Introduction Properties & Reactions Signs of chemical reactions, rates of reactions, & properties.

Physical properties Characteristics of a material that are observed without changing the identity of the substance. Synonyms: Examples: color, density, conductivity, malleability, melting point, boiling point, & solubility – whether a substance will dissolve in another substance.

Expectation 5: Physical change A change that results in a difference in the material, but does not change the type of material. The composition of the material is the same after the change as it was before. The material still has the same chemical and physical properties as before. Examples: tearing paper, crumpling paper, coloring on paper, breaking glass, dissolving salt in water, getting paper wet

Expectation 4: Signs of a chemical reaction Chemical rxns occur all around us everyday. A chemical reaction is typically indicated by one of the following signs: Absorption or release of energy, such as light or heat, w/o outside influence. Formation of a gas in the absence of heating. djamdance.googlepages.com www.glasgowsciencecentre.org

Expectation 4 :Signs cont’d Color change that is not the result of color blending. Formation of a precipitate (a solid that does not dissolve in the liquid). Change in odor.

Chemical properties Characteristics of a material that describe how the material acts in the presence of other types of material.

Expectation 5: Chemical changes A change that results in the production of new materials with new properties. The materials after the change are different than the materials present before the change. These new materials have different chemical & physical properties.

Expectation9: Why chemical rxns occur Chemical rxns are the result of collisions between molecules &/or atoms. When reactant molecules collide with enough force in the right position a rxn will occur.

Expectation 9: Criteria for a successful rxn 1) Proper orientation In order for a reaction to occur the molecules must hit each other in the right position. Collisions that do not have proper orientation do not cause a rxn. Analogy: Keys only work if you put them in the lock the correct way 2) Sufficient energy (Activation Energy) Molecules must have enough energy when they collide in order to cause a change. Collisions with too little energy do not cause a rxn. Analogy: car hitting tree at 5mph vs. 55mph

Expectation 9: Activation energy Amount of energy required for the collision to cause a rxn. chem.ufl.edu qwickstep.com

Expectation 10: Concentration At higher concentrations more molecules are present so the chances of collisions increases. Thus more collisions lead to a higher number of successful collisions or reactions. Analogy: 5 people running around in a room vs. 100 drmackay.org

Expectation 10: Surface area As the surface area increases the rxn rate is faster. Increasing surface area exposes more molecules. If more molecules are exposed then more molecules are available for collisions. More collisions result in a greater chance of successful collisions & thus a rxn.

Expectation 10: Nature of reactants Different substances have different reactivities. Gases react faster than liquids. Liquids react faster than solids.

Expectation 10/13: Temperature The effect of temperature on rxn rates is two fold. At higher temperatures molecules are moving faster so the chances of a collision increases. More collisions increases the rxn rate. At higher temperatures molecules have more energy, thus more collisions have the energy required for the reaction to occur. So the rxn rate is faster Analogy: 20 people running around in the room vs. walking.

Expectation 11: Catalyst A substance that increases the rxn rate. Provides an alternate path that requires less energy. Does not get consumed during the reaction & can be re-used. http://faculty.sdmiramar.edu