General Chemistry Chemical Reactions

Hi, welcome back to Educator.com.0000

Today's lesson in general chemistry is on chemical reactions; let's go over the outline.0002

We are first going to talk about something very fundamental to all of the physical sciences.0011

That is the law of conservation of mass.0017

We are going to see that in order to obey the law of conservation of mass,0020

anytime we have a chemical equation, we have to make sure that it is completely balanced.0024

After discussing this, we will then go on to something we call chemical equilibrium.0031

Because chemical equilibrium is going to be very applicable to our discussion of reactions in solution.0037

The solutions that we are specifically talking about are electrolyte solutions, nonelectrolyte solutions, and etc.0048

After talking about electrolytes and nonelectrolytes, we are then going to dive into a couple of different reactions that can occur in aqueous solutions.0058

The first one is what we call single replacement reactions.0067

The second one is what we call double replacement reactions.0072

The third one is neutralization reactions.0076

The fourth and final one is going to be reactions that produce unstable compounds0078

that are going to further decompose and form other compounds we cannot initially predict.0084

When this section is done, we are going to be able to predict0092

the compounds that are formed from any of these types of reactions.0095

We will wrap up the session with of course our summary followed by a pair of sample problems.0101

Let's go ahead and start.0107

The law of conservation of mass is fundamental to all of the physical sciences.0110

We always will be obeying this in chemical reactions.0114

Basically the law of conservation of mass tells us the following: that matter can neither be created nor destroyed.0118

In other words, what you have, you are going to have permanently.0126

It is coming from other sources.0131

You are never going to truly eliminate it or get rid of it.0134

But matter as we know can be transformed into different states, etc.0140

If we were to translate this into a chemical reaction before and after, the law of conservation of mass basically tells us the following.0146

That the total mass before a reaction is going to be equal to the total mass after a reaction.0153

In other words, the total mass of what you start with equals to the total mass of what you end up with.0162

Let's go ahead and look at a typical way of representing a chemical reaction.0170

Basically the general format of a chemical reaction is the following.0175

We have the reactants on the left side of the arrow.0180

We have the products on the right side of the arrow.0184

We can go ahead and verbalize this the following way.0186

That A combines with B to form C and D; or A and B yields C and D.0190

Or A and B mix C and D; whatever you are more comfortable with.0197

Basically if we were to apply the law of conservation of mass, this is telling us the following.0204

That the total mass combined of A and B will be equal to the total mass combined of C and D.0209

In order to obey the law of conservation of mass, we have to do what is called balancing a chemical equation.0222

You are going to be using this from now on pretty much in every lesson.0230

This is one of the most fundamental parts of general chemistry that you want to learn to master early on.0235

When we balance a chemical reaction, let's go ahead and look at the following.0242

N₂ plus H₂ goes on to form NH₃.0246

We want to look at the number of elements on both sides of the equation.0252

For example, here we notice that there are a total of two nitrogens on the left side.0255

But there is only one nitrogen on the right side.0262

In addition on the reactant side there are two hydrogens.0266

There are three hydrogens on the right side.0271

We say that this is an unbalanced chemical reaction.0274

We never want to leave a chemical equation unbalanced.0280

The way to go about this is the following.0285

We are going to put what are called coefficients right in front of each of the molecules to go ahead and balance this.0288

The coefficients are usually going to be whole numbers.0301

I want to give you a couple tips.0304

Whenever the reactants are all homonuclear, it doesn't matter which element you try to balance first.0306

What I mean by homonuclear is that it is basically a compound composed of just one element; one element type only.0312

Examples of homonuclear would be N₂, H₂, O₂, Cl₂, etc.0326

Those again are what we call homonuclear.0335

A lot of the time, it is trial and error; so let's go ahead and start.0339

N₂ plus H₂ goes on to form NH₃.0343

What I would like to do, I would like to maybe balance nitrogen first.0349

Again it doesn't matter what order you do it in.0352

I am going to go ahead and put a 2 in front of NH₃.0356

Then I ask myself: I have now two nitrogens on the left.0361

I have now two nitrogens on the right; the last thing to fix is hydrogen.0364

Now I have two hydrogens on the left; I have six hydrogens on the right.0370

Basically we ask ourselves what multiplied by two is going to give us six.0376

The answer is going to be three.0382

My balanced chemical equation is going to be N₂ plus 3H₂ goes on to form 2NH₃.0384

