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Unit 7: Chemical Reactions Part 1: Chemical Equations and Balancing Part 2: Types of Chemical Reactions Unit Synapsis In previous unit we looked at the some basics about substances. We addressed differences between molecular and ionic compounds and how to name these compounds. In this unit we will look at how these molecules and compounds react with one another and also introduce the dissolving process. Previous concepts vital to this unit include identifying parent ions as well as naming. Part 1: Chemical Equations and Balancing Corresponds to pages 282 to 288 in your textbook. Part 1: Chemical Equations & Balancing / Learning Targets “After this lesson I can… • …identify the parts of a chemical equation.” • …recite the law of conservation of mass.” • …balance a chemical equation.” • …translate a balanced chemical equation into words and vice versa.” Chemical Equations • A chemical equation is a model for a chemical change. • Here are three examples of chemical equations: • CH4(g) + 2 O2(g) 2 H2O(l) + CO2(g) • 3 H2(g) + N2(g) 2 NH3(g) • KCl(s) K+(aq) + Cl-(aq) • A chemical change is when chemical bonds are broken, atoms are rearranged, and new bonds are formed to create entirely new molecules or compounds • This is different than a nuclear change, which is when the atoms that make up the molecules change. • This is also different than a physical change, which is when the attractions between molecules or compounds are overcome or commence because of changes in temperature or pressure. • The parts of a chemical equation are defined in detail in a few slides but before we get to that let’s look at a major law all chemical equations must follow. The Law of Conservation of Matter • The law of conservation of mass states that in any type of change, matter cannot be created or destroyed. Another way of saying this is that the total amount of matter in the universe is constant. • One of the things this means is that chemical equations must be balanced. • A balanced chemical equation has the same number of atoms going into and out of the equation. • The reactants are the substances to the left of the arrow in a chemical equation, & the products are the substances to the right of the arrow. • If an equation is presented in it’s unbalanced form, it can be balanced by inserting coefficients in front of the chemical formulas. If a chemical formula does not have a coefficient in front of it, the coefficient is 1. • The coefficients technically represent the number of moles but thinking of them as just the number of molecules makes things easier. • When balancing, you must NEVER alter the chemical formulas. You can only change or add coefficients in front of the chemical formulas. Parts of a Chemical Equation Physical States • Note that on the previous slide there were exactly: • 7 carbon atoms in the products & reactants • 16 hydrogen atoms in the products & reactants • 22 oxygen atoms in the products & reactants • Another important part of a chemical equation is that the physical state is sometimes noted next to the chemical formulas. • When it comes to balancing the physical state is not important and sometimes it is left out. However, you should know what each letter means when you see it. • There is a letter for each physical state, and a 4th notation is used to describe something dissolved in water. • (l): liquid • (g): gas • (s): solid • (aq): aqueous (dissolved in water) Balanced & Unbalanced Chemical Equation Example #1 Balanced & Unbalanced Chemical Equation Example #2 Balanced & Unbalanced Chemical Equation Example #3 Balanced & Unbalanced Chemical Equation Example #4 Balanced & Unbalanced Chemical Equation Example #5 Balancing Chemical Equations Tips 1) Balance elements that appear twice on one side of the equation last. So if Oxygen is in two of the reactants and/or two of the products, save it for last. (This is a lot of equations so as a general rule leave oxygen for last) 2) Only whole numbers are allowed, but if you end up with half a number, you can get yourself