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Chemistry library
Course: Chemistry library > Unit 5
Lesson 5: 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
- Molecular, complete ionic, and net ionic equations
- 2015 AP Chemistry free response 3a
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Precipitation reactions
A lot of ionic compounds dissolve in water, dissociating into individual ions. But when two ions find each other that form an insoluble compound, they suddenly fall out of solution in what's called a precipitation reaction. In this episode of Crash Course Chemistry, we learn about precipitation, precipitates, anions, cations, and how to describe and discuss ionic reactions.
Writer: Kim Krieger
Chief Editor: Blake de Pastino
Consultant: Dr. Heiko Langner
Director/Editor: Nicholas Jenkins
Sound Designer: Michael Aranda
Graphics: Thought Cafe
Want to join the conversation?
- Why can the silver water ( yes that was a very lay-man term, please excuses me(: )
Turn you blue? What is the reaction that it causes?(14 votes)- As far as I'm aware, colloidal silver is blue. The change to blue skin takes a long time, so it could possibly just be a buildup of silver in the cells.(3 votes)
- At aroundHank mentions that the precipitate formed is silver chloride. Does this mean that there is no longer any salt (NaCl) present in the water, and that all of it has formed into silver chloride? 6:20(8 votes)
- Whether or not there is still salt in the water depends on how much Silver Nitrate (AgNO3) you put into the salt-water solution.
1. If you put just enough silver nitrate (AgNO3) to react with (NaCl) to produce AgCl, there will be no Cl ions (anions) left. That doesn't mean that Sodium (Na) reacted with Nitrate (NO3) to form Sodium Nitrate (NaNO3). Sodium and Nitrate ions stay dissolved in the solution separately because Nitrates are highly soluble in water ( at) 5:52
2. If you put less Silver Nitrate then there will be still salt left.
3. If you keep putting Silver Nitrate into the solution then after some time there will be no Cl ion left to react with Ag ion to precipitate AgCl. Silver and Nitrate ion would just dissolve like any Ionic compound would.(10 votes)
- Speaking of Iron... When doctors say that you don't have enough iron in your system, what do they mean? Like Is that Iron part of the periodic table? If so, then where?(4 votes)
- iron in our body is mostly present in the haemoglobin compound in the RBCs and around 2% of it is present as ferritin complexes that are present in all cells, but most commonly in bone marrow, liver, and spleen.
When a doctors says that you do not have enough iron in your body most of the time he means it in the literal sense but otherwise it can also mean protein deficiency.
Hope i was helpful to you.(7 votes)
- At, "researchers are now looking at the anti-microbial uses for silver nanoparticles" 8:36
What is a nanoparticle?
An atom is a partical right?, so isn't a silver atom/particle supposed to be the smallest thing of silver?(5 votes)- "Nanoparticles are of great scientific interest as they are, in effect, a bridge between bulk materials and atomic or molecular structures." http://en.wikipedia.org/wiki/Nanoparticle#Properties
Atoms aren't particles, but rather a composition of subatomic particles.
A silver atom is the smallest thing that can be defined as silver (47 protons)(4 votes)
- At, Hank mentions 100 million trillion dollars of gold in our hydrosphere. Wouldn't it be simpler to say 100 quintillion dollars or 1 X 10^20? 3:33(3 votes)
- I personally prefer 100 quintillion, but that number might be too big to comprehend. Most people would be familiar with million and trillion.(6 votes)
- The mole is the unit of measurement for amount of substance in the International System of Units. A mole of a substance or a mole of particles is defined as exactly 6.02214076×10²³ particles, which may be atoms, molecules, ions, or electrons. In short, for particles 1 mol = 6.02214076×10²³.(3 votes)
- How can we identify whether the product formed is soluble in water or not?(4 votes)
- You would need to check the solubility rules for each compound
Solubility rules (and solubility charts that summarise them) can be found on Google(2 votes)
- I was wondering why exactly precipitates get formed in a chemical reaction? I know certain compounds are insoluble and others are soluble, but why exactly does this happen?(3 votes)
- how can we predict from the reactants that the products in a reaction are going to be in a solid or aqueous state? how is the state of matter in these reactions predicted?(3 votes)
- How can we identify which chemical reactions are the one with precipitates ?(3 votes)
- Frankly speaking, finding that out involves many complex calculations. But, it is safe to assume that almost every double displacement reaction is a precipitation reaction.(1 vote)
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. (upbeat music) Precipitation: it's stuff
falling out of other stuff. 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. In fact, at today's market
value, the ocean contains about one hundred million
trillion dollars of gold. And that was not a stutter, a hundred million trillion. That's a hundred trillion
with six more zeros after it. So you could see why it
might be nice to master some precipitation reactions. There have been chemists
that have driven themselves crazy trying to figure
out how to economically extract gold from sea water. But thus far, none have done it. This solution here, of
silver nitrate, is similar to that ion-rich water that seeped through the Montana limestone
millions of years ago. And we can use it right here at this desk to recreate the ancient
reactions that deposited silver and veins across our landscape. But instead of the types of
salts found in limestone, we can use a very similar
and substantially more familiar ionic compound, table salt. Good old NaCl. Add some drops of sodium chloride, also known as your salt water
to the silver nitrate solution and there you see your precipitate. Oooo, gross. Now the question that we
immediately want to ask is, what is this white stuff down here? The key to understanding
what just happened here is that both of the compounds are ionic. You remember there are
two kinds of ions right? Cations are positively charged, and anions are negatively charged. Just like little bar magnets, they attract so cations only react with anions to form new compounds. And don't just think there's
one anion and one cation. The sodium ion and
sodium chloride will have chloride ions on all four sides which in turn are
