Stoichiometry & Balancing Equations Remember we stated in the previous chapter that stoichiometry is the study of the quantitative relationships between the amounts of reactants and products in chemical reactions. We use BALANCED equations to understand stoichiometric relationships of the elements and compounds within a chemical reaction.
The Balanced Equation 2Al (s) + 3Br 2(l) Al 2 Br 6(s) 2 mol of Al : 3 mol of Br 2 : 1 mol of Al 2 Br 6 2 atoms of Al = 2 atoms of Al 6 atoms of Br = 6 atoms of Br The number of the same atom of each element must be equal on each side of the equation.
A Closer Look at the Equation 2Al (s) + 3Br 2(l) Al 2 Br 6(s) The chemicals on the left are the reactants and the right are the products. The coefficient in front of the chemical denotes the stoichiometric relationship. The numerical subscript represents the number of atoms present in the molecule. The letter subscripted denotes the phase of matter.
Balancing Equations For example the following is balanced. CH 4 + 2O 2 CO 2 + 2H 2 O Try to balance the following: Fe 2 S 3 + O 2 Fe 2 O 3 + S Al + H 2 SO 4 Al 2 (SO 4 ) 3 + H 2 Ca + Al 2 Br 6 CaBr 2 + Al
Balanced Equations Check your answer from the previous slide: 2Fe 2 S 3 + 3O 2 2Fe 2 O 3 + 6S 2Al + 3H 2 SO 4 Al 2 (SO 4 ) 3 + 3H 2 3Ca + Al 2 Br 6 3CaBr 2 + 2Al
Combination Reactions A combination reaction is a reaction where two substances chemically combine to form another substance. A + B AB 2Na (s) + Cl 2 (g) 2NaCl (s) P 4 (s) + 6Cl 2 (g) 4PCl 3 (s)
Decomposition Reaction A decomposition reaction is when a single compound decomposes into two or more other substances. AB A + B 2KClO 3 (s) 2KCl (s) + 3O 2 (g) 2NO 2 (g) 2NO (g) + O 2 (g)
Displacement Reaction A displacement (single replacement) reaction is a reaction where one element displaces another element. A + BC B + AC Zn (s) + 2HCl (aq) ZnCl 2 (aq) + H 2 (g) Cu (s) + 2AgNO 3 (aq) Cu(NO 3 ) 2 (aq) + 2Ag (s)
Metathesis Reaction A metathesis (double replacement) reaction is a reaction where two compounds switch cations to form two new compounds. A + B - + C + D - A + D - + C + B - CaCl 2 (aq) + Na 2 CO 3 (aq) CaCO 3 (s) + 2NaCl (aq) AgNO 3(aq) + KCl (aq) ???
SOLUBILITY Solubility – the amount of a substance that can be dissolved in a given quantity of solvent (like water) at a specific temperature Unsaturated – amount of substance less than saturated Saturated – the exact amount at solubility Supersaturated – excess amount of substance
How Solubility Influences Rxn When a substance is soluble in water, it will appear with a subscript of (aq) meaning that the substance is broken up into its ions incorporated into the water lattice. When a substance is insoluble in water, it will be written with a subscript of (s), (l), or (g) and will precipitate out of solution.
