Science Chemistry library Chemical reactions and stoichiometry Types of chemical reactions. 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. Current timeTotal duration Google Classroom Facebook Twitter. Video transcript - [Hank Green] You're at dinner with your best friend.
Fine conversation, fine wine, some barbecued beef cheeks, you look outside to admire the full moon but when you glance back you realize that your friend has turned into a werewolf. Fortunately, the cutlery is made of silver and you know how to use it. Or perhaps you're in the bath one day and as you reach for the soap you notice a wart on your big toe.
Well, squeeze a little silver nitrate on that big boy and you'll be ready for sandal season in no time. Shiny electrically conductive, and oh so useful, silver has been valued since ancient times and has a reputation for purity and warding off evil whether in the form of werewolves or warts.
Silver was also a big driver of the settlement in the western United States, including Montana, where I live. And of course, all that silver got here because of chemistry. Specifically, it's here because of countless chemical reactions that took place over the eons called precipitation reactions.
When chemicals, in a solution, react to form a solid. Precipitation reactions are what create geological deposits in the earth as well as rings around your bathtub. They're what we use to make our waste water drinkable and they've been used by folks for thousands of years to get rich, because precipitation reactions happen to be one of the best ways to produce chemicals of the highest purity.
So they're not only the key to how silver was deposited in these mountains hundreds of millions years ago, they're also the key to getting that silver back out.
I can do it right here on this desk and all I need to get started is this. Water falling out of the sky. Solids falling out of solution. And for us here, it all comes down to a little thing called solubility. Water, as we've discussed here before, is pretty dang good at dissolving stuff.
Ionic compounds, in particular. A positively charged ion and a negatively charged ion held together by their charges might form a crystal when they're dry but add a bit of water and those little polar molecules slide their way between the ions dissolving massive amounts of ionic compounds. But some ionic compounds can overcome even the dissolving power of water. And when they form through reactions and solution, they fall out as a solid precipitate. Yes, precipitate is both a noun and a verb.
Get used to it. When we talk about an ionic compound that's fallen out of solution, I say precipitate to distinguish it from precipitate which is more the verby sound. And this is purely my preference because that's how my teacher said it when I was being taught. So the rich silver veins in Montana formed when water stuffed with ionic compounds ran through cracks and pillars that were limestone.
Where conditions were right, silver ions in the water reacting with ionic compounds or salts, in the limestone to make insoluble silver compounds that fell out of solution. And it looks a little bit like this. It actually looks exactly like this.
It's pretty cool because, you can't feel this, but it's extremely heavy because silver is a pretty heavy element. And it wasn't just the silver salts and the solution, all kinds of stuff; gold and potassium and copper salts, and most notably, sodium salts are dissolved as water rushes across the landscape. If these dissolved compounds stay in solution until they get to the ocean, they pretty much stay there forever. The water evaporates, leaving the salts behind in the ocean where over the eons, it has built up leaving the ocean super salty as we know it today.
And while sodium chloride, what we call salt when not doing chemistry, is the most common salt. There are also tons of other things dissolve in the ocean, including quite a lot of gold.
No precipitate forms. The solution stays light blue. What happened? The product that forms may be insoluble, in which case a precipitate will form, or soluble, in which case the solution will be clear.
You can automatically exclude the reactions where sodium carbonate and copper II chloride are the products because these were the initial reactants. The balanced chemical equation is:. You know that sodium chloride NaCl is soluble in water, so the remaining product copper carbonate must be the one that is insoluble. Notice how the sodium and chloride ions remain unchanged during the reaction.
They are called spectator ions. They can be removed from the equation yielding the overall precipitation reaction:.
The possible combinations of the ions are as follows:. Once again, the reactions where sodium sulfate and copper II chloride are the products can be excluded, since they were the initial reactants.
After balancing, the resulting equation is as follows:. Separate the species into their ionic forms, as they would exist in an aqueous solution.
Balance the charge and the atoms. Cancel out all spectator ions those that appear as ions on both sides of the equation. This particular example is important because all of the reactants and the products are aqueous, meaning they cancel out of the net ionic equation. There is no solid precipitate formed; therefore, no precipitation reaction occurs. Write the net ionic equation for the potentially double displacement reactions. Make sure to include the states of matter and balance the equations.
After dissociation, the ionic equation is as follows:. The ionic equation is after balancing :. This means that both the products are aqueous i. The ionic equation is:. After canceling out spectator ions, the net ionic equation is given below:.
Properties of Precipitates Precipitates are insoluble ionic solid products of a reaction, formed when certain cations and anions combine in an aqueous solution. Figure 1: Above is a diagram of the formation of a precipitate in solution. Precipitation and Double Replacement Reactions The use of solubility rules require an understanding of the way that ions react.
This can be thought of as "switching partners"; that is, the two reactants each "lose" their partner and form a bond with a different partner: Figure 2: A double replacement reaction A double replacement reaction is specifically classified as a precipitation reaction when the chemical equation in question occurs in aqueous solution and one of the of the products formed is insoluble.
Solubility Rules Whether or not a reaction forms a precipitate is dictated by the solubility rules. Bromides, chlorides, and iodides are soluble. Salts conta ining silver, lead, and mercury I are insoluble. Sulfides formed with group 2 cations and hydroxides formed with calcium, strontium, and barium are exceptions. Net Ionic Equations To understand the definition of a net ionic equation , recall the equation for the double replacement reaction.
Applications and Examples Precipitation reactions are useful in determining whether a certain element is present in a solution. Example 1 Complete the double replacement reaction and then reduce it to the net ionic equation.
Example 2 Complete the double replacement reaction and then reduce it to the net ionic equation. Practice Problems Write the net ionic equation for the potentially double displacement reactions. General Chemistry.
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