Thermochemistry

Energy is the capacity to do work Thermal energy is the energy associated with the random motion of atoms and molecules Chemical energy is the energy stored within the bonds of chemical substances There are also other forms of energy, such as nuclear, potential, and electrical

Heat is the transfer of thermal energy between two bodies that are at different temperatures. Energy Changes in Chemical Reactions Temperature is a measure of the thermal energy. Temperature = Thermal Energy 90 0 C 40 0 C greater thermal energy 6.2 Which one has greater thermal energy?

Law of Conservation of Energy: Energy can not be created or destroyed. It can only change form. What types of energy are involved when you eat a burger?

Thermochemistry is the study of heat change in chemical reactions. The system is the specific part of the universe that is of interest in the study. open mass & energyExchange: closed energy isolated nothing SYSTEMSURROUNDINGS 6.2

Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings. Endothermic process is any process in which heat has to be supplied to the system from the surroundings. 2H 2 (g) + O 2 (g) 2H 2 O (l) + energy Here’s a video of this reaction: Can you tell energy is a product? H 2 O (g) H 2 O (l) + energy energy + 2HgO (s) 2Hg (l) + O 2 (g) 6.2 energy + H 2 O (s) H 2 O (l)

Exothermic and Endothermic Exothermic Chemical  Thermal Releases heat! Endothermic Thermal  Chemical Absorbs heat! Firewood burning Ice melting

Product: 127 kJ/mol Reactants: kJ/mol Reactants: kJ/mol Products: 720 kJ/mol  H = H (products) – H (reactants)  H = heat given off or absorbed during a reaction Enthalpy (H) is used to quantify the heat flow into or out of a system in a process that occurs at constant pressure. Exothermic: negative ΔH Endothermic: positive ΔH

Reaction Endo or Exo? N 2 (g) + O 2 (g)  2NO (g) ∆H = 168 kJ/mol 2H 2 (g) + O 2 (g)  2H 2 O (l) ∆H = -571 kJ/mol 2 P(s) + 5 Cl 2 (g)  2 PCl 5 (s) ∆H = -880 kJ/mol H 2 O(l)  H 2 O(g) ∆H = 44 kJ/mol Endo! Exo! Try It! Endo!

The specific heat (s) of a substance is the amount of heat (q) required to raise the temperature of one gram of the substance by one degree Celsius. Heat (q) absorbed or released: q = mc  t  t = t final - t initial 6.4

I Do: How much heat is given off in Joules when an 869 g iron bar cools from 94 0 C to 5 0 C? Is this process endothermic or exothermic? q = mc  t 6.4

We Do: 71.5g of aluminum metal is heated from 20°C to 50°C in order to increase its malleability. What amount of heat is absorbed by the aluminum? Is this process endothermic or exothermic?_____________ q = mc  t

You Do: A 2.0-gram sample of graphite cools from 10°C to 0°C. How much heat was released from the graphite sample? Endothermic or exothermic? _______________ q = mc  t

Determining Endo/Exothermicity To determine if a reaction is exothermic or endothermic, we measure the temperature of the surroundings.

Calorimetry Calorimetry is the quantitative measurement of the heat required or evolved during a chemical process. For example, if the energy from an exothermic chemical reaction is absorbed in a container of water, the change in temperature of the water provides a measure of the amount of heat added.

The enthalpy of solution (  H soln ) is the heat generated or absorbed when a certain amount of solute dissolves in a certain amount of solvent.  H soln = H soln - H components 6.6 Which substance(s) could be used for melting ice? Which substance(s) could be used for a cold pack? LiCl and CaCl 2 NaCl, KCl, NH 4 Cl, NH 4 NO 3