How would you define Chemicals Energetics (Chemistry Topic)?

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  • 1 decade ago
    Favourite answer

    Chemical energetics is the branch of chemistry which studies the energy released/absorbed or mathematics of energy during the chemical reactions...

  • Anonymous
    1 decade ago

    Chemical energetics involves a lot of definitions and calculations.

    Definitions

    Enthalpy

    This is a measurement of the heat content of a reactant or product. It is given the symbol H. In a reaction it may increase or decrease and produce an enthalpy change (DH).

    Enthalpy Change

    This is the heat change that takes place under standard conditions. This is usually worked out by the following formula:

    Enthalpy Change (DH) = Hproducts - Hreactants

    A positive DH means that energy is absorbed during the reaction (ENDOTHERMIC) and a negative DH means that energy is released during the reaction (EXOTHERMIC).

    Standard Conditions

    Because the enthalpy change is affected by changes in temperature and pressure, we use standard conditions. These usually refer to 1 mole of a substance at 298 K and 1 atmosphere pressure.

    Enthalpy Change (DH) = Hproducts - Hreactants

    If the H of the products is more than that of the reactants then the enthalpy change will be a positive value. The reaction will be exothermic.

    If the H of the reactants is more than that of the products then the enthalpy change will be negative. The reaction will be exothermic.

    Hess's Law

    This states that the enthalpy of a reaction is the same regardless of the route taken.

    Route 1 = Route 2 + Route 3

    To make substance "C" from substance "A", you can either take route 1 or you can go via Route 2 followed by Route 3. Hesse's law states that the enthalpy change will be the same either way.

    Standard Enthalpy of Formation (DHfq)

    This is the enthalpy change 1 mole of a substance is formed from it's elements in their standard states. The standard enthalpy of formation of an element in it's standard state is zero because it has not been formed from something else.

    Standard Enthalpy of Combustion (DHcq)

    This is the enthalpy change when 1 mole of a substance is completely burnt in oxygen under standard conditions.

    Standard Enthalpy of Atomization

    This is the enthalpy change when 1 mole of a substance is converted to gaseous atoms under standard conditions.

    Bond Enthalpy

    This is equivalent to the strength of the bond and is the energy needed to make the bond. The bond dissociation enthalpy is the enthalpy change when 1 mole of bonds is broken. The bond enthalpy term is an average value of the bond dissociation enthalpies of a particular bond. Confused? Here is an example:

    In methane, CH4, there 4 C-H bonds. The energy needed to break the first C-H bond is not the same as that required to remove a hydrogen from a CH3, or a CH2 or a CH fragment. So the bond enthalpy term makes the assumption that the enthalpy of a bond is independent of the molecule it is in.

    The bond enthalpy term for the C-H bond is the average value from breaking all 4 C-H bonds in methane -416 KJmol-1. This value is taken from the value for the enthalpy of atomisation of methane.

    CH4(g) C(g) + 4H(g) -1662 KJmol-1

    Atomisation of methane involves breaking 4 C-H bonds so the energy needed to break one C-H bond must be -1662 KJmol-1/4.

    Lattice Enthalpy

    This is the enthalpy change when 1 mole of an ionic lattice is formed from its ions in their gaseous state under standard conditions.

    Ionisation Enthalpy

    This is the enthalpy change when 1 mole of electrons is removed from 1 mole of gaseous atoms under standard conditions.

    Electron Affinity

    This is the enthalpy change when 1 mole gaseous atoms gains 1 mole of electrons under standard conditions.

  • Ellie
    Lv 4
    1 decade ago

    Chemical Energetics is the study of the energy changes in a chemical reaction. Generally it covers enthalpy, Hess' law, leading onto entropy and gibbs free energy.

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