1. Changing {delta}H when you change a reaction. Let's look at an example, consider the reaction A-->B with {delta}H = 25kJ/mol. Positive {delta}H means this reaction is endothermic, so we have to add 25kJ of energy to convert each mol of A into a mol of B. If we reverse the order of the reaction, B-->A, the conversion of a mol of B into a mol of A will release 25kJ of heat, making it exothermic with {delta}H = -25kJ/mol. What if we triple the reaction, 3A-->3B? For each mol of A that is converted to a mol of B, the reaction still requires 25kJ of heat, but now the reaction as written is converting 3 mols of A into 3 mols of B, so {delta}H = 3(25kJ/mol) = 75kJ/mol.
2. Heat capacity and {delta}H for a phase change. When a substance freezes, melts, boils or condenses, it releases or absorbs heat without a change in temperature. This means that if I have a pot full of water on the stove and I start to boil it, no matter how hot I make the burner the boiling water will be 100 degC until all of it has boiled away. If you're looking at a problem that goes through a phase change and changes temperature, you can break the problem down into a couple smaller problems. For changing temperature without changing phase, it's a heat capacity problem; for changes in phase, it's {delta}H of phase change problem.
Remember, if you're looking at old exams we have not talked about quantum numbers or electron configuration or wavelengths yet, that's coming up in Chapter 7.
If you're in the mood for a study break, today at 4pm the MSUM Dragon volleyball team will be Digging for a Cure to raise money for cancer treatment and research. Wear purple and cheer on your Dragons as they crush Upper Iowa. Purple T-shirts will be sold at the game for $10.
Let me know if you have questions, I will answer to the blog.
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