption of
heat

System—chemicals and their reaction
Surroundings—everything else (beaker, air, etc)
Combined = universe



Direction of heat flow is always given from the point of view of the
system

+ q = endothermic—heat flows into the system from the
surroundings—heat is absorbed


System gains heat; surroundings cool down
-q = exothermic—system loses heat

System loses heat; surroundings heat up
Look at Page 294
Heat Capacity and Specific Heat
 calorie—quantity
of heat needed to raise
temperature of 1 g of pure H2O 1°C
1 Calorie = 1kc = 1000 calories
 Calorie—dietary
in food
use only, refers to the energy
 joule—SI
unit of heat and energy
1 J = .2390 calories
4.184 J = 1 cal
 heat
capacity—amount of heat needed to
raise the temperature of a whole sample of
substance exactly 1° C
– Depends on mass and composition
mass =
heat capacity

specific heat capacity—(C) amt of heat needed to
raise temperature of 1g of a substance 1° C
q = mC∆T
q = heat
∆T = Tf – Ti
C = specific heat (from table)
m = mass (g)
C = q__
m ∆T
See table 11.2 (p. 296) – Use values that contain J as a unit
Sample problems
When 435 J of heat is added to 3.4 g olive oil at 21ºC, the
temperature increases to 85ºC.
What is the specific heat of olive oil?
q = 435 J
m = 3.4 g
∆T = 85 °C – 21 °C = 64 °C
C=
435 J_____
3.4 g x 64 °C
= 1.999 J/g °C
= 2. 0 J/g °C
Measuring and Expressing Heat Changes

Calorimetry—the accurate and precise
measurement of heat change for chemical
and physical processes.
– The heat released by the system = heat gained by the surroundings
 Heat

lost = Heat gained
Calorimeter—device used to measure
heat changes
 styrofoam
cup—simple calorimeter, run at a constant
pressure
 See page 300
 bomb calorimeter –run at constant volume;
measures heat released from burning a compound
Enthalpy (H)

for systems at constant pressure, the heat
content (q) is the same as a property called
enthalpy (H) of the system
Enthalpy – the heat content of a system at
constant pressure.

heat changes symbolized as ∆H

so…

Exothermic Reaction--

Endothermic Reaction-- ∆H is +, (∆H > 0)
q = ∆H = mC∆T
∆H is - , (∆H < 0)
A student mixed 50.0 mL of water containing 0.50 mol HCl at
22.5ºC with 50.0 mL of water containing 0.50 mol NaOH at
22.5 ºC in a foam cup calorimeter. The temperature of the
resulting solution increased to 26.0ºC. How much heat in
joules (J) was released by this reaction?










50.0 mL HCl + 50.0 mL NaOH = 100.0 mL solution
100.0 mL x 1 g/mL = 100.0 g of solution
Tf= 26.0 °C
Ti = 22.5 °C
C of water = 4.18 J/g°C
∆H = ?
∆H
∆H
∆H
∆H
=
=
=
=
mc∆T
100.0 g ∙ 4.18 J/g°C (26.0 °C – 22.5 °C)
100.0 g ∙ 4.18 J/g°C ∙ 3.5 °C
1463 J = 1500 J
Thermochemical Equations

Thermochemical equations are equations that include the
heat change

Heat of Reaction—heat change for the equation exactly as
written




usually reported as ∆H
physical states must also be given
heat as reactant = endothermic
heat as product = exothermic
Exothermic
CaO(s)
+
H2O(l)
Endothermic
2NaHCO3(s)
+
→
129 kJ
Ca(OH)2
→
+
65.2 kJ
Na2CO3(s)
+
H20(l)
+
CO2(g)
How much heat is required to raise the temperature of
250.0 g of mercury 52ºC?
c Hg = 0.14 J/g °C - (chart value)
q=?
m = 250.0 g Hg
∆T = 52 °C
q = m c ∆T
q = 250.0 g ∙ 0.14 J ∙ 52 °C
g °C
q = 1820 J
q = 1.8 kJ