Another Example: Aluminum chloride, Al2Cl6, can be made by the reaction of aluminum with chlorine according to the following equation:                        2 Al  + 3 Cl2 → Al2Cl6 What is the limiting reactant if 20.0 grams of Al and 30.0 grams of Cl2 are used? How much Al2Cl6 can form? What is the fractional and percentage yield of the product? It is known that Al = 26.98 g/mol, Cl2 =  70.90 g/mol and Al2Cl6 =  266.7 g/mol

Moles of Al = 20.00 g/26.98 g/mol = 0.74 moles Moles of Cl2 = 30.00 g/70.90 g/mol = 0.42 moles Recall the balanced chemical reaction: 2 Al  + 3 Cl2 → Al2Cl6 Every two moles of Al produces one mole of product. If all the Al is consumed, we will get 0.74/2 = 0.37 moles of product. Every three moles of Cl2 produces one mole of product. If all the Cl2 is consumed, we will get 0.42/3 = 0.14 moles of product.

Moles of Al are in excess; the moles of Cl2 are insufficient for complete reaction; Cl2 is the limiting reagent. If we use up all the Cl2 available, all 0.42 moles, we will get 0.42/3 = 0.14 moles of Al2Cl6. Theoretically, if all Al had reacted, we would have had 0.37 moles of Al2Cl6. The fractional yield is actual yield divided by theoretical yield: 0.14/0.37 = 0.378 The percentage yield is the fractional yield times 100: 0.378 x 100 = 37.8%.

When is 2 + 2 not equal to 4? Dilution Problem: If you have available a 0.25 M NaOH solution and pure water, how much water needs to be added to make 10 liters of 0.2 molar solution of NaOH? Strategy: Moles of NaOH are not changed by dilution. In 10 L of 0.2 M NaOH the number of moles of NaOH present are 2 mol. So, you need to add 8 L of the 0.25 M NaOH to 2 L of pure water to arrive at the final desired solution concentration.

States of Matter: A Simple-Minded View DIGRESSION ◄► ◄► ◄► ◄► ◄► States of Matter: A Simple-Minded View ◄► ◄► ◄► ◄► ◄► The three principal states of matter are said to be: ● Gases ● Liquids ● Solids These are called different phases, and it is possible to go from one phase to another by a temperature change. Example: water as steam, liquid, ice

Space-Filling Property Gas – takes shape of container and fills it Liquid – takes shape of container without necessarily filling it Solid – has its own well-defined shape irrespective of container

Density Gas – 1019 molecules per cm3 at STP Chemical change occurs by means of binary collisions. Liquid – ~1022 molecules per cm3 Chemical change occurs in the “solvent cage”; followed by escape. Solid – ~1022 molecules per cm3 Chemical change occurs by hopping to vacancy sites; otherwise nothing happens!

Response to Shear Force Gas – inelastic ; little resistance Liquid – inelastic; viscous resistance Solid – elastic over finite range of force, then breaks shape.

Microscopic Structure Gas – disordered throughout Liquid – short-range order, long-range disorder Solid – short-range and long-range order Essentially random relative positioning of the gas molecules Approximately, close-packed ; first two or three shells around a given molecule are identifiable, but well-defined positional relationships die out with distance. Molecules have well-defined positions in a crystal lattice.

Not All Matter Fits the Three-Way Division ! Think about: liquid crystals amorphous solids polymers glasses, gels, foams, aerosols, etc. Conclusion: Chemistry is really more complex than you might first imagine … … and it does not fit neatly into boxes!

Adventures in Liquid Crystals The liquid crystal state is a distinct phase of matter observed between the crystalline (solid) and isotropic (liquid) states. There are many types of liquid crystal states, depending upon the amount of order in the material.

Nematic Phase The nematic liquid crystal phase is characterized by molecules that have no positional order but tend to point in the same direction (along the director).

Smectic Phase In the smectic state, the molecules maintain the general orientational order of nematics, but also tend to align themselves in layers or planes. Motion is restricted to within these planes, and separate planes are observed to flow past each other. The increased order means that the smectic state is more "solid-like" than the nematic.

Smectic Phase In the smectic-A phase, the director is perpendicular to the smectic plane and there is particular positional order in the layer.

Smectic Phase In the smectic-C phase, molecules are arranged as in the smectic-A phase, but the director is at a constant tilt angle measured with respect to the smectic plane.

Simulation of Phase Change in Liquid Crystals The material begins in the crystalline state, and as the user increases the temperature, it undergoes a phase change. The first liquid crystal phase is the smectic A, where there is layer-like arrangement as well as translational and rotational motion of the molecules. A further increase in temperature leads to the nematic phase, where the molecules rapidly diffuse out of the initial lattice structure and from the layer-like arrangement as well. At the highest temperatures, the material becomes an isotropic liquid. http://plc.cwru.edu/tutorial/enhanced/files/lc/phase/phase.htm