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Balancing ANY chemical Equation Elena Man By the end of this workshop you will be able to: 1) How to write chemical equations based on written descriptions 2) How to consistently and efficiently balance chemical equations 3) How to identify different types of reactions 4) How to predict products of chemical reactions 5) How to write balanced net ionic equations 2 Chemical Reactions: A new substance is created. Bonds that hold the atoms together are broken and new bonds form. Indications of a Chemical Reaction: 1. Production of Heat and/or Light 2. Production of a Gas ( CO2 , H2 ) 3. Precipitate (solid in a liquid) forms. We use balanced chemical equations to REPRESENT a chemical reaction. Balanced: EQUAL NUMBERS of atoms for each element involved in the reaction is represented on the reactant and product sides. This is a requirement the equation must satisfy to be consistent with the law of conservation of matter. Law of Conservation of Matter : Matter (atoms) is neither created nor destroyed. 3 How to write chemical equations based on written descriptions 1- Read each statement 2- Write the correct formula for the substance indicated 3- Replace each word “and”, “combine” or “plus” with the symbol “+” 4- Replace each phrase indicating change (make, produces, yield, etc.) with and arrow pointing toward the right. 5- Balance each equation 4 Additional information in Chemical Equations • The physical states of reactants and products in chemical equations very often are indicated with a parenthetical abbreviation following the formulas. Common abbreviations include s for solids, l for liquids, g for gases, and aq for substances dissolved in water. 2Na(s) + 2H2O(l)⟶2NaOH(aq) + H2(g) 5 Examples of writing equations from descriptions Lead (II) nitrate and potassium carbonate produce lead (II) carbonate and potassium nitrate: Pb(NO3)2 + K2CO3 → PbCO3 + KNO3 We need to balance this equation now. Pb(NO3)2 + K2CO3 → PbCO3 + 2KNO3 Potassium chlorate decomposes into potassium chloride and oxygen KClO3 → KCl + O2 We need to balance this equation now. 2KClO3 → 2KCl + 3O2 Gas ammonia is produced when nitrogen gas combines with hydrogen gas. N2 + H2 → NH3 We need to balance this equation now. N2 + 3H2 → 2NH3 Let’s look at our workshop outline and check our learning….(1) 6 How to balance ANY chemical equation: 𝐶𝑢(𝑠) + 𝐻𝑁𝑂3(𝑎𝑞) → 𝐶𝑢(𝑁𝑂3 )2 𝑎𝑞 + 𝑁𝑂2 𝑔 + 𝐻2 𝑂(𝑙) • Step 1: List out the elements present on BOTH the PRODUCT and REACTANT sides of the equation. Then, count how many of each atom are present initially. • Step 2: Determine which element to balance first. You want to balance the easiest elements first. The easiest elements to balance are the ones that appear in the fewest substances in the equation. Therefore, to determine balancing order, count how many substances each element appears in. 7 Remember, if at any point one of the easier elements to balance becomes unbalanced, rebalance that element before continuing to the harder elements. • Step 3: Only change coefficients (the numbers in front of the substance), NEVER CHANGE SUBSCRIPTS. • Step 4: When trying to balance an odd number of atoms on one side with an even number of atoms on the other side DOUBLE the coefficient of the compound that is causing the number of atoms to be ODD and rebalance. 𝐶𝑢(𝑠) + 𝟐𝐻𝑁𝑂3(𝑎𝑞) → 𝐶𝑢(𝑁𝑂3 )2 𝑎𝑞 + 𝟐𝑁𝑂2 𝑔 + 𝐻2 𝑂(𝑙) NO2 was causing the number of N’s to be odd on the product side, therefore I doubled the coefficient of NO2 from 1 to 2. 8 Balancing Nitrogen Continued I rebalanced N by changing the coefficient of HNO3 to 4. However, now I have unbalanced H again, therefore I must rebalance H before trying to balance O. • And now we have a balanced equation! 𝐶𝑢(𝑠) + 𝟒𝐻𝑁𝑂3(𝑎𝑞) → 𝐶𝑢(𝑁𝑂3 )2 𝑎𝑞 + 𝟐𝑁𝑂2 𝑔 + 𝟐𝐻2 𝑂(𝑙) Let’s look again at our workshop outline and check our learning….