Chapter 5: Chemical Reactions – Thermodynamics and Energy in GOB Chemistry

Thermodynamics

Introduction to Thermodynamics

Thermodynamics is the study of energy and heat exchange in chemical reactions. It is essential for understanding how energy is transferred and transformed in biological, organic, and general chemical systems.

• Energy: Defined as the capacity to do work. In chemical reactions, energy can be transformed between potential energy and kinetic energy.

• Nutrition labels list Calories, which measure the energy content of food.

• When food is consumed, molecules are broken down by acids and enzymes, releasing energy.

Heat of Reaction

Chemical reactions involve the exchange of energy, often in the form of heat. The direction of heat flow determines the type of reaction.

• Exothermic Reaction: Releases heat to the surroundings. The products have lower energy than the reactants. Example: $\text{NaC}_2\text{H}_3\text{O}_2(aq) \rightarrow \text{NaC}_2\text{H}_3\text{O}_2(s) + \text{Heat}$

• Endothermic Reaction: Absorbs heat from the surroundings. The products have higher energy than the reactants. Example: $\text{Heat} + \text{NH}_4\text{NO}_3(s) \rightarrow \text{NH}_4\text{NO}_3(aq)$

Change in Enthalpy ($\Delta H$)

Enthalpy is a thermodynamic quantity representing the heat content of a system. The change in enthalpy ($\Delta H$) indicates whether a reaction is exothermic or endothermic.

• Exothermic: $\Delta H$ is negative; heat is released.

• Endothermic: $\Delta H$ is positive; heat is absorbed.

• Formula: $\Delta H_{\text{reaction}} = H_{\text{products}} - H_{\text{reactants}}$

Change in Entropy ($\Delta S$)

Entropy measures the randomness or disorder in a chemical system. The change in entropy ($\Delta S$) reflects the difference in randomness between products and reactants.

• Increase in Entropy: $\Delta S$ is positive; system becomes more random.

• Decrease in Entropy: $\Delta S$ is negative; system becomes more ordered.

• Formula: $\Delta S = S_{\text{products}} - S_{\text{reactants}}$

Gibbs Free Energy ($G$) and $\Delta G$

Gibbs free energy ($G$) is the energy available to do work in a chemical system. The change in free energy ($\Delta G$) determines whether a reaction is spontaneous.

• Formula: $\Delta G = \Delta H - T(\Delta S)$ where $T$ is temperature in Kelvin.

• Exergonic Reaction: $\Delta G$ is negative; reaction is spontaneous and releases energy.

• Endergonic Reaction: $\Delta G$ is positive; reaction is nonspontaneous and requires energy input.

Key Terms and Concepts

• Reactants: Substances present at the start of a reaction (left side of the equation).

• Products: Substances formed by the reaction (right side of the equation).

• Arrow ($\rightarrow$): Indicates the direction of the reaction, meaning "yields".

Examples and Applications

• Food Energy: The breakdown of food molecules in the body is an example of chemical reactions involving thermodynamics.

• Exothermic Reaction Example: Combustion of glucose in cellular respiration.

• Endothermic Reaction Example: Photosynthesis in plants.

Additional info: The slides also introduce reaction kinetics, which is the study of the rate at which chemical reactions occur, and set the stage for further topics such as oxidation-reduction and organic reaction mechanisms.