Always, this is very important, always double check your work to make sure that you have done the balancing correct.0393

As you can see, we have two nitrogens on both sides of the equation.0397

And we have a total of six hydrogens on both sides of the equation.0401

Before we move on, another thing I just want to point out is the following.0409

When you have the coefficient and a subscript, we just multiply them together to get the actual number.0413

That is how we get six hydrogens there; coefficient multiplied by the subscript.0419

Let's go ahead and now move on to another chemical reaction where the reactants are not both homonuclear.0432

For example, CH₄ plus O₂ goes on to form CO₂ and water.0440

We have CH₄ which is going to be what we call heteronuclear.0449

That is the compound is made up of more than one element.0453

My advice I usually give is the following: you always save the homonuclear reactant for the last step.0457

That is going to make your life a lot easier.0469

Let's go ahead and try to balance this.0471

Again it is CH₄ plus O₂ goes on to form CO₂ and H₂O.0474

I am going to save oxygen for last because it is part of a homonuclear compound.0481

I am left with either carbon or hydrogen to start with.0485

Again it doesn't matter for those; let's just go ahead and pick carbon.0488

Carbon, we have one on each side of the equation.0493

We have one in the CH₄ and we have one in the carbon dioxide; so carbon is already balanced.0496

The only thing remaining is hydrogen now to work with before we tackle oxygen.0502

On the left side of the equation, I have a total of four hydrogens.0507

On the right side of the equation, I have a total of two hydrogens.0510

I am going to put 2 right in front of water.0515

Now my hydrogens are balanced; I have four on both sides of the equation.0518

Remember multiply the coefficient by the subscript, two times two gives us four.0522

Now we can go ahead and tackle oxygen.0527

I have a total of two oxygens on the left.0529

I have a total now of two oxygens in carbon dioxide.0534

I have a total of two oxygens in water; remember to carry through the coefficient.0538

So actually I have four oxygens total on the right side of the reaction.0545

I am going to put a 2 right in front of O₂.0553

That gives me a total now of four oxygens on both sides of the reaction.0556

That is it; that is our balanced chemical reaction.0560

Another piece of advice I usually give students is the following.0565

Balancing chemical reactions can be very frustrating.0569

You don't always get it right the first time; a lot of it is trial and error.0572

If going by one element the first time doesn't work, try another element until you get it.0577

But remember it always can be balanced.0582

It is a fundamental law of nature; it always can work.0585

Right now, I want to introduce you a chemical reaction where sometimes the coefficients0593

that we are going to use initially are not going to be all whole numbers.0599

Instead the coefficients that we are going to be using are going to be fractional.0603

Remember, remember what we have to do.0609

It is going to be the coefficient times the subscript.0613

That is going to give us the total number of that element in the compound; the total number of element in compound.0620

Let's go ahead and balance this; C₂H₆ plus O₂ goes on to CO₂ and water.0633

I have two carbons on the left side; I only have one carbon on the right side.0639

I am going to go ahead and put a 2 in front of CO₂; now my carbons are fixed.0645

Remember that I am leaving oxygen for last because oxygen is part of a homonuclear compound.0649

What we have to do now is fix hydrogens.0655

I have six hydrogens on the left; I have two on the right side.0658

Then we ask ourselves: what coefficent, what number times two is going to give me six?0663

That is going to be a three; so now my hydrogens are fixed; six on each side.0669

Now we have the following C₂H₆ plus O₂ goes on to form 2Cl₂ and 3H₂O.0675

Let's go ahead and look at the oxygen count right now.0685

I have two oxygens on the left side.0688

Here on the right side, I have a total of four oxygens in CO₂.0691

I have a total of three oxygens in water.0695

That means I have a grand total of seven oxygens on the right side and only have two on the left side.0700

This is when we have a case of an even-odd number.0710

If you look at the previous examples, we have always had an even number on one side and an even number on the other side.0713

In that case, it is always convenient just to use a whole number.0720

But when you have an even-odd pair like this, there is going to be a little trick we are going to do.0724

Basically you ask yourself the following.0729

You see that I am shy of oxygens on the left side.0732

We have to ask ourselves: what coefficient times two is going to give me seven?0736

