Chemical Reactions Chapter 7

What is a Chemical Reaction? A chemical reaction involves changing from one type of molecule to another. Reactants  Products

Chemical Reaction Another name for a chemical change New properties when you are done No new atoms are made –Atoms are rearranged New compounds can be made –Old bonds are broken –New bonds are formed

Evidence of Chemical Reactions look for evidence of a new substance visual clues (permanent) –color change –precipitate formation solid that forms when liquid solutions are mixed –gas bubbles –large energy changes container becomes very hot or cold emission of light other clues –new odor –whooshing sound from a tube –permanent new state

Evidence of Chemical Change Color Change Formation of Solid Precipitate Formation of a Gas Emission of Light Release or Absorption of Heat

All chemical reactions have two parts Reactants - the substances you start with Products- the substances you end up with The reactants turn into the products. Reactants  Products

How are Reactants Transformed into Products?

Starting a Reaction Always takes a little energy Energy goes into breaking bonds in the reactants Can use different forms of energy –Heat –Electricity –Light

Forming Bonds Makes Energy Releases energy –Energy is conserved Chemical Energy- energy stored in the bonds of the chemicals. Reactions have an energy change

Chemical Equations Shorthand way of describing a reaction Provides information about the reaction –Formulas of reactants and products –States of reactants and products –Relative numbers of reactant and product molecules that are required –Can be used to determine weights of reactants used and products that can be made

Symbols used in equations the arrow separates the reactants from the products –The arrow= “reacts to form” –The plus sign = “and” (s) after the formula –solid (g) after the formula –gas (l) after the formula –liquid (aq) after the formula - dissolved in water, an aqueous solution

Symbols used in equations indicates a reversible reaction shows that heat is supplied to the reaction is used to indicate a catalyst used supplied, in this case, platinum.

Skeleton Equation Uses formulas and symbols to describe a reaction doesn’t indicate how many. All chemical equations are sentences that describe reactions.

Convert these to equations Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (II) chloride and dihydrogen sulfide gas. Nitric acid dissolved in water, reacts with solid sodium carbonate to form liquid water, carbon dioxide gas and sodium nitrate dissolved in water.

The other way Fe(g) + O 2 (g)  Fe 2 O 3 (s) Cu(s) + AgNO 3 (aq)  Ag(s) + Cu(NO 3 ) 2 (aq) NO 2 N 2 (g) + O 2 (g)

Balancing Chemical Equations

Taking 20 kids to the zoo? What if you came home with only 18 kids? Parents are funny that way! What if you came home with 22 kids? At who’s house would you drop them off?

Balanced Equation Atoms can’t be created or destroyed Just like having to account for the children on a zoo trip, you have to account for the number of atoms used in a reaction and those atoms produced. –All the atoms we start with we must end up with –A balanced equation has the same number of each element on both sides of the equation.

Rules for balancing  Write the correct formulas for all the reactants and products  If the formula isn’t written correctly the equation won’t balance  Count the number of atoms of each type appearing on both sides of the equation

Rules for Balancing  Balance the elements one at a time by adding coefficients (the numbers in front)  You can’t balance equations by changing subscripts  Check to make sure it is balanced.

NEVER Change a subscript to balance an equation. –Changing the subscript changes the formula and if you change the formula you are describing a different reaction. H 2 O is a different compound than H 2 O 2 Never put a coefficient in the middle of a formula –2 NaCl is okay, Na2Cl is not.

Example H 2 +H2OH2OO2O2  Make a table to keep track of where you are at

Example H 2 +H2OH2OO2O2  Need twice as much O in the product RP H O

Example H 2 +H2OH2OO2O2  Changes the O RP H O

Example H 2 +H2OH2OO2O2  Also changes the H RP H O

Example H 2 +H2OH2OO2O2  Need twice as much H in the reactant RP H O

Example H 2 +H2OH2OO2O2  Recount RP H O

Example H 2 +H2OH2OO2O2  The equation is balanced, has the same number of each kind of atom on both sides RP H O

Example H 2 +H2OH2OO2O2  This is the answer RP H O Not this

Examples AgNO 3 + Cu  Cu(NO 3 ) 2 + Ag Mg + N 2  Mg 3 N 2 P + O 2  P 4 O 10 Na + H 2 O  H 2 + NaOH CH 4 + O 2  CO 2 + H 2 O

Types of Reactions Predicting the Products

Types of Reactions There are millions of reactions. –Can’t remember them all Fall into several categories. –Will be able to predict the products. –Will recognize them by the reactants

Combination Reactions Combine - put together –2 elements, or compounds combine to make one compound. We can predict the products if they are two elements. –Mg + N 2 

Write and balance Ca + Cl 2  Fe + O 2  iron (II) oxide Al + O 2  Remember that the first step is to write the formula Then balance

Decomposition Reactions decompose = fall apart –one reactant falls apart into two or more elements or compounds. NaCl Na + Cl 2 CaCO 3 CaO + CO 2

Decomposition Reactions Can predict the products if it is a binary compound –Made up of only two elements –Falls apart into its elements H 2 O HgO

Decomposition Reactions If the compound has more than two elements you must be given one of the products The other product will be from the missing pieces NiCO 3 H 2 CO 3 (aq) 

Single Replacement One element replaces another –Reactants must be an element and a compound. –Products will be a different element and a different compound. Na + KCl  K + NaCl F 2 + LiCl  LiF + Cl 2

Single Replacement Metals replace metals (and hydrogen) K + AlN  Zn + HCl  Think of water as HOH Metals replace one of the H, combine with hydroxide. Na + HOH 

Double Replacement Two things replace each other. –Reactants must be two ionic compounds or acids. –Usually in aqueous solution NaOH + FeCl 3  The positive ions change place. NaOH + FeCl 3  Fe +3 OH - + Na +1 Cl -1 NaOH + FeCl 3  Fe(OH) 3 + NaCl

Combustion –A compound composed of only C H and maybe O is reacted with oxygen If the combustion is complete, the products will be CO 2 and H 2 O. If the combustion is incomplete, the products will be CO and H 2 O.

How to recognize which type Look at the reactants E + E Combination CDecomposition E + CSingle replacement C + CDouble replacement Hydrocarbon + O 2 Combustion

Complete and balance CaCl 2 + NaOH  CuCl 2 + K 2 S  KOH + Fe(NO 3 ) 3  (NH 4 ) 2 SO 4 + BaF 2 

Complete and Balance H 2 + O 2  H 2 O  Zn + H 2 SO 4  HgO  KBr +Cl 2  AgNO 3 + NaCl  Mg(OH) 2 + H 2 SO 3 

Complete and Balance C 4 H 10 + O 2  C 3 H 8 + O 2  C 6 H 12 O 6 + O 2  C 8 H 8 +O 2 

Chapter 7 Summary

An equation Describes a reaction Must be balanced because to follow Law of Conservation of Energy Can only be balanced by changing the coefficients. Has special symbols to indicate state, and if catalyst or energy is required.

Reactions Come in 5 types. Can tell what type they are by the reactants.

The Process Determine the type by looking at the reactants. Put the pieces next to each other Use charges to write the formulas Use coefficients to balance the equation.