surrounded by four sodiums, and this pattern repeats
many many many times until we end up with the salt crystals that we dissolved in the water. But how do we know which ions are cations and which are anions? Well, sodium is positively
charged so it's a cation. And we know that it's positively charged because sodium is a metal
from the left side of the periodic table. And those are always
cations when they're alone. Silver is also a metal
and is also a cation. We know that chlorine is a gas from the right side of the periodic table so that is an anion. Now what about the nitrate? Also anion; nitrates,
sulfates and phosphates are really common and
they're always anions. Whenever you see an N S or P
followed by a bunch of oxygens, you know you're looking at an anion. With that in mind, look at the possible
products of this reaction. What we're looking for is a product that doesn't dissolve in water. So we know it's not sodium chloride. That was one of our reactants and it dissolves readily in water. Hence, the oceans. And it isn't silver
nitrate, our other reactant, or sodium nitrate because, as a rule, nitrates dissolve really easily in water so we know that's dissolved. So we're left with silver chloride. Just process of elimination. This makes sense because silver also makes insoluble compounds
with bromine and iodine which are in the same column of the periodic table as chlorine. Elements in the same column
often behave in similar ways and you'll notice, of
course, that we don't end up with, like, a huge nice
chunk of pure silver here. Now it's bonded to chlorine. Kind of like table salt, silver chloride is a crystal and solid. Unlike salt, though, it's
not very soluble in water. Now getting the silver
out of this compound will involve another kind of reaction; a redox reaction. Which we'll talk more about next week. In the mean time, we still
have to learn the language of describing this sort of reaction. Because of the neat, and
somewhat unique, interactions that are involved in
precipitation reactions, dissolved substances
producing solids, ions, dissociating and rebonding, there are special ways to write and balance them as equations. One way is to include
notations and parenthesis that tells you what state
the chemicals are in. AQ meaning aqueous or
insolution and S for solid meaning that it's your precipitate. This is called the molecular equation. Another way which tends
to give a clearer picture of what actually happens
during the reaction is to write everything out as ions. Here, you list the compounds
that dissolve completely insolution as ion. Makes sense because as soon
as the salts are dissolved, every ion is on its own and it doesn't really matter
where it originally came from. So the left side shows
silver and nitrate and sodium and chloride ions, all in one solution. And the right side shows
sodium and nitrate, still as separate ions insolution
with the silver chloride precipitated out as solid. Now, we don't care about
the complete equation and only want to see
the active precipitants we can write it in yet a third way. We just leave out the so
called spectator ions; nitrate and sodium, which don't participate
directly in the reaction, and end up with a net ionic equation showing just silver and chloride ions reacting to form silver chloride. This is nice and short and to the point, which is what chemists
love 'cause remember some of them have terrible writing skills and have to dictate
their stuff to their mom. Now as a side, remember
when I first brought up the weirdness of using Ag
to denote the word silver? Well, all that stems from
the fact that the Latin word for silver is argentum. And the ancient were, as
most people are today, obsessed with what silver represented; not just wealth but also health. Ancient and or Europeans
associated silver with purity and goodness. Hippocrates, the ancient Greek doctor, wrote about silver's
anti-disease properties. And there's good science
behind silver's medical uses. A lot of metals are toxic to
things like fungi and microbes. But, unlike say, lead, silver
isn't that toxic to humans. Silver nitrate and a compound
called silver sulfadiazine were used to disinfect
wounds in world war one before antibiotics were discovered. Silver sulfadiazine is
still used to dress burns. And researchers are now looking
at the antimicrobial uses for silver nano particles. Some people even take colloidal silver, basically just silver particles
in a liquid suspension, as a kind of general health booster. But there's not actually any evidence that it boosts health. It can turn you blue though. Now you want that silver
even more, don't you? Now as we always do, in order
to make a reaction practical, you have to go through the
final step of converting the formula equation into
a molar mass equation. If we wanted to get the
silver out of solution, how much salt would we need? Specifically, let's say that we wanna get one troy ounce of silver. The troy ounce is part
of the troy weight system which is used to weigh precious metals. It's derived from the
way the Romans measured bronze and silver bars
they used for currency two thousand years ago. And we are still stuck with it. But let's be a little
bit modern about this; a troy ounce equals 31 point one grams. So we want 31 point one grams of silver, the molar mass of silver is 107 point eight six eight grams per mole. Do the calculation and we find that 31 point one grams equals zero point two eight eight mols of silver. From the molecular
equation, we can see that in order for it to balance we'll need one mol of sodium chloride for every mol of silver. So to get zero point two
eight eight mols of silver, how much sodium chloride do we need? We've made it easy for you, it's zero point two eight eight mols. So then we convert zero
point two eight eight mols of sodium chloride into
units of mass and grams. Sodium chlorides molar mass is 58 point four five grams per mol. Multiply 58 point four
five grams per mol by zero point two eight eight and we find we need 16 point eight grams of sodium chloride to precipitate out one
troy ounce of silver out of silver nitrate solution. And look at that, you get a nice pile of silver
chloride in solution. Yes, it's not pure silver. Not yet. Just like real miners who
dig ors out of the ground that contained just a few percent silver, we need to do some refining. In our case, there's another
type of reaction necessary called a redox reaction. Redox is short for reduction-oxidation and that is what we'll be
talking about next time.