This may have been the largest Chem Talk reading we have ever done in this class. Section 4.8 covered all sorts of things including spontaneity, enthalpy changes, entropy changes, spontaneity in nature, polymers, and how temperature affects rubber bands. The book talks of systems and surroundings and how they deal with how a certain scientist wants to investigate. In order to see and consider change, a scientist must first understand what the system is doing and how the surroundings effect what happens. Open systems are systems that allow energy to pass through them, and closed ones contain energy within. An interesting concept is the law of conservation of energy, which states that energy is never destroyed or created, but transferred. The rest of this little section was a little overview of positive and negative changes in entropy and enthalpy which is a little bit of a challenge to grasp, but is accomplishable. There are certain combinations in nature that allow for spontaneous reactions, depending on whether both the entropy and enthalpy are negative or positive. The Gibbs free energy equation can usefully tell if a reaction would occur spontaneously or not, which is cool. If left with a positive answer after using the equation, the reaction is not spontaneous and vise versa. The talk ended with a discussion about the entropy and enthalpy of rubber bands when they are both stretched and contracted, which seemed pretty interesting, but we were not able to test this in class, which is unfortunate. All in all, this talk gave me a better picture around entropy and enthalpy.
- When a change occurs spontaneously, it means that once it has begun, it continues without an input of energy.
- The two questions are: does the change give off heat energy when it occurs or absorbs heat energy when it occurs, and does the change result in particles becoming more disordered or less disordered?
- The relative locations of the reactant energy and product energy indicate whether the overall enthalpy change is endothermic or exothermic.
- The energy in an exothermic reaction comes from a source that is supplied to the system in order to start the reaction. For example; water in the magnesium and iron reaction.
- Scientists figured out that the net change of the MRE reaction is exothermic, which means it looses energy to its surroundings. Scientists were able to take this data and make the MRE heat faster.
- A catalyst speeds up a reaction by providing a lower energy alternative pathway for the reaction to take. This means that more starting materials at a given temperature have enough energy to make it over the barrier, and in turn heat up quicker.
- A graph without the catalyst is steeper, meaning it gets to a higher temperature over a longer period of time, while one with a catalyst heats up quicker, but not to as a high of a temperature.
Chapter 4 section 7’s Chem Talk was quite a large one, packed with information. The talk mainly covered thermodynamics, heat energy changes, and energy diagrams. As information about spontaneous reactions and the explanation of the theories of thermodynamics were given in the talk, my mind wondered a bit, thinking more in-depth of the subject. Chemists today use thermodynamics and kinetics to design reactions that help to better human lives. Though after reading this talk, it would still be nice to get some kind of explanation of why certain things happen when they do. Next, the talk went more into detail about our experiment and the spontaneous reaction between magnesium and water to create heat. This is the same reaction that takes place in MRE’s, which are used by the military to heat food in the field. While the reaction is taking place, enthalpy change also takes place. This is the change in heat energy that occurs at a constant pressure. The talk then goes into energy diagrams. These diagrams basically show that you can start with something with low energy, and after a reaction end up with higher energy than you started with. Overall, this talk was a little confusing when going into the details about the reactions and enthalpy, but a cool topic.
Electrochemical cells and Half Reactions:
In our lab, we learned about batteries, conductivity in solutions, and electrodes. Electrolytes are substances that dissolve in water to make it conduct electricity. For a solution to be able to conduct electricity it must contain particles that can move around. The Chem Talk talks about ions and how they are formed by dissolved compounds. Though molecular compounds do not break apart though, for example sugar. When dissolved in water, they remain as sugar molecules. While on the other hand, ions are able to move around freely in water, making them able to conduct electricity. The path of electricity in batteries is a pretty simple thing as well. A battery has two half cells; one where oxidation takes place and one where reductions simultaneously occurs. The book then explains that if a metal is more reactive, the reaction will occur faster in the metal. The farther apart two metals, the stronger the metal ion reaction will be.
The beginning of the section 4.4 Chem Talk starts out with a basic review of what a metal is and certain characteristics that it possesses. Importantly, the book mentions hows the majority of metals listed on the periodic table are more reactive than hydrogen and are most commonly found in nature in their ionic forms. Next, the talk goes into the activity series, which was a pretty basic and understandable concept. Special reactions can take place between metals and metal ions. For example, rust, or oxidation can take place when a metal is giving up electrons. Hydrogen is included in the activity series because it is a natural reactant with metals, meaning it can quickly establish how reactive another metal is. This talk was pretty easy to grasp and somewhat interesting. Adios.