Enthalpy is the heat content of a system at constant pressure, but chemists almost always talk about change in enthalpy rather than total energy. Endothermic reactions have positive changes in enthalpy while exothermic reactions have negative changes in enthalpy.
Alright when in Chemistry and dealing with thermo chemistry you might come across the word called enthalpy. Enthalpy is what we're going to denote as h capital H, and it's the heat content of a system at constant pressure. So basically it's the amount of energy within a system okay, so there're 2 ways we can do this, we can say the total energy of a substance we're just going to call that H. This is actually impossible to do, Chemists can't do this, there's just too much involved in one particular substance that we don't actually even fully understand even now to figure out the actual energy of that content without changing it. So we're going to talk about delta H or changes for any energy from reactants to products, we're going to label that and you're going to see this way more often than just H in fact I doubt that you'll ever see H as H by itself without the triangle meaning change of. So we can talk about how the energy changes as we go from reactants to products or as we go from one system to another. And we're going to call that delta H which is much more valuable actually than just a single substance alone. So how do we figure that out? well a delta H is the heat of the final whatever we're dealing with minus the heat of the initial of whatever we're dealing with.
We're going through phase changes or going through a reaction whatever it may be. So you might actually, this might make more sense to you delta H of the reaction equals the heat of the product minus the heat of the reactants. Okay now change can be found using bomb calorimeters, using things of that nature so we can actually figure out the delta H. But we can't actually figure out the H of the products alone or the reactants alone, only the change that it undergoes. Alright so what I'm I talking about? So let's a look at this thermo chemical equation which is iron, solid iron is reacting with gas and it's giving us rust or iron III oxide. And we're also since we're dealing with thermo Chemistry we're going to talk about how much energy we're actually changing and this is actually releasing 1625 kilo joules of heat.
Notice the heat of the product is less than the heat of reactants, so this guy over here has the 1625 kilo joules within it, then it releases that energy and it's able to form this rust. So we're going to include that release in energy in our reaction. So if we're going to write this out properly in a thermo chemical equation we would say iron plus oxygen yields rust or iron III oxide and we're going to include the delta H and we're going to say releases 1625 kilo joules of heat. The products minus the reactants is going to be negative and we're going to call this exothermic because we're releasing heat, heat is the exiting the system. So when you see the negative of the delta H we're going to notice exiting is being released.
Okay let's go in the opposite; let's say we're talking about ammonium nitrate. So we have ammonium nitrate and we're going to add 27 kilo joules of heat to it. What that's going to do, is going to break apart, it's going to break that bond up and we're going to get these 2 irons, ammonium and nitrate. Now in this case the products were greater in energy than the reactants, so how we're going to write this thermo chemically would be we have the solid ammonium nitrate going into our aqueous solution and breaking up the irons and notice that it is positive 27 kilo joules we require, we need 27 kilo joules for this reaction to happen. That's why it's positive I'm going to put a little positive there to denote that I'm going to need that energy for this reaction to occur. So that's our change in energy, we're going to call this endothermic reaction meaning that we're entering, energy is entering the system. So if you see a positive sign in the delta H we're going to say endothermic entering the system. So delta H is extremely important when talking about thermo chemical equation, it actually tells us how much energy we're dealing with or if they're being released or absorbed and we're going to call that energy enthalpy and enthalpy has an H in it so you can remember that, that means heat.