Động lực và Nhiệt động của phản ứng hóa học (tiếp)


Example 2: Systems

The best way to understand thermodynamics is by realizing that anything that transfers, receives, or contains heat can be described as a system. Heat can enter or leave a system, which affects the amount of thermal energy it contains. Consider a kettle of water sitting on a stove. As it is heated, thermal energy is added to the system (the kettle with the water). As the stove is turned off, the kettle cools down as the heat diffuses back to the room; the kettle slowly equilibrates to room temperature. This is an example of the system losing thermal energy. To view an animated diagram of a thermodynamic system, click on ‘Outside Link’ number 2.

Thermodynamics vs. Kinetics

As mentioned above, the most stable states of a kinetic reaction are those of the reactants, in which an input of energy is required to move the reaction from a state of stability, to that of reacting and converting itself to products. Kinetics is related to reactivity. In contrast, the most stable state of a thermodynamically favorable reaction is the products, because the reaction occurs spontaneously, without the need for energy to be added. Thermodynamics is related to stability.

Therefore, something that is unreactive will desire to stay in the form of reactants, which will require an input of energy to cause the reaction to go forward, converting reactants into products. This is illustrated in example #3 below. A reactive species does not require an input of energy to be converted from reactants to products, because its most stable and preferred state is that of the products. Instead, a thermodynamically favorable reaction requires energy to be converted from products back to reactants.An energy source moves the reaction forward (kinetics corresponds to movement). The same is for thermodynamically favorable reactions, except that the reaction must be stimulated backward from products to reactants.

Example 3: ATP
Adenosine triphosphate, also known as ATP, provides the energy cells require in order to maintain metabolic pathways, DNA synthesis and repair, and any other cellular function necessary for survival. ATP itself is a reactive molecule that has three phosphate groups. Molecules tend toward stable states, converting to states of lower energies. Thus, ATP, a high-energy molecule, tends to lose a phosphate group and become adenosine diphosphate, ADP. In order for this to happen, an enzyme strips one phosphate group off of ATP, converting it to the more stable molecule ADP. This enzyme provides the energy of activation that enables ATP to become ADP, indicating that ATP is kinetically stable.

Example 4: Water and Sugar

The following example involves solvents and polarity: consider a simple situation, a spoonful of sugar is added to a cup of water. If the two are left to react, over time the sugar dissolves in the water, becoming the product of sugar+water. The natural charges and polarity of water causes the sugar molecules to react with it, eventually dissolving within the water. There is no required input of energy, indicating that this reaction is thermodynamically favorable, and therefore spontaneous. Clearly, the two reactants prefer to react and maintain stability as products.

Note: although this is a thermodynamically favorable or spontaneous reaction and does not require energy input, the use of kinetic energy will force this reaction to happen faster. If sugar is added to the cup of water and the system is heated, the kinetic energy of the reactants is increased by the thermal energy of the heat, which causes the molecules to react with one another at a much faster rate than if they been left alone at room temperature. This is an example of how thermodynamics and kinetics are closely related.

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