1. Unit 7
Thermochemistry

2. thermochemistry
the study of heat changes in chemical reactions and physical changes

3. SURROUNDINGS SYSTEM system—specific part being analyzed
surroundings—everything outside the system (usually the immediate area)
universe = system + surroundings
SURROUNDINGS
SYSTEM
open
closed
isolated
**Exchange:
mass & energy
energy
nothing

4. Energy is the capacity to do work
Thermal energy is the energy associated with the random motion of atoms and molecules
Chemical energy is the energy stored within the bonds of chemical substances
Nuclear energy is the energy stored within the collection of neutrons and protons in the atom
Electrical energy is the energy associated with the flow of electrons
Potential energy is the energy available by virtue of an object’s position

5. Energy Changes in Chemical Reactions
Heat is the transfer of thermal energy between two bodies that are at different temperatures.
Temperature is a measure of the thermal energy.
Temperature = Thermal Energy
400C
900C
greater thermal energy

6. Units for Measuring Heat Flow
calorie - the amount of heat needed to raise the temperature of 1 g of H2O by 1 oC.
-Used except when referring to food
Calorie (capital C) refers to the energy in food

7. Units for Measuring Heat Flow
1 Calorie = 1 kilocalorie = 1000 cal
SI unit of heat and energy = Joule
4.184 J = 1 cal
1 J = cal   

8. Energy Transformations
Heat – (variable is q) – energy that transfers from one object to another
only changes can be detected!
flows from warmer to cooler object

9. 2H2 (g) + O2 (g) 2H2O (l) + energy
Exothermic - process that gives off heat- energy goes from system to surroundings.
2H2 (g) + O2 (g) H2O (l) + energy
Energy is written on the product (right) side of the equation
q and ΔH are negative (-)
Δ is a Greek letter – means change in

10. exothermic
The products are lower in energy than the reactants
Thus, energy is released.
ΔH = -395 kJ
• The negative sign does not mean negative energy, but instead that energy is lost.

11. q and ΔH are positive (+)
Endothermic - process in which heat has to be supplied – energy goes from surroundings to system
energy + 2HgO (s) Hg (l) + O2 (g)
energy is written on the reactant (left) side of the equation
q and ΔH
are positive (+)

12. endothermic
The products are higher in energy than the reactants Thus, energy is absorbed.
ΔH = +176 kJ
• The positive sign means energy is absorbed

13. Changes of state
When a substance changes state the temperature remains constant during the actual phase change.- this is because the heat energy is being used up by the phase change process.

14. The specific heat (C) of a substance is the amount of heat (q) required to raise the temperature of one gram of the substance by one degree Celsius.
Specific heat formula
q = m X Dt X C
q = heat
m = mass ( in grams)
Dt = tfinal – tinitial
C = specific heat
q = DH

16. Dt = tfinal – tinitial = 50C – 940C = - 890C
How much heat is given off when an 869 g iron bar cools from 940C to 50C?
C of Fe = J/g • 0C m = 869 g
Dt = tfinal – tinitial = 50C – 940C = - 890C
q = mCDt
= 869 g x J/g • 0C x –890C
= -34,000 J
6.4

17. The specific heat of water is high due to hydrogen bonding

18. enthalpy changes are measured with a calorimeter
Calorimetry the measurement of the heat into or out of a system heat released = the heat absorbed
enthalpy changes are measured with a calorimeter
No heat enters or leaves!

19. The Law of Conservation of Energy
in any chemical or physical process, energy is neither created nor destroyed.
- All the energy is accounted for as work, stored energy, or heat.

20. thermo chemical equations
thermo chemical equations— equations that show heat changes
enthalpy (H)—heat content of a substance
Cannot measure directly but we CAN measure
change in enthalpy = ΔH;
heat change for a process; usually measured in kJ (kilojoules)

21. Heat of reaction
The physical states of all reactants and products must be specified
H2O (s) H2O (l)
DH = 6.01 kJ
H2O (l) H2O (g)
DH = 44.0 kJ
Equation –
= given mol x DH
# mol

23. Thermo chemical Equations
How much heat is evolved when 266 g of white phosphorus (P4) burn in air?
P4 (s) + 5O2 (g) P4O10 (s) DH = kJ
1 mol P4
123.9 g P4 x
266 g P4
3013 kJ
1 mol P4 x
= 6470 kJ

24. ΔHf = HEAT OF FORMATION = heat absorbed or released to make 1 mol of a compound from its elements
ΔHsoln = Heat of Solution = heat change caused by dissolving of one mole of substance

25. DHf for the formation of rust (Fe2O3) is –826 kJ/mol
DHf for the formation of rust (Fe2O3) is –826 kJ/mol. How much energy is involved in the formation of 5 grams of rust
1 mol Fe2O3
160 g Fe2O3 x
5.0 g Fe2O3 1
826 kJ
1 mol Fe2O3 x
= kJ

26. When 1. 0 g of solid NaOH (DHsoln = – 445
When 1.0 g of solid NaOH (DHsoln = – kJ/mol) dissolves in 10 L of water, how much heat is released?
1 mol NaOH
40.0 g NaOH x
445.1 kJ
1 mol NaOH x
1.0 g NaOH 1
= kJ

27. Molar Heat of Combustion = ΔHcomb = heat released in combustion of 1 mol of substance
Molar Heat of Fusion (DHfus.) = the heat absorbed by 1 mol of a substance in melting from a solid to a liquid
Molar Heat of Solidification (DHsolid.) = the heat lost when 1 mol of liquid solidifies (or freezes) to a solid

28. Molar Heat of Vaporization (DHvap
Molar Heat of Vaporization (DHvap.) = the amount of heat necessary to vaporize 1 mol of a given liquid.
Molar Heat of Condensation (DHcond.) = amount of heat released when 1 mol of vapor condenses to a liquid

29. Standard heat of reaction
Hess’s law of heat summation states that if you add two or more thermochemical equations you can add the heats of reaction to give the final heat of reaction

30. Standard Heats of Formation
The standard heat of formation (ΔHf0) of a compound is the change in enthalpy that accompanies the formation of one mole of a compound
Standard heat of formation for elements = 0

31. Standard Heat of Reaction
DH = H (products) – H (reactants)
Multiply the standard heat of formation by the number of moles for each reactant and product.
Add the reactants together
Add the products together
Subtract the sum of the reactants from the sum of the products

32. Calculate DH for the following reaction.
C2H4(g) + H2(g) C2H6(g)
DH for C2H4(g) = 52.5 kJ/mol;
DH for H2(g) = 0 kJ/mol; (free element)
H for C2H6(g) = –84.7 kJ/mol
= kJ