Predicting Reactions: A System

General Chemistry Index

Where are we going with this? This page will assist in developing the ability to predict products of simple reactions as listed in of reactions: synthesis (i.e., combination), decomposition, single displacement, double displacement, acid/base, and combustion.

Predicting Reactions: A System 
What happens if I mix this with that?

Predicting the products of chemical reactions is a process by which potential reactants are scrutinized to determine whether they will react and if so what product(s) will be formed.

What happens if I mix this baking soda with vinegar?
What happens if I let this spilled gasoline sit on the painted garage floor?
What happens if I pour bleach directly onto my clothes?

Predicting chemical reactions does not take place only in the lab; it is actually a part of everyday life! However, in the lab, we can be more specific and better isolate the this and the that.

So… I told you this would be long! Maybe another soda or cup of coffee?

Recall: the goal is to end up with neutral molecules. The goal is to determine the correct product formula so that the overall charge is zero.

Let's just say we are starting with known substances and correct chemical formulas on the reactant side, okay? Coming up with a system to predict the products is, at best, a starting place. 

Because of the vast scope of chemistry, there will be variations that this system will not cover. And there are always exceptions!

Back to the original question: If I mix this with that, what happens?

The system I'm going to offer expands on that question:

What do you have to start with?

This step identifies what kind of reaction you could be looking at. Is it synthesis? Decomposition? Single replacement? Double replacement? Combustion?

 

What does it become? 

1. Will it even react? 
2. What combination of atoms or ions is needed to form the correct product formula? For a neutral ionic compound, what subscripts are needed so that the overall charge is zero? (AKA what are the subscripts?) 
3. What is the FINAL balanced equation?

 

So, here is a "decision tree" for what to do to predict the products of a chemical reaction. 

Predicting the Products of Chemical Reactions Decision Tree

What do you have to start with?

• Two elements: goto Synthesis Reaction below (Click) 

• One compound: goto Decomposition Reaction below (Click)

• An element and a compound: goto Single Displacement Reaction below (Click) 

• Two compounds: goto Double Displacement Reaction below (Click) 

And also…

• Combustion: goto Combustion Reaction below (Click) 

What does it become?

Synthesis Reaction (background)

 

First off, will they react? 

For two elements to react in a synthesis reaction, they must be able to form a stable compound together. Depending on the elements involved, this may occur through ionic bonding or covalent bonding.

 

They need to have opposite valences. At an introductory level, we can think of one element as tending to give up or share electrons and the other as tending to gain or share electrons so that both reach more stable valence-electron arrangements.

 

As a simple starting point, atoms with only a few valence electrons often tend to lose or share electrons, while atoms with nearly full valence shells often tend to gain or share electrons.

 

 

Plot twist—some elements can have more than one valence. For example, sulfur is often described at an introductory level as having valences of 2, 4, and 6.

 

Secondly, if they will react, what is the correct formula of the compound formed in the product?

The subscripts show the ratio of atoms in the product formula. At an introductory level, the valences of the elements can often help predict that ratio. For ionic compounds, ion charges can be criss-crossed to determine the subscripts. For simple covalent compounds, the same ratio idea can be used with typical valences as a shortcut, but the valences should not be mistaken for actual ionic charges.

For this introductory system, think of valence as an element's typical combining capacity. The numbers can help us predict ratios, but the + and − signs we sometimes attach to them do not always represent actual ionic charges. 

For instance, take carbon and oxygen. 

Since both are nonmetals, they form a covalent compound rather than an ionic compound.

Oxygen has a valence of 2. Since it NEEDS 2 more electrons to complete its valence shell of eight, as an introductory shortcut, you can conceptualize its combining tendency as −2.

Carbon has a valence of 4. (Since we're conceptualizing oxygen's combining tendency as −2, we'll represent carbon's as +4 for this introductory shortcut.)

If we are combining carbon and oxygen, we start off with something like this:

C + O₂ → ??
So, the product has to be C?O?
As an introductory shortcut, criss-cross the valences: C₂O₄
Reduce the subscripts to the lowest whole-number ratio: C₂O₄ becomes CO₂
C + O₂ → CO₂

Thirdly, balance the equation using coefficients so that the same number of each type of atom is present on both sides:

C + O2 --> CO2 (already in balance)

Likewise:

• Magnesium has a valence of 2—it has 2 valence electrons and typically loses both when forming an ionic compound.
• Chlorine has a valence of 1—it has 7 valence electrons, so it NEEDS one more to complete its valence shell.
• Therefore, combining them gives the formula MgCl₂

The balanced equation would be:

Mg + Cl₂ → MgCl₂

______________________________

Decomposition Reaction (background)

 

First off, will it decompose?

Not everything will break apart easily. Some things only break apart at high temperatures.

It seems fair to presume that, if given a predicting-products exercise, the compound will, by some means, undergo a decomposition reaction and form simpler substances.

Secondly, if… Well, this one is pretty easy. Whatever you start with breaks apart. But… into how many pieces! (Probably two.) 

