Calorimetry and Enthalpy Calorimetry n a laboratory technique

Calorimetry and Enthalpy

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

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

Calorimeter n Insulated device used to measure heat flow • Either measures heat absorbed OR released

Question Time n n n What is calorimetry? What is the basis for calorimetry? What is a calorimeter?

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

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

Exothermic Reaction H is negative Hproducts<Hreactants Potential Enthalpy

Endothermic Reaction H is positive Hproducts>Hreactants Potential Enthalpy

Question Time n n 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?

Thermochemical Equation n equation that includes the physical states of all reactants and products (1 atm, 25 o. C) and the energy change, H n Endo) NH 4 NO 3(s) NH 4+(aq) + NO 3 -(aq) H = 27 k. J n Exo) Ca. O(s) + H 2 O(l) Ca(OH)2(s) H = -65. 2 k. J

Thermochemical Equations for Endothermic Reactions n n Endothermic : H is positive; heat is on the reactants side of the equation (Heat absorbed) NH 4 NO 3(s) NH 4+(aq) + NO 3 -(aq) H = 27 k. J NH 4 NO 3(s) + Heat NH 4+(aq) + NO 3 -(aq) NH 4 NO 3(s) + 27 k. J NH 4+(aq) + NO 3 -(aq)

Thermochemical Equations for Exothermic Reactions n n Exothermic : H is negative; heat is on the products side of the equation (Heat released) Ca. O(s) + H 2 O(l) Ca(OH)2(s) H = -65. 2 k. J Ca. O(s) + H 2 O(l) Ca(OH)2(s) + Heat Ca. O(s) + H 2 O(l) Ca(OH)2(s) + 65. 2 k. J

Heat of Combustion n the enthalpy change for the complete burning of a substance n CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O(l) H = -891 k. J CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O(l) + Heat CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O(l) + 891 k. J n n

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

Example 1 n 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. 17 J/g °C C = 6. 2 J/g °C

Example 2 n n How much heat is released by the combustion of 250. 0 g of octane, C 8 H 18? Octane ΔHcomb = -5471 k. J/mol Q = -11970 k. J or 11970 k. J is released