calorimetry. since we cannot know the exact enthalpy of the reactants and products, we measure h...
TRANSCRIPT
• 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.
ΔHrxn = -q of the calorimeter
• HEAT CAPACITY: The amount of energy required to raise the temperature of a substance by 1 K (1C). – 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 (1C).– 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?
• 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 AgNO3 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:
AgNO3(aq) + HCl(aq) → AgCl(s) + HNO3(aq)
• Calculate ΔH for this reaction in kJ/mol AgNO3, 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.