Chapter 15.4 & 15.5 ENTHALPY AND CALORIMETRY

 Thermochemistry = heat changes that accompany chemical reactions and phase changes  Energy released  Energy absorbed Example: Heat Packs 4Fe + O 2  2Fe 2 O kJ THERMOCHEMISTRY This provides energy for warming cold hands

 Universe = system + surroundings  System = contains the reaction you wish to study  Surroundings = everything other than the system THERMOCHEMISTRY System Surroundings

 Enthalpy (H) = heat content of a system  Changes in enthalpy can be described in two ways  Exothermic reactions – reactions release heat. Enthalpy changes for exothermic reactions are always negative  Endothermic reactions – reactions are always positive. The enthalpy is absorbed in the reaction WHAT IS ENTHALPY AND HOW IS IT MEASURED? ∆H rxn = H final – H initial ∆H rxn = H products - H reactants

 The Thermodynamics Charts ENTHALPY AND ENTHALPY CHANGES

 The overall reaction including energy of the reaction is called a Thermochemical Equation 4Fe (s) + 3O 2 (s)  2Fe 2 O 3(s) kJ 4Fe (s) + 3O 2 (s)  2Fe 2 O 3(s) ∆H rxn = kJ  You can see that the thermochemical equation can be written in two ways.  This reaction is exothermic because the heat is being released from the system to the surroundings and the ∆H rxn is negative and is written as a product  The container gets hot THERMOCHEMICAL EQUATIONS Thermochemical equations can be written 2 ways

 Endothermic Equations 27 kJ + NH 4 NO 3(s)  NH 4(aq) + NO 3(aq) NH 4 NO 3(s)  NH 4(aq) + NO 3(aq) ∆H rxn = + 27 kJ  This is an example of an endothermic reaction  This reaction is endothermic because the heat is being absorbed from the surroundings and the ∆H rxn is positive and it is written as a reactant  The container gets cold THERMOCHEMICAL EQUATIONS

 Calorimeter = an insulated device used for measuring the amount of heat absorbed or released during a chemical or physical process.  Equation for measuring heat gained or lost q = c x m x ∆T  Specific Heat of water =  A known mass of water is placed in an insulated chamber to absorb the energy released from the reacting system or to provide the energy released by the system  A bomb calorimeter is a kind of calorimeter used by food chemists CALORIMETRY

DETERMINING SPECIFIC HEAT We will use the foam cup calorimeter Used to determine the specific heat of an unknown metal 1)An initial temperature of 25.60°C is recorded for the 125 g of water in the calorimeter 2) A 50 g sample of an unknown metal is heated to 115°C and placed in the calorimeter 3) The metal transfers heat to the water until the metal and water are at the same temperature. The final temperature is 29.30°C.

 Steps 1-3 showed an experimental procedure. Note that the temperature in the calorimeter becomes constant at 29.30°C, which is the final temperature attained by both the water and the metal. Assuming no heat is lost to the surroundings, the heat gained by the water is equal to the heat lost by the metal. The quantity of heat can be calculated using: q = c x m x ∆T First, calculate the heat gained by the water q = x 125 x ( ) q = x 125 x 3.70 q = 1940 J The heat gained by the water = the heat lost by the metal = c x 50 x (29.30 – 115) = c x 50 x (-85.7) = c (50)(-85.7).453 J/g°C = c CALCULATING THE SPECIFIC HEAT OF AN UNKNOWN METAL