Chemical Reactions

1. Chemical Reactions
1. Why do reactions occur anyway?  To lower energy and entropy of the reactants.  Many reactions occur in water = aqueous chemistry.
2. There are three main types of reactions we will look at in the chapter:
1. precipitation reactions:  typically look like this:  (aq) + (aq)  g  (s) + (aq)  A precipitate solid is formed. (The cations swap anion partners)
2. acid base / neutralization reactions:  HA(aq) + MOH(aq)  g H₂O(l) + MA(aq)  
3. redox reactions:  when electrons are transferred so that oxidation states change:  Cu²⁺(aq) + Mg(s)  g Cu(s) + Mg²⁺(aq)
   
   
2. Electrolytes - solution containing lots of ions
1. Compare covalent compounds and ionic compounds in water:
1. Sugar dissolves in water.  It is a covalent compound.  When it dissolves the molecules stay intact - they just separate from each other.  The IMF (such as dipole dipole forces) between the molecules are broken, but NOT the covalent bonds (that hold the atoms within one molecule).  Note that to determine solubility of covalent compounds in a liquid the general rule is "like dissolves like" which means polar liquids dissolve polar compounds and nonpolar liquids dissolve nonpolar compounds.  Sugar water does not conduct electricity since there are no ions - it is a non-electrolyte.  
2. Salt also dissolves in water, BUT it goes one step further. Besides having the NaCl units separate from each other, they also break apart into the Na⁺ and Cl^- ions.  When ionic compounds dissolve in water they also dissociate = ionize = break into ions. The ionic bonds actually break so that the ion-dipole forces can form when each ion is surrounded by water.  If the ion-dipole forces will be more stabilizing than the original ionic bonds, a salt will dissolve.  Note that to determine solubility of ionic compounds in water we use solubility rules. Soluble ionic compounds put lots on ions into solution so they make strong electrolytes.  Insoluble ionic compounds only put a few ions into solution so they make weak electrolytes. Electrolytes conduct electricity.  
2. Watch this movie.  Here's another one if you click on Activities and Dissolution of NaCl movie. 
3. Acids break the generalization.  Acids are covalent molecules that actually ionize as though they were ionic compounds!  For example HCl (hydrochloric acid) in water can ionize into H⁺(aq) and Cl^-(aq) ions.  Practically all the HCl will ionize, thus putting lots of ions in solution - strong electrolyte.  Weak acids are acids that ionize just a little bit putting some ions in solution = weak electrolyte. Weak acids set up equilibrium since the reaction is never over - the rxn goes forwards and backwards forever. 
4. Summary Table 

Strong Electrolytes ionize > 70%	Weak Electrolytes ionize < 70%	Non Electrolytes ionize 0%
soluble ionic compounds	insoluble ionic compounds	most covalent compounds
strong acids	weak acids
strong bases	weak acids
conducts well	conducts a little	don't conduct at all

5. Watch these two electrolyte movies by clicking on Activities then Electrolytes movie and Strong and Weak Electrolytes movie.
6. How many ions are in solution if I have 0.444 moles of calcium carbonate in water?  0.444 moles CaCO₃ (2 ions / CaCO₃)(6.02 x 10²³ / mol) = 5.35 x 10²³ ions.  Note CaCO₃ contains 2 ions:  Ca²⁺ and CO₃^2-.  If the question had asked about nickel(III) nitrate there would have been 4 ions. 

3. Ionic and Net Ionic Reactions
1. Given this reaction:  copper(II) nitrate plus potassium carbonate in water.  Write and balance the reaction.  Draw pictures of the reactants and products.  Use your solubility rules to determine which of these ionic compounds are ionized in water and which are solids. Remember precipitation reactions are swap partner reactions (also called double displacement)

B. Answer:  Cu(NO₃)₂(aq) + K₂CO₃(aq)  g  CuCO₃(s) + 2 KNO₃(aq)  This is the molecular reaction.  Here are the beakers.  Water molecules are omitted for clarity. 