Usually, it will look like this:

AB → A + B

The products must be valid substances with the correct chemical formulas. They may be elements or compounds. And don't forget about those diatomic elements! For example, if decomposition produces elemental oxygen, students should write O₂, not O.

“Decompose into simpler substances” can suggest that a compound must separate into its original elements. That is not always true. A decomposition reaction produces two or more simpler substances, which may themselves be compounds.

For example:

CaCO₃ → CaO + CO₂

Neither product is simply an individual element. 

Thirdly, balance the equation using coefficients so that the same number of each type of atom is present on both sides:v

2H2O → 2 H2 + O2

It could be tricky, though!

CaCO₃ → CaO + CO₂

______________________________

Single Displacement Reaction (background)

First off, will they react? 

For one thing to replace another… Let's say it like this… For A to replace B, 

A + BC --> ?? + ????

If it WILL react, we get the standard single displacement pattern:

A + BC → AC + B 

For the reaction to occur, A has to be more highly reactive than B. 

 

How would anyone know that? There is a chart—the activity series!

So, sodium won't replace potassium in a compound because potassium is above sodium on the metal activity series. Etc.!

 

Also, either B or C could represent a polyatomic ion! Usually, the polyatomic ion stays together while the element in the compound is the part being replaced. 

 

That makes it harder to figure out what is being replaced. Look into the BC part and match one of them to the A with regard to location on the periodic table. There's a good chance that A will be in a family/group that is near the family or group of B or C. (You'll have to be open-minded about this claim when dealing with transition elements.)

If A is a metal, it will usually attempt to replace another metal or hydrogen. If A is a halogen, it will attempt to replace another halogen. Then use the appropriate activity series to see whether the replacement can occur. 

 

Secondly, if they will react, how many of each are needed to get the correct formula of the new compound in the product?

You know what you are starting with, so the reactant side is done. Let's do aluminum and HCl as an example…

Al + HCl --> ?? + ??

Since Al is above H on the activity series, Al can replace H. The product side will be: 

 

Al + HCl --> Al_?Cl_? + H_?

Product side subscript time:

This should be fun!

1. H is diatomic, so it will be H2

2. The aluminum chloride formula has to have an overall charge of zero.

The subscripts show the simplest whole-number ratio of ions needed to give the compound an overall charge of zero. You can "criss-cross" the charges of two elements (then reduce mathematically to the lowest whole-number ratio) to find the right numbers.

For instance, take aluminum and chlorine…

Al has a valence of 3 and commonly forms Al³⁺.

Cl has 7 valence electrons and a typical valence of 1, meaning it needs 1 more electron to complete its valence shell and commonly forms Cl⁻.

The correct formula will be AlCl3

Thirdly, balance the equation using coefficients so that the same number of each type of atom is present on both sides: 

 

2Al + 6HCl --> 2AlCl3 + 3H2

______________________________

Double Displacement Reaction (background)

First off, will they react?  

 

For one thing to replace another… Let's say it like this…  

 

AB + CD --> ???? + ???? 

In a double-displacement reaction, one element does not simply replace another. Instead, the positive and negative parts of two compounds exchange partners.

If it will react, then we arrive at the general form of the double replacement reaction:

AB + CD → AD + CB 

Deciding what is A, B, C, and D can be hard when polyatomic ions are involved. Really, the only way to get good at this is to do it a lot. Practice makes identifying the parts much easier.

Look at A and C first. Are they both the positive ions (cations)? Then look at B and D. Are they both the negative ions (anions)? (There is a chart!)? 

If A and C are positive and B and D are negative, you can identify the potential "swaps."

At this point, you have to answer the question! 

 

Will they react?

In real reactions, determining whether the ion exchange actually occurs can be more complicated.

Answering this question depends on whether the ion exchange produces something that drives the reaction forward. 

(CLICK HERE)

So… 

1. Does it form water? 

2. Does it form a gas?  

3. Does it form an insoluble solid—a precipitate?… easy! 

If the ion swap produces water, a gas, or a precipitate, the reaction can occur. 

Secondly, if they will react, what are the correct formulas of the new compounds in the products?

The work done in the first step should have resulted in you knowing what the A, B, C, and D parts are. The potential products will be AD and CB.

You know what you are starting with, so the reactant side is done. Let's do the following as an example…

Fe2(SO4)3 + KOH → 

So, there's some AB and CD up there? That looks like a sentence in a foreign language!

So this… 

 

AB          +   CD  → AD + CB

Fe2(SO4)3 + KOH →

(As a temporary step, it can help to put polyatomic ions in parentheses while you figure out the new subscripts.) 

Fe2(SO4)3 + K(OH)1 →

Fe2(SO4)3 + K(OH)1 → Fe?(OH)? + K?(SO4)?

Product side subscript time:

This should be… never mind!

1. Do that thing with the charges to get product formulas with an overall charge of zero. For instance…

K (from periodic table) commonly forms K⁺, with a charge of +1.

SO₄²⁻ has a charge of −2.

Criss-cross the charges to get K2(SO4)1

Chemistry "grammar" says we don't write a subscript of 1, and we don't use parentheses around a polyatomic ion unless more than one of that ion is needed.