out of that situation by doubling ALL the coefficients in the equation. (if you end up with .25 you can get yourself out of that situation by quadrupling all coefficients) 3) Take your time. Balancing is a skill that takes practice but with enough patience an answer will be found. Chemical Equations as Sentences • A chemical equation can be stated as a sentence and you should be able to translate back and forth between a sentence and an equation. Start by writing the equation down and then balance it after. Here are 3 examples: • “Magnesium reacts with oxygen gas (O2) to form Magnesium Oxide” • 2 Mg(s) + O2(g) 2 MgO(s) • “Sodium Metal reacts with Iron(III) Oxide to produce Sodium Oxide and Iron metal” • 6 Na(s) + 2 Fe2O3(s) 3 Na2O(s) + 4 Fe(s) • “Tetraphosphorus trisulfide reacts with oxygen gas (O2) to produce tetraphosphorus hexaoxide and Sulfur dioxide” • P4S3(s) + 6 O2(g) P4O6(g) + 3 SO2(g) Chemical Equations as Sentences • 2 Na3PO4 + 3 CaCl2 Ca3(PO4)2 + 6 NaCl • “Sodium phosphate reacts with calcium chloride to produce calcium phosphate and sodium chloride” • 2 N2O5 4 NO2 + O2 • “dinitrogen pentaoxide breaks down to produce nitrogen dioxide and oxygen gas” • 2 KClO3(s) 2 KCl(s) + 3 O2(g) • “Potassium Chlorate breaks down to form potassium chloride and oxygen gas” Part 1 Additional Resources • WikiHow on balancing chemical equations. • Khan Academy on Balancing Chemical Equation. Part 2: Types of Chemical Reactions Corresponds to pages: 276 to 284 in your textbook. Part 2: Types of Chemical Reactions / Learning Targets “After this lesson I can… • …Identify a chemical equation as representing a combustion reaction, single replacement reaction, double replacement reaction synthesis reaction, decomposition reaction, or an ionic compound dissolving in water. • …Define combustion reactions, single replacement reactions, double replacement reactions, synthesis reactions, and decomposition reactions. • …Write the equation for an ionic compound dissolving in water (simplified and complete). #1: Ionic Compounds Dissolving in Water • While it’s not exactly a chemical reaction, when an ionic compound dissolves in water it splits apart into it’s parent ions. • When the ionic compound splits apart it’s called ionization or dissociation. • For these equations you will have to use what you learned about parent ions in UNIT 6 and what you learned about balancing in part 1 of this unit. • Like physical states, charges do NOT matter for balancing. • Sample equations for ionic compounds dissolving in water: • NaCl(s) Na+(aq) + Cl-(aq) • CaBr2(s) Ca2+(aq) + 2 Br-(aq) • KNO3(s) K+(aq) + NO3-(aq) • Fe(NO3)3(s) Fe3+(aq) + 3 NO3-(aq) • How to Identify?: Look for 1 reactant and 2 products that are both ions and in the aqueous physical state. Ionic Compounds Dissolving in water Sample Problems Write the equation for the following compounds dissolving in water 1) FeBr3 2) Cu(NO3)2 3) LiBr 4) Na3PO4 5) MgSO4 6) BaCl2 Ionic Compounds Dissolving in Water • Sometimes chemists get lazy and don’t want to take the time to write out longer equations so they simplify using the “aq” symbol to indicate an ionic compound that’s been dissolved in water. • Sample Equations for Ionic compounds dissolving in water: • NaCl(s) Na+(aq) + Cl-(aq) could be written simply as: • NaCl(s) NaCl(aq) • CaBr2(s) Ca2+(aq) + 2 Br-(aq) could be written simply as: • CaBr2(s) CaBr2(aq) • Fe(NO3)3(s) Fe3+(aq) + 3 NO3-(aq) could be written simply as: • Fe(NO3)3(s) Fe(NO3)3(aq) Ionic Compounds Dissolving in Water • When dealing with ionic compounds dissolved in water it is important to start paying closer attention to the physical state symbols. • You must remember that: • “CuSO4(aq)”really means “Cu2+(aq) + SO42-(aq)” • “AlCl3(aq)” really means “Al3+(aq) + 3 Cl-(aq)” • “Na2CO3(aq)” really means “2 Na+(aq) + CO32-(aq)” #2: Synthesis Reactions (a.k.a. combination reactions) • In a synthesis reaction, two or more elements or compounds combine to form a larger, more complex compound. • Sample Chemical Equations: • 3 H2 + N2 2 NH3 • 2 H2 + O2 2 H2O • Mg + O2 MgO • CaO + H2O Ca(OH)2 • Generic Chemical Equation: • A+BC • How to identify?: Look for 1 product #3: Decomposition Reactions • In a decomposition reaction, A compound breaks down into two or more elements or compounds. This often occurs through heating but not always. • In decomposition equations the arrow actually means “breaks down to produce” or “decomposes to produce” • Sample Chemical Equations: • 2 H2O2 2 H2O + O2 • CuCO3(s) CuO(s) + CO2(g) • C3H5(NO3)3(l) 6 N2(g) + 12 CO2(g) + 10 H2O(g) + O2(g) • Generic Chemical Equation: • AB+C • How to identify?: Look for 1 reactant and 2 or more Products #4: Single Replacement Reactions • In a single replacement reaction two elements replace one another. In most cases, this involves 1 pure metal and 1 metal that is part of an ionic compound. • Sample Chemical Equations: • 2 Al(s) + 3 CuCl2(aq) 3 Cu(s) + 2 AlCl3(aq) • Fe(s) + CuSO4(aq) Cu(s) + FeSO4(aq) • Fe(s) + Pb(NO3)2(aq) Pb(s) + Fe(NO3)2(aq) • Generic Chemical Equation: • A + BC B + AC • How to identify?: Look for a pure element and an ionic compound in the reactants. In the products the pure element should take the place of the positive ion in the ionic compound. #5: Double Replacement Reactions • In a double replacement reaction, two positive ions in an ionic compound exchange their negative ions. • Sample Chemical Equations (Solid Precipitation) • 3 CaCl2(aq) + 2 Na3PO4(aq) 6 NaCl(aq) + Ca3(PO4)2(s) • 2 NaOH(aq) + CuBr2(aq) Cu(OH)2(s) + 2 NaBr(aq) • Li2CO3(aq) + Mg(NO3)2(aq) MgCO3(s) + 2 LiNO3(aq) • HCl(g) + NaOH(aq) H2O + NaCl(aq) • Generic Chemical Equation • AB + CD CB + AD • How to identify?: Look for two ionic compounds in the reactants. The products should also contain two ionic compounds but the parent cations should have switched their parent anions. #6: Combustion Reactions • As you might have already realized, combustion reactions are when carbon compounds burn or explode. • With one notable exception, which is the combustion of hydrogen gas, this produces carbon dioxide and water. • Combustion reactions produce light and heat and occasionally you will see “heat” written as a product. • Sample Chemical Equations: • CH4 + 2 O2 CO2 + 2 H2O • 2 C8H18 + 25 O2 16 CO2 + 18 H2O • 2 C2H6O + 7 O2 4 CO2 + 6 H2O • Generic Chemical Equation: • Organic Compound + O2 CO2 + H2O • How to identify?: Look for Carbon Dioxide and Water in the products. Oxygen should be a reactant. Other types of reactions • Not all reactions fit neatly into the six classifications listed above. Here are some examples of those equations: • Cu2S + 12 HNO3 Cu(NO3)2 + CuSO4 + 10 NO2 + 6 H2O • 2 K2MnF6 + 4 SbF5 4 KSbF6 + 2 MnF3 + F2 • It’s not one of our objectives that your able to place every single chemical reaction into a specific category, just that you are able to clearly identify the six mentioned on the previous slides. • There is also reactions that are in a “gray area” between two different classifications. • Hydrogen gas, for example, will explode and produce nothing but water: 2 H2 + O2 2 H2O • This could be classified as combination. Even though carbon dioxide is not a product, it could also be classified as combustion because it produces a flame, has water as a product, and oxygen gas as a reactant. Identifying Types of Chemical Reactions Summary • Combustion: Look for Carbon Dioxide and Water in the products. Oxygen should be a reactant. • Ionic compound dissolving in water: Look for 1 reactant and 2 products that are both ions and in the aqueous physical state. • Single Replacement: Look for a pure element and an ionic compound in the reactants. In the products the pure element should take the place of the positive ion in the ionic compound. • Double Replacement: Look for two ionic compounds in the reactants. The products should also contain two ionic compounds but the parent cations should have switched their parent anions. • Decomposition: Look for 1 reactant and 2 or more Products • Synthesis/Combination: Look for 1 product Part 2 Additional Resources • Quizlet on types of chemical reactions • Note: this does not cover Ionic compounds dissolving in water