Thermochemistry Chapter 12

Thermochemistry The study of the changes in the heat of chemical reactions. Heat – the energy that is transferred from one object to another due to a difference in temperature.

Bonds breaking requires energy Bonds forming releases energy Therefore… Almost all chemical reactions absorb or release energy as heat.

Exothermic Reaction – Gives off more energy forming bonds than breaking bonds. (feels hot) Endothermic Reaction – Requires more energy to break bonds than to form bonds. (feels cold)

Enthalpy The heat content of a chemical system. Enthalpy = the amount of heat released or absorbed when a chemical reaction takes place at constant pressure. Enthalpy depends on: temperature, physical state, and composition

Energy Vs. Enthalpy Very similar numerically. Enthalpy is energy in a system at a constant pressure. Enthalpy is much more useful, most everyday chemical rxns take place at constant pressure.

Enthalpy Change H = H Products - H Reactants Measured in KJ Comparisons are made at 1atm of pressure, 25 ° C (298K), & in its standard state of matter.

Standard State of an Element The most stable form of an element under standard conditions. Standard Enthalpy Change – change that is measured when reactants in their standard state change to products in their standard state - H °

Endothermic Vs Exothermic Energy is absorbed Rxn Vessel becomes cooler Temp inside vessel decreases Energy of Reactants < Energy of Products + H Energy is released Rxn Vessel becomes warmer Temp inside vessel Increases Energy of Reactants > Energy of Products - H

Writing Enthalpy Changes N 2 (g) + 3H 2 (g) ----> 2NH 3 (g) H = kJ Can Reverse Equation 2NH 3 (g) -----> N 2 (g) + 3H 2 (g) H = kJ _____________________________________________ 92.4 kJ of energy is released for every 1 mole of N 2 (g) & 92.4 kJ of energy is released for every 3 moles of H 2 (g) & 92.4 kJ of energy is released for every 2 moles of NH 3 (g) produced.

1.How much energy is released if only 1 mole of ammonia (NH 3 ) gas is produced? 2.How much energy is released if 10 moles of nitrogen (N 2 ) gas and 30 moles of hydrogen (H 2 ) gas is used in the reaction?

Hess’s Law Hess’s Law states that the heat of a whole reaction is equivalent to the sum of it’s steps.

Rules for Hess’s Law 1.If the coefficients of an equation are multiplied by a factor, the enthalpy change for the reaction is multiplied by the same factor. 2.If an equation is reversed, the sign of enthalpy change reverses also.

Examples C + O 2  CO 2 Can occur as 2 steps C + ½O 2  CO  H  = – kJ CO + ½O 2  CO 2  H  = – KJ C + CO + O 2  CO + CO 2  H  = KJ

C 2 H 4 (g) + H 2 O(l)  C 2 H 5 OH(l) Can Occur as 2 Steps C 2 H 4 (g) + 3O 2 (g)  2CO 2 (g) + 2H 2 O(l)  H  = KJ 2CO 2 (g) + 3H 2 O(l)  C 2 H 5 OH(l) + 3O 2 (g)  H  = KJ C 2 H 4 (g) + H 2 O(l)  C 2 H 5 OH(l)  H  = +44 KJ More ules/module3/lesson5/hessmore.html ules/module3/lesson5/hessmore.html

Calorimetry The study of heat flow and heat measurements Calorimetry experiments determine the enthalpy changes of reactions by making accurate measurements of temperature changes produced in a calorimeter.

Temperature change depends on the amount of heat released & the heat capacity of the surroundings. – Heat Capacity = Amt of heat necessary to raise the temp by 1 °C Heat capacity is effected by composition and mass -Specific Heat(C) = Heat capacity of 1g of a substance

Specific Heat of Water Water has one of the largest specific heats. Specific Heat of H 2 O = 4.184J/g°C 4.184KJ/g°C = 1 calorie calorie amt of heat needed to raise 1g of H 2 O, 1°C

Heat vs Temperature Change A transfer of heat is detected by measuring a temperature change, however, a small temperature change does not always signify a small transfer of heat.

Using Calorimeters q is used to denote heat measurements made in a calorimeter q of the surrounding is equal and opposite in magnitude to heat of the rxn : q rxn = -q sur To determine amt of heat absored by water q sur(water) = m x C x (T f - T i )