Chapter 8 Chemical Equations and Reactions 8.1 Describing Chemical Reactions

Chemical Reaction Chemical Reaction – the process by which one or more substances change to produce one or more different substances Evidence of chemical change Formation of a gas, E change, solid formed, color change in system

Chemical Reactions A new substance results from a chem rxn. to prove a rxn has occurred Reactants  Products Energy changes Collisions must occur Law of Conservation of Mass

Chemical Equation Chemical Equation – A representation of a chemical reaction that uses symbols to show the relationship between the reactants and products Word equations: Ethanol + oxygen  carbon dioxide + water

Chemical Equations Tells you… Number and type of atoms State of reactants and products (s,l,g) If it is dissolved in water (aq) If heat or catalyst are used

Equations C2H6 (g) C2H4 (g) + H2 (g) Physical Catalyst State Products 800 K C2H6 (g) C2H4 (g) + H2 (g) Physical Catalyst State Products Reactants Cr2O3

Assignment Do the section review on page 266 for homework. Do only numbers 1-5, This will be due on Monday.

Balancing Chemical Equations 8.2

Equations _CH3CH2OH + _O2  _CO2 + _H2O This eqn needs balanced Coefficients – integer/numeral used in a chemical eqn to indicate relative amts of reactants or products. Never change subscripts

Balancing an Equation Write the word eqn Write the unbalanced formula eqn. Count the # of each element Insert coefficients for atoms Check to make sure both sides are the same.

Tips for Balancing Equations Delay balancing elements (like H and O) that occur in several reactants or products. If the same polyatomic ions appear on both sides of the equation, treat them as single units, like monatomic ions.

Tips for Balancing Equations 3. After considering 1&2, balance elements left to right. 4. Remember, balancing 1 element may unbalance others. 5. For ionic equations, be sure charges are balanced.

Practice Balancing Ammonia plus oxygen yields nitrogen monoxide plus water plus heat. Ionic equations Need to make sure ionic charges are balanced HCl + H2O  Cl- + H3O+