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Enthalpy Its more than the energy of a substance!.

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Presentation on theme: "Enthalpy Its more than the energy of a substance!."— Presentation transcript:

1 Enthalpy Its more than the energy of a substance!

2 Enthalpy Total Energy of a substance Equals the sum of all the kinetic energy and the potential energy of its particles. Depends on the substances temperature, physical state, and composition. Enthalpy Is the substances energy Takes into account the pressure and volume of the substance Total Energy and Enthalpy are numerically very close (Enthalpy is greater) Total Energy of a substance Equals the sum of all the kinetic energy and the potential energy of its particles. Depends on the substances temperature, physical state, and composition. Enthalpy Is the substances energy Takes into account the pressure and volume of the substance Total Energy and Enthalpy are numerically very close (Enthalpy is greater)

3 Hesss Law The enthalpy change for a net reaction (sum of two or more reactions) is the sum of the enthalpy changes for the summed reactions.

4 Spontaneous Reactions Most exothermic reactions are spontaneous (especially those where large amounts of heat are released)

5 H, H Enthalpy is represented by the capital letter H Enthalpy change is represented by H H equals the heat absorbed or gained during a reaction In science, means change or a difference H means a change (or a difference) in enthalpy: H = enthalpy of the products minus enthalpy of the reactants Enthalpy is represented by the capital letter H Enthalpy change is represented by H H equals the heat absorbed or gained during a reaction In science, means change or a difference H means a change (or a difference) in enthalpy: H = enthalpy of the products minus enthalpy of the reactants

6 Heat Transfer and the Sign of H Sign of HProcessHeat ___________________________________________________________________ PositiveEndothermicAbsorbed NegativeExothermicReleased Sign of HProcessHeat ___________________________________________________________________ PositiveEndothermicAbsorbed NegativeExothermicReleased

7 H The amount of heat that a reaction absorbs or releases depends on the conditions under which the reaction is conducted: Temperature Pressure Physical states of the reactants and products The amount of heat that a reaction absorbs or releases depends on the conditions under which the reaction is conducted: Temperature Pressure Physical states of the reactants and products

8 Joule SI unit (SI International System of Units) Imagine lifting a medium-sized apple a distance of 1 meter against the force of gravity. 1 calorie (cal) = J 1 cal is the amount of heat needed to raise the temperature of 1 gram of water by 1 Celsius degree 1 Calorie (Cal) = 1 kilocalorie Joule SI unit (SI International System of Units) Imagine lifting a medium-sized apple a distance of 1 meter against the force of gravity. 1 calorie (cal) = J 1 cal is the amount of heat needed to raise the temperature of 1 gram of water by 1 Celsius degree 1 Calorie (Cal) = 1 kilocalorie

9 Standard State of a Substance 1 atm and 25 0 are the standard pressure and temperature for reporting enthalpy Standard state of an element is the most stable form of an element under standard conditions A standard enthalpy change is denoted with a superscript 0, H 0 1 atm and 25 0 are the standard pressure and temperature for reporting enthalpy Standard state of an element is the most stable form of an element under standard conditions A standard enthalpy change is denoted with a superscript 0, H 0

10 Chemistry problems and Enthalpy Similar to your previous stoichiometry problems The amount of heat absorbed or released in a reaction depends on the number of moles of reactants involved Enthalpy change for a reaction is proportionately smaller or larger depending on the quantities of the reactants and products involved Similar to your previous stoichiometry problems The amount of heat absorbed or released in a reaction depends on the number of moles of reactants involved Enthalpy change for a reaction is proportionately smaller or larger depending on the quantities of the reactants and products involved

11 Endothermic & Exothermic Reactions Look at the energy diagrams of Endothermic and Exothermic Reactions The energy of the reactants must be raised to the top of the energy barrier before it can drop to a lower level Look at the energy diagrams of Endothermic and Exothermic Reactions The energy of the reactants must be raised to the top of the energy barrier before it can drop to a lower level

12 Activation Energy The difference between the energy at the peak and the energy of the reactants is called the activation energy The activation energy is the energy needed to start the reaction When particles collide with sufficient energy (= activation energy), existing bonds may be disrupted and new bonds can form The difference between the energy at the peak and the energy of the reactants is called the activation energy The activation energy is the energy needed to start the reaction When particles collide with sufficient energy (= activation energy), existing bonds may be disrupted and new bonds can form


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