Basically 2 goes down stairs; 7 goes upstairs; it is very simple.0743

When you have an even-odd pair, you will always use a fractional coefficient.0750

Basically the bottom number is going to be the subscript from the homonuclear compound.0757

The top number is always going to be the greater number; what you are trying to get the homonuclear compound up to.0771

Once again when you have an even-odd pair, use fractional coefficients.0785

The fractional coefficient I am going to use is 7/2.0789

Remember a few sessions back, we talked about Dalton's atomic theory.0796

Part of his theory was that elements combine in small whole number ratios to form compounds.0802

What that means is the following: we never want to leave a chemical equation in a fractional form.0808

In other words, what we are going to do right now, we are going to eliminate the fraction.0816

We are going to be left with nothing but whole numbers.0819

The fraction we have 7/2; if we want to get rid of a fraction, we simply divide by its denominator.0825

But I just can't multiply oxygen by 2; if I change one element, I have to do it proportionally to everything.0833

My grand and final answer is going to be the following: it is going to be 2C₂H₆ plus 7O₂.0842

That is going to go on and form four carbon dioxides and six waters.0858

This gives us our balanced chemical equation with no fractions.0877

Let's just go ahead and do one final check, make sure we have done it right.0880

Let's go ahead and see if we have the same number of elements on each side.0883

I have 4 carbons total here; 4 carbons total here; carbons are good to go.0889

I have 12 hydrogens here; I have 12 hydrogens here; hydrogens are good to go.0895

Finally I have 14 oxygens here.0903

I have 8 oxygens in CO₂ and 6 oxygens in the water; giving me a grand total of 14 oxygens.0907

As you can see, this method does work.0920

Again the final thought is to never leave a coefficient as a fraction.0923

Now that we have learned how to balance chemical equations, let's move on to the next topic of the session.0932

This is what we call chemical equilibrium.0939

When an ionic compound fully dissociates... what I mean by fully dissociates is that it completely breaks up.0944

It forms individual cations and anions.0951

Whenever this happens, we use a single arrow in the forward direction.0954

For example, hydrochloric acid is going to be fully dissociating when dissolved in water.0959

That is going to break up into its respective cation and anion.0966

But not all ionic compounds do this.0972

A lot of ionic compounds really incompletely dissociate which means they don't fully break up into cation and anion.0976

When this occurs, we use a new type of arrow.0985

It looks like a pair of opposite arrows.0991

This is what we call an equilibrium arrow.0995

What the equilibrium tells us is that the reaction is reversible.1004

In other words... let's look at HF, as hydrofluoric acid breaks apart.1019

In other words, as we move in the forward direction, H⁺ and F^- instantaneously form.1024

However look at the reverse arrow; let's go ahead and translate that.1032

What that is telling us is that H⁺ and F^- are going to somehow find each other again.1037

They are going to recombine and reform the original hydrofluoric acid molecule.1042

This is what we call dynamic equilibrium.1048

That is it is a constant process; it is not static whatsoever.1052

Basically both the forward and the reverse reactions happen simultaneously.1057

In other words, as soon as a compound dissociates, the ions are going to recombine into the original compound.1063

Again you are going to call this an equilibrium arrow.1072

We are going to be using this quite heavily in the latter half of this general chemistry class.1075

Now we are going to talk about specific compounds when they dissolve in water.1085

They are called electrolytes and nonelectrolytes.1091

When you hear the word electrolytes, it obviously sounds like electricity or electron.1094

Basically electrolytes are going to be ionic compounds that form ions when dissolved in water.1101

For example, we can go ahead and take a salt, table salt, sodium chloride.1107

Sodium chloride is going to dissociate and form the respective ions; sodium cation and chloride anion.1112

The following is going to be a fundamental requirement for a solution to be conductive.1122

Basically a solution has to have free ions in order to conduct an electric current.1128

That is why they call them electrolytes.1133

Strong electrolytes completely dissociate.1138

For strong electrolytes which completely dissociate, we are going to use a regular single arrow in the forward direction.1143

Weak electrolytes incompletely dissociate.1151

Remember going back to the previous slide, for ionic compounds that incompletely dissociate, we use an equilibrium arrow.1155

