Chemical Reactions SC2. Obtain, evaluate and communicate information about how the Law of Conservation of Matter is used to determine chemical composition in compounds and chemical reactions. a. Use mathematics and computational thinking to balance chemical reactions (i.e., synthesis, decomposition, single replacement, double replacement, and combustion) and construct an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties. b. Plan and carry out an investigation to determine that a new chemical has been formed by identifying indicators of a chemical reaction (e.g., precipitate formation, gas evolution, color change, water production, and changes in energy to the system).

Chemical Reaction A chemical reaction occurs when one or more substances change into one or more new substances. Indicators of a chemical reaction include: Energy change (gain or loss of heat) Gas production (bubbles or odor) Precipitate production (liquids make a solid) Color change Water production

Chemical Equations REACTANTS  PRODUCTS A chemical reaction is represented in writing by a chemical equation. REACTANTS  PRODUCTS Multiple reactants and products are separated from each other by + signs Reactants are separated from products by a yield sign () Other notations include Δ, ↔, s, l, g, aq

Writing Chemical Equations Iron metal combines with oxygen gas in the presence of water to produce solid iron(III) oxide. Aqueous sodium hydroxide combines with aqueous copper(II) nitrate to form aqueous sodium nitrate and solid copper(II) hydroxide.

Reading Chemical Equations ZnS(s) + O2(g)  ZnO(s) + SO2(g) KClO3 (s)  KCl(s) + O2(g)

It’s the Law!

The Law Says… Law of Conservation of Matter: In a chemical reaction, matter can be neither created nor destroyed. In a chemical reaction, the amount of reactants equal the amount of products.

It’s the Law! How it applies to us now: Law of Conservation of Atoms – The number of atoms of each type of element must be the same on each side of the equation. This makes the equation BALANCED!

Matter Is Conserved Mg + Cl2 MgCl2 Total atoms = Total atoms Total Mass = Total Mass 1(24.3) + 2(35.5) 1(35.5 + 24.3 + 35.5) 95.3 g = 95.3 g

hydrogen + oxygen water Balancing Equations hydrogen + oxygen water H2 + O2 H2O FORMULAS MUST BE CORRECT! FORMULAS CANNOT BE CHANGED when you start balancing

Balancing Equations H2 + O2 H2O If the formulas cannot be changed, how can the atoms be made equal? Adjust the number of molecules by changing the coefficients.

Balancing Equations H2 + O2 H2O

Learning Check Fe3O4 + 4 H2 3 Fe + 4 H2O 1. Number of H atoms in 4 H2O A) 2 B) 4 C) 8 2. Number of O atoms in 4 H2O 3. Number of Fe atoms in Fe3O4 A) 1 B) 3 C) 4

Balancing Equations N2 + H2 NH3

Matter Is Conserved N2 + 3 H2 2 NH3 Total atoms = Total atoms Total Mass = Total Mass 2(14.0) + 6(1.0) 2[14.0 + (3x1.0)] 34.0 g = 34.0 g

Learning Check Balance each equation. The coefficients for each equation are read from left to right. 4. Mg + N2 Mg3N2 A) 1, 3, 2 B) 3, 1, 2 C) 3, 1, 1 5. Al + Cl2 AlCl3 A) 3, 3, 2 B) 1, 3, 1 C) 2, 3, 2

Balancing Equations Balancing Hints for Complex Eqns: Balance tallies with “1s” first. Balance the metals next. Balance the ion groups third. Save oxygen and hydrogen until last.

Balancing Equations Ca3(PO4)2 + H2SO4 CaSO4 + H3PO4

Balancing Equations NH3 + O2 NO + H2O

Balancing Equations Cu + H2SO4 CuSO4 + H2O + SO2

Learning Check 6. Fe2O3 + C Fe + CO2 A) 2, 3, 2, 3 B) 2, 3, 4, 3 C) 1, 1, 2, 3 7. Al + FeO Fe + Al2O3 A) 2, 3, 3, 1 B) 2, 1, 1, 1 C) 3, 3, 3, 1 8. Al + H2SO4 Al2(SO4)3 + H2 A) 3, 2, 1, 2 B) 2, 3, 1, 3 C) 2, 3, 2, 3

Types of Reactions There are six general types of reaction: Synthesis (also called combination) Decomposition Single replacement Double replacement Combustion Oxidation-reduction (REDOX)

Synthesis These reactions have two or more reactants that combine into a single product. A + B  AB Mg + O2  MgO Fe + S  Fe2S3

Decomposition These reactions have one reactant that breaks down into two or more products. AB  A + B HgO  Hg + O2 H2O  H2 + O2

