Thermochemistry Enthalpy & Hess Law Packet #35
Introduction I Thermochemistry, is the branch of chemistry that, investigates the amount of energy that is gained or lost during a chemical reaction.
Introduction II In exothermic, or endothermic, reactions, the energy lost, or gained, by the system is in the form of heat. A special energy function that is used to measure the amount of heat energy is enthalpy ( H). Enthalpy is the amount of energy that is lost or gained during a chemical reaction.
Introduction III In a lab setting, most chemical and physical changes are open to the atmosphere. This atmosphere, in the lab setting, will have these changes occur at a constant pressure. The heat content of a system at constant pressure can be defined as enthalpy ( H p ).
Introduction IV For a reaction occurring under conditions of constant pressure, the change in enthalpy ( H) is equal to the amount of heat released or absorbed. H p = heat (released/absorbed) p indicates that the process occurred under conditions of constant pressure.
Calorimeter A calorimeter is a device used to determine the amount of heat associated with a chemical reaction. The reaction is run in the calorimeter and the temperature change of the calorimeter is observed. Remember system vs. surroundings.
The Connection Enthalpy & Specific Heat (Capacity) q sys = H q sys = -q surr q sys = -m * C * T
Thermochemical Equations In a chemical equation, the enthalpy change for the reaction can be written as either a reactant or a product. CaO(s) + H 2 O(l) Ca(OH) 2 (s) + 65.2 kj The above is called a thermochemical equation. The heat of reaction is the enthalpy change ( H) for the chemical equation. In the equation above, heat energy was lost as a result of the reaction. Therefore, H= -65.2kJ
Problem Set I Using the thermochemcial equation 2NaHCO 3 (s) + 129 kj Na 2 CO 3 (s) + H 2 O(g) + CO 2 Calculate the amount of heat in, kj, required to decompose 2.24 mol NaHCO 3 (s). This question is a hybrid of the mole concept and enthalpy concept.
Problem Set II When 1 mole of methane CH 4 is burned at constant pressure, 890 kj of energy is released as heat. Calculate H for a process in which a 5.8-g sample of methane is burned at constant pressure.
HESS LAW
Introduction Important Note Enthalpy is a state function. Change in enthalpy, for a given process, is independent of the pathway for the process. Practical Example When one travels from Miami to Denver. Distance traveled vs. Change in elevation During a chemical reaction, it does not matter if the reaction requires one step, or multiple steps, to get to the end product(s) enthalpy will always be the same Much like the change in elevation when going from Miami to Denver. This principle is known as Hess Law.
Hess Law Conceptual Example The overall equation written in one step, where overall enthalpy change is represented by H 1 is N 2 (g) + 2O 2 (g) 2NO 2 (g) H 1 = 68 kj The overall reaction is carried out in two distinct steps N 2 (g) + O 2 (g) 2NO(g) H 2 = 180 kj 2NO(g) + O 2 (g) 2NO 2 (g) H 3 = -112 kj Net Overall Reaction: - N 2 (g) + 2O 2 (g) 2NO 2 (g) H 1 = 68 kj
Hess Law Conceptual Example
Enthalpy, Hess Law & The Connection to Energy Diagram Net Overall Reaction: - N 2 (g) + 2O 2 (g) 2NO 2 (g) H 1 = 68 kj Overall, the reaction is an endothermic reaction. 68 kj of heat was required to carry out the reaction. Heat was GAINED.
Enthalpy, Hess Law & The Connection to Energy Diagram Net Overall Reaction: - N 2 (g) + 2O 2 (g) 2NO 2 (g) H 1 = 68 kj The chemical reaction can also be written as N 2 (g) + 2O 2 (g) + 68 kj 2NO 2 (g) The reaction is an endothermic reaction.
CHARACTERISTICS OF ENTHALPY CHANGES
Characteristics of Changes To use Hess law to compute enthalpy changes for reactions, it is important to understand two characteristics of H for a reaction: - If a reaction is reversed, the sign of H is also reversed. The magnitude of H is directly proportional to the quantities of reactants and products in a reaction. If the coefficients in a balanced reaction is multiplied by an integer, the value of H is multiplied by the same integer.
HEAT OF COMBUSTION
Heat of Combustion The heat of reaction for the complete burning of one mole of a substance. See next slide CH 4 + 2O 2 (g) CO 2 (g) + 2H 2 O(l) + 890 kj OR CH 4 + 2O 2 (g) CO 2 (g) + 2H 2 O(l) H = -890 kj
Heat of Combustion List @ 25 C
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