Section 2.6—Chemical Reactions Objectives: Define the components of a chemical reaction Convert word equations to formula equations Define and predict products for double replacement and neutralization reactions

Watch as 2 H2 and 1 O2 undergo a chemical reaction Chemical Reactions When substances react, bonds are broken, atoms are rearranged, and bond are re-formed to create new compounds. 2 H2 + O2  2 H2O O H H Watch as 2 H2 and 1 O2 undergo a chemical reaction

Chemical Reactions Bonds and atoms are rearranged to form new compounds. 2 H2 + O2  2 H2O H O O H O H The substances in the end are different from those in the beginning. H Bonds are broken and re-formed between different atoms

Chemical Reactions In chemistry, the “sentence” (showing the starting materials and the final products of a chemical reaction) is called a chemical equation.

Examples of Chemical Equations Word equation: magnesium metal is reacted with aqueous hydrochloric acid to produce aqueous magnesium chloride and hydrogen gas  Formula equation: Mg (s) + 2 HCl (aq) MgCl2 (aq) + H2 (g) Visualization

Parts of a Chemical Equation The starting materials for the reaction—each compound is separated by a “+” Formed in the reaction Reactants Products NaCl (aq) + AgNO3 (aq)  AgCl(s) + NaNO3 (aq) States of matter Arrow s = solid l = liquid g = gas aq = aqueous (dissolved in water) Read as: Yields Produces Forms Makes etc.

Writing Chemical Equations You must write each chemical formula correctly first! (Recall how to write formulas from names - Section 2.2) “and”, “is mixed with” or “reacts with” = + “yield”, “produces” and “forms” =  Then after they are written, chemical equations have to be balanced – we’ll do that in Sec. 2.7!

Write the word equation into symbol form Let’s Practice #1 Aluminum metal is reacted with hydrochloric acid to form aluminum chloride and hydrogen gas Example: Write the word equation into symbol form

Write the word equation into symbol form Let’s Practice #1 Aluminum metal is reacted with hydrochloric acid to form aluminum chloride and hydrogen gas Example: Write the word equation into symbol form Al + HCl  AlCl3 + H2

Write the word equation into symbol form Let’s Practice #2 Copper (II) nitrate and sodium hydroxide form copper (II) hydroxide and sodium nitrate Example: Write the word equation into symbol form

Write the word equation into symbol form Let’s Practice #2 Copper (II) nitrate and sodium hydroxide form copper (II) hydroxide and sodium nitrate Example: Write the word equation into symbol form Cu(NO3)2 + NaOH  Cu(OH)2 + NaNO3

Double Replacement Reactions The cations from two compounds replace each other. NaCl + AgNO3  AgCl + NaNO3 Cl Ag Cl Na Ag O N Na O N Two ionic compounds switch ions

Double Replacement Reactions The general format of a double replacement reaction:

Products of a Double Replacement 1 Combine the cation of the first reactant with the anion of the second reactant CaCl2 + AgNO3 

Products of a Double Replacement 2 Combine the cation of the second reactant with the anion of the first reactant CaCl2 + AgNO3 

Products of a Double Replacement Remember to write cations first … 3 & balance charges with subscripts when writing formulas Remember NOT the change the subscripts that are part of any polyatomic ion! Ca Cl2 + Ag NO3 Ca Cl2 + Ag NO3 Ca(NO3)2 + AgCl

Neutralization Reactions Neutralization reactions are double replacement reactions where one cation is “H” and one anion is “OH” and water is formed. General format of a neutralization reaction: ACID BASE IONIC WATER COMPOUND

Products of a Neutralization Reaction Combine the cation of the first reactant (the acid) with the anion of the second reactant (the base). This gives you the water. 1 H2SO4 + NaOH 

Products of a Neutralization Reaction 2 Combine the cation of the second reactant with the anion of the first reactant. This gives you an ionic compound. H2SO4 + NaOH 

Products of a Neutralization Reaction 3 Remember to write cations first … & balance charges with subscripts when writing formulas Only leave subscripts that are in the original compound there if they are a part of a polyatomic ion! H2 SO4 + Na OH H2 SO4 + Na OH Na2SO4 + HOH You can write the water as “HOH” when you combine the cation & anion or you can change it to “H2O”

Write the products for this reaction Let’s Practice #3 Example: Write the products for this reaction Sr(OH)2 + HBr 

Write the products for this reaction Let’s Practice #3 Example: Write the products for this reaction Sr(OH)2 + HBr  SrBr2 + HOH

Write the products for this reaction Let’s Practice #4 Example: Write the products for this reaction HCl + Ca(OH)2 

Write the products for this reaction Let’s Practice #4 Example: Write the products for this reaction HCl + Ca(OH)2  CaCl2 + HOH