Energetics - Endo and Exothermic, Enthalpy, Measuring Changes.
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Endothermic | The system GAINS energy from the Surroundings (energy is taken in). Delta H is positive.
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Exothermic | The system LOSES energy to the surroundings (energy is given out), Delta H is Negative.
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When Bonds break Energy is: | Taken into the system (Endothermic).
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When Bonds Form energy is: | Given out from the system (Exothermic).
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Enthalpy Change: | The measure of heat energy change (Delta H) under constant pressure.
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Standard Enthalpy of Combustion: | The Energy released by one mole of a substance when it burns completely in Oxygen.
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Delta H is positive: | Endothermic Reaction (heat taken into the system).
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Delta H Is negative: | Exothermic Reaction (Heat given off from system).
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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)
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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).
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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.
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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.
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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.
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Change in Heat (q) = | Mass (m) x Specific Heat Capacity (c) x Change in Temp (Delta-T). Measured in Degrees C/K.
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Temperature: | The average Kinetic Energy of the particles in a System. It is independent of how much of a substance there is.
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Heat: | The Total energy of a given amount of a Substance.
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q = | m x c x (delta)T
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A Calorimeter: | An instrument used to measure the heat changes which accompany chemical changes. E.g. Burning a fuel.
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Hess's Law: | Change in Enthalpy of a Reaction = Change in Enthalpy of Formation of Products - Change in Enthalpy of Formation of Reactants.
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Enthalpy of Elements: | The Enthalpies of Elements in their standard states is taken to be 0.
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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.
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Bond Dissociation Enthalpy: | The enthalpy change required to break a covalent bond in all species in a gaseous state.
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