Law of Conservation of Mass

The Law of Conservation of Mass Chemist Antoine Lavoisier’s work in the 1700s resulted in the Law of Conservation of Mass. It states that: In a chemical reaction, the total mass of the products is always the same as the total mass of the reactants.

The Law of Conservation of Mass Reactant A + Reactant B  Product C + Product D 45 g 25 g 50 g ? Calcium + hydrochloric acid  calcium chloride + hydrogen gas 16 g 33 g 28 g ?

Consider the Electrolysis of Water H2O  H2 + O2 is NOT a balanced chemical equation. Why? Because the number of H and O atoms in the reactants does not equal the number of H and O atoms in the products. How can we make a balanced chemical equation?

Water  oxygen + hydrogen Example The electrolysis of water yields oxygen gas and hydrogen gas. Write the word equation Water  oxygen + hydrogen Write the skeleton equation H2O(l)  O2(g) + H2(g) Write the balanced chemical equation 2 H2O(l)  1 O2(g) + 2 H2(g)

REVIEW: How to Count Atoms The symbol of an element represents one atom of that element. E.g. Ca = 1 atom of calcium A subscript indicates more than one atom of the element. E.g. N2 = 2 atoms of nitrogen A subscript outside a bracket multiplies all the elements inside the brackets. E.g. Ba3(PO4)2 = 3 atoms of barium, 2 atoms of phosphorous and 8 atoms of oxygen

REVIEW: How to Count Atoms …cont’d A coefficient is a number written in front of a chemical symbol and indicates the number of atoms of that element. E.g. 3 C = 3 atoms of carbon A coefficient written in front of a chemical formula multiplies the number of atoms of each element in the formula. E.g. 4 Cu(NO3)2 = 4 atoms of copper, 8 atoms of nitrogen and 24 atoms of oxygen

Steps to Balancing Chemical Equations Write out the skeleton equation. Balance atoms that occur in the largest number first. Leave oxygen and hydrogen until the end. Balance polyatomic ions next (only if they are the same on both sides) and balance them as an ion unit (do not split into individual atoms). Balance hydrogen and oxygen last. Make sure you have the same number of each element on both sides of the equation.

Example Word equation: Methane + oxygen gas  water + carbon dioxide Skeleton equation: CH4(g) + O2(g)  H2O(g) + CO2(g) The chemical equation must obey the law of conservation of mass. Thus the total number of one type of atom must be the same on both sides of the equation. To ensure this one balances the equation by putting COEFFICIENTS in front of the substances. Balanced Chemical Equation: 1 CH4(g) + 2 O2(g)  2 H2O(g) + 1 CO2(g)

Rules for Balancing Chemical Equations We will work through a second example Word Equation Skeleton Equation Balanced Chemical Equation The rusting of iron (III)

More Examples Mg + HNO3  H2 + Mg(NO3)2 Na + H2O  NaOH + H2

Homework Read pg. 160-167 Define: reactant, product, chemical reaction, chemical equation, coefficient Pg. 164 #1-4 Pg. 165 #1-2