Measuring and Expressing Enthalpy in Thermochemical Equations I

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Measuring and Expressing Enthalpy in Thermochemical Equations

Measuring and Expressing Enthalpy in Thermochemical Equations

I. Enthalpy, ΔH • Definition: the amount of energy absorbed or released by a

I. Enthalpy, ΔH • Definition: the amount of energy absorbed or released by a system during a reaction at constant pressure – Use interchangeably with heat, q = ΔH • In a chemical equation, the enthalpy change for the reaction can be written as either a reactant or product – Ex: Ca. O + H 2 O Ca(OH)2 + 65. 2 k. J Heat or the enthalpy change is considered a product • A chemical equation that includes the enthalpy change is called a thermochemical equation

Examples H 2(g) + Cl 2(g) 2 HCl(g) (reactants) (products) • Individual H and

Examples H 2(g) + Cl 2(g) 2 HCl(g) (reactants) (products) • Individual H and Cl atoms are unstable so they immediately combine to form HCl. Energy is RELEASED – Forming bonds is EXOTHERMIC 2 HCl(g) H 2(g) + Cl 2(g) (reactants) (products) • During a collision, there must be a enough kinetic energy to break the bonds. Energy is ABSORBED – Breaking bonds is ENDOTHERMIC

Graphic Representations of Endo and Exo Reactions The overall reaction will either release energy

Graphic Representations of Endo and Exo Reactions The overall reaction will either release energy (exothermic) or absorb energy (endothermic) H = “-” = exothermic ** Nature favors rxns that release energy, exothermic q ( Hrxn) = Hproducts - Hreactants H = “+” = endothermic

Ex 10. 5: Enthalpy When 1 mol of methane (CH 4) is burned at

Ex 10. 5: Enthalpy When 1 mol of methane (CH 4) is burned at constant pressure, 890 k. J/mol 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. H = heat flow = -320 k. J

II. Calorimetry • Calorimetry is the precise measurement of the heat flow into or

II. Calorimetry • Calorimetry is the precise measurement of the heat flow into or out of a system during chemical or physical processes • The heat released by the system = heat absorbed by surroundings • The heat absorbed by a system = heat released by surroundings • Device used to measure absorption or release of heat is called a calorimeter

Example Calorimeters Styrofoam Cup is a simple calorimeter Bomb calorimeter—measures calories in food

Example Calorimeters Styrofoam Cup is a simple calorimeter Bomb calorimeter—measures calories in food

III. Expressing Enthalpy in Thermochemical Equations Exothermic Example: H 2(g) + Cl 2(g) 2

III. Expressing Enthalpy in Thermochemical Equations Exothermic Example: H 2(g) + Cl 2(g) 2 HCl(g) + k. J H = -184. 6 k. J Endothermic Example: N 2(g) + 2 O 2(g) + 66. 2 k. J 2 NO 2(g) H = +66. 2 k. J *Units: 1 k. J/mol = 1000 J/mol

Example Problem N 2(s) + 3 H 2(g) 2 NH 3(g) + 393. 5

Example Problem N 2(s) + 3 H 2(g) 2 NH 3(g) + 393. 5 k. J • How much heat was released if 0. 5 mol of hydrogen gas reacted? • How much heat was released if 46 grams of ammonia reacted?