17. Thermodynamics: Now What Happens When You Heat It Up?

This paragraph delves into the thermodynamics of the sodium bicarbonate decomposition reaction, commonly known as baking soda. The instructor explains the role of baking soda in baking, particularly in bread rising due to the gas formation it facilitates. The discussion then shifts to the thermodynamic values of the reaction, noting a positive delta H0 of 135.6 kilojoules indicating an endothermic process. The instructor guides the audience to understand that the reaction's entropy, delta S0, is positive due to the increase in disorder from solid to gas. The calculation of delta G0 at room temperature reveals a non-spontaneous reaction, which contrasts with the spontaneous reaction observed at higher temperatures, such as those in an oven. The segment emphasizes the influence of temperature on the spontaneity of reactions with both delta H and delta S being positive.

The second paragraph explores how temperature affects the spontaneity of chemical reactions, especially those with a positive delta H and delta S. The instructor illustrates this concept by plotting delta G0 against temperature, showing a linear relationship. Two calculated values of delta G0, one at room temperature (positive, indicating non-spontaneity) and one at 450 Kelvin (negative, indicating spontaneity), are used to demonstrate the transition point. The rearrangement of the Gibbs free energy equation is explained to derive the slope and y-intercept, which correspond to delta S and delta H, respectively. The concept of T star, the temperature at which a reaction switches from non-spontaneous to spontaneous, is introduced and calculated using the provided values for delta H0 and delta S0, resulting in a T star of 406 Kelvin.

This paragraph examines various scenarios where both delta H0 and delta S can be either positive or negative and their effects on the spontaneity of reactions. The instructor discusses four cases: reactions that are always spontaneous, never spontaneous, and sometimes spontaneous depending on the temperature relative to T star. The audience is engaged to predict the behavior of these reactions based on the signs of delta H and delta S. The importance of recognizing that temperature can either favor or disfavor spontaneity depending on the circumstances is highlighted.

The fourth paragraph transitions into the role of temperature in biological systems, particularly in relation to thermodynamics and kinetics. It emphasizes that increasing temperature generally accelerates reactions in biological systems. The paragraph then shifts focus to hydrogen bonding, a critical interaction in biology. Hydrogen bond donors and acceptors are defined, with a focus on their importance in identifying polar and non-polar bonds, a topic relevant for an upcoming exam. The structure of water and its ability to form hydrogen bonds due to its polar nature and lone pairs is discussed, highlighting the significance of hydrogen bonds in biological systems.

This paragraph delves deeper into hydrogen bonding, comparing it to covalent bonds and emphasizing its weaker yet crucial role in biology. Examples of hydrogen bond strengths are given, particularly between oxygen and nitrogen, and contrasted with the strength of covalent bonds. The paragraph explains that while hydrogen bonds are weaker, they are the strongest intermolecular interactions and are essential for the structure of proteins and DNA. The role of hydrogen bonds in protein folding and the stability of DNA's double helix is highlighted, illustrating their indispensable function in biological systems.

The focus of this paragraph is on the importance of hydrogen bonding in DNA and RNA, particularly in the context of base pairing and the double helix structure of DNA. The instructor explains the specific hydrogen bonds that occur between nitrogen and hydrogen in DNA's base pairs, enabling the recognition and pairing of genes. The paragraph also touches on the role of hydrogen bonds in RNA interference (RNAi), a gene silencing mechanism, and its potential therapeutic applications. The梦想 of customizing medicine based on individual DNA is discussed, with a nod to the ongoing research at MIT that contributes to this field.

The final paragraph examines the thermodynamics of ATP hydrolysis, a process that is spontaneous at body temperature due to its negative delta G0. The instructor explains how the hydrolysis of ATP can be coupled with non-spontaneous reactions to drive them forward, using the example of adding a phosphate group to glucose. The concept of coupling reactions is introduced, where the overall spontaneity is determined by the sum of individual delta G values. The paragraph concludes by addressing the stability of ATP, which despite being prone to hydrolysis, is kinetically slow enough to serve as an effective energy storage molecule in biological systems.