Chemical ReactionsSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Chemical ReactionsChemistry is about reactions with molecules colliding and forming new molecules.A number of reactions can be classified as "types”, allowing some general predictions on outcomes of reactions.Understanding the mechanism of reactions is important to our understanding of processes such as acid rain, corrosion, stain removers. Dissolution ReactionsPrecipitation ReactionsAcid-Base ReactionsOxidation-Reduction ReactionsDissolution ReactionsIn dissolution reactions two or more compounds disperse into each other to form a homogenous phase. The starting compounds could be different phases (e.g. a solid and a liquid), but the outcome of dissolution is a homogenous phase; a SOLUTION In dissolution reactions, the compound of lower concentration is called the SOLUTE and the higher concentration component is the SOLVENT.During dissolution the solvent interacts with the solute such that for the solute, the interactions between the solute and solvent dominate over the solute-solute interactions and solvent-solvent interactions.Dissolution reactions are considered to be intermediate between a chemical and physical process. In terms of it being considered to be a chemical process, solute-solute interactions are broken up and replaced by solute-solvent interactions. On the other hand the solution that results cannot be expressed as a chemical formula and hence the outcome of dissolution cannot be represented as a typical chemical equation.To write an equation for a dissolution reaction the solvent is left out and the change in state of the solute denoted.For example: dissolving sucrose in water C12H22O11(s)  C12H22O11 (aq)s - solid aq - aqueous solution.Note: for dissolution reactions, the solvent need not be water, nor necessarily a liquidOther examples of common liquid solvents are , but benzene (C6H6), acetone (CH3COCH3), carbon tetrachloride (CCl4), methanol (CH3OH)Dissolution of Ionic CompoundsMost ionic compounds dissolve easily in water. As we have seen ionic compounds, like NaCl, have rigid lattices defined by the oppositely charged ions.For the ionic compound to dissolve in water, the water molecules must overcome the strong interactions that exist between the oppositely charged species so that ion-water (solute-solvent) interactions dominate over ion-ion interactions (solute-solute)The negative end of the water molecule (O) interacts with the positive ions in the crystal and the positive end of water (H) interacts with the negative endWhen a water molecule encounters an ion it orients itself so that the appropriate "side" of the water molecule interacts with the ion (for negative ions, the H points toward the ion and for positive ions, the O).Having oriented itself in this way, the water molecule essentially pulls this ion out of the crystal lattice. Other water molecule surround this one ion and screens the ion from the oppositely charged ions in the crystal. Hence the water molecules, solvate the ion, and the solvated ion then moves through the solution. By this process the ionic compound dissolves in water and is said to DISSOCIATE INTO ITS IONS.Dissolution reactions for ionic compounds are written as:NaCl(s)  Na+(aq) + Cl-(aq)Dissolution of Covalent compoundsNeutral molecules do not have charges by which they can interact with the solvent. However, they can interact with the solvent through their polarity. Since the polarity of a molecule is due to a charge separation within the molecule, solute molecules that are polar can dissolve in polar solvents in much like the way ions dissolve in water (a polar solvent).For polar solutes, the polar solvent molecules orient themselves around the solute molecules so that the more positive end of the solvent is oriented towards the more negative solute molecule and the more negative end of the solvent orients itself towards the more positive end of the molecule.In this way the solute molecules are solvated replacing the solute-solute interactions by solute-solvent interactions. The solute molecules remain intact, but each solute molecule is solvated by solvent molecules. Dissolution of molecular compounds can be written as:C12H22O11(s)  C12H22O11 (aq)For a solute molecule to go “into solution”, the solvent molecules must solvate the solute molecule so that the solute-solvent interaction dominate over the solute-solute interactions.Hence, solute molecules dissolve in solutions of the same polarity - “like dissolves like”.So polar solutes dissolve in polar solvents, non-polar solutes dissolve in non-polar solventsSoaps or surfactants are “designed” so that the soap molecule dissolves in water, yet can interact with a non-polar oil molecule in a grease staina) Dissolution of an ionic compoundb) Dissolution of a covalent compoundSolubilitiesEthanol and water dissolve in each other and are said to be miscible.As more ethanol is added, at some point the ethanol concentration is larger than the water concentration and the solute and solvent switch.Water and ethanol have infinite solubilties in the other.Solubility is defined as the amount of a solute that can dissolve in a fixed amount of solvent, at a given temperatureSolubility varies with temperature - generally higher the temperature larger is the solubility.As NaCl is added to water, a point is reached when the NaCl does dissolve, but remains as a solid in the salt solution.The point at which the NaCl stops dissolving in the salt solution, defines the solubility of NaCl in water, at that temperature.The solution is said to be saturated with NaCl. For a liquid solute dissolved in a liquid solvent, at the saturation point, a new layer is formed, with the new layer contains the solute with some of the original solvent dissolved.Electrolytes and Non-Electrolytesdeionized, pure, water+-bulb+batterydeionized water + NaCl+-bulb+batteryNa+Cl-NaCl(s) --> Na+(aq) + Cl-(aq)When the battery is turned on the Na+ ions flow toward the negative plate (anode) and the Cl- ions to the positive plate (cathode).The flow of ions constitutes a current. The circuit is now complete, current flows through the circuit, and the bulb turns on.NaCl is called an