3. Notice the molecular reaction doesn't really reflect reality.  All the (aq) species are ions in water - not bonded together.  So we can write ionic reactions which show all the ionized species as ions:  Cu²⁺(aq) +  2 NO₃^-(aq) + 2 K⁺(aq) + CO₃^2-(aq)  g  CuCO₃(s) + 2 NO₃^-(aq) + 2 K⁺(aq)  This is much closer to reality and matches the picture above much better.  It is just a pain to write it all out!    YOU have to remember that the (aq) state really means dissolved and ionized and surrounded by water for ionic compounds. 
4. Now we can write the net ionic reaction where we cross out the ions that appear on both sides called spectator ions since they don't do anything and are not part of the reaction really.  What are the spectator ions?  See the potassium and nitrate ions are on both sides - they did nothing.  Cross them out.   Cu²⁺(aq) + CO₃^2-(aq)  g  CuCO₃(s) 

4. Precipitation Reactions
1. Defined above. Cations swap anion partners = double displacement.  Remember to look at solubility rules to determine if the ionic compounds are ionized in water or solid. Now that we have already seen one reaction worked out - here are the steps to take when writing reactions:
1. Will a reaction occur or is it no reaction (NR)?  If nothing changes at all there is no reaction.
2. Write the correct product formulas so that compounds add up to zero. (don't even pay attention to the reactant formulas - they have nothing to do with it) Review oxidation states and ionic formulas on the Help page if needed. 
3. Write the state:  solid, liquid, gas or aqueous
4. last of all balance (because of the Law of Conservation of Mass) 
2. Write and balance the molecular, ionic and net ionic reaction between lithium nitrate and sodium sulfate.  Well hum,  Check it out - we have four kinds of ions in solution before reaction - Li⁺, NO₃^-, Na⁺, and SO₄^2-.  The minus ions won't react - they repel each other.  The positive ions won't react - they repel each other.  Li⁺ and NO₃^- don't react or they would not have dissolved in the first place.  Na⁺ and SO₄^2- don't react or they also would not have dissolved in the first place.  So the only possible products are lithium sulfate and sodium nitrate - both are soluble and ionized in water!  So we end with the same ions in solution.  Nothing happened.  NR.  
3. Write and balance the molecular, ionic and net ionic reaction between potassium sulfide and magnesium iodide.  
1. First imagine the products - potassium iodide and magnesium sulfide.  One of those is insoluble so there is a rxn. 
2. K₂S + MgI₂ g  KI + MgS  (these are the correct formulas:  KI since +1-1=0 and MgS since +2-2=0)
3. K₂S(aq) + MgI₂(aq)  g  KI(aq) + MgS(s)    from solubility rules
4. K₂S(aq) + MgI₂(aq)  g  2 KI(aq) + MgS(s)   balanced! 
5. Imagine a picture - draw it yourself!  In one beaker there is two K⁺ and one S^2- ions.  In the second beaker there is one Mg²⁺ and 2 I^- ions.  Pour them together and we get MgS solid sinking to the bottom with two K⁺ and two I^- ions still ionized in the water. The ionic reaction reflects this. 
6. Ionic rxn:  2 K⁺(aq) + S^2-(aq) + Mg²⁺(aq) + 2 I^-(aq)  g  2 K⁺(aq) + 2 I^-(aq) + MgS(s)
7. Net ionic rxn:     S^2-(aq) + Mg²⁺(aq)  g MgS(s)   (spectator ions were potassium and iodide)
4. Write the molecular, ionic and net ionic reactions for adding an aqueous solution of sodium sulfate with a solution of lead(II) nitrate.  
1. Figure out the products - NaNO₃ and PbSO₄   Are they soluble?  NaNO₃ is soluble but PbSO₄ is not.  
2. Now write the three versions of the reaction, don't forget to balance them.  Draw it if you want to. 
1. Na₂SO₄(aq) + Pb(NO₃)₂(aq) g 2 NaNO₃(aq) + PbSO₄(s)
2. 2 Na⁺(aq) + SO₄^2-(aq) + Pb²⁺(aq) + 2 NO₃^-(aq) g 2 Na⁺(aq) + 2 NO₃^-(aq) + PbSO₄ (s)
3. SO₄^2-(aq) + Pb²⁺(aq)  g  PbSO₄(s) 
5. Write and balance the molecular, ionic and net ionic reaction between silver nitrate and sodium chloride. 
1. NaCl(aq) + AgNO₃(aq) g AgCl (s) + NaNO₃(aq)
2. Na⁺(aq) + Cl^-(aq) + Ag⁺(aq) + NO₃^-(aq) g AgCl (s) + Na⁺(aq) + NO₃^-(aq)
3. Cl^-(aq) + Ag⁺(aq) g AgCl (s)   Note that Na⁺ and NO₃^- cancelled out - they are the spectator ions
6. Precipitation movie. 

