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How to Solve Single Replacement Reactions - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

Let's talk about how to write a single replacement reaction. So single replacement reactions are essentially the backbone. It's when you have an element, and it reacts with the compound, and you have an ionic compound; meaning there's a metal, and a non-metal. B is the metal. Y is the non-metal.

Then what happens is, if they switch partners, or essentially A comes in and removes B from this division. So A now is with Y, and B is alone. You might think of it as the dance. If you're at the school dance; B and y are dancing together. A is by itself. A comes along and cuts in and now in the products, B is by itself, and A dances with Y. You can also have it this way. B now dances with x, and y is alone. If A is a metal, it will replace B. If X is a non-metal, it will replace Y.

The next question we should ask ourselves is, when will a metal replace another metal? There are situations, where actually no reaction happens, meaning, that the metal does not replace the other metal. We have to look at our activity series. In our activity series is, there is one for metals, and there is one for non-metals. Let's look at the one for metals.

Lithium is the most active or most reactive. It won't react with anything. If it comes in contact with any compound, a reaction will take place, because Lithium is the most active metal in the periodic table. This is an abbreviated activity series. So in my activity series, Zinc is the least active. Zinc is hardly reactive at all. If it comes across a compound, it will be unlikely that it will react.

So this is order from the most active metal, to least active metal. Metal can replace anything that's listed beneath it, but nothing that's listed above it. So, the Calcium here, can replace Sodium, Aluminum, Manganese, and Zinc, but cannot replace Potassium and Lithium. No reaction will +happen. Let's look at the first reaction.

We have Potassium, and it's hoping to react with Aluminum Hydroxide. Aluminum is a metal, so Potassium is looking to replace Aluminum, and bond with Hydroxide. If we look at our periodic table, Potassium is here. Aluminum is down here. So yes, a reaction will take place. So Potassium will now react with the Hydroxide, and make sure that we know Potassium is a +1, Hydroxide is a -1. So it will just be KOH, we can ignore that subscript. Aluminum is going to be the one by itself. If we balance this reaction, we get 3 Potassiums react with one Aluminum Hydroxide to produce 3 Potassium Hydroxide, and Aluminum. This is Aluminum solid, not an Aluminum ion.

We can also do a complete ionic equation for that. Because Aluminum Hydroxide is aqueous, we ignore that it separates into its ions in solution. So in reality, it looks like this, what's really happening is this. So we have 3 Potassium, plus Aluminum ions, plus 3 Hydroxide ions. It's going to produce Potassium Hydroxide is soluble. So it will break up in solution. So it will produce 3 Potassium ions plus 3 Hydroxide ions plus Aluminum solid.

So again, we can look for spectator. We cannot cross out Potassium ion with Potassium, they are different. So we can't cross out the +3. Though you can cross out Aluminum with Aluminum solid, very different too. These are not different. The Hydroxides are not different, they are spectator ions.

So now our net ionic equation would look something like this. This would be our ionic. We also might be familiar if we're learning about redox. This is redox reaction. If you're not learning about redox right now, then just ignore this what I'm saying.

All single replacement reactions are also redox reactions, because there's a change of charge. Potassium is going from Potassium with no charge to Potassium with a +1 charge. So there's exchange of electrons going on here. If you're not learning about redox right now, let's ignore that. We'll talk about that another time.