Chemical Reactions Chapter 7 Pg. 192-219

Describing Reactions Chapter 7 Section 1 Pg. 192-198

Chemical Equations Reactants- the substances that undergo change in a chemical reaction Products- the new substances formed as a result of that change Chemical equation- a representation of a chemical reaction in which the reactants and products are expressed as formulas.

Chemical Equations Example:

Conservation of Mass The law of conservation of mass states that mass in neither created nor destroyed in a chemical reaction. During chemical reactions, the mass of the products is always equal to the mass of the reactants. Number of atoms on the left side of the equation equals the number of atoms on the right.

Balancing Equations In order to show that mass is conserved during a reaction, a chemical equation must be balanced. Balance a chemical equation by changing the coefficients (the number that appears before the formulas) Never change the subscripts in a formula when balancing equations.

Balancing Equations Step 1: count the number of atoms of each element on each side of the equation Step 2: change one or more coefficients until the equation is balanced CH4 + O2 CO2 + H2O unbalanced C = 1 C = 1 H = 4 H = 2 O = 2 O = 3 balanced

Balancing Equations H2 + O2  H2O H2 + N2  NH3 Al2O3  Al + O2

Counting with Moles Because chemical reactions often involve large numbers of small particles, chemists use a counting unit called the mole to measure amounts of a substance. A mole (mol) is an amount of a substance that contains approximately 6.02 x 1023 particles (atoms, molecules, or ions) of that substance. This number is known as Avogadro’s number.

Counting with Moles Molar Mass The mass of one mole of a substance The molar mass is the same as its atomic mass expressed in grams. (Ex: C is 12amu, so molar mass of C is 12 g) A CO2 molecules is composed of one C atom (12 amu) and 2 O atoms (2 x 16amu = 32amu). So CO2 has a molar mass of 44 grams. (add the two amu totals together)

Counting with Moles Mole-Mass Conversions Once you know the molar mass, you can convert moles of that substance into mass or vice versa. Example: the molar mass of CO2 is 44 grams, which means that one mole of CO2 has a mass of grams. This relationship yields the following conversion factors. 44 g CO2 1mol CO2 1mol CO2 44g CO2

Counting with Moles Mole-Mass Conversion (continued) Suppose you have 55 grams of CO2. To calculate how many moles of CO2 you have, multiply the mass by the conversion factor. 55g CO2 X 1mol CO2 = 1.25 mol CO2 44g CO2 You can check your answer by using the other conversion factor 1.25mol CO2 X 44g CO2 = 55g CO2 1mol CO2