1. Reaction Types and Chemical Equations Chemical Reactions

2. Describing a Chemical Reaction Indications of a Chemical Reaction Evolution of heat, light, and/or sound Evolution of heat, light, and/or sound Production of a gas Production of a gas Formation of a precipitate Formation of a precipitate Color change Color change

3. Signs of Chemical Reactions There are five main signs that indicate a chemical reaction has taken place: change in colorchange in odorproduction of new gases or vapor input or release of energy release input formation of precipitate

4. Chemical Reactions The starting substances turn into different substances by rearranging the atoms. The starting substances turn into different substances by rearranging the atoms. Mass is conserved in chemical reactions. Mass is conserved in chemical reactions. Starting substances = reactants Starting substances = reactants Ending substances = products Ending substances = products

5. 5 Major Types of Chemical Reactions Synthesis two or more substances combine to form one substance two or more substances combine to form one substance A + B → AB Decomposition one substance breaks down into 2 or more substances one substance breaks down into 2 or more substances AB → A + B

6. Replacement/Displacement Single Replacement one element switches places with an element in a compound one element switches places with an element in a compound A + BC → AC + B Double Replacement two elements in two compounds exchange places two elements in two compounds exchange places AB + CD → AD + CB

7. Combustion of a Hydrocarbon Combustion is the burning of a substance in the presence of oxygen. Combustion is the burning of a substance in the presence of oxygen. This reaction is very specific. This reaction is very specific. O 2 should always be a reactant. O 2 should always be a reactant. CO 2 and H 2 O should always be products. CO 2 and H 2 O should always be products. CH 4 + 2O 2 → CO 2 + 2H 2 O

8. Chemical Equations A chemical reaction can be described by a chemical equation. A chemical reaction can be described by a chemical equation. As mass is conserved in a chemical reaction, the amount of each element involved must be equal on both sides. As mass is conserved in a chemical reaction, the amount of each element involved must be equal on both sides. Example: Example:

9. The Law of Conservation of Mass Mass is neither created nor destroyed during a chemical reaction. Mass is conserved. The law was first shown to be true by French scientist Antoine Lavosier (1743- 1794) by making careful measurements with sensitive analytical balance. Equation: Mass reactants = Mass products

10. Skeleton Equation Uses chemical formulas, but is not balanced Uses chemical formulas, but is not balanced Na (s) + H 2 O (l) → NaOH (aq) + H 2(g) Na (s) + H 2 O (l) → NaOH (aq) + H 2(g) Subscripts in parentheses are states of matter. Subscripts in parentheses are states of matter. s = solidl = liquidg = gas aq = aqueous solution

11. Balancing Chemical Equations Add coefficients in front of the chemical formulas. The coefficients act as multipliers. Add coefficients in front of the chemical formulas. The coefficients act as multipliers. Never change the subscripts when balancing and equation! Never change the subscripts when balancing and equation! __ Na (s) + __ H 2 O (l) → __ NaOH (aq) + __H 2(g)

12. Laws about the Composition of Pure Substances Law of Definite Composition A given compound always contains the same, fixed ratio of elements. A given compound always contains the same, fixed ratio of elements. Law of Multiple Proportions Elements can combine in different ratios to form different compounds. Elements can combine in different ratios to form different compounds. If two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers. If two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers. Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

13. Pure Substances For example… Two different compounds, each has a definite composition. Carbon, C Oxygen, O Carbon monoxide, CO Carbon, C Oxygen, O Oxygen, O Carbon dioxide, CO 2 Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

14. Example Problem If 39.098 g of potassium reacts completely with 35.453 g of chlorine, what mass of potassium chloride is formed?

15. Example Problem Determine if compound I and II are the same compound. Compound Itotal mass = 76.00 g mass of Fe = 53.46 g mass of O = 22.54 g Compound IItotal mass = 56.00 g mass of Fe = 43.53 g mass of O = 12.47 g