#ARE COMPOUNDS IN SIMPLE STATES 0 KJ PROFESSIONAL#
These experiments have been repeatedly carried out over a number of years by a variety of professional chemists. Even though those two values cannot be measured, we can measure the difference (H 2 minus H 1 is called ΔH) in an experiment using a calorimeter. The product(s) have some unknown absolute enthalpy value (call it H 2) and the reactant(s) have another value (also unknown), called H 1. (Notice I discuss enthalpy changes, since absolute enthalpy values for a given substance cannot be measured.) For example, here is the formation reaction for carbon dioxide: What all this means is that EACH formation reaction has an enthalpy change value associated with it. The reason for the sign convention has to do with chemistry's viewpoint of the system and the surroundings. Context is all important!!!!)Įxothermic chemical reactions will have a negative ΔH and endothermic reactions have a positive ΔH. (There are other uses of a subscripted "f," however the differences in context will be obvious.) The symbol "°" is taken to mean "standard conditions." (Yes, I know that symbol is also used for degrees Celsius. The subscripted "f" is taken to mean formation when used in the thermochemistry area. The symbol for the standard enthalpy of formation is:Īll chemical reactions involve a change in enthalpy (defined as the heat produced or absorbed during a reaction at constant pressure). Hydrogen peroxide is made a different way, but you can still write the formation reaction. You can't make hydrogen peroxide by reacting hydrogen and oxygen directly. (3) Formation reactions sometimes are "fake" reactions, in that they cannot possibly happen as written. Once again, a formation reaction involves making a substance from its elements and ONLY the elements. Here is the formation reaction for C 6H 12O 6:ĦC(s, graphite) + 6H 2(g) + 3O 2(g) -> C 6H 12O 6(s) Here is an example of a chemical reaction that IS NOT a formation reaction:ĦCO 2(g) + 6H 2O(ℓ) -> C 6H 12O 6(s) + 6O 2(g) Look again at the definition of formation. What is being written is a formation reaction. (2) There is never a compound on the reactant side, only elements. This is because Br (monoatomic gas) is not bromine in its standard state. The enthalpy of formation for Br (monoatomic gas) is 111.881 kJ/mol.
There is also P 4 and S 8 (which are not diatomic, but you get the point).Īn example: the enthalpy of formation for Br 2 in its standard state is zero. However, there are some elements for which the standard state is diatomic (H 2 and Cl 2 are two of the seven diatomics, with two others in the previous paragraph.). (Yes, the standard state for mercury is liquid.). Examine the various compounds and elements for (s), (l), and (g) markings, as well as (aq).Īlso, please note that for many elements, the standard state is monoatomic. One hint: there will be a table of standard enthalpies of formation somewhere in your text. You will need to pick up these facts on your own in most classes. For example, the standard state for carbon is graphite (remember, a solid), not diamond!!! The standard state for the element bromine (Br 2) is liquid and iodine (I 2) is solid. You need to know the specifics of a substance's standard state, but it is not something that gets taught. and 25 ☌, these substances are in the physical state specified. Thus, the standard state for carbon is solid, for water is liquid and for hydrogen is gas. This is the physical state (solid, liquid, or gas) that a substance would be in under standard conditions. (1) Every substance is shown in its standard state. Three points to be made about these examples: Here are some examples:Ĭ(s, graphite) + 2H 2 (g) + 1⁄ 2O 2(g) -> CH 3OH(ℓ)īy the way, here is the discussion on enthalpy, if you missed it. The substance in question is always written with a coefficient of one. (2) Formation - this word means a substance, written as the product of a chemical equation, is formed DIRECTLY from the elements involved. If a solution is being discussed, then everything in solution will be at a 1.00-molar concentration. (1) Standard - this means a very specific temperature and pressure: one atmosphere and 25 ☌ (or 298 K). You'll understand it better in a few moments.įirst two definitions of chemistry words with very specific meanings: Go on and study the concept and then return to the above statement. When you move on to calculating various values, the above piece of information becomes quite important. Now I know you don't know exactly what that means, but please remember it. The standard enthalpy of formation for an element in its standard state is zero. Standard Enthalpy of Formation Standard Enthalpy of Formation Return to the Thermochemistry menu.īefore explaining the concepts involved, here is an important idea:
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