Presentation on theme: "Enthalpy The energy with the fun-to-say name. Active Sources of Heat q = mc T can only work for passive sources or destinations for heat. Consider your."— Presentation transcript:
Active Sources of Heat q = mc T can only work for passive sources or destinations for heat. Consider your body. If we just do mc T, we get m body *c body * T body + m air *c air * T air = 0 Which means before too long, you should be the same temperature as the air, and dying of hypothermia. Obviously, something else is going on...
Active Sources of Heat...Your body is constantly making new heat by doing reactions. It converts compounds into other compounds, and in the process also makes heat. The same is true of any reaction; it either actively makes new heat, or actively uses it up. mc T just won't cut it here.
q for Reactions Let's take a hypothetical reaction: A (aq) --> B (aq) If this reaction is done in 50mL of water, which gets heated from 20 o C to 25 o C, then: q water + q reaction = 0 50g * 4.184 J/g* o C * 5 o C + q reaction = 0 q reaction = -1046 J If the water absorbed 1046 J, then the reaction must have released 1046 J.
q for Reactions There's only one problem: You did this reaction with 2 moles of A and got q reaction = -1046 J Your classmate did it with 4 moles of A and got q reaction = -2092 J Another classmate used 3.52 moles of A and got q reaction = -1841 J Probably we should have some sort of better comparison.
q for Reactions Let's define a standardized number: H = q reaction /moles -1046 J/2 moles = -2092 J/4 moles = -1841 J/3.52 moles H = -523 J/mole This number ( H) is referred to as the “enthalpy” of the reaction, but it's just a standardized amount of heat. This is like ERA in baseball—you compare pitchers over 9 innings, because the guy who gives up 3 runs in seven innings is better than the guy who gives up 2 in one inning.
q for Reactions This equation is usually written the other way around: If H = q/molesthen q = H*moles So now we have two things that q is equal to: q = mc Tobjects q = H*molesreactions
q for Reactions The sign conventions stay the same: positive H or q is endothermic negative H or q is exothermic KEY POINT! Your thermometer is never measuring the reaction itself—the thermometer will only measure the water that the reaction happens in.
Energy Hills Reactions are often shown as hills: reactants on the left, products on the right. To get the ball up the hill, we have to put energy in— the H is positive, and this is endothermic. HH
Energy Hills If the reaction is exothermic, the starting materials are higher energy than the products, so we get energy out (negative H) Energy that can be used to do things, like crush Indiana Jones. HH
Summary mc T is only for passive objects Reactions actively generate or use heat For reactions, q = H*moles H is just a standardized amount of heat, so you can compare your results to other people's. Enthalpies are often drawn as energy hills.