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Energetics Chap 7

Energetics - Endo and Exothermic, Enthalpy, Measuring Changes.

Endothermic The system GAINS energy from the Surroundings (energy is taken in). Delta H is positive.
Exothermic The system LOSES energy to the surroundings (energy is given out), Delta H is Negative.
When Bonds break Energy is: Taken into the system (Endothermic).
When Bonds Form energy is: Given out from the system (Exothermic).
Enthalpy Change: The measure of heat energy change (Delta H) under constant pressure.
Standard Enthalpy of Combustion: The Energy released by one mole of a substance when it burns completely in Oxygen.
Delta H is positive: Endothermic Reaction (heat taken into the system).
Delta H Is negative: Exothermic Reaction (Heat given off from system).
In an Enthalpy Level Diagram, in terms of stability, what are the products, if they are at a lower level of Enthalpy? They are more stable, Energy has been given out from the system as stronger bonds have been formed. (exothermic Reaction)
In an Enthalpy Level Diagram, in terms of stability, what are the products, if they are at a higher level of Enthalpy? They are less stable, energy was added to the system in order to break bonds. (Endothermic Reaction).
Standard enthalpy of Combustion: The change in Enthalpy when one mole of a substance is completely burnt in Oxygen under standard conditions, with the reactants and products in their standard states.
Standard Enthalpy of Formation: Is the Enthalpy change when one mole of a substance is formed from its basic elements under standard conditions, with the reactants and products in their standard states.
Specific Heat Capacity: The energy required to raise the temperature of 1g of a substance by one degree K/C. Measured in Joules per Gram per Degree.
Change in Heat (q) = Mass (m) x Specific Heat Capacity (c) x Change in Temp (Delta-T). Measured in Degrees C/K.
Temperature: The average Kinetic Energy of the particles in a System. It is independent of how much of a substance there is.
Heat: The Total energy of a given amount of a Substance.
q = m x c x (delta)T
A Calorimeter: An instrument used to measure the heat changes which accompany chemical changes. E.g. Burning a fuel.
Hess's Law: Change in Enthalpy of a Reaction = Change in Enthalpy of Formation of Products - Change in Enthalpy of Formation of Reactants.
Enthalpy of Elements: The Enthalpies of Elements in their standard states is taken to be 0.
Mean Bond Enthalpy: The average energy required to break a covalent bond for a species (eg C-H). As bond enthalpies are usually slightly different in different molecules.
Bond Dissociation Enthalpy: The enthalpy change required to break a covalent bond in all species in a gaseous state.
Created by: mjwilson1988



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