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Energetics. Energy changes in chemistry. Enthalpy. What is enthalpy? Enthalpy (H) is the total energy of system. It is in the molecules, the bonds, and stored chemically. Enthalpy change ( Δ H) is the measure of the change in the potential energy of the bonds. Endothermic vs exothermic.

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Energetics

Energetics

Energy changes in chemistry.


Enthalpy
Enthalpy

  • What is enthalpy?

    • Enthalpy (H) is the total energy of system. It is in the molecules, the bonds, and stored chemically.

    • Enthalpy change (Δ H) is the measure of the change in the potential energy of the bonds.


Endothermic vs exothermic
Endothermic vs exothermic

  • What is the difference between the two?

    • Endothermic reactions have more energy stored in the new bonds (products) than were in the old bonds (reactants) which takes in energy.

    • Exothermic reactions have more energy in the old bonds (reactants) than in the new bonds (products).


Enthalpy change
Enthalpy change

  • What is the value of enthalpy change for exothermic reactions?

    • Exothermic reaction enthalpy change is negative due to Δ H = enthalpy products – enthalpy reactants.

    • Endothermic reaction would be the opposite because of the same equation.


Enthalpy change1
Enthalpy change

  • How do we show enthalpy change of a reaction?

    • It is usually written with the equation.

    • + represents endothermic, - represents exothermic

    • It is given in the units of kJ/mol (or just kJ) because it changes with the amount of reactants and limiting reactants.

    • This is at thermochemicalstandard conditions which are 25 degrees C, 1 atm and solution concentrations of 1 mol/dm3.


Hess s law
Hess’s Law

  • What does Hess’s Law say?

    • It states that if you can add 2 or more different equations to produce the final equation, you can add the individual enthalpy changes to find the total.

    • This means that if you break down the steps involved, the overall change will be the same.


Hess s law1
Hess’s Law

  • 2H2O2(l) 2H2O(l) + O2(g)

    • 2H2(g) + O2(g) 2 H2O(l)ΔH= -572 kJ

    • H2(g) + O2(g)  H2O2(l)ΔH= -188 kJ

      • Where do we start?


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