Single Replacement A + BX  B + AX These reactions involve an elemental reactant that takes the place of another element in a compound. A + BX  B + AX K + H2O  KOH + H2 Zn + Cu(NO3)2  Cu + Zn(NO3)2

Single Replacement Y + BX  X + BY These reactions involve an elemental reactant that takes the place of another element in a compound. Y + BX  X + BY NaI + Br2  NaBr + I2 F2 + KCl  KF + Cl2

Single Replacement Not all combinations result in chemical reactions Successful replacement depends on where the exchanging elements are placed on the reactivity series

Reactivity Series More reactive elements will displace less reactive elements from compounds Hydrogen (below = unreactive with acids)

Double Replacement AY + BX  AX + BY These reactions involve aqueous reactant compounds that swap ions to produce other compounds. AY + BX  AX + BY Na2S + Cd(NO3)2  CdS + NaNO3 FeS + HCl  H2S + FeCl2

Double Replacement Neutralization and precipitation reactions are specific examples of double replacement reactions that can occur: Neutralization – when acids and bases combine to produce a salt and water NaOH + HCl  NaCl and H2O

Double Replacement SOLUBILITY RULES Precipitation – when solutions combine and a precipitate forms How can you predict if one will form? SOLUBILITY RULES

Solubility Rules NAG SAG PMS CastroBar Soluble: Nitrates Acetates Group 1 Exceptions: Sulfates ** * Lead, Mercury, Silver Ammonium * Calcium, Strontium, Barium Group 17 * NAG SAG PMS CastroBar

Combustion CH4 + O2  CO2 + H2O These reactions involve a fuel (typically a hydrocarbon) burning in oxygen gas to produce water and carbon dioxide. CH4 + O2  CO2 + H2O

Oxidation-Reduction If the oxidation states of any elements in a reaction change, the reaction is an oxidation-reduction reaction. Oxidation state refers to the charge on an atom if it were bonded ionically Oxidation states are identified for each element, per atom, in a compound with numbers like charges, but with sign first

Oxidation-Reduction Rules for Assigning Oxidation State: 1. Elements in a free, uncombined state, or as diatomic molecules, have an oxidation state of zero 2. Monatomic ion charge = oxidation state 3. Sum of all oxidation states in a neutral compound is zero; sum in a polyatomic ion is charge on ion

Oxidation-Reduction Rules for Assigning Oxidation State: 4. Oxidation state of elements in Group 1 = +1, Group 2 = +2 5. Oxidation state of oxygen is -2, unless as peroxide (-1) or with fluorine (+1) 6. Oxidation state of hydrogen is +1, unless as hydride (-1)

Oxidation-Reduction Rules for Assigning Oxidation State: 7. Oxidation state of fluorine is always -1; other halogens are usually -1, unless with oxygen or fluorine MnO2 + HCl → MnCl2 + Cl2 + H2O Al + Fe2O3 → Fe + Al2O3

Oxidation-Reduction Oxidation is the process by which an atom’s oxidation state increases For an atom’s “charge” to increase, what has to happen? So, loss of electrons = oxidation (LEO) Or, oxidation = loss of electrons (OIL) The atom that undergoes oxidation is called the reducing agent

Oxidation-Reduction Reduction is the process by which an atom’s oxidation state decreases For an atom’s “charge” to decrease, what has to happen? So, gain of electrons = reduction (GER) Or, reduction = gain of electrons (RIG) The atom that undergoes reduction is called the oxidizing agent

Oxidation-Reduction MnO2 + HCl → MnCl2 + Cl2 + H2O Al + Fe2O3 → Fe + Al2O3 H2O2 → O2 + H2O

Net Ionic Equations Net ionic equations represent what actually “happens” in a chemical reaction Only the species directly involved in the reaction are included Spectator ions are not included Any substance that is a solid, liquid, or gas remains “as is” in the net ionic equation

Net Ionic Equations To write a net ionic equation: Write the balanced molecular equation. Write the balanced complete ionic equation. Cross out the spectator ions that are present. Write the "leftovers" as the net ionic equation.

Net Ionic Equations (NH4)2CO3 (aq) + Al(NO3)3 (aq)  NH4NO3 (aq) + Al2(CO3)3 (s) NaOH (aq) + H2SO4 (aq)  Na2SO4 (aq) + H2O (l) Mg (s) + HCl (aq)  MgCl2 (aq) + H2 (g) Zn (s) + CuSO4 (aq)  ZnSO4 (aq) + Cu (s) Na2CO3 (aq) + HNO3 (aq)  NaNO3 (aq) + H2O (l) + CO2 (g)