Solubility of Ionic Compounds in Water Sodium, potassium, and ammonium compounds Acetates and nitrates Halides (chlorides, bromides,Lead(II), silver, and mercury (I) and iodides)halides are insoluble SulfatesCalcium, strontium, barium, and Lead(II) sulfates are insoluble Soluble Compounds Exception s
Insolubility of Ionic Compounds in Water Carbonates and phosphatesSodium, potassium, and ammonium compounds are soluble. HydroxidesSodium, potassium, calcium, strontium, and barium compounds are soluble SulfideSodium, potassium, calcium, and ammonium compounds are soluble Insoluble CompoundsExceptions
A Look at Metathesis Again Looking back at slide 10 to the first reaction: when the cations rearranged, the CaCO 3 being insoluble by our definition is recorded as CaCO 3 (s). The CaCO 3 would precipitate out of solution as a solid. Looking at AgNO 3(aq) + KCl (aq) ?, we can rearrange the cations and conclude that the AgCl is a solid and will precipitate out of solution
Types of Metathesis Reactions Three classifications of metathesis reactions –Precipitation reaction - formation of a solid Pb(NO 3 ) 2 (aq) + Na 2 CO 3 (aq) PbCO 3 (s) + 2NaNO 3 (aq) –Neutralization reaction - formation of water HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O (l) –Gas formation reaction - CO 2, H 2 S, SO x, & NO x are typically formed
Precipitation Reactions A solid precipitate is produced in the rearrangement of cations as follows: Pb(NO 3 ) 2 (aq) + Na 2 CO 3 (aq) PbCO 3 (s) + 2NaNO 3 (aq) The Ionic Equation is expressed as: Pb +2 + 2NO 3 - + 2Na + + CO 3 -2 PbCO 3 (s) + 2Na + + 2NO 3 - After neglecting the spectator ions, the net ionic equation will look like: Pb +2 (aq) + CO 3 -2 (aq) PbCO 3 (s)
Reviewing Ionic Compounds Ca +2 + 2Cl - CaCl 2 Each ion comes together based on charge to form an overall neutral ionic compound. 3Ca +2 + 2PO 4 -3 Ca 3 (PO 4 ) 2 The cation and the polyatomic ion come together based on charge to form an overall neutral ionic compound.
Net Ionic Equations (NIE) If you were given the reactants Ca(NO 3 ) 2 and Na 3 PO 4 you should be able to predict the precipitate and write a balance equation, the ionic equation, and the net ionic equation (NIE) for this reaction. The NIE for these reactants is as follows: 3Ca +2 (aq) + 2PO 4 -3 (aq) Ca 3 (PO 4 ) 2(s)
Common Polyatomic Ions carbonate ion CO 3 -2 sulfate ion SO 4 -2 sulfite ion SO 3 -2 hydroxide OH - phosphate PO 4 -3 permanganate MnO 4 - chromate CrO 4 -2 dichromate Cr 2 O 7 -2 ammonium NH 4 + oxalate C 2 O 4 -2 bicarbonate HCO 3 - cyanide ion CN - acetate C 2 H 3 O 3 -
Neutralization Reaction A neutralization reaction is a reaction that occurs between an acid and a base with the production of a salt and water. HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O (l) acid base salt water
Gas Formation Reaction A gas formation reaction is a metathesis reaction that generates a gas as a product. –Metal carbonates or bicarbonates + acid –Metal sulfides + acid –Metal sulfites + acid –Ammonium salts and strong base
Metal Carbonates Metal carbonates or bicarbonates when combined with an acid form salt, water and carbon dioxide gas. Na 2 CO 3(aq) + 2HCl (aq) 2NaCl (aq) + H 2 O (l) + CO 2(g) –Where CO 2 gas is given off
Metal Sulfides Metal sulfides when combined with an acid form salt and hydrogen sulfide gas. Na 2 S (aq) + 2HCl (aq) 2NaCl (aq) + H 2 S (g) –Where H 2 S gas is given off
Metal Sulfites Metal sulfites when combined with an acid form salt, water, and sulfur dioxide gas. Na 2 SO 3(aq) + 2HCl (aq) 2NaCl (aq) + H 2 O (l) + SO 2(g) –Where SO 2 gas is given off
Ammonium Salts Ammonium salts when combined with a base produce salt, water and ammonia. NH 4 Cl (aq) + NaOH (aq) NaCl (aq) + H 2 O (l) + NH 3(g) –Where ammonia gas is given off
Combustion Reactions A combustion reaction is a reaction with molecular oxygen to form products in which all elements are combined with oxygen. CH 4 + 2O 2 CO 2 + 2H 2 O
Limiting Reactants One of the reactants is in limited supply and thus restricts the amount of product formed. Think of it as: If you wanted to bake a batch of peanut butter cookies and the recipe calls for 1 cup of peanut butter and all you have is ½ a cup, even though you have all the other ingredients, you can at most make ½ a batch of cookies.
