EnthalpyThe energy with the fun-to-say name

Active Sources of Heatq = mcT can only work for passive sources or

destinations for heat.

Consider your body. If we just do mcT, we get

mbody

*cbody

*Tbody

+ mair

*cair

*Tair

= 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.

mcT just won't cut it here.

q for ReactionsLet's take a hypothetical reaction:

A (aq) --> B (aq)

If this reaction is done in 50mL of water, which gets heated from 20oC to 25oC, then:

qwater

+ qreaction

= 0 50g * 4.184 J/g*oC * 5 oC + q

reaction = 0

qreaction

= -1046 J

If the water absorbed 1046 J, then the reaction must have released 1046 J.

q for ReactionsThere'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 ReactionsLet'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 ReactionsThis equation is usually written the other way around:

If H = q/moles then q = H*moles

So now we have two things that q is equal to:

q = mcT objects

q = H*moles reactions

q for ReactionsThe sign conventions stay the same:

positive H or q is endothermicnegative 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 HillsReactions 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.

H

Energy HillsIf 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.

H

Summary mcT 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.