Chemical Reactions

Empirical formula The simplest ratio between the atoms. The formulas for ionic compounds must be written as empirical formulas. For molecular substances sometimes the ratio between the atoms in a molecule is not in the simplest ratio. When the molecular and empirical formula are different then the molecular formula is a multiple of the empirical formula.

DO NOW Which compound contains a greater % of Oxygen. Justify your answer with a calculation Li2O, CaO, PbO2

All chemical reactions… have two parts: Reactants = the substances you start with Products = the substances you end up with The reactants will turn into the products. Reactants → Products

Products Reactants

How to Describe a Reaction A reaction can be described several ways: #1. In a sentence every item is a word Copper reacts with chlorine to form copper (II) chloride. #2. In a word equation some symbols used Copper + chlorine → copper (II) chloride #3. In a chemical equation only chemical equations are used Cu + Cl2 → CuCl2

Symbols in Equations (s) after the formula = solid: Fe(s) (g) after the formula = gas: CO2(g) (l) after the formula = liquid: H2O(l) (aq) after the formula = dissolved in water, an aqueous solution: NaCl(aq) is a salt water solution

Symbols used in equations the arrow → separates the reactants from the products (arrow points to products) Read as: “reacts to form” or yields or produces The plus sign + means “and” ↑ used after a product indicates a gas has been produced: H2↑ ↓ used after a product indicates a solid has been produced: PbI2↓

Symbols used in equations double arrow indicates a reversible reaction (more later) shows that heat is supplied to the reaction is used to indicate a catalyst is supplied (in this case, platinum is the catalyst)

What is a catalyst? A substance that speeds up a reaction, without being changed or used up by the reaction. Enzymes are biological or protein catalysts in your body.

The Skeleton Equation All chemical equations are a description of the reaction. A skeleton equation uses formulas and symbols to describe a reaction but doesn’t indicate how many; this means they are NOT balanced

Write a skeleton equation for: Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas. Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.

Write a skeleton equation for: Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas. FeS(s) + HCl(g) → FeCl2(s) + H2S(g) Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water. HNO3(aq) + Na2CO3(g) → CO2(g) + H2O(l)

Now, read these equations: Fe(s) + O2(g) → Fe2O3(s) Cu(s) + AgNO3(aq) → Ag(s) + Cu(NO3)2(aq) NO2(g) N2(g) + O2(g)

ANSWERS TO WORKSHEET 24. Crystal is a hydrate. Heating removes the water of hydration 25 Hydrated crystal is blue Anhydrous crystal is white 26 The anhydrous compound is pure. The hydrated crystal contains 50% water so it is more expensive (you are paying for water in the bottle!!!) 3 2 1

Balanced Chemical Equations According to the Law of Conservation of Mass: atoms aren’t created or destroyed in a chemical reaction, they are just rearranged. All the atoms we start with in the reactants we must end up with in the products (meaning: balanced!) A balanced equation has the same number of each element on both sides of the equation.

Rules for balancing: Assemble the correct formulas for all the reactants and products, using “+” and “→” Count the number of atoms of each type appearing on both sides Balance the elements one at a time by adding coefficients (the numbers in front) where you need more - save balancing the H and O until LAST! (hint: I prefer to save O until the very last) Double-Check to make sure it is balanced.

Never Never change a subscript to balance an equation (You can only change coefficients) If you change the subscript (formula) you are describing a different chemical. H2O is a different compound than H2O2 Never put a coefficient in the middle of a formula; they must go only in the front 2NaCl is okay, but Na2Cl is not.

