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Calorimetry

Calorimetry. Since we cannot know the exact enthalpy of the reactants and products, we measure  H through calorimetry , the measurement of heat flow. Calorimetry.

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Calorimetry

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  1. Calorimetry

  2. Since we cannot know the exact enthalpy of the reactants and products, we measure H through calorimetry, the measurement of heat flow.

  3. 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

  4. 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.

  5. Example of specific heats

  6. 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

  7. 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?

  8. Two Kinds of Calorimeters Constant Pressure Calorimeter Constant Volume (Bomb) Calorimeter

  9. Sample Question • 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.

  10. 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.

  11. 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.

  12. Write the thermochemical equations for the previous 2 reactions.

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