Regents Chemistry Chemical Reactions: An Introduction

Physical vs. Chemical Properties Substances are described by their physical and chemical properties Physical properties describe how it looks, smells and feels. No change in composition occurs! ex – color, odor, volume and state of matter Chemical properties describe the substances ability to form new substances ex – ability of wood to burn, metal to rust, food to digest

Examples of Properties Classify each as a physical or chemical property 1. Boiling point of ethanol 2. The hardness of a diamond 3. Sugar ferments to form alcohol 4. A metal wire conducts electricity

Physical vs. Chemical Changes A physical change is a change in one or more physical properties, but no change in the fundamental components that make up the substance. Most common are changes of state A Chemical change is a change in fundamental components, a change into a new substance. Called reactions!

Examples Classify each of the following as a physical or chemical change 1. Melting of ice into boiling of water 2. Electrolysis of water 3. Iron metal is melted 4. Tarnishing of silver 5. Breaking a rock into pieces  Worksheet

Regents Chemistry Chemical Reactions

What’s a Chemical Reaction? Chemical reactions are part of our everyday life Natural gas is burned to heat houses Steel on cars rusts Plastic is made for all kinds of uses Mini-hot packs are used to keep Us warm outdoors!

How Do We Know a Reaction Occurs? Chemical reactions often give visible clues Demos silver nitrate and potassium chloride Methane gas and oxygen Mini-heat packs

Evidence of a Reaction Some of the BIG clues.. 1. The color changes Bunsen Burner 2. A solid forms silver nitrate and potassium chloride 3. Bubbles form zinc metal and HCl 4. Heat and/or a flame is produced, or heat is absorbed Mini-heat packs

What’s involved in a reaction? Chemical reactions have (2) parts The Reactants and the Products Reactants are on the left side of the equation and shows what goes into the reaction! Products are on the right side of the equation and shows what comes out of the reaction! K + H 2 O  H 2 + KOH Example

Reactants/Products Have Physical States! We look at our previous example… K + H 2 O  H 2 + KOH (s)(l)(g)(aq) We place the abbreviation for the state After each chemical formula SymbolState (s) solid (l)Liquid (g)gas (aq)aqueous (dissolved in water)

Writing Unbalanced Chemical Equations We will start by writing basic chemical equations from word problems Example: Solid carbon reacts with gaseous oxygen to form gaseous carbon dioxide C (s) + O 2 (g)  CO (g) Reactants = solid carbon + gaseous oxygen Products = gaseous carbon dioxide

Example #2 Solid zinc metal reacts with hydrochloric acid to produce bubbles of hydrogen gas and aqueous zinc chloride Zn (s) + HCl (aq)  H 2 (g) + ZnCl 2 (aq) You must use the naming compound rules To figure out how to write the chemical formulas Remember- some gases are always diatomic H 2, O 2, Cl 2 worksheet

Exothermic and Endothermic Processes Exothermic processes release energy Example: Burning of wood Endothermic processes absorb energy Example: Cold packs!

Regents Chemistry Balancing Chemical Equations

BALANCING CHEMICAL EQUATIONS RECAP All forward equations are written with reactants of the left an products on the right Example: 2H 2 + O 2  2H 2 O Coefficient means 2 molecules of H 2, 4 atoms total Subscript means 2 atoms of H 4 atoms of H in product

BALANCING CHEMICAL EQUATIONS Atoms are neither created or destroyed… so all equations must be balanced! RULES FOR BALANCING EQUATIONS 1.Cannot change subscripts 2. Can change coefficients

Examples – Balance this equation Na (s) + H 2 O (l)  NaOH (aq) + H 2(g) 2Na (s) + 2H 2 O (l)  2NaOH (aq) + H 2(g) Worksheet BALANCING CHEMICAL EQUATIONS

Regents Chemistry  Writing and Balancing Equations

Regents Chemistry  Types of Reactions

Types of Reactions  Although we cannot classify all reactions into distinct categories…there are four major types of reactions you should know –Synthesis (combination) Reactions –Decomposition (analysis) Reactions –Single Replacement Reactions –Double Replacement Reactions

Synthesis (combination) Reactions  A reaction is classified as a synthesis reaction when two or more reactants combine and a single product is formed B + G  BG 4Fe (s) + 3O 2 (g)  2Fe 2 O 3 (s)

Decomposition (analysis) Reactions  Decomposition reactions are the reverse of combination reactions  Occur when one substance breaks down into simpler substances BG  B + G CaCO 3 (s)  CaO (s) + CO 2 (g)

Regents Chemistry Single and Double Replacement Reactions

Single Replacement Reactions  Involves an element and a compound  Occurs when a metal displaces the metal in a compound B 2 + B 1 G  B 2 G + B 1 Not all metals will displace (react with) a metal in a compound..so how do we know if a reactions will occur? …we use our Table J in our reference tables! Cu (s) + 2AgNO 3 (aq)  Cu(NO 3 ) 2 (aq) + 2Ag (s)

