- Oxidation–reduction (redox) reactions
- Worked example: Using oxidation numbers to identify oxidation and reduction
- Balancing redox equations
- Dissolution and precipitation
- Precipitation reactions
- Double replacement reactions
- Single replacement reactions
- Molecular, complete ionic, and net ionic equations
- Molecular, complete ionic, and net ionic equations
- 2015 AP Chemistry free response 3a
Definition and examples of double replacement reactions. Predicting and balancing neutralization and precipitation reactions.
What is a double replacement reaction?
Double replacement reactions—also called double displacement, exchange, or metathesis reactions—occur when parts of two ionic compounds are exchanged, making two new compounds. The overall pattern of a double replacement reaction looks like this:
You can think of the reaction as swapping the cations or the anions, but not swapping both since you would end up with the same substances you started with. The solvent for a double replacement reaction is usually water, and the reactants and products are usually ionic compounds—but they can also be acids or bases.
Here is an example of a double replacement reaction:
In this example, the cations are and , and the anions are and . If we swap the anions, or cations, we get as our products and .
Precipitation and neutralization reactions
Identifying double replacement reactions is usually fairly straightforward once you can recognize the pattern. Predicting whether the reaction will occur can be trickier; it helps to be able to recognize some common types of double replacement reactions. In this article we will be discussing precipitation reactions and neutralization reactions.
A precipitation reaction is when two aqueous ionic compounds form a new ionic compound that is not soluble in water. One example is the reaction between lead (II) nitrate and potassium iodide. Both compounds are white solids that can be dissolved in water to make clear, colorless solutions. When you combine the two clear solutions, you get the following reaction:
We made a beautiful golden solid from two clear solutions! In real life, your reaction flask might look something like the picture below.
The insoluble product compound is called the precipitate. The solvent and soluble components of the reaction are called the supernatant or supernate. We can use solubility rules to predict whether a precipitation reaction will take place. The formation of a solid precipitate is the driving force that makes the reaction proceed in the forward direction.
Concept check: What is in our supernatant?
Neutralization reactions are a type of double replacement reaction that occurs between an acid and a base. The following is an example of a neutralization reaction:
An aqueous neutralization reaction generally produces water and a new ionic compound, also called a salt. The hardest part of identifying a neutralization reaction is recognizing that you have an acid and a base for the reactants. Once you know you have a neutralization reaction, you can generally predict the reaction will occur in the forward direction as long as you have a strong acid and/or a strong base as a reactant.
A fun neutralization reaction that you may have tried is the combination of baking soda—sodium bicarbonate, —and vinegar—mostly water with acetic acid, )—which produces carbonic acid——and sodium acetate—. If you have tried this reaction at home, you probably remember a lot of fizzing because the neutralization reaction is accompanied by a gas-producing reaction, where the carbonic acid decomposes into carbon dioxide gas—bubbles!—and water.
Note that double replacement reactions can be written as molecular, complete ionic, or net ionic equations. In this article we are only writing out the molecular equation, but you probably want to be familiar with writing the other forms of the equation as well.
Example: predicting and balancing a double replacement reaction
Let’s take a look at an example where we don't know the products:
First, we can identify the cations and anions that will get swapped. The cations are and , and the anions are and . Swapping anions gives the products and :
We can see that our double replacement reaction is also a neutralization reaction since we are reacting sulfuric acid, a strong acid, with barium hydroxide, a strong base. What is the state of the product barium sulfate? If we check our solubility rules, we see that barium sulfate is insoluble and should precipitate out of solution. That means our reaction is a precipitation reaction, too! We can also include that information in our equation by adding the symbol after the .
We aren’t quite done yet, though. Our reaction is not balanced since we have unequal numbers of hydrogens and oxygens on both sides of the arrow. We can fix this by multiplying by to give our final, balanced molecular equation:
Double replacement reactions have two ionic compounds that are exchanging anions or cations. Precipitation reactions and neutralization reactions are two common types of double replacement reactions. Precipitation reactions produce an insoluble product from two aqueous reactants, and you can identify a precipitation reaction using solubility rules. Neutralization reactions occur when the reactants are an acid and a base, and neutralization reactions are usually favorable as long as the reaction involves a strong acid and/or a strong base.
Want to join the conversation?
