1. Unit 5 Energetics

2. Enthalpy [159]
Many forms of energy are absorbed or released during chemical reactions
Most energy involved in a reaction is heat
Heat energy is known as enthalpy
Symbol is H

3. System vs. surroundings [160]

4. Exothermic and Endothermic Reactions [160-161]
Type of reaction
Heat energy change
Temperature change
Relative enthalpies
Sign of H
exothermic
Heat evolved or released
Becomes hotter
Hproducts <Hreactants
negative
endothermic
Heat absorbed
Becomes colder
Hproducts >Hreactants
positive
Page 161 exercises 1 and 2

5. Exothermic energy diagram [160]
Common exothermic reactions are combustion and acid base neutralization

6. Endothermic energy diagram [160]

8. Enthalpy conditions [161]
Standard enthalpy changes are measured under defined conditions (standard state)
25°C or 298K
101.3 kPa or 1 atm
Solution concentrations 1 mol dm-3 or 1 M
Elements and compounds are in their normal physical states at these conditions

9. Calculation of Enthalpy Changes - stoichiometry
2 Mg (s) + O2 (g)  2 MgO (s) H = kJ What is H for the reverse reaction? How much heat is released when 4 mole of MgO is formed? How much heat is released when 10 g of O2 is consumed? How much heat is released when 25 g of MgO is formed?

10. Calculation of Enthalpy Changes – calorimetry [161-168]
Temperature  Heat
When temperature changes, heat must be absorbed or released
Calorimetry is used to measure heat
H or H or q = mCpT
m = mass
Cp = specific heat of substance (J/gK or J/g°C), amount of heat needed to raise the temperature of 1 g by 1°C
T = change in temperature

11. Calculation of Enthalpy Changes – calorimetry [161-168]
Worked example page 162
Exercises 3 and 4 page 162
Exercise 5 page 164
Exercise 6 page 165

12. Calculation of Enthalpy Changes – calorimetry [161-168]
Enthalpy of solution example
\When a 9.55 g sample of NaOH dissolves in g of water, the temperature rises from 23.6 to 47.4C. Calculate the H (in kJ/mole NaOH) for the solution process.

13. Calculation of Enthalpy Changes – calorimetry [161-168]
Specific heat of a metal example
We are going to determine the specific heat of a metal using experimental data. In this experiment, we used a "coffee cup" calorimeter and gathered the following data:
Mass of empty cup 2.31 g
Mass of cup + water g
Mass of cup + water + metal g
Initial temperature of water 17.0 °C
Initial temperature of metal 52.0 °C
Final temperature of system 27.0 °C

14. Calculation of Enthalpy Changes – Hess’ Law [168-170]
Hess's Law states the change in enthalpy (H), in a reaction is the same regardless whether the reaction occurs in a single step or in several steps. 

15. Calculation of Enthalpy Changes – Hess’ Law [168-170]
Worked example page 170
Exercises 8 and 9 page 170

16. Calculation of Enthalpy Changes – Bond Enthalpy [170-174]
Bond enthalpy is defined as the energy needed to break one mole of bonds in gaseous molecules under standard conditions
Bond enthalpies measure strength of covalent bond

17. Calculation of Enthalpy Changes – Bond Enthalpy [170-174]
Breaking bonds require energy – endothermic
Forming bonds release energy – exothermic
Worked example page 172
Exercises page 172

18. Calculation of Enthalpy Changes – Bond Enthalpy [170-174]
To calculate bond enthalpy
Hreaction = Hbonds broken - Hbonds formed
Worked example page 173
Exercises page 174
If the bonds being broken are weaker that those being made, the reaction will have a _______ H and be ____________.
(+ or -) (exo or endothermic)