Chemical Reactions Vocabulary

Transcription

1 Chemical Reactions Vocabulary Chemical Reaction Chemical Equation Process in which the physical and chemical properties of the original substances change as a new substance with different physical and chemical properties is formed. An expression in which symbols and formulas are used to represent a chemical reaction(rxn) Reactant Substances entering into a chemical rxn. (What you start with) Product Substances produced by a chemical rxn. ( What you end with) Exothermic Rxn that produces heat energy in the form of light, heat or both. (Releases heat) Endothermic Rxn that requires heat, absorbs heat, feels cool. Activation Energy Energy needed to start a chemical rxn. Catalyst Substances that usually decrease the activation energy needed to begin a chemical rxn. It is not affected by the reaction or used up in the rxn. Enzymes are an example.

2 Word and Formula Equations I. Word equation a. an equation in which the reactants and products in a chemical reaction are represented by words. EX: methane and oxygen react to yield carbon dioxide and water II. III. Formula equation a. Represents the reactants and products of a chemical reaction by their symbols or formulas. EX: CH 4 (g) + O 2 (g) CO 2 (g) + H 2 O(g) (not balanced) Symbols used in Chemical Reactions Yields Reversible reaction (s) Solid state / precipitate (l) Liquid state (g) Gaseous state (aq) Aqueous state (dissolved in water) Reactants are heated MnO 2 Reaction uses a catalyst Law of conservation of mass The total mass of the products must equal the total mass of the reactants (must have the same # of atoms of each element in the products as there are/were in the reactants)

3 Balancing Chemical Equations Rules: 1.Determine the correct formulas for all the reactants and products in the reaction. 2.Write the formulas for the reactants on the left and the formulas for the products on the right with an arrow in between. If two or more reactants or products are involved, separate their formulas with plus signs. 3.Once the correct skeleton equation is written balancing will begin. A polyatomic ion appearing unchanged on both sides of the equation is counted as a single unit (this is where we always begin!) 4.Balance the elements one at a time using coefficients. When no coefficient is written, it is assumed to be 1. After the polyatomic ions, it is best to begin with an element other than hydrogen or oxygen. These two elements often occur more than twice in an equation. DO NOT attempt to balance an equation by changing the subscripts in the chemical formula of a substance! 5.Check each atom or polyatomic ion to be sure that the equation is balanced.

4 6.Make sure all coefficients are in the lowest possible ratio. Types of Chemical Reactions I. Combination Reaction (Synthesis) a. Reactants: generally 2 elements, or 2 cpds b. Products: a single cpd EX: 2Mg + O 2 2MgO II. Decomposition Reaction a. Reactants: generally 1 binary or ternary cpd b. Products: 2 elements (for binary) or 2+ elements and/or cpds for ternary) EX: 2H 2 O 2 2H 2 O + O 2 III. Single-Replacement Reaction a. Reactants: an element and a cpd b. Products: a different element and a new cpd. c. An element replaces another element from a cpd in an aqueous solution d. The element that is displaced must be less active than the element that is doing the displacing. EX: Zn + 2HCl ZnCl 2 + H 2 IV. Double-Replacement Reaction a. Reactants: 2 ionic cpds i. 2 ionic cpds react by exchanging cations to form two different cpds b. Products: 2 new cpds i. Driven by the formation of a precipitate, gaseous product, or water.

5 EX: K 2 CO 3 + BaCl 2 2KCl + BaCO 3 (s) V. Combustion Reaction a. Reactants: O 2 and a cpd of C, H, O (usually any hydrocarbon with oxygen) b. Products: CO 2 and H 2 O c. Complete combustion produced in the presence of an adequate oxygen supply EX: CH 4 + 2O 2 CO 2 + 2H 2 O d. Incomplete combustion produced when oxygen is deficient; two products formed are CO and H 2 O EX: 2CH 4 + 3O 2 2CO + 4H 2 O DIATOMIC MOLECULES (know these) Molecules in which the element exists as a pair of the original element ex. Oxygen O 2. When you see them named in reactions they probably need to be treated as diatomics, here are some of the elements that normally exist as diatomics: Hydrogen H 2 Nitrogen N 2 Oxygen O 2 Fluorine F 2 Chlorine Cl 2 Bromine Br 2 Iodine I 2

6 Net ionic equations Net ionic equations- reactions that occur in an aqueous solution (dissolved in water). DR or SR rxns that occur between cpds in water. (at least one is aqueous) A. Rxns are often represented by writing their formula units. AgNO 3 (aq) + NaCl(aq) AgCl(s)+NaNO 3 (aq) B. Most ionic compounds dissociate, or separate into cations and anions when dissolved in water. C. Realistically this type of rxn should be initially written as a complete ionic equation. A reaction written showing each individual ion with its charge, separated in the (aq) solution. Ag + (aq) + NO 3 - (aq) + Na + (aq) + Cl - (aq) AgCl(s) + Na + (aq) + NO 3 - (aq) This is called a complete ionic equation: This equation can be simplified and made more useful by eliminating the ions that do NOT participate in the rxn. Cancel the free ions that appear on both sides of the equation. D. Spectator ions-ions that are not directly involved in a rxn. ( the ions that are cancelled) E. Net ionic equation-the equation that indicates only those particles that take part in the rxn. Ag + (aq) + Cl - (aq) AgCl(s) In writing net ionic equations, you must also balance the ionic charge. In the previous ex. the net ionic charge is zero and therefore balanced. This may not be the case for other rxns. Consider the following rxn. Pb(s) + AgNO 3 (aq) Ag(s) + Pb(NO 3 ) 2 (aq)