Chapter 6, Section 2 Describing Chemical Reactions

Chemical Equations A short, easy way to show a chemical reaction Uses symbols instead of words An equation tells the substances you start with (reactants), and those you end up with (products) This would be read as, “Sodium plus chlorine “yields” sodium chloride reactants product

Reactant + reactant = product + product It is possible to have more than one product:

Conservation of Matter During a chemical reaction, matter is neither created or destroyed The total mass of the reactants must equal the total mass of the products Table Talk: If you want to measure all the matter before and after a reaction, what must you do?

Balancing Chemical Equations To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation It’s not THIS bad ;)

Steps to balancing a Chemical Equation 1.Write the equation with the correct formulas for the reactants and products: H 2 + O 2  H 2 O 2.Count the Atoms: there are 2 H atoms on the left and two on the right, but 2 O atoms on the left, but only one on the right. This is not a balanced equation 3.Use coefficients to balance atoms: 2 H 2 + O 2  2 H 2 O 4.Look back and check Table Talk: Why are the Hydrogen and Oxygen reactants written with subscripts?

Now you try it! 1.Write the equation Mg + O 2  MgO 2.Count the atoms 3.Use coefficients 4.Look back and check 2Mg + O 2  2MgO

Classifying Chemical Reactions There are 3 types of chemical reactions: 1.Synthesis = when two or more elements combine to make a more complex substance (ex: 2 H 2 + O 2  2 H 2 O) 2.Decomposition = When compounds break down into simpler products 3.Replacement = when one element replaces another in a compound or when two elements in different compounds trade places See BrainPop, “Chemical Equations” 4:21 video on balancing an equation end