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Ch. 11 Thermochemistry- Heat and Chemical Change

Ch. 11 Thermochemistry- Heat and Chemical Change. I. The Flow of Energy-Heat A. Energy Transformations. 1. Physical and chemical changes are always accompanied by an energy change.

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Ch. 11 Thermochemistry- Heat and Chemical Change

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  1. Ch. 11 Thermochemistry- Heat and Chemical Change

  2. I. The Flow of Energy-HeatA. Energy Transformations 1. Physical and chemical changes are always accompanied by an energy change. 2. Energy is the capacity for doing work or supplying heat and the most common form of energy change involves heat. 3. Energy transfers from the matter at higher temperature to lower temperatures. 4. The energy transferred as a result of a temperature difference is called Heat (q).

  3. B. Exothermic and Endothermic Process 1. Energy changes occur between the system and its surroundings. 2. Chemical changes are always accompanied by a change in energy. 3. If energy is absorbed in a reaction, it is an endothermic reaction. 4. If energy is released in a reaction, it is an exothermic reaction and can tend to take place spontaneously. 5. Both endothermic and exothermic reactions require a certain minimum amount of energy to get started which is called the activation energy.

  4. C. Heat Capacity and Specific Heat 1. A calorie is defined as the quantity of heat needed to raise the temperature of 1 g of pure water 1°C. (A joule is also related to the calorie) 2. Heat capacity is the amount of heat needed to increase the temperature of an object exactly 1°C. 3. Specific heat, Cp , is the heat needed to raise the temperature of 1g of a substance by 1°C. 4. Heat is calculated by multiplying the mass of the substance by the temperature difference and by the specific heat of the substance. Heat = (mass) (∆ T) (Cp) 5. Heat is expressed in Joules (J) or Calories (Cal).

  5. II. Measuring and Expressing Heat ChangesA. Calorimetry 1. Calorimetry is the accurate and precise measurement of heat change for chemical and physical process. 2. When energy is added to a solid substance, it’s temperature increases until melting point is reached. 3. When more energy is added, the substance begins to melt but the temperature does not change.

  6. II. Measuring and Expressing Heat ChangesA. Calorimetry 4. The increased energy increases the kinetic energy until melting point at which it changes the position of the particles. 5. Enthalpy is an energy quantity. a. Enthalpy of fusion is the energy needed to melt one gram of a substance at melting point. b. Enthalpy of vaporization is the energy needed to vaporize one gram of a substance at boiling point.

  7. B. Thermochemical Equations 1. An equation that includes the heat change is called a thermochemical equation. 2. A heat of reaction is the heat change for the equation exactly as it is written. 3. Heat of reaction is reported as ∆H, which is the heat change at constant pressure (101.3 kPa). 4. The physical states of the reactants and products must also be given at room temperature (25°C). 5. Exothermic reactions are negative values and endothermic reactions are positive values. 6. The heat of combustion is the heat of reaction for the complete burning of one mole of a substance.

  8. III. Heat in Changes of StateA. Heats of Fusion and Solidification 1. The heat absorbed by one mole of a substance in melting (a solid to a liquid) at constant temperature is is the molar heat of fusion. 2. The heat absorbed by one mole of a substance in solidifying (a liquid to a solid) at constant temperature is is the molar heat of solidification. 3. The formula for solving for heat is q = m ∆ H fus , where m is molality of a substance ∆ H is the enthalpy. 4. Heat of fusion is a positive value and Heat of solidification is a negative value.

  9. B. Heats of Vaporization and Condensation 1. The heat needed by one mole of a substance in boiling or evaporation from a liquid to a gas at constant temperature is is the molar heat of vaporization. 2. The heat absorbed by one mole of a substance and condenses from a gas to a liquid at constant temperature is the molar heat of condensation. 3. The formula for solving for heat is q = m ∆ Hvap, where m is molality of a substance ∆ H is the enthalpy. 4. Heat of vaporization is a positive value and Heat of condensation is a negative value.

  10. C. Heat ofSolution 1. Heat changes can also occur when a solute dissolves in a solvent. 2. The heat change is caused by dissolution of one mole of a substance 3. This is called the heat of solution (ΔHsoln). 4. The heat of solution can be endothermic or exothermic

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