Finally there are compounds that when dissolved in water, they do not form ions whatsoever.1162

These are going to be nonelectrolytes; these are going to be typically molecular compounds.1171

For example, if you take ordinary glucose for example, C₆H₁₂O₆; you go ahead and dissolve it in water.1176

You don't form individual ions; it just becomes aqueous.1190

You see that there are no ions whatsoever.1197

Glucose is going to be a nonelectrolyte solution.1200

Now that we have gotten those vocabulary terms out of the way,1205

what we are going to do right now, we are going to study specific types of aqueous reactions.1208

We are going to learn how to predict the products.1215

The first type of aqueous reaction is what we call a single replacement reaction; also known as single displacement.1218

A single replacement reaction is always going to have the following general format.1228

A... this is usually going to be a metal... plus B and C.1233

Remember this is going to be the metal; this is going to be the nonmetal; usually.1240

Or the cation and the anion goes on to form AC and B.1247

You notice that A has come in and essentially displaced B from its bond with C.1252

We go on to form AC and B; this is called a single replacement.1259

Let's go ahead and look at a couple of examples.1263

You take lithium metal and you react it with copper(II) chloride.1266

Lithium is going to come in and knock out copper; we form lithium chloride instead.1273

Copper is also going to come out as copper solid; however let's reverse the process.1279

If we take copper solid and go on and react it with lithium chloride, we get no reaction at all.1286

Let's go ahead and see why this is going to occur and how you are going to be able to predict this.1295

All of the experiments have been done already.1301

The results are summarized in what is called an activity series.1303

Basically an activity series is the following.1307

You see that lithium is right at the first element.1310

We call lithium a very active metal.1314

It can come in and knock out essentially any other element.1318

Let's look at sodium; the way to use this activity series is the following.1325

Sodium can knock out any element below it, but it cannot knock out any element above it.1329

For example, sodium will not be able to knock out calcium.1335

But it will be able to displace magnesium in a single replacement reaction.1340

Depending on your instructor, you may or may not have to memorize this activity series; definitely be on the lookout for that.1346

Another very common type of single replacement reaction occurs between the following: a metal and acid.1353

For example, when zinc solid is going to get dissolved in hydrochloric acid, we get the formation zinc chloride and hydrogen gas.1363

Essentially zinc has come in and knocked out hydrogen.1373

Let's go ahead and see if we can find hydrogen on this activity series.1377

We know that zinc is going to be right here and right below it is going to be H₂.1381

Yes, this activity series does predict that zinc is going to come in and displace hydrogen in a single replacement reaction.1389

Here is the thing; when hydrogen gets displaced, it always forms hydrogen gas.1398

When displaced in a single replacement reaction, it is going to form H₂ gas.1405

This is how we use an activity series again for single replacement reactions.1417

The next type of reaction we want to talk about is going to be called a double replacement reaction.1422

It is also known as double displacement.1428

Basically instead of one quote and quote knockout, two occur.1432

We are going to have two swaps; I always like to show brackets with this.1437

Basically A is now going to hook up with D and now C is going to hook up with B to form AD and CB.1443

Please note how the cation always hooks up with the other anion, etc.1452

An example of this is going to be the reaction between sodium chloride and lead(II) nitrate.1459

Sodium is the cation; it is going to hook up now with the other anion which is nitrate.1465

Lead, Pb²⁺, is going to hook up with the other anion chloride.1471

We get sodium nitrate and lead(II) chloride.1474

Please note... this is very important when you do this... that the charges never change, especially for the transition metals.1479

Remember when we went back to nomenclature and you learned that transition metals can have different oxidation numbers.1488

Here this is going to be Pb²⁺; there should be really a 2 there; sorry about that.1496

Here in the product side, it is going to remain Pb²⁺.1505

Please do not ever change a metal's charge in a double displacement reaction.1509

Pb²⁺ is going to remain Pb²⁺; that is why it is PbCl₂ and not just PbCl for example.1523

The next type of equation though, something very related to this, is the following.1532

That is called a net ionic equation.1537

A net ionic equation is usually written to show the formation of the precipitate.1539

A precipitate is just a fancy name for the solid that is formed in a reaction.1544

Basically a net ionic equation shows the formation of the precipitate, in this case, lead(II) chloride, from its constituent ions.1550