- How do you know whether or not there will be no reaction?(17 votes)
- You can use a solubility chart, or solubility rules. The solubility chart is used based on the products - if the combination of ions that are produced results in a down arrow on the solubility chart, it means it precipitates, and there is a reaction. If it says AQ, it means it's aqueous. If both products are aqueous, there is no reaction. If a product isn't on the chart, assume that it is aqueous.(25 votes)
- Where did that third OH come from in the first practice example? Are we assuming the charges would balance because it's in water with an abundance of oxygen and hydrogen atoms?(13 votes)
- It is because we know that the iron ion, Fe(III) has a charge of 3+, so it has to combine with 3 OH- ions to make a neutral compound.
We know the iron has a charge of 3+ from looking at the starting material, FeCl3. Since FeCl3 is a neutral compound and chloride has a charge of -1 (and there are three chlorides), then Fe must have a 3+ charge in the starting material and product.(20 votes)
- How do you know when or IF the many reactions will occur and when/if they will then decompose?
What is a salt? (just any ionic compound?)
How do you know if something is an acid or base just from the formula? (or do you need to memorize these)
Is everything that is soluble considered to be in aq state?
How do you know what medium the reaction takes place in or what medium it needs? (water, non-polar solvent, in air, in space, etc etc)(5 votes)
- A salt is generally any ionic compound, though I have also seen it defined as an ionic compound that is formed when you react an acid and a base. The cation (or positively charged ion) of the salt comes from the base, and the anion (or negatively charged ion) comes from the acid.
It is helpful to have the strong acids and bases memorized, since they have special reactivity. Luckily, there aren't that many strong acids and bases, and you can learn morem about this from this video: https://www.khanacademy.org/science/chemistry/acids-and-bases-topic/acids-and-bases/v/strong-acids-and-strong-bases
Anything that is soluble in water and dissolved (separated into individual cations and anions) is in the aqueous state. Double replacement reactions always occur in water, with the reactants in the aqueous state. Water is a really great solvent whenever you want to have ions around.
In general, it's tricky to predict for any random reaction what medium it might need. The more you know about how the reaction occurs, and the more you know about the properties of different solvents (like their polarity), the more educated of a guess you can make! For example, in double replacement reactions, we know that the solubility of the reactants is important because we need free ions around. So we might predict that a non-polar solvent that doesn't dissolve salts would be a bad solvent for a double replacement reaction.
I know that's a lot of information, I hope you find some of it helpful!(14 votes)
- This may be an obvious question but, what classifies a chemical as a salt? For example, when an acid-base neutralization reaction occurs and water and a salt is formed, what are the characteristics of the salt formed that makes it a salt?(5 votes)
- What is the name of the equations which produce salt water and carbon dioxide?
I cant seem to find this topic on here.
Please help.(3 votes)
- They are double replacement reactions followed by decomposition.
Double replacement: 2HCl(aq) + Na₂CO₃(aq) → 2NaCl(aq) + H₂CO₃(aq)
Decomposition: H₂CO₃(aq) → H₂O(l) + CO₂(g)(15 votes)
- How do you know that Fe has a 3+ charge? It's not in groups 1,2,13,14,15,16,17,or 18 so how can we predict the charge without knowing it's roman numeral?(2 votes)
- You have to apply the rules to whatever else it is bonded to and go from there.
In this case it is Cl which is in group 17 and so forms Cl- anions, so if the formula is FeCl3 then the Fe must be 3+ to have no charge overall.(9 votes)
- What are the solubility rules?(4 votes)
- what would have to be the situation in which there would be no possible reaction in a double displacement ?(3 votes)
- When you're predicting the products of the reactions, like in the example, how can you recognize that as double replacement and not synthesis or something else?(2 votes)
- If its two ionic compounds, it is usually a double replacement. If it is an element and a compound it is single, if it has O2 in the reactants and produces H2O and CO2 then its combustion. When there is a single product it is a synthesis.(4 votes)
- In the first example, how was it determined that Barium and Sodium are the cations? Are cations always written before anions in a neutral compound? Is there a list or set of rules that chemists memorize?(1 vote)
- Barium and Sodium are both metals, so they have few numbers of valence electrons. Barium has two and Sodium has only one. So, to achieve their perfect 8 electrons in their outer shell (octet rule), they will lose electrons to make their full previous shells their new valence electrons. When an atom loses electrons, it becomes a positive ion, or a cation. I hope that helped!(2 votes)