1. Enthalpy Calculating Heats of Reaction
Thermochemistry
Enthalpy
Calculating Heats of Reaction

2. Warm up!
A 3.54 g piece of aluminum is heated to 96.2 ºC and allowed to cool to room temperature, 22.5 ºC. Calculate the heat (in kJ) associated with the cooling process. The specific heat of aluminum is J/ g · K.
– 236
+ 236
– 0.236
– 0.638

3. Learning Check:
Classify each process as exothermic or endothermic. The system is underlined in each example.
Your hand gets cold when you touch ice.
The ice gets warmer when you touch it.
Water boils in a kettle being heated on a stove.
Water vapor condenses on a cold pipe.
Ice cream melts.
Candle burns.

4. Enthalpy of Reaction
The change in enthalpy, ΔH, is the enthalpy of the products minus the enthalpy of the reactants:
ΔH = Hproducts − Hreactants
ΔH is called the enthalpy of reaction, or the heat of reaction. 4

5. ΔH = heat given off or absorbed during a reaction
Enthalpy (H) is used to quantify the heat flow into or out of a system in a process that occurs at constant pressure.
ΔH = H (products) – H (reactants)
ΔH = heat given off or absorbed during a reaction
Hproducts < Hreactants
Hproducts > Hreactants
ΔH < 0
ΔH > 0

6. Thermochemical Equations
Is ΔH negative or positive?
System absorbs heat
Endothermic
ΔH > 0

7. Thermochemical Equations
Is ΔH negative or positive?
System gives off heat
Exothermic
ΔH < 0

8. Bond Breaking - Endothermic
Energy is always required to be inputted to break a bond. Bond breaking is always endothermic.

9. Bond Making - Exothermic
Energy is always released when a bond is formed. Bond making is always exothermic.

10. Enthalpy (ΔH) ΔHrxn = Hf (products) – Hf (reactants)

11. Enthalpies of formation
Enthalpy of formation (ΔHf) = enthalpy change associated with formation of compound from its constituent elements.
Magnitude depends on conditions of temperature, pressure, state of reactants and products
Standard set of conditions for comparison:
Pure form of a compound/substance, 1 atm, 25°C = 298K
Standard enthalpy of formation (ΔH°f) = enthalpy change for reaction that forms 1 mol of compound from its elements with all substances in their standard states
By definition: standard enthalpy of formation (ΔH°f) of most stable form of any element is zero (ΔH°f = 0) 11

12. Standard Enthalpies of Formation

13. ΔH°f and enthalpies of reactions: ΔH°rxn
ΔH°rxn = Σn ΔH°f (products) - Σn ΔH°f (reactants)
n = coefficients of chemical reactions of each reactant and product (varies)
combustion of ethanol
C2H5OH (l) + 3 O2 (g) → 2 CO2 (g) H2O (l)
Example: Calculate the enthalpy change for combustion reaction of 1 mol of ethanol.
Use standard enthalpies of formation table on previous slide 13

14. ΔH°f and enthalpies of reactions: ΔH°rxn
ΔH°rxn = Σn ΔH°f (products) - Σn ΔH°f (reactants)
n = coefficients of chemical reactions of each reactant and product (varies)
Example: combustion of ethanol
C2H5OH (l) + 3 O2 (g) → 2 CO2 (g) H2O (l)
Calculate the enthalpy change for combustion reaction of 1 mol of ethanol:
ΔH°rxn = [2x ΔH°f (CO2) + 3 ΔH°f (H2O)] – [ΔH°f (C2H5OH) + 3x ΔH°f (O2)]
Use standard enthalpies of formation table
ΔH°f C2H5OH(l) = kJ/mol
ΔH°f CO2(g) = kJ/mol
ΔH°f H2O(l) = kJ/mol
ΔH°f O2(g) = 0 (by definition)
ΔH°rxn = [2x(-393.5) + 3x(-285.8)] – [ ] = kJ 14

15. Try this one on your own..
Calculate the standard enthalpy change for the reaction that occurs when ammonia is burned in air to make nitrogen dioxide and water
4NH3(g) + 7O2(g) → 4NO2(g) + 6H2O(l)

16. using enthalpy of formation equation: