1. Chapter 5 Changes in Matter and Energy

2. Heat and Energy Changes
Thermochemistry: Energy changes that accompany changes in matter.
Thermal Energy: a form of kinetic energy that results from the motion of the particles.
Heat: the quantity of energy transferred between substances. q= quantity of heat
Temperature vs. Heat:
Temperature is the measure of the average kinetic energy of an object.
Heat ≠ temperature

3. System: object or collection of objects being studied
In lab, the system is the chemicals inside the beaker
Surroundings: everything outside of the system that can exchange energy with the system
The surroundings are outside the beaker
Universe: system plus surroundings
Exothermic: heat transferred from the system to the surroundings
Endothermic: heat transferred from the surroundings to the system

4. Types of energy Kinetic energy Potential energy Vibrational
Translational
Rotational
Bond energy

5. Endothermic reactions absorb energy from the surroundings.
D H>0
Heat
Potential energy

6. Exothermic reactions release energy to the surroundings.
DH <0
Heat
Potential energy

7. Enthalpy Changes for Chemical Reactions
Exothermic reactions generate specific amounts of heat
Because the potential energies of the products are lower than the potential energies of the reactants
Endothermic reactions consume specific amounts of heat
Potential energies of
the reactants are lower
than the products
DH for the reverse reaction
is equal, but has the
opposite sign to the forward
reaction

8. Calorimetry: Measuring q
Measuring the quantity of heat.
Use a calorimeter. Two kinds
Constant pressure calorimeter (called a coffee cup calorimeter)
or a bomb calorimeter

9. Bomb Calorimeter thermometer stirrer full of water ignition wire
Steel bomb
sample

10. Calorimetry: Calculating q
q=mcDT OR
q= CDT
m=mass in g
c=specific heat capacity in J /g.K
DT = change in temperature in K
C= heat capacity of calorimeter (J/K)

11. Specific Heat Capacity (C)
amount of heat required to raise the temperature of 1 gram of a substance by 1 kelvin
SI Units: Specific heat capacity = J /g.K
Specific heat of water = 4.184
SI unit: Joule (J)
1 calorie = J
English unit = BTU J
g.K

12. Heat Transfer
A 1.6 g sample of metal that appears to be gold requires 5.8 J to raise the temperature from 23°C to 41°C. Is the metal pure gold? J
g.K
Specific heat of gold is 0.13
Therefore the metal cannot be pure gold. 23

13. Example
The specific heat of graphite is 0.71 J/gºC. Calculate the energy needed to raise the temperature of 75 kg of graphite from 294 K to 348 K.
G: c= 0.71 J/gºC m= 75 kg DT= 348 K -294K= =54K
R: q
A: q=mcDT
S: q= g (0.71 J/gºC )(54K)
=2.9 x 103 KJ
Review Significant figures and Scientific notation!

14. Example- mixing of 2, Hot and COLD
A 46.2 g sample of copper is heated to 95.4ºC and then placed in a calorimeter containing 75.0 g of water at 19.6ºC. The final temperature of both the water and the copper is 21.8ºC. What is the specific heat of copper?
NOTE:
Assume the experiment is done correctly so that the system is CLOSED and insulated…ie a calorimeter is used…
Thus…heat lost by the hot object=heat gained by the cold object.
Water is used since the c of water is well known. The “c” of another substance can be found by comparing to water.
-1 x q lost= 1 x q gained
-1 x (m c DT) lost= (m c DT) gained

15. D H Every energy measurement has three parts. A unit ( Joules ).
A number how many.
and a sign to tell direction.
negative - exothermic
positive- endothermic

16. Molar Enthalpies
The enthalpy change associated with the change on one mole of a substance.
Denoted by a subscript (X) with D H
Example: B2H6 (g) + 3O2(g) B2O3(s + 3H2O (g)
Hr= kJ
(enthalpy for one mole )
If enthalpy DH is without a subscript, it does not denote per mole.
D Hx

17. Example D Hx = D H/n or n x DHx = DH
What is the molar enthalpy of reaction of methane with chlorine gas if 32.6 g absorbs KJ of energy?
By definition…
D Hx = D H/n or n x DHx = DH

18. Liquid to gas or back- D Hvap or L vap
How much energy is added when g of ammonia is vapourized?
Change of state…check tables at back of textbook ---
Solid to liquid or back - D Hfus or L fus
Liquid to gas or back- D Hvap or L vap
DH= DHfus n, DH= DHvap n, DH= Lfus m, DH= Lvap m

20. Representing Enthalpy Changes
Method 1:
Thermochemical Equations with Energy Terms (Example is endothermic)
Method 2: Thermochemical Equations
with ∆H Values (Example is exothermic)

21. Method 3: Molar Enthalpies of Reaction (Example is exothermic)

22. Method 4: Potential Energy Diagrams (Example left is exothermic and
Right is endothermic)