AP Chem Unit 4

physical change

When a material undergoes a change that does not affect its chemical composition

ex: change in shape/size(tearing, folding, moving), phase changes(melting, boiling etc)

chemical change

when one or more substances are converted into new substances

ex: burning, oxidizing, rusting, digesting

signs that a chemical reaction has taken place:

Change in:
-color
-odor
-texture
Production of:
-light
-heat
-gas(bubbles, smoke)
-a precipitate

how do we describe chemical reactions?

We use chemical equations

What must reactions obey?

-Reactions must obey the Law of Conservation of Mass and be balanced

-the number of atoms on one side of the equation must equal the number on the other side

In AP Chem, most reactions are…

in solution, in which ions (not necessarily molecules) are reacting

Although water is the universal solvent…

some substances dissolve better than others

Solubility rules for ionic compounds:

-All nitrates(compounds ending in NO₃^-)

-All compounds w alkali metal ions(Li, Na, K, Rb, Cs) and the ammonium ion (NH₄⁺)

-All acetates(compounds that end with C₂H₃O₂^-)

-All chlorides(Cl), bromides(Br), and iodides(I), except for compounds with Ag, Pb, Hg_2²⁺

-All sulfates(SO₄^2-), except when it’s with Ag, Pb, Hg, Ca, Sr, Ba

Insolubility rules:

-All chromates(compounds ending with CrO₄^2-), except for when with alkali metals and NH₄⁺ chromates

-All phosphates(ending with PO₄^3-) except when with alkali metal and NH₄⁺ phosphates

-All carbonates(ending w CO₃^2-) except when with alkali metals and NH₄⁺ carbonates

-All hydroxides(ending with OH^-) except when with alkali metal hydroxides and Ba, Sr, Ca

soluble compounds exist…

as seperate ions in water solution, meaning ions will dissociate and there will be seperate ions swimming around in the water solution to make a reaction

ex: NaNO₃(s)(soluble) → Na⁺(aq) + NO₃^-(aq) - seperates

Insoluble compounds exist…

as complete compounds in water solution, wont dissociate and will sink down to the bottom of the beaker

ex: AgCl(s) → AgCl(s) (insoluble and wont seperate)

What are the 3 special cases for reactions? and name the reactions

The following 3 substances DO NOT EXIST at normal temps or pressures. Instead of being produced, they undergo a gas evolution reaction:

H₂CO₃ → H₂O(l) + CO₂(g)

H₂SO₃ → H₂O(l) + SO₂(g)

NH₄OH → H₂O(l) + NH₃(g)

strong acids:

-HCl, HBr, HI, HNO₃, H₂SO₄, HClO_{4
-all other acids are weak}

_{—-weak - do not ionize, so in ionic equations just leave as normal}

Other Synthesis reactions:

-a metal oxide will combine w water to produce a metallic hydroxide

—solid potassium oxide is added to water

—K₂O(s) + H₂O(l) → 2K⁺(aq) + 2OH^-(aq)

-a nonmetallic oxide will combine w water to produce an acid

—sulfur trioxide gas is bubbled into water

—SO₃(g) + H₂O(l) → H⁺(aq) + HSO₄^-(aq)

decomposition reactions:

-a metallic carbonate, when heated vigorously

, will decompose into a metallic oxide and CO₂ gas

—solid calcium carbonate is heated over a flame in a crucible

—CaCO₃(s) → CaO(s) + CO₂(g)

-a metallic chlorate, when heated vigorously, will decompose into a metallic chloride and O₂ gas
—Solid sodium chlorate in a test tube is heated vigorously

—NaClO₃(s) → NaCl(s) + O₂(g)

-Acids can decompose into nonmetallic oxides, while bases can decompose into metallic oxides when heated

—carbonic acid decomposes at normal atmospheric pressures

—H₂CO₃(aq) → H₂O(l) + CO₂(g)

—solid sodium hydroxide is heated in a test tube

—2NaOH(s) → Na₂O(s) + H₂O(g)

Is solid salt dissolving in water a chemical or physical change?

