Chemical Reactions Balancing Equations

n In order to show that mass is conserved during a reaction, a chemical equation must be balanced n You do this by changing the coefficients, the #’s before the formulas, NEVER change the subscripts

Balancing Steps 1. Write the unbalanced equation. 2. Count atoms on each side. 3. Change coefficients to make #s equal. Coefficient  subscript = # atoms 4. Reduce coefficients to lowest possible ratio, if necessary.

Counting With Moles n Because chemical reactions usually involve large #’s of small particles, chemists use a counting unit called the mole to measure amounts of substances n A mole is an amount of a substance that contains 6.02 X 10 to the 23 rd power particles of that substance (Avogadro’s #)

Molar Mass n A mole of carbon has a different mass than a mole of sulfur n The mass of one mole of a substance is called a molar mass

Molar Mass n For an element, the molar mass is the same as its atomic mass expressed in grams (example atomic mass for carbon is 12.0 amu, so the molar mass is 12.0 grams)

Molar Mass n For a compound, you add up the atomic masses of its component atoms, and than express them in grams

Mole Mass Conversions n Once you know the molar mass, you can convert moles of that substance into mass, or a mass of that substance into moles n For either calculation you need to express the molar mass as a conversion factor

Chemical Calculations n In chemical reactions, the mass of a reactant or product can be calculated by using a balanced chemical equation and molar masses of the reactants and products