CHM152Chapter 16 - 6

Chemistry (MasteringChemistry Series)

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

View Full Document Right Arrow Icon

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

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: 1 1 Chapter 16 Chapter 16 Thermodynamics Thermodynamics • Study of energy changes in chemical reactions ¡ useful to understand the nature of chemical changes • Recall thermochemistry (study of heat changes) from Chapter 8 • You are responsible for information from CHM 151, specifically sections 2, 5, 7, 9, and 10 (except “work”). Thermochemistry Review • 1st Law of Thermodynamics: Energy is conserved, it cannot be created or destroyed ¡ If the system gives off heat, the universe must absorb it. ¡ Heat flow (enthalpy, Δ H) always defined with reference to the system • System absorbs heat, Δ H > 0, endothermic • System gives off heat, Δ H < 0 , exothermic Calculating Δ H o rxn • Δ H o rxn = Σ Δ H o f products - Σ Δ H o f reactants ¡ Heat of formation values, Appendix B ¡ Δ H o f is reported as kJ/mol The o symbol refers to the standard state 1 00 • The symbol refers to the standard state, 1.00 atm pressure, 25.0 o C, 1.00 M for solutions • Review heat of formation reactions (making 1 mole of substance from its elements in the standard state) ¡ Δ H o f for elements is 0 (no energy needed!) • Write heat of formation rxn for MgCl 2 (s), H 2 O(l). Thermodynamics • How do we know if a chemical reaction will occur? • How can we tell how far a reaction will go? • Thermodynamics helps to answer these questions. • Enthalpy is only one part of the equation. • Entropy is another. Spontaneous Processes • In chemistry, we define a spontaneous process as one that proceeds on its own without any external influence. C id th i f i t ¡ Consider the expansion of a gas into a vacuum. This happens spontaneously. The reverse process does not! • Figure 16.1 2 2 Spontaneous Chemical Reactions Spontaneous Chemical Reactions • Spontaneous processes ¡ hot object cools ¡ gases expand ¡ iron rusts ¡ Cs reacts with H 2 O • What do all these processes have in common? • They happen with no external intervention. ¡ Nature tends toward lower energy! Usually exothermic reactions are spontaneous (but not always)!! Spontaneous Processes • Spontaneous does not mean the same thing as “fast” ¡ Rusting of iron is slow but spontaneous • Thermodynamics just tells us whether or not a reaction will occur, not how fast it will occur ¡ Kinetics (Ch. 12) looks at that! • Problem 16.1 Entropy (S) • Section 8.13: Nature tends toward disorder (increase in entropy). ¡ Who wants to play 52 card pick-up? • Entropy is a measure of disorder or randomness. ¡ A process is usually spontaneous if it increases the disorder (change in entropy is positive: Δ S > 0) ¡ S: J/K Which has the greater entropy? Which has the greater entropy?...
View Full Document

Page1 / 11

CHM152Chapter 16 - 6 - 1 1 Chapter 16 Chapter 16...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online