-ionic(chemical) bonds are being broken

-ion dipole forces are forming, and they are quite strong

-the structure of the ionic crystal lattice is being broken, and ions are definitely being rearranged

-this line between a physical change and a chemical reaction is sometimes blurry and not as obvious as it seems

what are the 3 steps for converting quantities of substance in a reaction?

1. Balance the equation

2. Convert to moles

3. mole ratio

4. convert to final unit (atoms, grams, molecules etc)

strong acids have to be written…

-in their ionized(dissociated) forms in a chemical equations - they dissociate 100% into ions in aqueous solution into hydrogen ions and their conjugate base anions

-H⁺(aq) + HSO₄^-(aq)

-Meanwhile, weak acids remain mostly neutral molecules in water

strong bases:

• Lithium hydroxide (LiOH)

• Sodium hydroxide (NaOH)

• Potassium hydroxide (KOH)

• Rubidium hydroxide (RbOH)

• Cesium hydroxide (CsOH)

• Calcium hydroxide (Ca(OH)₂)

• Strontium hydroxide (Sr(OH)₂)

• Barium hydroxide (Ba(OH)₂)

A net ionic equation has no reaction when…

every product is aqeous or all the ions cancel out and nothing is left

what is a metal oxide?

its oxygen with a metal, such as, CaO

what is a metal hydroxide?

a metal with a hydroxide, NaOH

how do you balance/find the products or reactants of the equation when shown drawn out?

-on the reactant or product side, count the total number of atoms of each type

-then look for excess reactants/products (molecules that never reacted), which dont match the type of compound shown in the product side of the equation

-place them in the other side

-pull molecules from the reactant/product side until the equaion is even

if you have an ionic compound and attempt to dissolve it…

-ionic(chemical) bonds are being broken

-The only way it can dissolve in water is if the ion-dipole forces between the water and ions are stronger than the ionic bonds that hold the crystal together

-this process has a blurry line between a physical change and a chemical reaction

limiting reactant

the reactant that is consumed first in a chemical reaction; the reactant that produces less product

excess reactant

the reactant left over at the end of a reaction

How do we find the limiting reactant of an equation?

-Make sure the initial equation is balanced
-convert the 2 reactants to the amount of the product asked for

How do we find the amount of excess reactant(amount left unreacted)?

-find the limiting reactant

-using the original amount of the limiting reactant, convert it into the excess reactant

—-using the actual amount used to find the amount used of the excess substance

-subtract the amount used of the excess substance from the original amount of the excess reactant to find the amount left over of the excess reactant

How do you calculate percent yield?

-actual yield / theoretical yield X 100

actual yield

amount of product actually produced in a reaction — determined experimentally

theoretocal yield

the amount of product calculated that should be produced in a reaction — determined using a stoichiometry problem

How can I find the theo yield/actual yield based off have the percent yield and one of those?

-place the percent yield as it is, ex: 72% to 72.0

-divide 100 on each side to make the percent .72

-if theo yield, move the x to the other side then solve

-if actual, multiply actual by percent yield

How do you solve stoich equations with molarity(M=n/V)

-if given mL, convert it to L

-use the L to: L X mol(molarity) / L

—the molarity given counts as the mol amount used in the conversion above

-complete the mole conversion with the compound for the M and mL and convert it into the compound asked for

Pertaining to excess and limiting reactants, how do you tell which ions have the lowest and greatest concentrations?

-it may say that there is the excess of a compound

—if 2 of the choices are in the excess then look at which is most abundant

—ex: excess Na₂SO₄ → Na has 2 molecules, while sulfate has only 1, so Na would have the greatest concentration w sulfate coming in 2nd

In stoich problems, if asked to list for an appropriate observation that there is a chemical reaction between the compounds, what should i try to say first?

a solid precipitate is formed when the 2 reactants are mixed

How do you find the diagram that best represents the results after a stoich solution?

-look for the one with the precipitate

-using the limiting reactant, you can solve that one of the elements would not exist in the diagrams

-using the excess reactant, you can solve that some elements would exist/exist the most in the diagrams

How do you find the molar concentration of an element after the reaction is complete in a stoichiometry problem?

-use one of the reactants that have the element asked for

-then do mole conversion between the general compound and the individual mole amount within the compound