Chemical Reactions Chapter 11

Describing Chemical Change: To understand chemical reactions we need to describe them in writing. Reactants vs. Products Reactants = those substances that are reacting or changing Products = those substances that are being formed from the reactants. Reactants Products Yields

Combination (Synthesis) Reactions Two or more substances combine to form a new compound. A + X  AX Must only have one product!!

Decomposition Reactions A single compound undergoes a reaction that produces two or more simpler substances AX  A + X Must only have one reactant!!

Single Replacement Reactions A + BX  AX + B BX + Y  BY + X Must have an element reacting with a compound MUST use activity series… more on this next week

Double Replacement Reactions The ions of two compounds exchange places in an aqueous solution to form two new compounds. AX + BY  AY + BX In order for this reaction to work, you must have a liquid, gas, or solid as a product. If not, it is not a reaction. More on this next week

Combustion Reactions The burning of natural gas, wood, gasoline Oxygen gas must be in the reactants. Carbon dioxide and water will be in the products. C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g)

Describing a Chemical Change: Remember: All chemical reactions must follow the law of conservation of mass What is this law?

Describing Chemical Change: Skeleton Equation: Does not indicate relative mole amounts Shows only the formulas of the reactants and the products Example: Fe + O2 Fe2O3 Not Balanced!! Can also be written as a word equation: Iron plus oxygen yields Iron (III) oxide

Describing a Chemical Change You can also indicate the state of each substance by using symbols to represent solid (s), liquid (l), gas (g) and aqueous (aq) Example: Fe(s) + O2 (g) Fe2O3(s)

Balancing Chemical Equations Balanced equations usually contain coefficients Coefficients = numbers placed in front of the symbol/formula for each substance Represents the number of moles Coefficients are always whole numbers and in the lowest whole number ratio

Balancing Chemical Equations: A balanced chemical equation has the same number of atoms of each element on both sides of the equation. Example: 2H2 + O2 2H2O Coefficients

Balancing Chemical Equations: Some helpful tips: 1. Solve the more complicated compounds first. Leave hydrogen and oxygen for the end. 2.Polyatomic ions present on both sides of the equation count as one unit. 3. When an element is present in 2 different spots on the same side, balance last.

Practice from worksheet 1. ___CaO + ___H2O  ___Ca(OH)2 14. ___Al + ___O2  ___Al2O3 27. ___AgNO3 + ___Ni  ___Ni(NO3)2 + ___Ag 35. ___Al I3 + ___HgCl2  ____AlCl3 + ____HgI2 48. ___CH4 + ___O2  ___CO2 + ___H2O 49. ___C2H6 + ___O2  ___CO2 + ___H2O