CHEMICAL REACTIONS

Chemical Reactions A chemical reaction is a process in which the physical and chemical properties of the original substances change as new substances with different chemical and physical properties are formed. Remember: a new substance is always formed. The properties and energies of the substances always change when a chemical reaction takes place.

Characteristics A. There are always two kinds of substances: 1. Reactant - substance that enters into a chemical rxn 2. Product - substance produced by a chemical rxn. Reactants  Products

Characteristics B. There is always a change in energy in a chemical rxn. Exothermic - energy is given off. (Something that is burning or gives off light) Endothermic - energy is absorbed. (Photosynthesis or test tube gets colder)

Characteristics C. There is always activation energy Activation energy is the energy needed to form short-lived, highly energetic, extremely unstable, intermediate molecules that are rearranged to form products in the chemical rxn.

Characteristics D. Bonding Capacity In order for a chemical rxn to occur, the reactants must have the ability to combine with substances to form products. Must follow the rules for bonding. Remember: An atom bonds with another atom to complete its outermost energy level.

Characteristics E. Conservation of Mass Atoms can be neither created nor destroyed. The number of reactant atoms must equal the number of product atoms. This is important when balancing chemical equations.

BALANCING CHEMICAL EQUATIONS Chemical reactions involve a rearrangement of atoms. Chemical equations are expressions in symbols and formulas that represent a chemical reaction. EXAMPLE: Magnesium + Oxygen yields Magnesium Oxide + energy (word equation) Mg + O2  MgO + heat (skeleton equation) 2 Mg + O2  2 MgO + heat (balanced equation) A balanced chemical equation has the same number of atoms of each element on both sides of the equation.

RULES TO FOLLOW Change coefficients only. Never change a symbol, formula, or subscript. Place coefficients in front of the entire chemical formula.

STEPS TO BALANCING AN EQUATION Write the chemical equation for the reaction. Verify that the chemical symbols and formulas are correct.   Count the number of atoms of each element on both sides of the arrow. If the atom numbers are equal, then the equation is balanced. Balance the equation using coefficients. A coefficient is a whole number placed in front of a chemical formula to indicate the number of molecules present. Balance metals, then nonmetals, then hydrogen, then oxygen Look for the least common multiple (LCM). Check your work by recounting the number of atoms on each side of the arrow.

BALANCING EQUATIONS (Part One) 1. ______ Al + ______ O2  ______Al2O3 Al - O - Al - O -

DO NOW The Balancing Equations II handout is homework. Due tomorrow. You are not required to show your work of counting atoms, but if you get the problem wrong and have not shown your work, I cannot give partial credit.

Word Equations Write the correct formulas for each of the chemicals named. Determine which are the reactant(s) and which are the product(s). Write out the chemical equation for the reaction. Count the number of atoms of each element on both sides of the arrow. Balance the equation using coefficients Check your work by recounting the number of atoms on each side of the arrow.

Helpful Hints Symbols you should know: means “yields” (s) means solid Ag (s)   (g) means gas H2 (g) (l) means liquid H2O (l) (aq) means aqueous NaCl (aq) dissolved in water

Helpful Hints All metals are singular Non-noble gases are diatomic – travel in pairs. The diatomic gases are N2, O2, F2, Cl2, Br2, I2, H2

Practice 1. copper (II) hydroxide yields copper (II) oxide and water.   2. ammonia gas and hydrochloric acid yields ammonium chloride.

Practice 3. potassium metal and water yields potassium hydroxide and hydrogen gas. 4. propane (C3H8) and oxygen yields carbon dioxide and water.

Types of Chemical Reactions There are five basic types of chemical reactions. Not all reactions will take these five forms. Other classes of reactions will be considered later. net ionic oxidation-reduction neutralization

SYNTHESIS Reaction sometimes called Combination two or more simple substances combine to form a new, more complex substance. What to look for: one product EX. 2 Mg (s) + O2 (g)  2 MgO (s) 4 Fe (s) + 3 O2 (g)  2 Fe2O3 (s) metal corrosion due to oxidation

SYNTHESIS Reaction

DECOMPOSITION Reaction a complex substance breaks down into two or more simpler substances. What to look for: one reactant H2SO4 Ex. C12H22O11(s) 11 H2O(g) + 12 C(s) sucrose  water and carbon