5. Acid Base / Neutralization Reactions
1. Acids - species that ionize and lose H⁺ ions in water.  Generic acid HA g H⁺(aq) + A^-(aq).  
1. Strong acids are those in which almost all of the molecules lose H⁺ ions in water.   Strong acids are soluble in water and are strong electrolytes since they ionize almost 100%.  These are strong acids:  HCl = hydrochloric acid, HBr = hydrobromic acid, HI - hydroiodic acid, HNO₃ = nitric acid, and H₂SO₄ = sulfuric acid. HCl g H⁺(aq) + Cl^-(aq). 
2. Weak acids ionize much less than 100% so are weak electrolytes since they make few ions. Acetic acid in vinegar CH₃COOH and HF are weak acids. They set up equilibrium in water.  HF D H⁺(aq) + F^-(aq).  
3. Please note that H⁺ does not really exist - it is shorthand for H₃O⁺ because H⁺ jumps on a water molecule. Remember acids lose H⁺ by "people that take acids are losers"    Watch this acid movie by clicking on Activities then Intro to Acids movie. 
4. What makes something acidic is H⁺ ions in water. 
2. Bases - species that make OH^- ions in water. Most are metal hydroxides. Generic base MOH g M⁺(aq) + OH^-(aq).  
1. Strong bases are soluble and ionize in water.  These are strong bases:  NaOH = sodium hydroxide, KOH = potassium hydroxide, LiOH = lithium hydroxide, and Ba(OH)₂ = barium hydroxide.  Strong bases are strong electrolytes since they ionize almost 100%  KOH g  K⁺(aq) + OH^-(aq)
2. Weak bases ionize much less than 100% so they are weak electrolytes since they make just a few ions.  Magnesium hydroxide and ammonia are weak bases.  Mg(OH)₂ is only slightly soluble in water (Milk of Magnesia).  Ammonia does not even contain the OH^- ion but it reacts with water like this;  NH₃(aq) + H₂O(l)  D NH₄⁺(aq) + OH^-(aq).  So it creates some OH^-ions.  
3. What makes something basic is hydroxide ions being present. Watch this base movie by clicking on Activities then Intro to Bases movie. 
3. Now the neutralization reaction is when we add an acid already in water to a base already in water.  A typical acid base reaction is:  acid + base g  ionic salt + water.  In other words, the H from the acid combines with the OH from the base to make a water molecule and the other elements combine to form an ionic salt which is usually soluble.  Note that this is a double displacement reaction.
4. Example - write and balance the molecular, ionic, and net ionic reaction between hydrochloric acid and sodium hydroxide: 
1. HCl(aq) + NaOH(aq)  g NaCl(aq) + H₂O(l)
2. the ionic is H⁺(aq) +  Cl^-(aq) + Na⁺(aq) + OH^-(aq)  g  Na⁺(aq) +  Cl^-(aq) + H₂O(l) 
3. and finally the net ionic reaction is    H⁺(aq) +  OH^-(aq)  g   H₂O(l)         and the spectator ions are sodium and chloride.
5. Example - write and balance the chemical, ionic, and net ionic reaction between sulfuric acid and lithium hydroxide:  
1. 2 LiOH(aq) + H₂SO₄(aq)  g  Li₂SO₄(aq) + 2 H₂O(l) is the molecular reaction.  
2. 2 Li⁺(aq) +2 OH^-(aq) + 2 H⁺(aq) + SO₄^2-(aq)  g  2 Li⁺(aq) + SO₄^2- (aq) + 2 H₂O(l) is the ionic reaction.  
3.  OH^-(aq) +  H⁺(aq) g  H₂O(l) is the net ionic reaction. (the 2 coefficients cancel out)

