Chapter6LEcture

# What the other conditions are notably whether the

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3. What the other conditions are, notably whether the change is “at constant p” or “at constant V” 4. Whether you have data per mole or per gram. The symbol above has superscripts to show that it means C for heating at constant p and that the number is J per mole. You have to remember that it also depends on T. However, for many things over a range near room T, C p,m is almost constant even if T changes somewhat. Just be sure there is no phase change.

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6 Copyright: 2010 Prof. Magde Chapter 6: Heating to Raise T H (for a T increase) = +C p,m T For many things over a range near room T, C p,m is almost constant. For water: C p = about 4.18 J g -1 or about 75.3 J mol -1 So let’s boil 36 g of ice: Note that the phase transitions dominate. Why are the phase transitions done in moles but the T rise is in grams? Just because of the numbers we knew. total 1 1 1 1 H H(melting) H(war ming) H(evaporation) 1mole 36g 15kJ mol 18g 36g 4.18Jg deg 100 1mole 36g 45.7kJ mol 136kJ 18g      
6 Copyright: 2010 Prof. Magde Chapter 6: Enthalpies in Reactions Physical changes, like phase transitions, have an associated enthalpy. So do chemical changes. Combustion reactions give off energy by heating the surroundings. That’s why we burn wood or coal to keep warm or heat our food. C(s) + O 2 (g) CO 2 (g) H = -393.5 kJ The H is for the reaction as written, for however moles are shown. When you add equations, you add enthalpies (just like adding for melting and evaporation). 2C(s) + O 2 (g) 2CO(g) H = -221.0 kJ 2CO(g) + O 2 (g) 2CO 2 (g) H = -566.0 kJ Add: 2C(s) +2O 2 (g) + 2CO(g) 2CO(g) + 2CO 2 (g) Cancel 2CO(g) on each side: 2C(s) +2O 2 (g) 2CO 2 (g) H = -787.0 This is double the reaction at the top, as it should be, because it is the same but with 2 moles.

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6 Copyright: 2010 Prof. Magde Chapter 6: Enthalpies in Reactions C(s) + O 2 (g) CO 2 (g) H = - 393.5 kJ •So if we have a good set of reactions, like this one, we can combine them to learn the enthalpy changes in other reactions without having to measure them all! This is the biggest single thing the US government spent money on for technical support of the economy prior to the modern era. •Mostly, you do combustion reactions, because they are fairly easy to do. Burn everything! •If you turn the reaction around, the sign of the enthalpy change switches: CO 2 (g) C(s) + O 2 (g) H = +393.5 kJ •You have to put energy in, either by doing work or by heating the CO 2 in order to break the bonds and make the atoms come apart. That makes sense.
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