That is basically going to be Pb²⁺ combining with two chlorides.1558

It is always aqueous plus aqueous goes on to form the solid.1563

Now that we have covered net ionic equations, how do you predict that a compound is going to be solid?1570

Or how do you predict if it is going to remain aqueous in these reactions?1579

This is something you may or may not have to memorize also.1584

So please be on the lookout for that from your instructor.1586

This is what we call the solubility rules for ionic compounds.1591

Basically this table is divided into two areas; first the compounds that are usually soluble.1595

What soluble means is that the compound is going to remain broken up into ions and dissolved.1602

Something that is soluble is going to be essentially aq or aqueous in the chemical reaction.1608

You are going to find ammonium salts, group 1 salts, nitrates, etc.1616

What is important in this solubility table is to pay attention to the exceptions.1623

For example, acetates are usually soluble; for example, sodium acetate will be aqueous in water.1627

However there are exceptions such as silver, Ag¹⁺, and aluminum, Al³⁺.1635

While sodium acetate is going to be aqueous, silver acetate and aluminum acetate are exceptions.1640

They are going to be solid; that is they are going to be insoluble.1647

So you always have to pay attention to the exceptions.1654

The same goes for the other side of this table; compounds are usually insoluble.1657

They are going to be remaining intact and not break up relatively speaking.1662

They are going to remain as solids; so carbonates, phosphates, etc, are going to be solids.1667

However there are exceptions.1673

Something like magnesium carbonate is going to be mostly a solid in water.1676

But an exception is ammonium; ammonium carbonate is going to remain aqueous in water.1681

Let's go ahead and apply another type of double displacement reaction.1688

That is what we call a neutralization reaction.1697

A neutralization reaction always has the generic formula.1700

An acid is going to react with a base.1703

It is always going to form a salt, water, and it is always going to give off heat.1706

Anytime you mix an acid and a base, it is going to feel warm to the touch.1713

A typical reaction would be between hydrochloric acid and sodium hydroxide.1720

Neutralization reactions are going to be typically double displacement reactions.1725

Hydrogen is going to hook up with hydroxide.1729

Chloride is going to hook up with sodium.1733

Let's do the salt; the salt is going to be sodium chloride.1736

From the table of solubility rules which we just covered, this type of chloride is going to be soluble which is aqueous.1742

You see that when hydrogen combines with hydroxide, that is essentially going to form water.1750

That is going to be a liquid.1757

There we have it; a double displacement reaction in the form of what we call a neutralization reaction.1759

Let's do one last example; that is going to be between sulfuric acid and potassium hydroxide.1765

Sulfate is going to hook up with potassium.1772

Again hydrogen is going to hook up with hydroxide.1775

This time the salt is going to be potassium sulfate.1778

I need two potassiums to balance sulfate's 2- charge.1786

Look at the table of solubility rules; that is going to be aqueous.1790

Again I am going to always form water; let's go ahead and balance it.1794

I am going to need two potassium hydroxides to go ahead and react with the sulfuric acid.1799

I am also going to form two waters as a result.1808

Remember always balance your chemical reactions.1811

We saw that a neutralization reaction is a type of double displacement reaction.1817

There are a few exceptions where using the table of solubility rules1823

and simply going by a generic format for double replacement does not work.1829

These three reactions here are going to form compounds initially that are unstable.1835

These unstable compounds are going to in turn produce additional products which we just can't predict.1842

We are going to now introduce these to you.1850

The first type, number one, is going to be the reaction between a metal carbonate...1852

This is usually going to be a group 1 metal... reacting with an acid, HA.1858

Usually A is going to be the following: Cl, Br, iodine, nitrate, and also can be sulfate.1868

Sulfuric acid, nitric acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, etc.1885

These are what we call the strong acids.1894

Again depending on your instructor, you may have to know a number of different ones.1897

So please definitely be on the lookout for that.1902

When we do the double displacement format, we are going to get H₂CO₃, carbonic acid, in combination with an HA aqueous.1905

It doesn't stop there; what happens is the following; that carbonic acid is actually unstable in water.1915

It is going to decompose to form CO₂ and liquid water; that is one example.1920

Again a metal carbonate reacting with a strong acid goes on to form carbon dioxide gas and liquid water.1928