Since we cannot know the exact enthalpy of the reactants and products, we measure H through calorimetry, the measurement of heat flow.
Calorimetry A chemical reaction occurs in a chamber (calorimeter) designed to absorb as much of the heat energy released by the reaction as possible (if it is exothermic). The heat energy absorbed by the chamber is presumed to be equal to the heat energy lost by the reaction. ΔH rxn = -q of the calorimeter
HEAT CAPACITY: The amount of energy required to raise the temperature of a substance by 1 K (1 C). – Heat capacity is mass dependent. SPECIFIC HEAT: The amount of energy required to raise the temperature of 1 g of a substance by 1 K (1 C). – Specific heat is mass independent.
Determining heat flow Heat will flow from hot to cold. To measure, you need to know. – Mass of substance (m) – Specific heat of the substance (c) – Temperature change of the substance (Δt) q = m c Δt
Sample Question (a) How much heat is needed to warm 250 g of water (about 1 cup) from 22 °C (about room temperature) to near its boiling point, 98 °C? The specific heat of water is 4.18 J/g-K. (b) What is the molar heat capacity of water?
Two Kinds of Calorimeters Constant Pressure CalorimeterConstant Volume (Bomb) Calorimeter
When a student mixes 50 mL of 1.0 M HCl and 50 mL of 1.0 M NaOH in a coffee-cup calorimeter, the temperature of the resultant solution increases from 21.0 °C to 27.5 °C. Calculate the enthalpy change for the reaction in kJ/mol HCl, assuming that the calorimeter loses only a negligible quantity of heat, that the total volume of the solution is 100 mL, that its density is 1.0 g/mL, and that its specific heat is 4.18 J/g-K. Sample Question
Another Question When 50.0 mL of 0.100 M AgNO 3 and 50.0 mL of 0.100 M HCl are mixed in a constant-pressure calorimeter, the temperature of the mixture increases from 22.30 °C to 23.11 °C. The temperature increase is caused by the following reaction: AgNO 3 (aq) + HCl(aq) → AgCl(s) + HNO 3 (aq) Calculate ΔH for this reaction in kJ/mol AgNO 3, assuming that the combined solution has a mass of 100.0 g and a specific heat of 4.18 J/g °C.
Writing Thermochemical Equations Coefficients are given in moles. States of products and reactants must be given. A ΔH value is given to the right of the reaction. That value is the amount of heat energy given off or absorbed if that number of moles reacts. It is acceptable to have fractions for coefficients in thermochemical equations You can have ½ mole of one of the reactant or products.
Write the thermochemical equations for the previous 2 reactions. HCl (aq) + NaOH (aq) H 2 O (l) + NaCl (aq) ΔH= - 2.7kJ AgNO 3 (aq) + HCl(aq) → AgCl(s) + HNO 3 (aq) ΔH= -68 kJ/mol