(2) 9 How to identify 5 types of reactions •Synthesis Reactions •Decomposition Reactions •Combustion Reactions •Single Replacement (Displacement): •Double Replacement (Displacement) 10 Synthesis Reactions Easiest to recognize – 1 product is formed: Formula: A + B AB Ex: 4Al + 3O2 2Al2O3 11 Decomposition Reactions ONE substance breaks down into simpler substances. Opposite of synthesis – 1 reactant. Formula: AB A + B Ex: 2CaCO3 2CaO + 2CO2 12 Combustion Reactions Hydrocarbons (contain C & H) combine with O2 and produce CO2 and H2O. Release energy in form of light and heat 2 C6H14 + 19 O2 12 CO2 + 14 H2O 2 C2H6 + 7 O2 4 CO2 + 6 H2O 13 Single Replacement (Displacement) One substance replaces another in a compound. Creates a “free” element B. These are redox reactions in which one or more elements change its oxidation number. Formula: A + BC AC + B Ex1: Zn + CuSO4 ZnSO4 + Cu Ex2: Cl2 + 2KBr --> 2KCl + Br2 14 Double Replacement (Displacement) Metathesis reactions Cations of two compounds exchange places to form two new compounds. Metathesis (Exchange) Reactions General Form: 𝑨𝑩 + 𝑪𝑫 → 𝑨𝑫 + 𝑪𝑩 - Involve the exchange or “changing out” of certain ions for others. - Positive ions switch places and get new “partners” or negative ions. - The new products are formed based on the charges of the ions being put together . Ex: BaCl2 + Na2SO4 2NaCl + BaSO4 15 Conditions for metathesis reactions to occur Must be some net change to the system (i.e. the removal of ions from the system). There are four conditions under which a metathesis reaction will be “driven forward”: 1. The formation of a gas. (gas evolution reaction) Example: 𝟐𝑯𝑪𝒍 𝒂𝒒 + 𝑵𝒂𝟐 𝑺 𝒂𝒒 → 𝟐𝑵𝒂𝑪𝒍 𝒂𝒒 + 𝑯𝟐 𝑺(𝒈) 2. The formation of a liquid. (acid-base neutralization rxn) Example: 𝑯𝑪𝒍 𝒂𝒒 + 𝑵𝒂𝑶𝑯 𝒂𝒒 → 𝑵𝒂𝑪𝒍 𝒂𝒒 + 𝑯𝟐 𝑶 𝒍 3. The formation of a solid. (precipitation reaction) Example: 𝑲𝟐 𝑪𝑶𝟑 𝒂𝒒 + 𝑴𝒈𝑰𝟐 𝒂𝒒 → 𝟐𝑲𝑰 𝒂𝒒 + 𝑴𝒈𝑪𝑶𝟑 (𝒔) 4. The formation of a weak electrolyte. Example: 𝑵𝒂𝑭 𝒂𝒒 + 𝑯𝑩𝒓 𝒂𝒒 → 𝑯𝑭 𝒂𝒒 + 𝑵𝒂𝑩𝒓(𝒂𝒒) 16 Predicting products of chemical reactions The solubility guidelines may be used to predict whether a precipitation reaction will occur. When solutions of soluble ionic compounds are mixed together we need to identify all the ions present in the solution and then consider if possible cation/anion pairing could result in an insoluble compound. 17 Solubility Guidelines Soluble Compounds contain Exceptions include: -Group 1 metal cations (Li+, Na+, K+, Rb+, Cs+) and ammonium ion NH4+ -Halide ions (Cl−, Br−, and I−) -Acetate(C2H3O2−), -Bicarbonate(HCO3-) ion -Chlorate (ClO3−) ion -Sulfate (SO42−) ion -Nitrate (NO3-) ion -Halides of Ag+, Hg22+ and Pb2+ are insoluble +, Ba2+ ,Ca2+, Hg 2+ -Sulfates of Ag Pb2+and Sr2+ are insoluble 2 18 Solubility table continued… Insoluble Compounds contain - Carbonate ions (CO32−), Phosphate ions(PO43−), Chromate ions(CrO42−), Sulfide ions (S2−) Hydroxide ion (OH−) Exceptions -Compounds of ALL these anions with group IA and ammonium ion (NH4+)are soluble. -Compounds of Chromate ions (CrO42−), are also soluble with Ca+2 and Mg +2 -Sulfide ions (S2−) are also soluble with Group IIA -Hydroxides are also soluble Sr+2 , *Ca+2, and Ba2+. 19 What is a weak electrolyte? • Electrolytes: Substances that form ions when dissolved in solution. Electrolytes can be weak or strong. • Strong Electrolytes: Substances that completely separate into their component ions when dissolved. (All soluble ionic compounds and strong acids are strong electrolytes.) • Weak Electrolytes: Substances that exist in solution mostly as neutral molecules, with only a small fraction separating into ions. (These are mostly weak acids and bases.) • Nonelectrolytes: Substances that do NOT form ions when dissolved in solution. These are usually molecular. Let’s look again at our workshop outline and check our learning…(3,4,5) 20 Net Ionic Equations For metathesis reactions it is useful to express the net change of a chemical reaction, including only the species involved in the chemical reaction. These are: NET IONIC EQUATIONS How to Write a Net Ionic Equation 1. Write and balance the molecular equation 𝑲𝟐 𝑪𝑶𝟑 + 𝑴𝒈𝑰𝟐 → 𝟐𝑲𝑰 + 𝑴𝒈𝑪𝑶𝟑 2. Determine the phases of each substance in the reaction. For ionic substances, use the solubility rules to determine if the substance is soluble (aq) or insoluble (s). 𝑲𝟐 𝑪𝑶𝟑 𝒂𝒒 + 𝑴𝒈𝑰𝟐 𝒂𝒒 → 𝟐𝑲𝑰 𝒂𝒒 + 𝑴𝒈𝑪𝑶𝟑(𝒔) 3. Write the complete ionic equation by breaking up any aqueous strong electrolytes into separated ions. The phases of all separated ions should be aqueous. 𝟐𝑲+ (𝒂𝒒) + 𝑪𝑶𝟑 𝟐− (𝒂𝒒) + 𝑴𝒈𝟐+ (𝒂𝒒) + 𝟐𝑰− (𝒂𝒒) → 𝟐𝑲+ (𝒂𝒒) + 𝟐𝑰− (𝒂𝒒) + 𝑴𝒈𝑪𝑶𝟑(𝒔) 4. Cancel out any ions that are present in the same form on both the product side and the reactant side of the equation. These ions are called spectator ions since they are not involved in the reaction. Rewrite what’s left. 𝑪𝑶𝟑 𝟐− (𝒂𝒒) + 𝑴𝒈𝟐+ (𝒂𝒒) → 𝑴𝒈𝑪𝑶𝟑 (𝒔) Let’s look again at our workshop outline and check our learning….(6,7) 22