When dealing with chemical reactions, it is important to understand that, at least for the case of AP Chemistry, the vast majority of them will be taking place in an aqueous solution, that is, dissolved in water💧. Therefore, we will not be dealing with molecules, but rather their constituent particles that are created when they dissolve.
Let's take NaCl🧂 for example. When you dissolve table salt in water, it is essentially ripped apart by the water molecules. It is important to understand that water is a polar molecule. Polar molecules have a partial positive end and a partial negative end. Therefore, when an ionic compound like NaCl dissolves, the water molecules rip it apart and trap it, per say.
In other words, ionic compounds dissociate into their ions when dissolved in water.
We can write out this dissolving of salt as: NaCl (aq) --> Na+ (aq) + Cl- (aq). Thus, if we have a reaction between two compounds in water, it will be the ions/parts of the molecules that dissolve that react together.
💡Na+ attracts to the negative oxygen atoms. Cl- attracts to the positive hydrogen atoms.
Let's look at a specific class of such reaction:
Sometimes, when two reactants react, one of the products is insoluble. To be insoluble means that it will NOT be ripped apart by water like NaCl would be.
One example of this is the precipitation of lead (ii) iodide:
2KI (aq) +Pb(NO3)2 (aq) --> 2KNO3 (aq) + PbI2 (s).
Because lead (ii) iodide is not soluble in water (we've went over solubility rules in earlier guides [link above]), it precipitates and falls out of solution.
Here is another table of solubility rules:
Image Courtesy of Regents Chemistry Reference Table
When we're dealing with reactions in aqueous solution, we only care about some of the ions reacting. Others are called spectator ions, and are essentially just there for the ride. They react, but they don't actually take true part in the reaction. Let's see this in action with our previous reaction: 2KI (aq) + Pb(NO3)2 (aq) --> 2KNO3 (aq) + PbI2 (s)
If we write this reaction out with all of the ions, we get:
2K+ + 2I- + Pb2+ + 2NO3- --> 2K+ + 2NO3- + PbI2 (s)
💡Note that PbI2 does NOT dissociate, since it is insoluble. Be on the lookout for this! Only dissociate soluble compounds. Also, don't dissociate weak acids and bases🍊 (you'll go over this more in the future).
This equation, with the ions written out, is called the complete ionic equation or the total ionic equation.
We see through this that potassium and nitrate go in and come out the same way, so we can cross them out and cancel them. Potassium and Nitrate were the spectator ions👓! This leaves us with:
2I- + Pb2+ --> PbI2(s).
This is called the net ionic equation for the reaction, and tells us what really happens. Potassium and nitrate took place, but they really didn't do much for us in terms of reaction. They could have been any suitable ion and it would happen the same.
💡Remember when writing net ionics that coefficients (the numbers before each reactant) are distributed between ions.
To memorize, here are the general steps to writing a net ionic equation:
Figure out which compounds are soluble and insoluble using solubility rules
Balance the equation
Complete Ionic Equation - Write soluble compounds into ions (complete the dissociation)
Net Ionic Equation - Cross out spectator ions and write the final result!
Try writing the net ionic equation for the following reaction:
KOH + Fe(NO3)3 --> KNO3 + Fe(OH)3
Using solubility rules, you should find that: Fe(OH)3 is the solid, or the precipitate
KOH (aq) + Fe(NO3)3 (aq) --> KNO3 (aq) + Fe(OH)3 (s)
You should notice that there is only 1 OH on left and 3 OH on the right. Same goes for NO3, but vice versa. This means we have to balance the equation.
3KOH (aq) + Fe(NO3)3 (aq) --> 3KNO3 (aq) + Fe(OH)3 (s)
📝Read: AP Chemistry - Balancing Chemical Equations
Break them up into their ions
Net Ionic Equation =
Spectator Ions: 3K+ + 3OH- + Fe+3 + 3NO3- --> 3K+ + 3NO3- + Fe(OH)3 (s)
Answer: 3OH- (aq) + Fe+3 (aq) --> Fe(OH)3 (s)
🎥Watch: AP Chemistry - Net Ionic Equations
✍️ Free Response Questions
AP Chemistry Free Response Questions
⚛️ Unit 1: Atomic Structure and Properties
1.1Moles and Molar Mass
1.2Mass Spectroscopy of Elements
1.3Elemental Composition of Pure Substances
1.4Composition of Mixtures
1.5Atomic Structure and Electron Configurations
1.6Photoelectron Spectroscopy & Graph Interp.
🤓 Unit 2: Molecular and Ionic Compound Structures and Properties
2.0Unit 2 Overview: Molecular and Ionic Bonding
2.1Types of Chemical Bonds
2.2Intramolecular Force and Potential Energy
2.3Ionic Bonding and Ionic Solids
2.4Metallic Bonding and Alloys
2.5Lewis Dot Diagrams
2.6Resonance and Formal Charge
🌀 Unit 3: Intermolecular Forces and Properties
3.0Unit 3 Overview: Intermolecular Forces and Properties
3.2Properties of Solids
3.3Solids, Liquids, and Gases
3.4The Ideal Gas Law
3.5The Kinetic Molecular Theory of Gases
3.6Deviations from the Ideal Gas Law
3.7Mixtures and Solutions
3.8Representations of Solutions
3.9Separation of Solids/Mixtures
3.10Solubility and Solubility Rules
3.11Spectroscopy and the Electromagnetic Spectrum
3.12Quantum Mechanics and the Photoelectric Effect
🧪 Unit 4: Chemical Reactions
4.0Unit 4 Overview: Chemical Reactions
4.1Recognizing Chemical Reactions
4.2Net Ionic Equations
4.4Physical vs. Chemical Changes
4.5Stoichiometry & Calculations
4.6Titrations - Intro and Calculations
4.8Intro to Acid-Base Neutralization Reactions
👟 Unit 5: Kinetics
5.0Unit 5 Overview: Kinetics
5.1Defining Rate of Reaction
5.2Introduction to Rate Laws
5.3Rate and Concentration Change
5.4Writing Rate Laws
5.5Collision Model of Kinetics
5.6Reaction Energy and Graphs w/ Energy
5.7Reaction Mechanisms and Elementary Steps
5.8Writing Rate Laws Using Mechanisms
🔥 Unit 6: Thermodynamics
6.0 Unit 6 Overview: Thermochemistry and Reaction Thermodynamics
6.1Endothermic Processes vs. Exothermic Processes
6.2Energy Diagrams of Reactions
6.3Kinetic Energy, Heat Transfer, and Thermal Equilibrium
6.4Heat Capacity and Coffee-Cup Calorimetry
6.5Phase Changes and Energy
6.6Introduction to Enthalpy of Reaction
6.7Bond Enthalpy and Bond Dissociation Energy
6.8Enthalpies of Formation
⚖️ Unit 7: Equilibrium
🍊 Unit 8: Acids and Bases
8.0Unit 8 Overview: Acids and Bases
8.1Introduction to Acids and Bases
Unit 9: Applications of Thermodynamics
🤺 AP Chemistry Essentials
🧐 Multiple Choice Questions
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