{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

Lecture 37

# Lecture 37 - Topics for the day Administrative stuff Where...

This preview shows pages 1–4. Sign up to view the full content.

Topics for the day Administrative stuff Where were we? Standard states Hess’s law Combining formation enthalpies Bond enthalpy Calorimetry Constant pressure Administrative stuff We probably won’t get to the last two questions on your Quest homework until Friday. Luckily it isn’t due till Sunday. ALEKS objective is due on Monday Questions about CH302 ALEKS is available, even if the instructor doesn’t use it for homework I don’t teach CH302. Take Research Methods! Where were we? Definitions: Open / Closed / Isolated systems, Potential / Kinetic energy Universe = system + surroundings State functions (e.g., P, V, T, H, E) are properties of a system that are independent of pathway. Thermodynamics looks at the change in state functions: Δ X = X final – X initial Gains to the system are positive and losses from the system are negative. The first law of thermodynamics is the conservation of energy law: Δ E system = – Δ E surroundings Heat is the flow of thermal energy from a higher temperature to lower temperature. Exothermic / Endothermic The heat capacity of a material is how much energy is needed to raise the temperature of that material. Enthalpy change Δ H is the flow of thermal energy under constant P. 1 2 3

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
A. Large piece of metal B. Small piece of metal C. It wouldn’t matter because they would be the same temperature. iClicker Time Thought experiment: Aluminum + blow torch. Which of these two pieces of metal would you be more willing to touch? s(Al) = 0.89 J ˚C –1 g –1 Same q, same s. Smaller m means larger Δ T q s m Δ T = Using the units to deduce the equation, s = Δ Tm q q = sm Δ T Standard State The standard state of any chemical is its pure form under the following conditions: Solids: Pure solid Liquid: Pure liquid Gases: 1 bar pressure* Solutions: 1 M at 1 bar pressure Elements: the most stable form at 1 bar pressure and 25˚C The standard value of a property X is denoted with a ˚, like this: X˚ * Most tables of data are still for the old definition of standard state (1 atm). The small change in standard pressure makes a negligible difference to most numbers. Unless your application is critical, it is usually safe to use data for 1 atm instead of 1 bar. A. H (g) B. H 2 (g) C. H 2 (l) D. H (s) iClicker Time What is the standard state of hydrogen? Remember you need to know which elements are diatomics in their standard state: H 2 , O 2 , N 2 , Cl 2 , Fl 2 , Br 2 , I 2 Possible mnemonics include: BrINClHOF (brinklehoff) HONClFIBr (honklefibber) Have No Fear Of Ice Cold Beer I Have No Bright Or Clever Friends Standard state of an element = most stable form at 1 atm and 25˚C 4 5 6
Recall Δ H depends on the physical state of each reactant and product. The standard molar enthalpy of reaction Δ rxn of a substance is the enthalpy change for the reaction in which all reactants and products are in their standard states.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### Page1 / 9

Lecture 37 - Topics for the day Administrative stuff Where...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online