Chapter 17: Measuring and Expressing Enthalpy Changes Chemistry 122 Chapter 17: Measuring and Expressing Enthalpy Changes

Calorimetry Calorimetry measures heat flow into or out of a system for chemical or physical processes The heat released by a system is equal to the heat absorbed by the surroundings The heat absorbed by the system is equal to the heat released by the surroundings The device itself is called a calorimeter

Constant-Pressure Calorimeters The heat content of a system at constant pressure is the same as a property called enthalpy (H) The amount of heat released or absorbed from a chemical reaction at constant pressure is referred to as a change in enthalpy (ΔH) of the system Heat and enthalpy change are the same thing q = ΔH

Same formula from before… When you measure the temperature of the water in the calorimeter before and after the chemical reaction, enthalpy can be calculated as follows: qsurr = m x C x ΔT This translates to: 'heat absorbed by the surroundings' Since the surroundings is water, the known values of C is used (4.18 J/g·°C) in calculations involving the heat transfer

The opposite for the system… Conversely, if you are measuring the heat lost or gained by the system, the enthalpy change is written as follows: qsys = ΔH = -qsurr = -m x C x ΔT The sign for an exothermic reaction (heat released) is negative and positive for an endothermic reaction (heat absorbed)

Bomb Calorimeters A sample is burned in a constant-pressure chamber in the presence of oxygen at high pressure The heat released by the reaction warms the water By measuring the increase in temperature, it is possible to calculate the amount of heat released during the combustion

Complete questions 12 – 13, pg. 513 Example Remember to write down all that is known in the example. Add up any volumes (or masses). Look for conversions. Rely on known constants. Complete questions 12 – 13, pg. 513

Thermochemical Equations IN A CHEMICAL EQUATION, THE ENTHALPY CHANGE FOR THE REACTION CAN BE WRITTEN AS EITHER A REACTANT OR A PRODUCT – depending on whether it is an exothermic or endothermic reaction Ex. CaO(s) + H2O(l) → Ca(OH)2(s) + 65.2kJ A chemical equation that includes the enthalpy change is called a thermochemical equation

Heat of Reaction The enthalpy change for the chemical equation exactly as it is written Exothermic: CaO(s) + H2O(l) → Ca(OH)2(s) ΔH = -65.2kJ Endothermic: 2NaHCO3(s) → Na2CO3(s) + H2O(l) + CO2(g) ΔH = 129kJ Refer to page 17.7, p. 515

This relates to stoichiometry (remember stoichiometry?) The amount of moles factors into the amount of enthalpy released or absorbed The states of matter are important as well This is because different physical states of the same substance can have different ΔH values Ex. Water is 4.184J/g∙˚C yet ice is 2.01J/g∙˚C

Example of Heat of Reaction – page 516 Read through the question to identify what is known This is a simple conversion question You know how much heat is absorbed when 2 moles of sodium bicarbonate is heated. However, the amount you want to investigate is 2.24 mol Questions 14 – 15, p. 516

Heat of Combustion Refers to the amount of heat released when one mole of substance is burned. Ex. CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) ΔH=-890kJ Unless otherwise indicated, standard conditions are being used in these equations. This means the temperature is 25°C and pressure is 101.3kPa.

Review Select a substance from 17.2. Write the thermochemical equation. Why is the ΔH negative?

3. Draw an enthalpy diagram for this reaction. 4. Is heat flowing into or out of the system? 5. What must the total energy (potential and kinetic) equal in any chemical or physical process?

For the remainder of class: Work on guided reading – section 17.2 Questions 16 – 20 need to be completed to check for understanding Section review 17.2 Practice Problems ?