1. Endothermic & Exothermic Reactions
Enthalpy- (H) Heat
Enthalpy Change- (ΔH) The change in heat for a reaction

2. Endothermic reactions
Energy is absorbed
Heat goes into the system from the surroundings
the surroundings will feel colder
Temperature of surroundings goes down
+ΔH
Exothermic reactions
energy is released
Heat goes out of the system into the surroundings
Temperature of the surrounding will increase
-∆H

3. Endo- and Exothermic ENDOTHERMIC EXOTHERMIC Heat Heat Surroundings
System
Surroundings
System
Heat
DEMONSTRATION
example of an ENDOTHERMIC reaction
- hydration of ammonium nitrate
(NH4)+(NO3)- (s) + H2O -> NH4 (aq) + NO3 (aq)
(cools surroundings enough to freeze water)
WHY SPONTANEOUS ?
ENDOTHERMIC
EXOTHERMIC

4. Endothermic & Exothermic Reactions

5. Endothermic & Exothermic Reactions

6. Endothermic & Exothermic Reactions
Are the following reactions endothermic or exothermic?
CO + 3H2  CH4 + H2O H= -206kJ
Exothermic (H is negative)
I add magnesium metal to some hydrochloric acid. The temperature goes from 23C to 27 C
Exothermic – temperature goes up
I mix together some vinegar & baking soda. The temperature goes from 28C to 23C
Endothermic – temperature goes down

7. Thermochemical equations
An equation that gives us information about H …
S + O2  SO2 H = – kJ
If we change the equation, then the H also changes …
SO2  S + O2 H = kJ
If the reaction is reversed the sign is reversed
Also, if the coefficients in the equation change, so will the amount of energy produced/absorbed:
2S + 2O2  2SO2 H = – kJ

8. Given the following thermochemical equation:
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
ΔH = kJ/mol
calculate the heat evolved when g of CH4 reacts
The chemical equations are always written in terms of moles, but the problem is stated in terms of mass (g).
First, convert grams to moles.
Then, convert moles to kJ (kilojoules)

9. CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l) ΔH = -890.4 kJ/mol
20.00g CH4
1 mole CH4 kJ
16.04g CH4
1 mole CH4
(from the coefficient in the equation) kJ

10. CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (l) ∆H = - 890.4 kJ
How much energy is given off when 2.00 mol of CH4 are burned?
How much energy is released when 22.4g of CH4 are burned?
If you were to attempt to make 45.0g of methane from CO2 and H2O , how much energy would be required?

11. -1780 kJ
-1240 kJ
2.50 x 103 kJ