Calorimetry

• 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.

Example of specific heats

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 Calorimeter Constant 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 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.

• Write the thermochemical equations for the previous 2 reactions.

HCl (aq) + NaOH (aq) H2O (l) + NaCl (aq) ΔH= - 2.7kJ

AgNO3(aq) + HCl(aq) → AgCl(s) + HNO3(aq) ΔH= -68 kJ/mol