6. Oxidation and Reduction
1. Redox reactions are so very important:
1. formation reactions (element + element g compound)
2. combustion reactions (C_xH_y + O₂(g) g  CO₂(g) + H₂O(g))
3. single replacement reactions  (Sn(s) + Cu(NO₃)₂(aq) g  Sn(NO₃)₂(aq) + Cu(s))
4. corrosion reactions  (4 Fe(s) + 3 O₂(g)  g 2 Fe₂O₃(s))
5. respiration, bleaching, and batteries also! 
2. It's all about charges (oxidation states) changing!  You MUST KNOW THE OXIDATION STATES.
1. oxidation is losing electrons so the charge goes up
2. reduction is gaining electrons so the charge is reduced (goes down)
3. You must have both together - and electron is transferred from what is getting oxidized to what is getting reduced
4. What are the oxidation states for:
1. H₂O  (H is +1 and O is -2)
2. CO₂  (C is +4 and O is -2)
3. N₂  (N is 0)  all elements are zero
4. Ni₂O₃  (O is -2 to that means Ni must be +3)

7. Redox Reactions
1. Watch this base movie by clicking on Activities then oxidation and reduction part I movie.
2. Example of a redox reaction:  Co (s) + Pb²⁺(aq) g Pb (s) + Co²⁺(aq)  is a single replacement rxn
1. Co goes from zero to +2
2. Pb²⁺ goes from +2 to zero
3. What is oxidized?  Co (s)
4. What is reduced?  Pb²⁺(aq)
5. What is the oxidizing agent?  Pb²⁺(aq)
6. What is the reducing agent? Co (s)
7. This is already a net ionic reaction by the way.  You can't actually have positive charges without negative charges.  
3. Example:  CH₄(g) + 2 O₂(g) g CO₂(g) + 2 H₂O(g)  a combustion reaction
1. C in CH₄ goes from -4 to a +4 in CO₂
2. O in O₂ goes from zero to -2 in CO₂ and H₂O
3. What is oxidized?  C in CH₄
4. What is reduced?  O in O₂
5. What is the oxidizing agent?  O₂
6. What is the reducing agent? CH₄
7. Note also that the answer to those four questions is never a product since they are being formed. There is not ionic rxn here since there are NO IONS!
4. Example:  2 Na (s) + 2 HCl (aq) g 2 NaCl(aq) + H₂(g)    is a single replacement rxn
1. Na goes from zero in the solid to +1 in NaCl
2. H goes from +1 in HCl to zero in hydrogen gas
3. What is oxidized?  Na (s)
4. What is reduced?  H in HCl(aq)
5. What is the oxidizing agent?  HCl(aq)
6. What is the reducing agent? Na (s)
7. What is the net ionic reaction?  2 Na (s) + 2 H⁺(aq) g 2 Na⁺(aq) + H₂ (g)  (Cl was just a spectator)

8. Activity Series
1. How did I know if those single replacement reactions would occur?  Well the activity series lets you know.  The MORE active metal or cation wants to be an ion in a compound.  The LESS active metal or cation wants to be in its elemental state. Look at example D and B above again and look at the activity series.
2. Practice these fabulous examples
3. Watch the formation of silver crystals movie here by clicking on Activities. It is a single replacement rxn
4. Write the reaction between solid copper and lead(II) nitrate.  Actually NR since Cu is less active and want to be elemental solid copper anyway.
5. Write the molecular, ionic and net ionic reaction between solid zinc and lead(II) nitrate.  Remember the 4 steps?  
1. Yes there is a reaction since zinc is more active and wants to be in the compound more than lead.
2. Products would be solid lead Pb(s) and zinc nitrate Zn(NO₃)₂(aq)  (remember Zn is always +2 charged)
3. Zn(s) + Pb(NO₃)₂(aq) g Pb(s) + Zn(NO₃)₂(aq)
4. already balanced 
5. ionic reactions:  Zn(s) + Pb²⁺(aq) + 2 NO₃^-(aq) g Pb(s) + Zn²⁺(aq) + 2 NO₃^-(aq)
6. net ionic:  Zn(s) + Pb²⁺(aq) g Pb(s) + Zn²⁺(aq)    (nitrate was a spectator ion)
7. What is oxidized, reduced?  What are the agents?  oxidized is solid Zn as its charge goes up, reduced is lead 2+ ion as its charge goes down, the ox agent is lead 2+ ions and the red agent is solid zinc.
6. The most active metal is Li.  why is it active anyway?  Well because Li WANTS to be +1 charged so it can be like a noble gas.  Na is also active because it wants to be +1 charged. Remember the metal atoms goal in life - to be s²p⁶ and they do that by losing electrons.  The transition metals are not as active since changing their d electrons is not that important to them. 

Done!!!  Study this chapter hard.  It is so important.