1. Chapter 16: Energy and Chemical Change
Table of Contents
Chapter 16: Energy and
Chemical Change
16.2-3: Thermochemistry

2. Practice specific heat equation On a separate sheet of paper…
1) If the temperature of 34.4 g of ethanol increases from 25.0°C to 78.8°C, how much heat has been absorbed?
2) g of gold [c = J/(g·°C)] absorbed J of heat. If the initial temperature was 25.0°C, what was the final temperature?
4.52 x 103 J
5.00 x 102 °C

3. Practice specific heat equation On a separate sheet of paper…
Endothermic or exothermic?
Melting ice
Boiling water
Freezing water
Making water from steam
endo
endo
exo
exo

4. Endo- or exothermic?

5. Chemical Energy and the Universe
Energy and Chemical Change: Basic Concepts
Chemical Energy and the Universe
Thermochemistry: the study of heat changes that accompany chemical reactions and phase changes.
Enthalpy (H): the heat content of a system at constant pressure.
Enthalpy (heat) of reaction (∆Hrxn) = the change in enthalpy for a reaction.
always measured as change.

6. Thermochemical Equations
Exothermic
4Fe(s) + 3O2(g) → 2Fe2O3(s) kJ
4Fe(s) + 3O2 → 2Fe2O3(s); ∆H = kJ
Endothermic
27 kJ + NH4NO3(s) → NH4+(aq) + NO3-(aq)
NH4NO3(s) → NH4+(aq) + NO3-(aq); ∆H = 27 kJ

7. Standard enthalpy changes have the symbol ∆H°.
Energy and Chemical Change: Basic Concepts
Enthalpy changes
Standard enthalpy changes have the symbol ∆H°.
Standard conditions:
1 atm pressure
298 K (25°C)
Units of enthalpy:
kJ released
1 mol substance

8. ∆Hcomb is exothermic (-)
Energy and Chemical Change: Basic Concepts
Enthalpy changes
Types of enthalpy
Enthalpy (heat) of combustion (∆Hcomb) = enthalpy change for the complete burning of one mole of the substance.
∆Hcomb is exothermic (-)

9. PHASE CHANGES
TEMPERATURE
boiling
evaporating
gas BP
condensing
liquid
melting MP
freezing
solid
ENERGY

10. ∆Hvap and ∆Hfus are endothermic (+)
Energy and Chemical Change: Basic Concepts
Changes of state
Molar enthalpy (heat) of vaporization (∆Hvap) = heat required to evaporate one mole of a liquid (liquid → gas)
Molar enthalpy (heat) of fusion (∆Hfus) = heat required to melt one mole of a solid (solid → liquid)
∆Hvap and ∆Hfus are endothermic (+)

11. ∆Hcondensation = -∆Hvap ∆Hsolidification = -∆Hfus
Energy and Chemical Change: Basic Concepts
Changes of state
Molar enthalpy (heat) of condensation (∆Hcond) = heat required to condense one mole of a liquid (gas → liquid)
Molar enthalpy (heat) of solidification (∆Hsolid) = heat required to solidify one mole of a solid (liquid → solid)
∆Hcondensation = -∆Hvap
∆Hsolidification = -∆Hfus

12. Energy and Chemical Change: Basic Concepts
The standard enthalpy (heat) of formation (∆H°f) of a compound is the change in enthalpy for the formation of one mole of a compound

13. PHASE CHANGES
TEMPERATURE
ΔHchange
boiling
evaporating
gas BP
condensing
liquid
melting MP
freezing
q=c∙m∙ΔT
solid
ENERGY

14. Converting Energy Units
Energy and Chemical Change: Basic Concepts
Converting Energy Units
Ex 9) How much heat is released when 82.1 g of methanol is burned?
Goal = heat released Known = 82.1 g methanol
82.1 g CH3OH
1 mol CH3OH
(-) 726 kJ
32.05 g CH3OH
1 mol CH3OH
Use ENTHALPY to convert mol to kJ
convert
g to mol
MOLAR MASS
q = kJ
1860

15. Practice finding heat ∆H = enthalpy = (+) endothermic = (-) exothermic
WHY? It tells us if it’s endo- or exothermic
q = heat
= always (+)
WHY? Because it tells us HOW MUCH energy there is, NOT if it’s endo- or exothermic

16. Practice finding heat 2.58 kJ 377 kJ 232 g
Ex 10) Calculate the heat required to melt 25.7 g of solid methanol.
Ex 11) How much heat is evolved when 275 g of ammonia gas condenses to a liquid?
Ex 12) What mass of methane must be burned in order to release 12,880 kJ of heat?
2.58 kJ
377 kJ
232 g