Chemistry Ch 9 Chemical Reactions Glencoe

Transcription

1 Chemistry Ch 9 Chemical Reactions Glencoe

2 Section 9.1 Reactions and Equations Chemical Reaction -is the process by which one or more substances are rearranged to form different substances. Five Evidences of chemical reaction (change): 1) release of heat or light, 2) color change, 3) odor change, 4) release of a gas, 5) formation of a solid (precipitate).

3 Reactants and Products Reactants are the starting substances in a chemical reaction. Products are the substances formed during a chemical reaction.

4 Word Equations Word Equation example: Aluminum and bromine react to produce aluminum bromide. Skeleton equations: -use chemical formulas to identify the reactants and products -indicate the state of the reactant or product (such as liquid, solid, gas, etc.) Know Table 9.1 symbols with meanings.

5 Chemical Equations Chemical Equations (balanced equation) -are a statement that uses chemical formulas and states of reactants and products and shows the identities and relative amounts (numbers/coefficients) of reactants and products.

6 Balancing Chemical Equations A Coefficient in a chemical equation is the number written in front of a reactant or product. -see p.285 Fig 9.5 for an example

7 Steps for Balancing Equations 1. Write the skeleton equation for the reaction. 2. Count the atoms of the elements in the reactants. 3. Count the atoms of the elements in the products. 4. Change the coefficients to make the number of atoms of each element equal on both sides of the equation. 5. Write the coefficients in their lowest possible ratio (then check your work). NOTE: see table 9.2

8 9.1 Balanced Equations Obey the Law of Conservation of Mass All chemical reactions obey the law that matter is neither created or destroyed (Law of Conservation of Matter). The number of atoms of each element are the same on both sides of a balanced equation.

9 9.2 Classifying Chemical Reactions There are five basic reaction types: A) Synthesis B) Decomposition C) Single Replacement (single displacement) D) Double Replacement (double displacement) E) Combustion

10 9.2 Synthesis Reaction In a synthesis reaction two or more reactants combine to form a product. A + B AB Two elements can combine to form a compound. One element and a compound can combine to form a compound.

11 9.2 Decomposition Reaction In a decomposition reaction a compound breaks down into two or more elements or compounds. AB A + B Decomposition often requires energy to initiate the reaction.

12 9.2 Single-Replacement Reaction In a single-replacement reaction one element replaces another element in a compound. A + BC AC + B Metals may replace hydrogen in water. Metals may replace another metal in a compound (depending upon their position in the activity series). Know how to use an activity series. A specific metal can replace any metal listed below it in the activity series. See p.293 Nonmetals can replace nonmetals based upon the same principle (using the activity series).

13 9.2 Double-replacement Reaction In a double-replacement reaction there is an exchange of ions in two compounds. AB + CD AD + CB Ex. CaCl 2 + NaF CaF 2 + NaCl In the example provided, the calcium ion exchanges places with the sodium ion to form two new compounds. Sometimes a product will be a precipitate (which is an insoluble or solid product). Refer to p.297 Table 9.3 for guidelines for writing double replacement reactions.

14 9.2 Combustion Reaction In a combustion reaction oxygen combines with another element/compound and produces heat and light. Ex. C x H y + O 2 CO 2 + H 2 O -NOTE: C x H y is a hydrocarbon (made up of hydrogen and carbon) Hydrogen and oxygen may combine to form water. This is both a combustion and a synthesis reaction.

15 9.3 Reactions in Aqueous Solutions An aqueous solution contains one or more substances (solutes) that are dissolved in water (the solvent which is the most abundant substance in the solution). Some compounds that dissolve are molecular and remain molecules when dissolved. Some molecular compounds form ions when dissolved in water they ionize (Ionization). Other molecular compounds form hydrogen ions when dissolved in water and are called acids.

16 Ionic Compounds in Solution When ionic compounds dissolve in water, their ions can separate a process called dissociation. Three products can form from double replacement reactions: a) a precipitate, b) water, or c) a gas.

