![]() ![]() First, notice that the symbol for a standard enthalpy change of reaction is ΔH° r.Here is a simple reaction between hydrogen and oxygen to make water: The standard enthalpy change of a reaction is the enthalpy change which occurs when equation quantities of materials react under standard conditions, and with everything in its standard state. Remember that an enthalpy change is the heat evolved or absorbed when a reaction takes place at constant pressure. Standard enthalpy change of reaction, ΔH° r The O 2 form is far more energetically stable than O 3, so the standard state for oxygen is the common O 2(g). Similarly, under standard conditions, oxygen can exist as O 2 (simply called oxygen) or as O 3 (called ozone - but it is just an allotrope of oxygen). Graphite is energetically slightly more stable than diamond, and so graphite is taken as the standard state of carbon. Oxygen's standard state is the gas, O 2(g) - not liquid oxygen or oxygen atoms.įor elements which have allotropes (two different forms of the element in the same physical state), the standard state is the most energetically stable of the allotropes.įor example, carbon exists in the solid state as both diamond and graphite. That means that the standard state for water, for example, is liquid water, H 2O(l) - not steam or water vapour or ice. That is the physical and chemical state that you would expect to find it in under standard conditions. \)įor a standard enthalpy change everything has to be present in its standard state. ![]()
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