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HSC Chemistry
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Module 5: Equilibrium and Acid Reactions5.1 Static and Dynamic Equilibrium5 Topics

5.2 Factors that Affect Equilibrium2 Topics

5.3 Calculating the Equilibrium Constant2 Topics

5.4 Solution Equilibria

Module 6: Acid/Base Reactions6.1 Properties of Acids and Bases7 Topics

6.2 Using Brønsted–Lowry Theory2 Topics

6.3 Quantitative Analysis1 Topic

Module 7: Organic Chemistry7.1 Nomenclature2 Topics

7.2 Hydrocarbons2 Topics

7.3 Products of Reactions Involving Hydrocarbons

7.4 Alcohols1 Topic

7.5 Reactions of Organic Acids and Bases

7.6 Polymers2 Topics

Module 8: Applying Chemical Ideas8.1 Analysis of Inorganic Substances3 Topics

8.2 Analysis of Organic Substances

8.3 Chemical Synthesis and Design

Working ScientificallyWorking Scientifically Overview1 Topic
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Lesson 1, Topic 5
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Entropy and Enthalpy in Nonequilibrium Systems
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 Enthalpy (H) is the heat content of a system.
 Entropy (S) is a measure of the distribution of available energy, but is also a measure of disorder.
 These determine whether a reaction is:
 Spontaneous – Proceeds without needing a continual input of energy from an external source.
 Nonspontaneous – Requires a continual input of energy from an external source.
The Gibbs free energy formula predicts the spontaneity of a reaction.
\Delta G\degree = \Delta H\degree  T\Delta S\degree
 If [katex]\Delta G\degree > 0[/katex], reaction is nonspontaneous.
 If [katex]\Delta G\degree = 0[/katex], system is at equilibrium.
 If [katex]\Delta G\degree < 0[/katex], reaction is spontaneous.
Combustion
Consider the complete combustion of octane.
C_8H_{18 (l)} + \frac{25}2 O_{2 (g)} \rightarrow 8CO_{2 (g)} + 9H_2O_{(l)}
 Combustion is exothermic as it always releases heat, i.e. [katex]\Delta H\degree < 0[/katex]
 Entropy can either be positive or negative, depending on the number of moles of gas on each side of the reaction
 For octane, entropy decreases as there are fewer moles of gas resulting from the reaction, i.e. [katex]\Delta S \degree < 0[/katex]
However, [katex]\Delta G\degree < 0[/katex] for all combustion reactions, regardless of temperature of molar ratios, as enthalpy is always extremely negative. Therefore, combustion is a spontaneous reaction.
Photosynthesis
6CO_{2 (g)} + 6H_2O_{(l)} \rightarrow C_6H_{12}O_{6 (aq)} + 6O_{2 (g)}
 Photosynthesis is endothermic, absorbing energy from the sun, i.e. [katex]\Delta H\degree > 0[/katex]
 Entropy decreases as there are 6 moles of gas on each side but more overall particles on the reactants size, i.e. [katex]\Delta S\degree < 0[/katex]
For photosynthesis, [katex]\Delta G\degree > 0[/katex] so it is a nonspontaneous reaction which requires a constant input of energy to proceed.