Chemical Equations: Balancing and Translating

Transcription

1 PASTE INTO PAGE 58 OF YOUR NOTEBOOK - PASTE INTO PAGE 58 OF YOUR NOTEBOOK Name Period: North Shore Chemistry Date: Chemical Equations: Balancing and Translating Reading adapted from Sarquis s Modern Chemistry Introduction A chemical reaction is the process by which one or more substances are changed into one or more different substances. In any chemical reaction, the original substances are known as the reactants, and the resulting substances are known as the products. According to the Law of Conservation of Mass, the total mass of reactants must equal the total mass of the products for any given chemical reaction. Chemical reactions are described by chemical equations. A chemical equation represents with symbols and formulas, the identities and relative molecular amounts of the reactants and products in a chemical reaction. For example, the following chemical equation shows the chemical reaction that happens when Alka-Seltzer tablets are dissolved in water: citric acid combines with sodium hydrogen carbonate (baking soda) to produce water, carbon dioxide, and sodium citrate. H3C6H5O7 + 3 NaHCO3 3 H2O + 3 CO2 + Na3C6H5O7 Balancing Equations Atoms are neither created nor destroyed in ordinary chemical reactions. Thus, the same elements are found on both the reactant and product sides of a chemical equation, the atoms of these elements are only rearranged through the breakage of bonds in reactants and making of new bonds in products when a chemical reaction takes place, and the same number of atoms of each element must appear on each side of a correct chemical equation. To balance number of atoms, add coefficients where necessary. A coefficient is a small whole number that appears in front of a formula in a chemical equation. Placing a coefficient in front of a formula specifies the relative number of molecules of a substance. If no coefficient is written, it s assumed to be 1. *KEY IDEA: It is important to note that the only way to balance a chemical equation is to change the coefficients of the equation not the subscripts of a chemical formula. For example, if we wanted to find a way for water, H 2O, to supply 2 atoms of oxygen, we would have to put a 2 as a coefficient, as in 2 H 2O. If we changed the subscript of oxygen in water to reflect 2 oxygen atoms, as in H 2O 2, this gives us a chemical entirely different from water. H 2O 2 is actually hydrogen peroxide, a chemical applied to externally to cuts to prevent bacterial infection.

2 Balancing Equations Using the Chain Method Step 1: The arrow goes between the reactants and products and acts like an equal sign. Draw a line where the arrow is to indicate that whatever s on the left must equal whatever s on the right. Reactants Products Cu + AgNO3 Ag + Cu(NO3)2 Step 2: Draw out what each compound looks like by chaining the atoms/ions together. Remember that polyatomic ions act as a unit they never come apart! Notice that there may not be the same number of atoms of each element. Reactants Products Cu + AgNO3 Ag + Cu(NO3)2 Cu Ag-NO 3 Ag Cu-NO 3 -NO 3 Step 3: Pick an element/polyatomic to balance. It usually helps to balance metals first, then nonmetals or polyatomic ions. Add more of the element/polyatomic that you want to balance on the side that has less of it. Remember that elements/polyatomics must be chained to whatever they are bonded with. Reactants Products Cu + AgNO3 Ag + Cu(NO3)2 Cu Ag-NO 3 Ag Cu-NO 3 -NO 3 Ag-NO 3 Step 4: Repeat step 3 with all elements/polyatomic ions until everything is balanced between the left and right sides. Reactants Products Cu + AgNO3 Ag + Cu(NO3)2 Cu Ag-NO 3 Ag Cu-NO 3 -NO 3 Ag-NO 3 Step 5: Count how many compounds you drew. These will be the coefficients to your equation. Reactants Products 1 Cu + 2 AgNO3 2 Ag + 1 Cu(NO3)2 Ag 1 Cu 1 Ag-NO 3 1 Ag 1 Cu-NO 3 -NO 3 2 Ag-NO 3 2 Ag

3 Directions: Follow along with the video, and balance the following examples using the Chain Method. Example 1) Ca + Pb (NO3)2 Pb+ Ca(NO3)2 Example 2) K + N2 K3N Example 3) AlCl3 + H2SO4 Al2(SO4)3+ HCl Example 4) NaOH + HCl NaCl+ H2O Example 5) H2O H2+ O2

