17-2 Describing Chemical Reactions

CHEMICAL EQUATION H2 + O2 H2O a shorter, easier way to show chemical reactions, using symbols, not words, for the reactants and the products hydrogen molecules react with oxygen molecules to form water molecules H2 + O2 H2O reactant + reactant “YIELDS” products

Law of Conservation of Mass total mass of reactants must equal total mass of products matter is NOT destroyed or created in a chemical reaction =

Chemical Formulas & Counting Atoms ELEMENTS – represented by a one or two-letter symbol (letter) C Na H COMPOUNDS– represented by a chemical formula (word) which uses subscripts to show the ratio of elements in the compound H2O2 CO2 C3H8O HOW MANY? coefficient in front of the chemical formula tells “how many” atoms or molecules 3H2O2 4CO2 2C3H8O

Balancing Chemical Equations Add the subscript “2” to all diatomics that are ALONE in the equation (N, O, F, Cl, Br, I, H) (note: only time a subscript can & must be changed) H + O     H2O Count the number of atoms that are present in the basic equation and record Pick an element that is not balanced on both sides of the equation ( H & O last) Add a coefficient in front of the chemical formula to adjust the count and record the new values Continue adding coefficients to obtain the same number of atoms of each element on both sides of the equation (change coefficient, then change counts)

Balancing Chemical Equations Add the subscript “2” to all diatomics that are ALONE in the equation (N, O, F, Cl, Br, I, H) (note: only time a subscript can & must be changed) Mg + O     MgO Count the number of atoms that are present in the basic equation and record Pick an element that is not balanced on both sides of the equation ( H & O last) C + Cl CCl4 Add a coefficient in front of the chemical formula to adjust the count and record the new values Continue adding coefficients to obtain the same number of atoms of each element on both sides of the equation (change coefficient, then change counts)

Building Equations (basic equation is given) Mg + O2 ------> MgO C + Cl2 ------> CCl4

Building w/Marshmallows Al + O2 Al2O3 Note: add like elements on the same side of the equation SO2 + O2 SO3

Balancing Chemical Equations Add the subscript “2” to all diatomics that are ALONE in the equation (N, O, F, Cl, Br, I, H) (note: only time a subscript can & must be changed) Al + O Al2O3 Count the number of atoms that are present in the basic equation and record Pick an element that is not balanced on both sides of the equation ( H & O last) SO2 + O SO3 Add a coefficient in front of the chemical formula to adjust the count and record the new values Continue adding coefficients to obtain the same number of atoms of each element on both sides of the equation (change coefficient, then change counts)

Classifying Chemical Equations SYNTHESIS – two or more substances combine to make a more complex substance AB + C ABC (synthesize means to put together) DECOMPOSTION - a complex substance breaks down into simpler substances ABC AB + C (decompose means to break down) REPLACEMENT – two elements in two different compounds trade places AB + CD AC + BD (replace means to find a new partner) 2 SO2 + O2 + 2 H2O 2 H2SO4 Synthesis of Acid Rain 2 H2O2 2 H2O + O2 Decomposition of Hydrogen Peroxide 2 CuO + C 2 Cu + CO2 Isolating Copper by Replacement

Synthesis, Decomposition, or Replacement