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Introduction

Section 2 - Measuring and Expressing Enthalpy Changes. Introduction. A burning match releases heat to its surroundings in all directions. How much heat does this exothermic reaction release?

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Introduction

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  1. Section 2 - Measuring and Expressing Enthalpy Changes Introduction • A burning match releases heat to its surroundings in all directions. • How much heat does this exothermic reaction release? • You will learn to measure heat flow in chemical and physical processes by applying the concept of specific heat.

  2. 5. Calorimetry • Calorimetry is the precise measurement of the heat flow into or out of a system for chemical and physical processes. • In calorimetry, the heat released by the system is equal to the heat absorbed by its surroundings. • Conversely, the heat absorbed by a system is equal to the heat released by its surroundings. • The insulated device (on the left) used to measure the absorption or release of heat in chemical or physical processes is called a calorimeter.

  3. Calorimetry (cont.) • Constant-Pressure Calorimeters • Because most reactions are open to the atmosphere the changes occur at constant pressure. • The heat content of a system at constant pressure is the same as a property called the enthalpy(H) of the system. • The heat released or absorbed by a reaction at constant pressure is the same as the change in enthalpy (∆H). • Heat and enthalpy change are used interchangeably, therefore q = ∆H.

  4. To measure enthalpy change for a reaction in aqueous solution: • Dissolve the reacting chemicals (system) in known volumes of water (surroundings). • Then measure the initial temperature of each solution and mix the solutions in the calorimeter. • After the reaction is complete, measure the final temperature of the mixed solutions. • Because you know the initial and final temperatures and the heat capacity of water, you can calculate the heat absorbed by the surroundings (qsurr) • qsurr = m x C x ∆T • Where m is mass of water, C is specific heat of water, and • ∆T = Tf – Ti. Calorimetry (cont.)

  5. Calorimetry (cont.) • Because the heat absorbed by the surroundings is equal to, but has the opposite sign of, the heat released by the system, enthalpy change (∆H) for the reaction can be written as follows. • The sign of ∆H is negative for an exothermic reaction and positive for an endothermic reaction.

  6. Calorimetry (cont.) • Constant-Volume Calorimeters • Calorimetry experiments can be performed at a constant volume using a bomb calorimeter. • A sample of a compound is burned in a constant-volume chamber in the presence of oxygen at high pressure. • The heat that is released warms the water surrounding the chamber. • By measuring the temperature increase of the water, it is possible to calculate the quantity of heat released during the combustion reaction. Nutritionists use bomb calorimeters to measure the energy content of the foods you eat, expressed in Calories per serving.

  7. Calorimetry (cont.) • Example: • Step 1:

  8. Calorimetry (cont.) • Step 2:

  9. 6. Thermochemical Equations • In a chemical equation, the enthalpy change for the reaction can be written as either a reactant or a product. • A chemical equation that includes the enthalpy change is called a thermochemical equation. • The heat of reaction is the enthalpy change for the chemical equation exactly as it is written. • Reported as ∆H, equal to heat flow at constant pressure. • Standard conditions are 101.3 kPa at 25 ⁰C, physical states must be given. Exothermic Reaction

  10. Thermochemical Equations (cont.) • The amount of heat released or absorbed during a reaction depends on the number of moles of the reactants involved. • The reaction on the left shows the decomposition of 2 mols of sodium bicarbonate. • The decomposition of 4 mols of sodium bicarbonate requires twice as much heat as 2 mols. • In endothermic reactions the potential energy is higher than the potential energy of the reactant. • Physical state must be stated because different states of the same matter have different ∆H values. Endothermic Reaction

  11. Thermochemical Equations (cont.) • Example: • Step 1:

  12. Thermochemical Equations (cont.) • Step 2:

  13. Thermochemical Equations (cont.) • The heat of combustion is the heat of reaction for the complete burning of one mole of a substance. • Like other heats of reaction, heats of combustion are carried out at 101.3 kPa of pressure and at 25 ⁰C • The combustion of methane is an exothermic reaction used to heat homes. Ch4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g) + 890 kJ • It can also be written: Ch4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g) ∆H= -890 kJ • Burning 1 mol of methane releases 890 kJ of heat. The heat of combustion (∆H) is -890 kJ per mole of methane.

  14. END OF SECTION 2

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