Monday, Aug. 26 th : “A” Day Tuesday, Aug. 27 th : “B” Day Agenda  Collect Student Safety Agreements – 5 XC pts  8.1: “Describing Chemical Reactions”  Safety Project Work Time Homework: 1. SSA signed/returned – NO XC 2. Sec. 8.1 review, pg. 266: # “The Big Idea” worksheet 4. Concept Review: “Describing Chemical Reactions” Next Time: Safety Projects Due Sec. 8.1 Quiz

Sec. 8.1: “Describing Chemical Reactions”  Chemical Reaction: the process by which one or more substances change to produce one or more different substances.

Chemical Change 2H 2 + O 2 2 H 2 O  Reactants: a substance or molecule that participates in a chemical reaction Reactants are on the left side of the arrow. Reactants are used up in the reaction.  Products: a substance that forms in a chemical reaction Products on the right side of the arrow. Products are made in the reaction.

Evidence of Chemical Change  For proof of a chemical change, you need a chemical analysis to show that at least 1 new substance forms.  The properties of the new substance must differ from those of the original substances.

Evidence of a Chemical Change

Chemical Reaction vs. Physical Change  Chemical reaction: new substances are formed with different properties Example: 2 H 2 + O 2 2 H 2 O  H 2 and O 2 are both gases at room temperature, but H 2 0 is a liquid.

Chemical Reaction vs. Physical Change  Physical changes: The identity of the substance doesn’t change. All changes of state are PHYSICAL changes.** (evaporation, condensation, melting, freezing) Ice, liquid water, and steam are all still H 2 O.  Examples: dissolving sugar in tea, crushing a rock, mixing paint colors

Reactions and Energy Changes  Energy can be released in a chemical reaction. methane + oxygen  carbon dioxide + water + energy CH 4 O 2 CO 2 H 2 O  Energy is a product. This is an EXOTHERMIC reaction.

Reactions and Energy Changes  Energy can be absorbed in a chemical reaction. dinitrogen tetroxide + energy  nitrogen dioxide N 2 O 4 NO 2  Energy is a reactant. This is an ENDOTHERMIC reaction.

Constructing a Chemical Equation  Chemical equation: a representation of a chemical reaction that uses symbols to show the relationship between the reactants and the products.  Writing a chemical equation requires 3 steps: 1.Write a word equation 2.Write a formula equation 3.Balance the chemical equation

Writing a Word Equation 1.Write the names of the reactants, separated by a “+” sign. 2.Add an arrow pointing to the right 3.After the arrow, write the products, again separated by a “+” sign. Example: Methane + oxygen carbon dioxide + water

Writing a Formula Equation Methane + oxygen carbon dioxide+ water  Change the words in the equation to formulas: Methane: CH 4 Oxygen: O 2 Carbon dioxide: CO 2 Water: H 2 O ?CH 4 + ?O 2 ?CO 2 + ?H 2 O (The ? means we don’t know how many molecules of each reactant and product we have yet)

Balancing a Chemical Equation ?CH 4 + ?O 2 ?CO 2 + ?H 2 O  The numbers of atoms of each element must be the same on both sides of the arrow!  Balance each element one at a time… CH O 2 CO H 2 O (Section 8.2 is all about balancing chemical equations!)

Equations are Like Recipes  Like a recipe, any instructions shown in a chemical equation can help a chemist be sure the reaction turns out the way it should.  A balanced equation indicates the relative amounts of reactants and products in the reaction.

Equations sometimes show the physical states of the reactants and products NaHCO 3 (s) + HC 2 H 3 O 2 (aq)  NaC 2 H 3 O 2 (aq) + CO 2 (g) + H 2 O(l) solid aqueous solutions gas liquid (s) = solid (l) = liquid (g) = gas (aq) = aqueous solution (dissolved in water)

Equations sometimes show the reaction conditions 350˚C; 25,000 kPa N 2 (g) + 3 H 2 (g) 2 NH 3 (g) catalyst  This reaction is know as the “Haber Process”.  The double arrow means that reactions occur both in the forward and reverse direction and the final result is a mixture of all 3 substances.  The reaction occurs at 350˚C and at a pressure of 25,000 kPa.  A catalyst is used to speed up the reaction.

State Symbols and Reaction Conditions

Homework/ Safety Project Work Time  You have the rest of the class period to work on your safety projects and/or the homework assignments: Homework: 1. SSA’s signed/returned 2. Sec. 8.1 review, pg. 266: # “The Big Idea” worksheet 4. Concept Review: “Describing Chemical Reactions”

Next Time Quiz over Sec. 8.1 Safety Projects Due