{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

chem21-22Heat - Module 21 Energy Calculations Calculations...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Module 21 — Energy Calculations Page i Calculations In Chemistry * * * * * Modules 21 and 22 Phase Changes, Energy, and ( H) Module 21 – Phases Changes and Energy .................................................................. 559 Lesson 21A: Phases and Phase Changes ............................................................................... 559 Lesson 21B: Specific Heat Capacity and Equations ............................................................ 572 Lesson 21C: Water, Energy, and Consistent Units .............................................................. 580 Lesson 21D: Calculating Joules Using Unit Cancellation ................................................... 585 Lesson 21E: Calorimetry ......................................................................................................... 591 Module 22 – Heats Of Reaction ( Δ H) .......................................................................... 599 Lesson 22A: Energy, Heat, and Work .................................................................................. 599 Lesson 22B: Exo- And Endothermic Reactions .................................................................. 607 Lesson 22C: Adding Δ H Equations (Hess’s Law) ............................................................... 612 Lesson 22D: Heats of Formation and Element Formulas ................................................... 618 Lesson 22E: Using Summation to Find Δ H .......................................................................... 626 For additional modules, visit www.ChemReview.Net
Background image of page 1

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

View Full Document Right Arrow Icon
Module 21 — Energy Calculations ©2011 ChemReview.net v. e4 Page 559 Module 21 — Energy Calculations Timing : Begin this module when you are assigned problems which involve phase changes or calculations involving joules or calories . Prerequisites : This module for the most part does not require many prior topics. You should be able to do most of the calculations if you have completed Modules 2, 4, 5, and 8. Lesson 21A: Phases and Energy Three Phases In first-year chemistry, we are initially concerned with three phases for pure substances: solid, liquid, and gas. In covalently bonded molecules, the forces that hold the atoms together within the molecule are relatively strong, but there are also relatively weak forces of attraction between molecules. These weak attractions mean that molecules are a bit “sticky:” they tend to attract each other somewhat like two weak but attracting magnets (these weak molecular attractions are electrical, but the behavior is similar). In the solid phase, molecules vibrate , but the weak attractions between molecules hold the molecules in a crystal structure where they are limited in the extent to which they can rotate, and they can translate (move from place to place) only very slightly. In their liquid phase, the molecules gain some freedom: they can vibrate, rotate, and translate. However, the liquid phase molecules are still very close together: they have minimal space between them. This is why solids and liquids do not compress (or compress only very slightly) when pressure is applied. In the gas phase, molecules are separated by a considerable distance. In a gas at room temperature, the distance between molecules is typically about 10 times the diameter of the molecule. This means that 99.9% of the gas is empty space. Gases can be compressed because the empty space between the molecules can be reduced. In a gas, the molecules remain weakly attractive. If a gas is highly compressed, or if its temperature is lowered (which slows down the speed at which the molecules move), the stickiness of the molecules becomes a larger factor. All gases condense into a liquid or a solid at some point as pressure is increased and temperature is decreased.
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}