1. Calorimetry and Enthalpy

2. Calorimetry a laboratory technique used to measure heat flow Based on:
the law of conservation of energy
Idea that heat released by system EQUALS heat absorbed by surroundings or vice versa
-q = q
-(m(DT)Cp) = mDTCp

3. Calorimetry Example An ice cube is added to a warm cup of water.
The amount of heat used to melt the ice cube is the same amount of heat lost by the warm water.

4. Calorimeter Insulated device used to measure heat flow
Either measures heat absorbed OR released

5. Question Time What is calorimetry? What is the basis for calorimetry?
What is a calorimeter?

6. Enthalpy and Enthalpy Changes
Enthalpy (H) is the heat content of a system at constant pressure
Thermochemistry uses the change in enthalpy(H) to study heat changes
At constant pressure, Q = H
Heat of reaction (Hrxn) – The change of enthalpy in a chemical reaction

7. Endo, Exo, Q, and DH
All chemical and physical changes release or absorb heat
Exothermic: releases heat, Q is negative, ∆H is negative
Endothermic: absorbs heat, Q is positive, ∆H is positive

8. Exothermic Reaction H is negative Hproducts<Hreactants Potential
Enthalpy

9. Endothermic Reaction H is positive Hproducts>Hreactants Potential
Enthalpy

10. Question Time
What is the sign of the heat of reaction if it is exothermic?
What is the sign of the heat of reaction if it is endothermic?

11. Thermochemical Equation
equation that includes the physical states of all reactants and products (1 atm, 25oC) and the energy change, H
Endo) NH4NO3(s)  NH4+(aq) + NO3-(aq) H = 27 kJ
Exo) CaO(s) + H2O(l)  Ca(OH)2(s) H = kJ

12. Thermochemical Equations for Endothermic Reactions
Endothermic : H is positive; heat is on the reactants side of the equation (Heat absorbed)
NH4NO3(s)  NH4+(aq) + NO3-(aq) H = 27 kJ
NH4NO3(s) + Heat  NH4+(aq) + NO3-(aq)
NH4NO3(s) + 27kJ  NH4+(aq) + NO3-(aq)

13. Thermochemical Equations for Exothermic Reactions
Exothermic : H is negative; heat is on the products side of the equation (Heat released)
CaO(s) + H2O(l)  Ca(OH)2(s) H = kJ
CaO(s) + H2O(l)  Ca(OH)2(s) + Heat
CaO(s) + H2O(l)  Ca(OH)2(s) kJ

14. Heat of Combustion
the enthalpy change for the complete burning of a substance
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) H = -891kJ
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) + Heat
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) + 891kJ

15. Question Time Is the following reaction endothermic or exothermic?
4Fe(s) +3O2(g)  2Fe2O3(s) H = -1625kJ
Which side of the equation would you write the heat for in the previous question?
What is needed as a reactant for a combustion reaction?

16. Example 1
A hot piece of metal (at 155 °C) with a mass of 4.68 g is placed into 53.9 grams of water at 22 °C. The water (Cp =4.184 J/g °C) heats up to 37.1 °C. What is the specific heat of metal?
C = 6.17J/g °C
C = 6.2 J/g °C

17. Example 2
How much heat is released by the combustion of g of octane, C8H18?
Octane ΔHcomb = kJ/mol
Q = kJ
or kJ is released