Section 2.6—Chemical Reactions If we’re going to do chemical reactions with antacids, we’d better know how to write them!

Diatomic Molecules This is not in the notes so you should pay attention, and add it in!!!!!! Some elements are so chemically reactive that, when they are not bonded to any other elements, they find another atom of their same kind to bond with.

Diatomic Molecules To write a formula for a diatomic molecule you write the symbol of the element and use a two as a subscript. Which elements form diatomic molecules? Well, I am glad you asked!

Diatomic Molecules There are seven diatomic molecules: ElementMolecule HydrogenH2H2 NitrogenN2N2 OxygenO2O2 FluorineF2F2 ChlorineCl 2 BromineBr 2 IodineI2I2

Diatomic Molecules There are two good ways to remember the diatomic molecules First – Remember a funny word! HOFBrINCl

Diatomic Molecules Second – Remember a funny phrase: The six that make a seven plus hydrogen.

Now back to our regularly scheduled notes.

Watch as 2 H 2 and 1 O 2 undergo a chemical reaction O O O O Bonds and atoms are rearranged to form new compounds. H H H H H H H H 2 H 2 + O 2  2 H 2 O Chemical Reactions

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

Chemical Equations The “sentence” of chemistry that shows how the starting materials and the final products of a chemical reaction

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

Parts of a Chemical Equation NaCl (aq) + AgNO 3 (aq)  AgCl(s) + NaNO 3 (aq) Reactants States of matter Arrow Products The starting materials for the reaction—each compound is separated by a “+” s = solid l = liquid g = gas aq = aqueous (dissolved in water) Read as: Yields Produces Forms Makes etc. Formed in the reaction

You must write each chemical formula correctly first! (Section 2.2) “and”, “is mixed with” or “reacts with” = + “yield”, “produces” and “forms” =  Writing Chemical Equations We will learn to balance equations in the next section, so don’t worry about it now!

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

Let’s Practice #1 Al + HCl  AlCl 3 + H 2 Example: Write the word equation into symbol form

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

Let’s Practice #2 Cu(NO 3 ) 2 + NaOH  Cu(OH) 2 + NaNO 3 Example: Write the word equation into symbol form

NaCl + AgNO 3  AgCl + NaNO 3 Double Replacement Reactions The cations from two compounds replace each other. Cl Na Ag O O O O N N O O Cl Ag Na O O O O N N O O Two ionic compounds switch ions

Double Replacement Reactions General format of a double replacement reaction:

Combine the cation of the first reactant with the anion of the second reactant CaCl 2 +AgNO 3 1 Products of a Double Replacement

Combine the cation of the second reactant with the anion of the first reactant CaCl 2 +AgNO 3 2 Products of a Double Replacement

& balance charges with subscripts when writing formulas Remember to write cations first … AgCl CaCl 2 +AgNO 3 3 Ca(NO 3 ) 2 + CaCl 2 AgNO 3 + Products of a Double Replacement Only leave subscripts that are in the original compound there if they are a part of a polyatomic ion!

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

Combine the cation of the first reactant with the anion of the second reactant H2H2 SO 4 +NaOH 1 Products of a Neutralization Reaction

Combine the cation of the second reactant with the anion of the first reactant H2H2 SO 4 +NaOH 2 Products of a Neutralization Reaction

& balance charges with subscripts when writing formulas Remember to write cations first … HOH H2H2 SO 4 +NaOH 3 Na 2 SO 4 + H2H2 SO 4 NaOH + Products of a Neutralization Reaction Only leave subscripts that are in the original compound there if they are a part of a polyatomic ion! You can write the water as “HOH” when you combine the cation & anion or you can change it to “H 2 O”

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

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

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

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