DECOMPOSITION Reaction

COMBUSTION reaction Also called an oxidation reaction when compounds react with oxygen. What to look for: a compound and oxygen (reactants) which should yield two compounds. EX. Hydrocarbon + O2  water + carbon dioxide CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (g) 2 C4H10 (g) + 13 O2 (g)  8 CO2 (g) + 10 H2O (g)

COMBUSTION reaction According to your book, the examples below are called combustion reactions. We will call them SYNTHESIS. Metal + oxygen gas  metal oxide 4 Fe(s) + 3 O2 (g)  2 Fe2O3 (s) Nonmetal + oxygen gas  nonmetal oxide 2 H2 (g) + O2 (g)  2 H2O (l)

SINGLE REPLACEMENT Rxn Also called single displacement reaction one element replaces another element in a compound. Use the activity series of metals to determine if the metal can be replaced or not. What to look for: uncombined elements on both sides of the arrow

SINGLE REPLACEMENT Rxn

ACTIVITY SERIES

SINGLE REPLACEMENT A. An active metal will displace the metallic ion in a compound of a less active metal. Need to look at activity series. EX. Cu (s) + AgNO3 (aq)  Cu(NO3) 2 (aq) + Ag (s)   B. Some active metals (all alkali metals and some alkaline earth metals) will react with water to produce a metallic hydroxide and hydrogen gas. EX. 2 Na (s) + 2 H2O (l)  2 NaOH (aq) + H2 (g)

SINGLE REPLACEMENT C. Some active metals, such as Al, Zn, and Fe, will displace the hydrogen in acids to give a salt and hydrogen gas. EX. Mg (s) + 2 HCl (aq)  MgCl2 (aq) + H2 (g)   D. Halogens (active nonmetals) will displace less active halogens. Fluorine is the most reactive, iodine is the least. EX. Cl2 (g) + 2 NaBr (aq)  2NaCl (aq) + Br2 (g)

DOUBLE REPLACEMENT Rxn Also called double displacement reaction different atoms in two different compounds replace each other. two compounds react to form two new compounds – either a gas, precipitate, or water What to look for: two compounds on each side Ex. HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l) NaCl (aq) + AgNO3 (aq)  2 NaNO3 (aq) + AgCl (s)

DOUBLE REPLACEMENT Rxn

Practice 1. Br2 + NaI  NaBr + I2 2. SO3 + H2O  H2SO4 3. Zn + HCl  ZnCl2 + H2 4. Ga2O3  Ga + O2 5. H2O2  H2O + O2 6. Ba(NO3) 2 + Na2SO4  BaSO4 + NaNO3 7. P2O5 + BaO  Ba3(PO4) 2 8. C4H8 + O2  CO2 + H2O 9. PdCl2 + HNO3  Pd(NO3)2 + HCl 10. C2H6 + O2  CO2 + H2O

DO THESE ON SCRAP PAPER Classify the following reactions: 2 Ag + S  Ag2S C4H8 + 6O2  4CO2 + 4H2O 2K + 2H2O  2KOH + H2 HCl + NaOH  H2O + NaCl

AQUEOUS SOLUTIONS A lot of reactions take place in water. Whenever something is dissolved in water, it is called AQUEOUS. There are a variety of solutes that can be dissolved in water. Most chemical reactions involving aqueous solutions end up with a precipitate, gas, or water as one of the products. In order to determine if a reaction will produce a precipitate, you must be familiar with the solubility rules.

SOLUBILITY RULES All common salts of the group 1A elements and ammonium are soluble. All common acetates and nitrates are soluble. All binary compounds of group 7A elements (other than F) with metals are soluble except those of silver, mercury (I), and lead. All sulfates are soluble except those of barium, strontium, lead, calcium, silver, and mercury (I) Except for those in rule 1, carbonates, hydroxides, oxides, sulfides, and phosphates are insoluble. See a more complete table, pages 920

DETERMINING SOLUBILITY - Use your solubility rules to determine whether or not a precipitate forms when the solutions listed below are mixed. - Write the formulas for the PRODUCTS. In the last blank, write the formula for the precipitate if one is formed, or write NP if no precipitate is formed. Potential Products Solid Formula/NP 1. NaBr + AgNO3  ________________ _______ 2. Ca(C2H3O2)2 + FeCl3 ________________ _______ 3. H2SO4 + Hg2(NO3)2 ________________ _______