Practice Balancing Examples _AgNO3 + _Cu → _Cu(NO3)2 + _Ag _Mg + _N2 → _Mg3N2 _P + _O2 → _P4O10 _Na + _H2O → _H2 + _NaOH _CH4 + _O2 → _CO2 + _H2O

Practice Balancing Examples 2AgNO3 + _Cu → _Cu(NO3)2 + 2Ag 3Mg + _N2 → _Mg3N2 4P + 5O2 → _P4O10 2Na + 2H2O → _H2 + 2NaOH _CH4 + 2O2 _CO2 + 2H2O

Types of Reactions How? We recognize them by their reactants There are probably millions of reactions. We can’t remember them all, but luckily they will fall into several categories. We will learn: a) the 5 major types. We will be able to: b) predict the products. For some, we will be able to: c) predict whether or not they will happen at all. How? We recognize them by their reactants

also called Combination Reactions The general equation is A + B  AB Synthesis Reaction also called Combination Reactions 2 substances combine to make one compound A B A B + The general equation is A + B  AB

also called Combination Reactions #1 Synthesis Reactions also called Combination Reactions 2 substances combine to make one compound the general equation is : A + B → AB Ca + O2 → CaO element + element SO2 + O2 → SO3 compound + element CO2 + H2O → H2CO3 compound + compound We can predict the products, especially if the reactants are two elements. Mg + N2 → _______ Mg3N2

HCl(g) + NH3(g)  NH4Cl(s) Synthesis Reaction HCl(g) + NH3(g)  NH4Cl(s)

Complete and balance: Ca + Cl2 → Fe + O2 → (assume iron (II) oxide is the product) Al + O2 → Remember that the first step is to write the correct formulas – you can still change the subscripts at this point, but not later while balancing! Then balance by changing the coefficients only

The general equation is : AB → A + B Decomposition Reaction A B A B + The general equation is : AB → A + B A reaction where a more complex molecule breaks down to form two or more simpler products

2NH4NO3(s)  4H2O(g) + 2N2(g) + O2(g) + energy Decomposition Reaction 2NH4NO3(s)  4H2O(g) + 2N2(g) + O2(g) + energy Timothy McVeigh bombing, 1995 Regular building demolition with ammonium nitrate explosives

#2 - Decomposition Reactions one reactant breaks apart into two or more elements or compounds. the general equation is : AB → A + B H2O H2 + O2 CaCO3 CaO + CO2 CuSO4•5H2O CuSO4 + 5H2O 2NaHCO3(s) Na2CO3(s) + H2O(l) + CO2(g) Note that energy (heat, sunlight, electricity, etc.) is usually required

#2 - Decomposition Reactions We can predict the products if it is a binary compound (which means it is made up of only two elements) It breaks apart into the elements: H2O HgO mercury mercury (II) oxide cinnabar

Single Displacement Reaction + B C A + BC  AC + B A reaction where an element displaces another element in a compound, producing a new compound and an element A metal will replace a cation (metal or H) A non-metal will replace an anion (non-metal)

Zn(s) + 2 HCl (aq) → ZnCl2 (aq) + H2 (g) zinc metal and hydrochloric acid react to form zinc chloride and hydrogen gas in this single-displacement reaction.

#3 - Single Displacement Reactions One element replaces another the reaction follows the form of: compound + element → compound + element Reactants must be an element and a compound. Products will be a different element and a different compound. Na + KCl → K + NaCl (cations switched) F2 + LiCl → LiF + Cl2 (anions switched)

#3 Single Displacement Reactions                 #3 Single Displacement Reactions Metals will replace other metals (and they can also replace hydrogen) Zn(s) + 2 HCl (aq) → ZnCl2 (aq) + H2 (g) Cu(s) + 2AgNO3 → 2Ag(s) + Cu(NO3)2 Think of water as: HOH Metals replace the first H, and then combines with the hydroxide (OH). 2Na (s) + 2H2O(l) → 2 NaOH(aq) + H2(g)

#3 Single Displacement Reactions We can even tell whether or not a single displacement reaction will happen: More ‘active’ element replaces less active The Activity Series of Metals lists metals (and hydrogen) in order of activity. Elements higher on the list replaces those lower on the list.

Single Displacement Reaction Hg(NO3)2(aq) + 2Ag(s)  Hg(s) + 2AgNO3(aq) Will this reaction occur? H2(g) + ZnCl2(aq)  2HCl(aq) + Zn(s) What about this reaction? Both of these reactions do not occur.

