Molecules

Physical Chemistry

• Thermochemistry - It is the application of the first law of thermodynamics (conservation of energy) to processes such as: chemical reactions, phase changes, e.g., boiling and melting, and the formation of solutions.

  	The first thermochemistry law (proposed in 1780) states that: the quantity of heat required to decompose a compound into its elements is equal to the heat evolved when that compound is formed from its elements. The second law of thermochemistry (also known as Hess law) states that the heat change in a chemical reaction is the same whether it takes place in one or
  Figure 12-19 Hess Law [view large image]	several stages. That is, the heat change depends on the initial and final states only (Figure 12-19).

  Chemical Kinetics - Chemical kinetics study reaction rates in a chemical reaction. It is measured by the amount of reactant used up, or the amount of product formed, in a certain period of time. Reactions with low activation energies go faster than reactions with high activation energies. The rate of a reaction can be affected by the temperature, the amounts of reactants in the container, and catalysts (Figure 12-20). Reactions almost always go faster at higher temperatures, because the increase in kinetic energy makes the reactants move faster and collide more often. The rate of a reaction also increases when reactants are added (resulting in higher concentration) since there are more collisions between the reactants. Another

  Figure 12-20 Reaction Rate [view large image]

  way to speed up a reaction is to lower the energy of activation. This can be done by adding a catalyst, which speeds up the reaction but is not itself changed or used up.

  
  
  
• Solution - A solution is a mixture in which one substance called the solute (the component that changes state upon dissolving in smaller amount) is uniformly dispersed in another substance called the solvent. Because the solute and the solvent do not react with each other, they can be mixed in varying proportions (see Figure 12-21). Solutes and solvents may be solids, liquids, or gases (see Figure 12-22). Gases form solutions easily because their particles are moving so rapidly that they are far apart and attractions to the other gas particles

  		are not important. When solids or liquids form solutions, there must be an attraction between the solute particles and the solvent particles. Otherwise, the particles do not mix and no solution forms. Compounds containing nonpolar molecules such as iodine, oil, or grease do not dissolve in water because water is polar. The polarities of a solute and a solvent must be similar in order to form a solution. It is found as early as the 1800s that whenever a substance is dissolved in a liquid the vapor pressure (the pressure at which the rate of evaporation is equal to the rate of condensation)
  Figure 12-21 Solution	Figure 12-22 Types of Solution [view large image]	of the latter is lowered. Such behaviour has the effect of rising the boiling point and lowering the freezing point of the solution. It also produces

  osmosis, which is the flow of a solvent, usually water, through a semipermeable membrane into a solution of higher solute concentration, i.e., diluting the solution.



• Electrochemistry - For the subject of physical chemistry the most important conductors are those of the electrolytes; they are distinguished from electronic conductors, such as metals, by the fact that the passage of an electric current is accompanied by the transfer of matter.

  		There are two main groups of electrolytic conductors; the first consists of pure substances, e.g., fused salts, and the second of solutions. The most thoroughly studied examples of the latter are solutions of acids, bases and salts in water. Sodium chloride (NaCl) is a strong electrolyte, which dissociates in water into hydrated ions of Na+(aq) and Cl-(aq). As shown in Figure 12-23a, the light bulb glows as these ions provide a path for current flow in a circuit of : light bulb - battery - electrolytic solution - light bulb. HF is a weak electrolyte only generates a dim light. While nonelectrolytic substances such as sugar block the passage of electricity altogether. Figure 12-23b shows the essential parts of a
  Figure 12-23a Electro- lyte [view large image]	Figure 12-23b Battery [view large image]	lead storage battery. When the switch in the battery circuit is closed, reactions take place at the anode and the cathode as shown in the diagram. The battery

  can store energy because the system of lead, lead dioxide, and sulfuric acid is in a higher energy state than those of the chemical substances they can react to form, namely, lead sulfate and water. When fully charged, the voltage between the terminals of a lead storage battery is 2.0 volts. Most automobile starters require either 6 or 12 volts to function, so the car batteries consists of 3 or 6 individual lead cells connected together to provide the proper voltage. By connecting the terminals of the battery to a source of direct current, which forces electric flow in the direction opposite to that of normal battery current, the battery can be recharged. (see also "modern rechargable battery")
  
  
• Phase Change - The term phase as used here relates to the fact that matter exists either as a solid, liquid, or gas such as ice, water, and steam for H₂O. Transitions from one phase to another are accompanied by the absorption or liberation of heat and usually by a change in volume. Figure 12-24 uses H₂O as an example to illustrate the process of phase change. The experiment puts ice (at -25^oC) into a container, which is surrounded by a heating coil (generating an uniform heating rate). Temperature of the ice rises from a to b until it reaches 0^oC - the melting point. Ice turns into water from b to c. The heat spent for the phase change (per unit mass) is called the heat of

  fusion. The temperature remains constant during the change of phase. Once the conversion is complete, the temperature of the water starts to rise again from c to d with a different rate since the specific heat (capacity to absorb heat) of water is greater than that of ice. Similar phase change occurs from d to e for water changing into steam. It takes longer time for the phase change from d to e, because the heat of vaporization (~ 540 cal/gm) is about 7 times higher than the heat of fusion. If the heating process continues from e to f, the gas would be called "superheated steam". The process reversed when heat is removed from the container.

  Figure 12-24 Phase Change [view large image]

  
  
• Surface Tension - A molecule in the interior of a liquid is completely surrounded by other molecules, and so, on the average, it is attracted equally in all directions. On a molecule in the surface, however, there is a resultant attraction inwards, because the number of molecules is greater in the bulk of the liquid than in the vapor. As a consequence of this inward pull the surface of a liquid always tends to contract to

  the smallest possible area; it is for this reason that drops of liquid and bubbles of gas in a liquid become spherical, a needle floats on the top of water, certain water bugs can travel across the surface of a pond or lake (see Figure 12-25a). When compounds called surfactants (Figure 12-05b) are added to water, they disrupt the hydrogen bonding between the water molecules. As a result, the surface tensions decreased and the water spreads out rather than forming drops or curved surface (Figure 12-25c). Soap, detergents, shampoos, and fabric softeners, are example of surfactants we use every day.

  Figure 12-25a Surface Tension [view large image]

  		Surfactant is a class of organic chemicals call amphiphiles (amphi-, meaning both), which has a hydrophilic (water loving) head and a hydrophobic (water hating) tail. An example of sodium dodecyl sulfate (SDS) molecule is shown in Figure 12-25b. When these molecules are dissolved in water the tails manage to avoid the water molecules by either forming a thin layer on the surface with the tail sticking out or wrapping into a sphere with the head facing the water molecules (Figure 12-25c). Oils and fats, which dislike
  Figure 12-25b Surfactant [view large image]	Figure 12-25c Surfactant in Water [view large image]	contact with water, are easily incorporated into the sphere and washed away in our daily cleaning actions.

Go to Next Section
 or to Top of Page to Select
 or to Main Menu