Predicting if a Reaction Will Occur  Table J (Activity Series) on p. 4 is arranged so that a metal listed on the table will react with the compound of a metal that is below it  For Example: Zn will react with a compound of copper Zn (s) + Cu(NO 3 ) 2  Cu (s) + Zn(NO 3 ) 2 (aq) But because Cu is below Zn on the table, it will not react with compounds of Zn Cu (s) + Zn(NO 3 ) 2  no reaction

So..  So..above it will bump it… below it will not!  The MORE ACTIVE will bump the LESS ACTIVE

 There is one element on the table that is not a metal…Hydrogen ion (H + )!  All metals above hydrogen will react with acids to release hydrogen gas and produce a salt Predicting if a Reaction Will Occur Mg (s) + 2HCl (aq)  H 2 (g) + MgCl 2 (aq)

 In the 2nd column of Table J is a list of nonmetals  A nonmetal will replace a less active nonmetal in a compound according to the equation Predicting if a Reaction Will Occur G 1 + BG 2  BG 1 + G 2 For example..Fluorine is listed as the most active nonmetal, and it will replace chlorine, bromine and iodine from other binary compounds F 2 (g) + 2NaCl (aq)  Cl 2 (g) + 2NaF (aq) Cl 2 (g) + NaF  no reaction Cl is below F so it will not react!

Practice Problems  Predict if reactions will occur and write the the products or NR for NO REACTION:  Ca (s) + AgNO 3 (aq)  Pb (s) + Al(NO 3 ) 3 (aq)  Cr (s) + Pb(NO 3 ) 2 (aq)  Co (s) + HCl (aq) --> CaNO3 (aq) + Ag (s) --> NR --> Cr(NO 3 ) 2 (aq) + Pb(s) --> CoCl 2 (aq) + H 2 (g)

Double Replacement Reactions  Double replacement reactions generally involve two soluble ionic compounds that react in solution to produce a precipitate, a gas, or a molecular compound such a water  Represented by the equation: B 1 G 1 + B 2 G 2  B 1 G 2 + B 2 G 1

Predicting if a Reaction Will Occur in a Double Replacement Reaction  Three general situations in which a double replacement reaction will occur  1. If a solid (precipitate) is formed –We look at Table F (p.2) in the reference tables and check the solubility of the two compounds Ex: AgNO 3 (aq) + NaCl (aq)  AgCl (s) + NaNO 3 (aq) The reaction will occur because AgCl is insoluble meaning, it will not stay dissociated in solution

Soluble vs. Insoluble  Soluble means the substance will stay as ions in the solution (just floating around!)…no reaction will happen!  Insoluble means it will form a solid, gas or molecular compound (such as water) and not stay as ions…a reaction will occur!

Predicting a Reaction...  2. If a gas is formed Na 2 S (aq) + 2HCl (aq) H 2 S (g) + 2NaCl (aq)  If a molecular substance, such as H 2 O is formed NaOH (aq) + HCl (aq) H 2 O (l) + NaCl (aq)

Practice Predicting..Will a Reaction Occur? PbSO 4 (s) + NaNO 3 (aq) Pb(NO 3 ) 2 (aq) + Na 2 SO 4 (aq) Ca(NO 3 ) 2 (aq) + KCl (aq) CaCl 2 (aq) + KNO 3 (aq) = NR

REGENTS CHEMISTRY Determining Missing Mass in Equations

The Law of Conservation of Mass The Law of Conservation of mass states that matter cannot be created nor destroyed This is very useful when considering chemical reactions WHY? When given a balanced equation in which either the reactant or product is missing, we can determine the formula of the missing substance..this we have done!

Missing Formulas in Equations K + ___________ = KCl There must be a Cl in the reactants because it appears in the products

Missing Mass in Equations Just as the formula of a missing reactant or product can be determined, the mass of a missing substance can also be found! TOTAL MASS BEFORE = TOTAL MASS AFTER HOW???? USING SIMPLE MATH!

EXAMPLE If g of potassium chlorate is decomposed to form 62.7 g of potassium chloride and oxygen gas, how many grams of oxygen are formed? 2KClO 3 (aq) --> 2KCl (aq) + 3O 2 (g) 1. Find the total mass of the reactants 2. The total mass of the reactants must equal the total mass of the products

EXAMPLE cont.. 2KClO 3 (aq) --> 2KCl (aq) + 3O 2 (g) mass of KCLO 3 = mass of KCl + mass of O g = 62.7g + mass O 2 mass O 2 = 103.0g g mass O 2 = 40.3g

Example #2 What mass of carbon dioxide will be produced if 144g of carbon react with 384 g oxygen gas? C (s) + O 2 (g) --> CO 2 (g) Mass C + mass O 2 = mass CO 2 144g C + 384g O 2 --> mass CO 2 528g = mass CO 2 worksheet

Unknown Reactants and Products Just like we can find missing mass, we can also identify missing reactants or products in a formula Remember 2 steps Whatever is on the left side must also be on the right side..this tells you what us missing! Think about the type of formula..this will help you complete the equation!