1 / 11

Chapter 6: Thermochemistry

This chapter explores the principles of Thermochemistry, which involves the relationships between heat and other forms of energy in chemical reactions. Topics covered include heat of reaction, system and surroundings, thermal equilibrium, enthalpy, thermochemical equations, stoichiometry and heats of reaction, heat capacity, specific heat, measuring heat of reaction, Hess's law, and standard enthalpies of formation.

suzettes
Download Presentation

Chapter 6: Thermochemistry

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Chapter 6: Thermochemistry Chemistry 1061: Principles of Chemistry I Andy Aspaas, Instructor

  2. Thermochemistry • Thermodynamics: relationships between heat and other forms of energy • Thermochemistry: an area of thermodynamics that involves heat transferred due to a chemical reaction • Energy: potential or capacity to move matter • Kinetic energy, Ek: macroscopic energy associated with an object’s movement • Potential energy, Ep: macroscopic energy assiciated with an object’s position in a field of force (only relative values) • Internal energy, U: microscopic sum of energy contained in a substance’s particles • Etot = Ek + Ep+ U

  3. Heat of reaction • System: collection of substances in which the thermodynamic change is happening • Surroundings: everything outside the system, includes the flask, the room, and the universe • Heat, q: energy that flows into or out of a system because of a difference in temperature • Thermal equilibrium: heat flows from areas of high temperature to areas of low temperature until the temperatures are equal

  4. Energy added or subtracted from a system • The sign of q is viewed from the perspective of the system, not the surroundings • Exothermic process, the reaction vessel warms, so energy must have left the system: q is – • Endothermic process, the reaction vessel cools, so energy must have been added to the system: q is +

  5. Enthalpy • Enthalpy, H: an extensive property (depends on quantity) which is related to the amount of heat that can be absorbed or evolved in a chemical reaction • H is a state function (only depends on present state, independent of any history) • ∆H = change in enthalpy = H(products) - H(reactants) • ∆H = qp(enthalpy change = reaction heat at constant pressure) • An exothermic reaction might have qp= -400 kJ • So, ∆H = -400 kJ

  6. Thermochemical equations • Thermochemical equation: balanced molar chemical equation, with enthalpy of reaction given after the equation 2Na(s) + 2H2O(l)  2NaOH(aq) + H2(g); ∆H = -368.6 kJ • 368.6 kJ of heat is evolved when 2 mol Na react with 2 mol H2O to form 2 mol NaOH and 1 mol H2 • Phase labels are important, ∆H may be different depending on phase of product • If a thermochemical equation is multiplied by anything, ∆H is also multiplied • If a thermochemical equation is reversed, the sign of ∆H is also reversed

  7. Stoichiometry and heats of reaction CH4(g) + 2O2(g)  CO2(g) + 2H2O(l); ∆H = -890.3 kJ • In excess oxygen, how many kJ of heat can be obtained from burning 36.0 g CH4? • Use ∆H as a conversion factor from mol to kJ

  8. Heat capacity • Heat capacity, C: quantity of heat required to raise temperature of a substance one degree Celsius q = C∆t where ∆t = tf - ti • Specific heat, s: quantity of heat required to raise temperature of one gram of a substance by one degree Celsius q = sm∆t • Water has a very high specific heat: 4.18 J / (g · °C)

  9. Measuring heat of reaction • Calorimeter: device used to measure heat of reaction • Insulated reaction vessel, with thermometer to record temperature change • Two nested styrofoam cups works well • First find amount of heat absorbed by calorimeter q = sm∆t and q = C∆t • Heat absorbed by surroundings is opposite of heat given off by system qcalorimeter = -qreaction • Divide qreaction by correct number of moles of limiting reactant to set up a thermodynamic equation

  10. Hess’s law • Adding reactions together will also add ∆H for each reaction • So, ∆H can be found for reactions where it would be difficult to measure experimentally • Remember, multiplying a reaction by a constant also multiplies the ∆H by that constant, and reversing a reaction reverses the sign of ∆H

  11. Standard enthalpies of formation • Standard enthalpy of reaction, ∆H °: ∆H at 25° C and 1 atm • Standard enthalpy of formation, ∆Hf°: enthalpy change for formation of one mole of the substance in its standard state from its elements ∆H ° =  n ∆Hf°(products) -  n ∆Hf°(reactants)

More Related