VARIOUS ENTHALPY CHANGE DEFINITIONS

VARIOUS ENTHALPY CHANGE DEFINITIONS This page explains what an enthalpy change is, and then gives a definition and brief comment for three of the various kinds of enthalpy change that you will come across. You will find some more definitions on other pages in this section. It is essential that you learn the definitions. You aren't going to be able to do any calculations successfully if you don't know exactly what all the terms mean. Enthalpy changes Enthalpy change is the name given to the amount of heat evolved or absorbed in a reaction carried out at constant pressure. It is given the symbol ΔH, read as "delta H".
	Note:  The term "enthalpy change" only applies to reactions done at constant pressure. That is actually how most lab reactions are done - in tubes or flasks (or whatever) open to the atmosphere, so that the pressure is constant at atmospheric pressure. The phrase "at constant pressure" is an essential part of the definition but, apart from that, you are unlikely to need to worry about it if you are doing a UK-based exam at the equivalent of A level.
Standard enthalpy changes Standard enthalpy changes refer to reactions done under standard conditions, and with everything present in their standard states. Standard states are sometimes referred to as "reference states". Standard conditions Standard conditions are: 298 K (25°C) a pressure of 1 bar (100 kPa). where solutions are involved, a concentration of 1 mol dm-3
	Warning!  Standard pressure was originally defined as 1 atmosphere (101.325 kPa), and you will still find that in older books (including my calculations book). At the time of writing (August 2010) there was at least one UK-based syllabus that was still talking in terms of "1 atmosphere". It is essential to check your syllabus to find out exactly what you need to learn.
Standard states For a standard enthalpy change everything has to be present in its standard state. That is the physical and chemical state that you would expect to find it in under standard conditions. That means that the standard state for water, for example, is liquid water, H2O(l) - not steam or water vapour or ice. Oxygen's standard state is the gas, O2(g) - not liquid oxygen or oxygen atoms. For 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. For example, carbon exists in the solid state as both diamond and graphite. Graphite is energetically slightly more stable than diamond, and so graphite is taken as the standard state of carbon. Similarly, under standard conditions, oxygen can exist as O2 (simply called oxygen) or as O3 (called ozone - but it is just an allotrope of oxygen). The O2 form is far more energetically stable than O3, so the standard state for oxygen is the common O2(g). The symbol for standard enthalpy changes The symbol for a standard enthalpy change is ΔH°, read as "delta H standard" or, perhaps more commonly, as "delta H nought".
	Note:  Technically, the "o" in the symbol should have a horizontal line through it, extending out at each side. This is such a bother to produce convincingly without the risk of different computers producing unreliable results, that I shall use the common practice of simplifying it to "o".
Standard enthalpy change of reaction, ΔH°r Remember that an enthalpy change is the heat evolved or absorbed when a reaction takes place at constant pressure. 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. That needs exploring a bit. Here is a simple reaction between hydrogen and oxygen to make water: First, notice that the symbol for a standard enthalpy change of reaction is ΔH°r. For enthalpy changes of reaction, the "r" (for reaction) is often missed off - it is just assumed. The "kJ mol-1" (kilojoules per mole) doesn't refer to any particular substance in the equation. Instead it refers to the quantities of all the substances given in the equation. In this case, 572 kJ of heat is evolved when 2 moles of hydrogen gas react with 1 mole of oxygen gas to form 2 moles of liquid water. Notice that everything is in its standard state. In particular, the water has to be formed as a liquid. And there is a hidden problem! The figure quoted is for the reaction under standard conditions, but hydrogen and oxygen don't react under standard conditions. Whenever a standard enthalpy change is quoted, standard conditions are assumed. If the reaction has to be done under different conditions, a different enthalpy change would be recorded. That has to be calculated back to what it would be under standard conditions. Fortunately, you don't have to know how to do that at this level.