Limiting Reactants (cont.) Consider the combustion reaction: CH 4 + 2O 2 CO 2 + 2H 2 O How much CO 2 can be produced if you have 0.13g of methane and 0.45g of O 2 ?
Percent Yield The maximum amount of product that can be obtained from a chemical reaction is the theoretical yield. The actual amount produced in a chemical process is the actual yield. The percent yield is equal to the actual yield divided by the theoretical yield times 100%.
Redox Reactions Oxidation of an element takes place when electrons are lost from the valence shell of the element. Reduction of an element takes place when electrons are added to the valence shell of the element. Redox reactions show the transfer of electrons that takes place during oxidation and reduction.
Redox Reactions (cont.) All oxidation and reduction reactions involve transfer of electrons between substances. View CD-ROM screen 5.12 Ag+ accepts electrons for Cu and is reduced to Ag and Cu loses electrons to Ag+ and is oxidized to Cu+2 in the following redox rxn: 2Ag + (aq) + Cu (s) 2Ag (s) + Cu +2 (aq)
Redox Reactions (cont.) The oxidation half reaction is : Cu (s) – 2e - Cu +2 (aq) The reduction half reaction is: 2Ag + (aq) + 2e - 2Ag (s) Cu is called the reducing agent because it caused Ag + to be reduced; and Ag + is called the oxidizing agent because it caused Cu to be oxidized.
Determining Oxidation Numbers Each atom in a pure element has an oxidation number of zero. For monoatomic ions, the ox. number is equal to its ionic charge. F is always –1, other halogens are –1 as well except with oxygen or fluorine. The ox. number for H is +1 except with hydrides (CaH 2 ).and O is –2 except with peroxides (H 2 O 2 ). The ox.# must = 0 for a compound or = to the overall charge of polyatomic ion being considered.
Balancing Redox Reactions We can use the balance of electrons transferred in a redox reaction to help us balance the overall equation. Consider the unbalanced equation: Zn (s) + HCl (aq) ZnCl 2 (aq) + H 2 (g) The balanced equation takes into consideration the oxidation of the Zn and the reduction of the H +. Zn (s) + 2HCl (aq) ZnCl 2 (aq) + H 2 (g)
Molarity Molarity = Moles of Solute Liters of Total Solution Symbol for molarity is M Units are moles/Liter
Solution Preparation To prepare a 1.0M solution of NaCl, you would determine how many grams of NaCl is contained in 1.0 moles of NaCl and then dissolve that amount in a 1.0L volumetric flask. You would then qs with distilled H 2 O. 1.0M NaCl = 1 mole (or 58.44g) NaCl 1.0L of solution How much NaCl would you use to make a 0.1M solution of NaCl? As 1/10 of a mole = 5.844g NaCl, you would dissolve 0.1mole (5.844g) of NaCl in 1.0L of solution.
Acids An acid is defined as follows: Arrhenius – releases H + when dissolved in H 2 O Bronsted-Lowrey – a substance that can donate a proton to another substance Lewis – a substance that can accept a pair of electrons from another atom to form a new bond
Bases A base is defined as follows: Arrhenius – releases OH - when dissolved in H 2 O Bronsted-Lowrey – a substance that can accept a proton from another substance Lewis – a substance that can donate a pair of electrons to another atom to form a new bond
pH and Concentrations of Acids and Bases pH = -log [H + ] 1 – acidic – 7 – basic – 14 When dealing with [H + ] less than 0.1M (pH=1), we use activity coefficients instead of pH.
pH of Household Items pH of vinegar = 2.80 pH of soda = 2.90 pH of orange juice = 3.80 pH of pure water = 7.00 pH of blood = 7.40 pH of ammonia = 11.00 pH of oven cleaner = 11.7
Titration A method for quantitative analysis of a substance by essentially complete reaction in solution with a measured quantity of a reagent of known concentration. Often used in redox reactions –Many redox reactions go rapidly to completion in aqueous media to determine the equivalency point. Typically used for neutralization reactions. –Acid is titrated with a base using an indicator to determine the equivalency point of the neutralization reactions.