NET IONIC EQUATIONS A chemical equation that involves compounds dissolved in water is written differently than a balanced chemical equation. It is called a NET IONIC EQUATION. This is an equation that shows all the soluble ionic solutions as ions. Original reaction: Cu(NO3)2 + 2NaOH  Cu(OH)2 (s) + 2 NaNO3   Reaction with ions: Cu+2 + 2 NO3-1 + 2 Na+1 + 2 OH-1  Cu(OH)2 (s) + 2 Na+1 + 2 NO3-1 (all are aqueous)

NET IONIC EQUATIONS There are ions that do not take part in a chemical reaction and are found in solution before the reaction and after. These ions are called SPECTATOR IONS. They do not participate. (Na+1 and NO3-1 in the above example). A net ionic equation includes only those compounds and ions that undergo a chemical change in an aqueous solution. Write only those ions that take part in the reaction. Reaction with ions: Cu+2 + 2 NO3-1 + 2 Na+1 + 2 OH-1  Cu(OH)2 (s) + 2 Na+1 + 2 NO3-1 Net ionic reaction: Cu+2 (aq) + 2 OH-1 (aq)  Cu(OH)2 (s)

NET IONIC EQUATIONS Binary acids: Strong - HCl, HBr, HI - Change into ions. All others are weak (including HCN) and written in molecular form. Ternary acids: Strong - the number of oxygen atoms exceeds the number of hydrogen atoms by at least two - change into ions. All organic carboxylic acids are weak (acetic). Polyprotic acids: (These acids contain more than one ionizable hydrogen.) The second ionization and any ones that follow are weak. These acids lose one H+ at a time. Example: H2SO4

NET IONIC EQUATIONS Bases: Strong - hydroxides of group (IA) and (IIA) elements (exception: beryllium) – change into ions. All others are weak. Salts: Written in ionic form if soluble and molecular form if insoluble. See solubility rules, page 920. Oxides: Always written in molecular form. (ex. BaO, H2O) Gases: Always written in molecular form. (Ex. H2, Cl2)

NET IONIC EQUATIONS So there basically are four categories. When you look at an equation, you need to determine if the formula is: Molecular (two nonmetals bonded together) Ionic (metal + nonmetals; contains polyatomic ions) Acid (starts with an H; not H2O or H2O2) Base (some hydroxides)

PRACTICE 2AgNO3 + H2SO4  Ag2SO4 + 2HNO3 4 HCl + 2Cr(NO3)2 + 2HgCl2  2 CrCl3 + Hg2Cl2 + 4 HNO3 H2SiO3 + 2NaOH  Na2SiO3 + 2H2O 2HCl + Ba(OH)2  BaCl2 + 2H2O

PREDICTING PRODUCTS One of the five sections on the AP test is on equations We will learn to predict the products of basic equations only. You will learn much more in AP.

DECOMPOSITION will have one reactant and multiple products  1. A compound may break down to form two elements. NaCl    2. A compound make break down to produce an element and a compound. H2O2  3. A compound may break down to produce two compounds.  MgCO3 

SYNTHESIS will have multiple reactants and one product   1. Group IA or IIA metals combine with a nonmetal to make a salt  Li + N2  2. Two nonmetals combine to form a molecular compound.   P4 + 6Cl2  3. Two compounds combine to form a single compound. SO2 + CaO 

SINGLE REPLACEMENT one element displaces another in a compound MUST USE ACTIVITY SERIES   1. Active metals replace less active metals or hydrogen from their compounds in an aqueous solution. Use an activity series to determine if it will replace something in the compound. Mg (s) + FeCl3(aq)  2. Active nonmetals replace less active nonmetals from their compounds in an aqueous solution.   Cl2(q) + KI(aq) 

DOUBLE REPLACEMENT Two compounds react to form two new compounds; all double replacement reactions must have a driving force that removes a pair of ions from solution. MUST USE THE SOLUBILITY RULES 1. Formation of a precipitate – a precipitate is an insoluble substance formed by the reaction of two aqueous substances. Two ions bond together so tightly that water cannot pull them apart. Know the solubility rules. KI (aq) + Pb (NO3)2 (aq) 

DOUBLE REPLACEMENT 2. Formation of a gas – gases may form directly in a double replacement reaction or can form from the decomposition of a product such as carbonic acid. HCl(aq) + K2CO3(aq)  3. Formation of a Molecular Substance – when a molecular substance such as water is formed, ions are removed from solution to form a molecular substance and the reaction “works”. LiOH(aq) + HBr(aq) 

COMBUSTION hydrocarbons combine with oxygen gas to form carbon dioxide gas and water vapor. CH4 (g) + O2 (g) 