17 9.3 Ionic Equations Complete Ionic Equations show all the particles in a solution as they exist. Net Ionic Equations only include the particles that participate in a reaction (precipitate, water, gas). Spectator Ions are those ions which do not participate in the reaction (they remain in solutions as aqueous ions). Net Ionic Equations are written from complete ionic equations by removing all spectator ions.

18 9.3 Reactions that form Precipitates See p.302 Example Problem 9.3 NOTE: those ions that are aqueous on both the reactant and product side of the reaction are not included in the net ionic equation because they are spectator ions.

19 9.3 Steps for Writing a Net Ionic Equation A) Write the skeleton equation B) Balance the skeleton equation (chemical equation) C) Show the ions of the reactants and products with their states (complete ionic) D) Cross out or remove the spectator ions E) Write the net ionic equation

20 9.3 Reactions that form Water Another type of double replacement reaction occurs in aqueous solution and produces water molecules. See p.304 Problem 9.4 HBr(aq) + NaOH(aq) H 2 O(l) + NaBr(aq) This reaction includes the reaction between an acid and a base. No evidence of a chemical reaction is observable because water is colorless and odorless and already makes up the solution.

21 9.3 Reactions that form Gas Double replacement reactions can form a gas (carbon dioxide, hydrogen cyanide or hydrogen sulfide).

22 Reaction between Vinegar and Baking Soda (sodium hydrogen carbonate) A double displacement reaction (also called a double decomposition or metathesis reaction) has the form: AB + CD A + BD where A, B, C, and D are atoms or ions. The reaction between vinegar and baking soda can be written as a double displacement reaction if carbonic acid (H 2 CO 3 ) is considered a product: HC 2 H 3 O 2 (aq) + NaHCO 3 (aq) NaC 2 H 3 O 2 (aq) + H 2 CO 3 (aq) Here A, B, C, and D are H +, C 2 H 3 O 2-, Na +, and HCO 3-, respectively. However, carbonic acid decomposes into carbon dioxide and water, H 2 CO 3 (aq) CO 2 (aq) + H 2 O() and the carbon dioxide can escape from the solution as a gas. The combined equation is HC 2 H 3 O 2 (aq) + NaHCO 3 (aq) NaC 2 H 3 O 2 (aq) + H 2 O() + CO 2 (g)

23 Reaction between Vinegar and Baking Soda (sodium hydrogen carbonate) A double displacement reaction (also called a double decomposition or metathesis reaction) has the form: AB + CD AD + CB where A, B, C, and D are atoms or ions. The reaction between vinegar and baking soda can be written as a double replacement reaction if carbonic acid (H 2 CO 3 ) is considered a product: HC 2 H 3 O 2 (aq) + NaHCO 3 (aq) NaC 2 H 3 O 2 (aq) + H 2 CO 3 (aq) Here A, B, C, and D are H +, C 2 H 3 O 2-, Na +, and HCO 3-, respectively. However, carbonic acid decomposes into carbon dioxide and water, H 2 CO 3 (aq) CO 2 (aq) + H 2 O() and the carbon dioxide can escape from the solution as a gas. The combined (overall) equation is HC 2 H 3 O 2 (aq) + NaHCO 3 (aq) NaC 2 H 3 O 2 (aq) + H 2 O() + CO 2 (g)

24 9.3 Overall Equations An overall equation is one that combines two reactions.

25 9.3 Important HCO 3 - Reactions Hydrogen ions react with bicarbonate (also called hydrogen carbonate) ions to produce carbon dioxide and water. H + + HCO3 - CO 2 + H 2 O This reaction is now occurring in the blood vessels in your lungs to remove carbon dioxide from your body. This reaction occurs in products with baking soda (sodium hydrogen carbonate). It helps whiten teeth and keeps your breath fresh. It helps baked goods rise. It is used as an antacid and in deodorants to absorb moisture and odors. It is used in cleaning and scrubbing. It is used in refrigerators to absorb odors and keep your refrigerator fresh.