CALCULATION OF ENTHALPY CHANGES

Molar Enthalpy  the enthalpy change for 1 mole of a substance associated with a chemical, physical or nuclear change  the units will J or kJ per mole

Example A common refrigerant, Freon-12, has a molar mass of g/mol. The molar enthalpy to vaporize Freon is kJ/mol. If 500 g of Freon is vaporized, what is the enthalpy change? On Board

Specific Heat Capacity  the quantity of heat needed to change the temperature of 1 g of substance by 1˚C  the symbol is C  the units are J/g˚C or Jg -1 ˚C -1 EX. C water = 4.18 J/g˚C

Specific Heat Capacity  the quantity of heat needed to change the temperature of 1 g of substance by 1˚C  the symbol is C  the units are J/g˚C or Jg -1 ˚C -1 EX. C water = 4.18 J/g˚C Specific heat capacity is a measure of how ‘easily’ a substance changes temperature. A substance with a high heat capacity can absorb more energy without changing temperature than a substance with low specific heat.

Heat Transfer Problems ΔH= mCΔT Enthalpy Change (J) Mass of the substance changing temperature (g) Specific Heat Capacity (Jg -1 °C -1 ) Change in Temperature T f –T i (°C)

Heat Transfer Problems  When T f is greater than T i : -energy is absorbed - the process is ENDOthermic - Δ H is positive ΔH= mCΔT Enthalpy Change (J) Mass of the substance changing temperature (g) Specific Heat Capacity (Jg -1 °C -1 ) Change in Temperature T f –T i (°C)

Heat Transfer Problems  When T f is less than T i : -energy is released - the process is EXOthermic - Δ H is negative ΔH= mCΔT Enthalpy Change (J) Mass of the substance changing temperature (g) Specific Heat Capacity (Jg -1 °C -1 ) Change in Temperature T f –T i (°C)

Examples 1) A 150 g sample of Pb at °C is plunged into a beaker containing 50 g of water at 22.0 °C. The final temperature of the mixture is 28.8 °C. a) What is the energy change for the water? b) How much energy is lost by the Pb? c) Calculate the C Pb On Board

Examples 2) While running an adult expends 5.0 x 10 5 J/mile. If this energy could be transferred as heat to a beaker of water, how many grams of water could be heated from 25.0 °C to °C? On Board

CALORIMETRY Calorimeter – an insulated device used to determine temperature changes during a chemical or physical process.  the calorimeter is part of the surroundings Two Kinds: 1) Simple2) Bomb

Simple Calorimetry  the process occurs at constant pressure  RECALL that Δ H is + for an ENDOTHERMIC change and – for an EXOTHERMIC Δ H observed = mC Δ T = - Δ H rxn

Assumptions  for a dilute aqueous solution, use C water = 4.18 J/g°C as the heat capacity for the solution  1 mL of solution = 1 g of solution  no heat is transferred to the outside of the calorimeter( i.e., the calorimeter is perfectly insulated)

Diagram insulated lid thermometer insulated cup reaction mixture

Example 1) 5.00 mL of 1.0 mol/L NaOH and 5.00 mL of 1.0 mol/L HCl are mixed in a simple calorimeter. The initial temperature of each substance is 23.0 °C. After mixing, the temperature rose to 27.5 °C. a) What is the enthalpy change for this rxn? b) Calculate the energy change per mole of HCl. On Board

Example 2) 1.50 g of NH 4 NO 3 is added to 35.0 g of water in a simple calorimeter. The initial temperature of the water is 22.7°C and the final temperature of the mixture is 19.4 °C. Calculate the enthalpy change per mole of salt.

Bomb Calorimetry

Not Really !

Bomb Calorimetry  the process occurs at constant volume  used to measure the heat released in a combustion reaction

Diagram

Bomb Calorimetry  the process occurs at constant volume  used to measure the heat released in a combustion reaction  NOTE: q is used in place of Δ H for an energy change at constant volume q observed = C’ΔT = - q combustion J °C -1

Example A food manufacturer wishes to know the number of calories per serving of a new dessert. They hire a chemist at the Dep’t of Consumer Affairs to help them out. a) The chemist places a 1.00 g sample of the dessert in a bomb calorimeter. The heat capacity of the calorimeter is kJK -1. The temperature increases by °C. Calculate the number of food calories (kcal) in the sample of dessert. b) A single serving of the dessert weighs g. How many food calories are in the dessert? On Board