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AS Chemistry - Enthalpy

Updated on April 7, 2013
enthalpy change
enthalpy change

Enthalpy (H) is the content of heat that is stored in a specific chemical system. To measure the enthalpy of something you measure the energy released/absorbed in the surroundings during a reaction or a chemical change.

The most common form of energy from chemical changes is heat energy. The law of conservation of energy is when heat changes into another energy form however, heat cannot just be lost.

Looking at the picture to the right you can see that enthalpy change is expressed as a triangle (delta) and a H. The H stands for the heat exchange with the surroundings and the triangle stands for the difference between the enthalpy of the products/reactants during a chemical reaction.

The average bond enthalpy (KJ Mol-1) for the following bonds:

  • C-H is +413
  • O-H is +463
  • O=O is +497
  • H-H is +463
  • C=C is +612

Bonds Enthalpies And Exo/Endothermic Reactions

Bond enthalpies will tell you how much energy you will need to break that specific chemical bond (which stores chemical energy) and thus you compare the strengths of different bonds.

Energy is needed to break the bonds and this change is endothermic when bonds form the same energy is released and this change is exothermic.

In exothermic reactions:

  • The enthalpy of the products will be smaller than that of the reactants.
  • Heat is lost to the surroundings.
  • The enthalpy change has a negative sign.

In endothermic reactions:

  • The enthalpy of the reactants will be smaller than that of the products.
  • Heat is gained from the surroundings.
  • The enthalpy change has a positive sign.

In every reaction there will be bonds broken and made. Energy is needed to break the reactant's bonds and energy is released during the making of bonds within the products, so how do you determine whether the reaction is overall endothermic or exothermic? The answer is the relative strength of the bonds that are broken and made. If the bonds that form have a higher bond enthalpy than the ones broken then the reaction will be exothermic and vis versa.

Standard Enthalpy Changes

Enthalpy change varies depending on the conditions in which the reaction happens. To overcome this chemists use something known as standard conditions to ensure that the conditions don't have an impact on the enthalpy change.

The standard conditions are:

  • Pressure of 100 kPa
  • 25oc
  • For reactions with an aqueous solution a concentration of 1 mol dm-3 is used.

The sign for standard enthalpy change is shown to the right where the H means enthalpy the triangle (delta) means change and the small circle shows standard conditions.

Standard enthalpy change of reaction is the enthalpy change that occurs in a reaction when one mole of substance is transformed by a chemical reaction when all the reactants and products are in their standard states and under standard conditions. To write the symbol for the standard enthalpy change of reaction use the picture to the right but replace the x with a small r.

Standard enthalpy change of combustion is the enthalpy change that occurs when 1 mole of oxygen reacts completely with one mole of a substance when the reactants and products are all in their standard states under standard conditions. To write the symbol for this replace the x with a small c.

Standard enthalpy change of formation is the enthalpy change that occurs when the constituent elements of one mole of a compound form from it's constituent elements under standard conditions and in their standard states. To write the symbol for this replace the x with a small f.


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