The “Activity Series” of Halogens Higher Activity Halogens can replace other halogens in compounds, provided they are above the halogen they are trying to replace in the periodic table. Fluorine Chlorine Bromine Iodine Lower Activity 2NaCl(s) + F2(g)  2NaF(s) + Cl2(g) ??? MgCl2(s) + Br2(g)  No Reaction! ???

#3 Single Replacement Reactions Practice: Fe + CuSO4 → Pb + KCl → Al + HCl →

Double Displacement Reaction AD + BC  AC + BD + B C A D Two compounds switch parts to make two new compounds the general equation is : AB + CD → AD + CB

Double Displacement Reaction sodium chloride and silver fluoride react to form sodium fluoride and silver chloride in this double displacement reaction

#4 - Double Replacement Reactions Two compounds switch parts to make two new compounds the reaction is: compound + compound → compound + compound NaOH + FeCl3 → The positive ions change place. NaOH + FeCl3 →→ Fe+3 OH- + Na+1 Cl-1 = NaOH + FeCl3 →→ Fe(OH)3 + NaCl

Double Displacement Reaction Pb(NO3)2(aq) + 2KI(aq)  PbI2(s) + 2KNO3(aq)

#4 - Double Replacement Reactions Have certain “driving forces”, or reasons Will only happen if one of the products: a) doesn’t dissolve in water and forms a solid (a “precipitate”), or b) is a gas that bubbles out, or c) is a molecular compound (which will usually be water).

Complete and balance: assume all of the following reactions actually take place: CaCl2 + NaOH → CuCl2 + K2S → KOH + Fe(NO3)3 → (NH4)2SO4 + BaF2 → NaOH(aq) + HCl(aq) → H2O(l) + NaCl(aq)

How to recognize which type? Look at the reactants: E + E OR C + C = Synthesis C = Decomposition E + C = Single displacement C + C = Double displacement

Practice Examples: H2 + O2 → → H2O → → Zn + H2SO4 → → HgO → → KBr + Cl2 → → AgNO3 + NaCl → → Mg(OH)2 + H2SO3 →→

+ fuel O2 Combustion Reaction XyOz + energy A reaction of a fuel with oxygen, releasing energy in the form of heat and/or light

Combustion Reaction

Element + O2  “oxide” + energy Combustion Reaction Element + O2  “oxide” + energy 2Mg(s) + O2(g)  2MgO(s) + energy P4(s) + 5O2(g)  P4O10(g) + energy

#5 – Combustion Reactions Combustion is a fast reaction of a substance with oxygen to make compounds called oxides. the general equation is : fuel + oxygen → oxides + energy the three things that must be present for combustion to happen are: fuel oxygen spark / heat

Combustion Reaction Examples: What is the main purpose for which fuels are burned around the world? The following equations show what happens when different carbon-based fuels are burned. C(s) + O2(g) → CO2(g) + energy CH4(g) + 2O2(g) → CO2(g) + 2H2O + energy ethanol CH3CH2OH(l) + O2(g) → CO2(g) + 3H2O(l) C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(g) + energy

Combustion of other Fuels Some fuels do not contain carbon. The products are oxides of each element in the fuels. Mg(s) + O2(g) → MgO(s) Cu(s) + O2(g) → CuO(s) H2(g) + O2(g) → H2O(l) P4(s) + 5O2(g) → P4O10(g) S(s) + O2(g) → SO2(g) As you see, many combustion reactions may also be classified as synthesis.

SUMMARY: An equation... Describes a reaction Must be balanced in order to follow the Law of Conservation of Mass Can only be balanced by changing the coefficients. Has special symbols to indicate the physical state, if a catalyst or energy is required, etc.

Reactions Come in 5 major types. We can tell what type they are by looking at the reactants. Single Replacement happens based on the Activity Series Double Replacement happens if one product is: 1) a precipitate (an insoluble solid), 2) water (a molecular compound), or 3) a gas.