4 Tricks and Tips for Balancing Chemical Equations Hint 1 # atoms on reactant side = # atoms on products (ALWAYS!!) Look for atoms that are not equal on both sides Hint 2 Balance metals first. Hint 3 Balance nonmetals next. Hint 4 Whenever possible, balance by groups (keep polyatomic ions TOGETHER!!!) instead of by individual elements. o Ex: 3 Ca(NO3)2 + 2 H3PO4 Ca3(PO4)2 + 6 HNO3 Balance the NO3 group and then the PO4 group Hint 5 Balance hydrogen (not in a polyatomic) next. Hint 6 Balance oxygen (not in a polyatomic) last. Hint 7 If you go from one side of the equation to the other and your numbers just keep going up o You re doing it wrong! o Erase it and start again. Balancing Equation Tutorial

5 Independent Practice: Balancing Chemical Equations Directions: Follow the Law of Conservation of Mass and balance the following equations. You may work independently or use your peers for help. The answer key will be posted on Moodle. 1) NaBr Na + Br2 2) Zn + Sn(NO3)4 Zn(NO3)2 + Sn 3) Br2 + CuF CuBr + F2 4) KBr + Pb(NO3)2 KNO3 + PbBr2 5) Na2CO3 + CaCl2 CaCO3 + NaCl

6 6) Na3PO4 + BaCl2 Ba3(PO4)2 + NaCl 7) Mg(OH)2 + H2SO4 MgSO4 + H2O 8) LiCl + (NH4)2CO3 NH4Cl + Li2CO3 9) NH4OH + CrCl3 NH4Cl + Cr(OH)3 10) Al2(SO4)3 + BaCl2 BaSO4 + AlCl3

7 Translating Chemical Equations Now that you know about chemical equations and how to name compounds, you can put these two skills together in order to translate chemical equations. But first, let s review what the differences between covalent and ionic bonds are. Directions: Follow along with the video and fill in the following table. Compounds No Is the first element a metal? Yes Covalent (molecular compound) particle = molecules Ionic (no true molecule formed) particle = formula unit Bonding Polar Definition Nomenclature Notes Binary Nomenclature Notes Ternary Nomenclature Notes Nonpolar Definition

8 Directions: Using the graphic organizer on the preceding page, find the IUPAC chemical name or chemical formula for each of the following compounds and state whether they are ionic or covalent compounds. IUPAC Chemical Name Chemical Formula Ionic or Covalent Magnesium oxide MgO Ionic Magnesium hydroxide Mg(OH)2 Ionic AlCl3 Carbon tetrahydride CH4 Covalent Lead(II) nitrate Potassium chloride Dinitrogen pentoxide CO2 NaNO3 Now, let s translate the following balanced equation: 1 MgO + 1 H 2 O 1 Mg(OH) 2 The coefficients in a balanced chemical equation can represent the number of atoms, formula units, molecules or moles for a substance. For a single element use the term: atom For a diatomic element use the term: molecule For a molecular compound use the term: molecule For a ionic compound use the term: formula unit Directions: Give a verbal description to each substance using the guidelines above. Substance MgO H2O Mg(OH)2 Verbal Description 1 formula unit of magnesium oxide 1 molecule of dihydrogen monoxide 1 formula unit of magnesium hydroxide So, the entire chemical equation should read: 1 formula unit of magnesium oxide reacts with 1 molecule of dihydrogen monoxide to produce 1 formula unit of magnesium hydroxide.

9 Let s try another equation: 3 Ba + 2 AlCl 3 3 BaCl Al Substance 3 Ba 2AlCl3 3 BaCl2 2 Al Verbal Description So, the entire chemical equation should read: Again: CH O 2 CO H 2 O Substance CH4 Verbal Description 2 O2 CO2 2 H2O So, the entire chemical equation should read:

10 Independent Practice: Putting It All Together Directions: Balance each equation and then translate it into a word equation. Reminder: The coefficients represent the number of atoms, formula units, or molecules for a substance. For a single element use the term: atom For a diatomic element use the term: molecule For a molecular compound use the term: molecule For a ionic compound use the term: formula unit Example: 2 Na + Pb(NO3)2 2 NaNO3 + Pb The equation is balanced and the coefficients are 2, 1, 2, 1. 2 atoms of Na react with 1 formula unit of lead(ii) nitrate to produce 2 formula units of sodium nitrate and 1 atom of lead. 1. AlBr3 + Cl2 AlCl3 + Br2 2. Na3PO4 + CaCl2 Ca3(PO4)2 + NaCl

11 3. KClO3 KCl + O2 4. Al + HCl AlCl3 + H2 5. Ca(OH)2 + H3PO4 H2O + Ca3(PO4)2