Thus, we get:

K2SO4 

Using the example from above, working through the process of getting both products to a neutral charge, we get the unbalanced (but each part is a neutrally charged molecule) equation:

?Fe2(SO4)3 + ?KOH → ?K2SO4 + ?Fe(OH)3

Thirdly, balance the equation using coefficients so that the same number of each type of atom is present on both sides.

So, for the above example…

Fe2(SO4)3 + 6KOH → 3K2SO4 + 2Fe(OH)3

 In this example, the reaction occurs because Fe(OH)₃ is insoluble and forms a solid precipitate.

______________________________

Combustion Reaction (background)

First off, what do you have to start with?

In the combustion reactions we will use, look for a substance containing carbon and hydrogen reacting with oxygen (O₂).

A compound made only of carbon and hydrogen is called a hydrocarbon.

The basic pattern is:

Hydrocarbon + O₂ → ?? + ??

For these introductory problems, assume that the reaction WILL occur and that there is enough oxygen for complete combustion.

Secondly, what does it become?

This part is actually pretty predictable.

In the complete combustion of a hydrocarbon:

Carbon ends up in CO₂.

Hydrogen ends up in H₂O.

So:

Hydrocarbon + O₂ → CO₂ + H₂O

Yep. That's pretty much the product-prediction part.

For example, start with methane:

CH₄ + O₂ → ?? + ??

Methane contains carbon and hydrogen and is reacting with oxygen, so the products are:

CH₄ + O₂ → CO₂ + H₂O

Notice that we did not criss-cross charges or change subscripts to invent the products. For the complete combustion of a hydrocarbon, the expected products are CO₂ and H₂O.

Thirdly, balance the equation using coefficients so that the same number of each type of atom is present on both sides.

Start by balancing carbon and hydrogen. Save oxygen for last because oxygen appears in both products.

CH₄ + 2O₂ → CO₂ + 2H₂O

Check it:

Carbon: 1 on each side

Hydrogen: 4 on each side

Oxygen: 4 on each side

Balanced!

The Big Combustion Shortcut

If you see:

hydrocarbon + O₂

think:

CO₂ + H₂O

Then balance the equation using coefficients.

One small catch: If there is not enough oxygen, incomplete combustion can occur and products such as carbon monoxide (CO) or carbon may form. For our introductory predicting-products problems, however, assume complete combustion unless told otherwise. 

 

 

Predicting Reactions: Overview

General Chemistry Index

Where are we going with this? This page will assist in developing the ability to predict products of simple reactions as listed in of reactions: synthesis (i.e., combination), decomposition, single displacement, double displacement, acid/base, and combustion.

Predicting Reactions: Overview 
What happens if I mix this with that?

This comes up every week. Someone asks me what happens if I mix two things together. Or they ask what happens if they eat/drink/swallow something.

The answer to that question is the very heart of predicting the products of a chemical reaction. Predicting the products of chemical reactions is a process by which potential reactants are scrutinized to determine whether they will react and if so what product(s) will be formed.

What happens if I mix this baking soda with vinegar?
What happens if I let this spilled gasoline sit on the painted garage floor?
What happens if I pour bleach directly onto my clothes?

Predicting chemical reactions does not take place only in the lab; it is actually a part of everyday life! However, in the lab, we can be more specific and isolate the this and the that more.

So… here we go! This is going to be long, so get comfy. Maybe a soda or cup of coffee?

So, a very quick review of chemical reactions… 

• You start off with some reactants.
• Something happens.
• There are some products.
  
  
• Chemical reactions follow the law of conservation of mass such that…

• The number and type of each atom on the reactant side are equal to the number and type of each atom on the product side.
• The total mass on the reactant side is equal to the total mass on the product side (ignoring mass/energy E=mc² stuff) For ordinary chemical reactions, any mass-energy difference is far too small to matter in our calculations

• Compounds form in fixed, specific ratios of atoms. 

Okay, back to that what happens stuff…

You have a couple of reactants. You want to know what happens if they combine (assuming they will). If they DO combine, then a few things need to be considered.

If the compound formed is a neutral compound, the overall charge must equal zero. That goes back to all those bonding types and such. So, at a simplistic level, you can think of it as they are trying to fill their valence electron orbitals. To be a little more specific, they are combining to achieve more stable valence-electron arrangements.

You are going to rely on the periodic table to provide information about those charges! 

Oh… and those polyatomic ions… A CHART would be nice!

Okay, this is a little shifty, here… About those subscripts… Remember that a subscript tells how many atoms of an element—or how many polyatomic ions—are represented in a chemical formula. Changing the subscripts changes what you have:

H2O is not the same as H2O2. The first is water. The second is hydrogen peroxide. You can die if you DON'T drink the first. You can die if you DO drink the second.

So, you have two things on the table. You know what they are. (The bottle is labeled!) So, you KNOW the formula for those substances. You CANNOT change the subscripts of the things on the reactant side.

When they react (if they react), they are going to form NEW things. (Pretty much the definition of a chemical reaction.) So, the NEW things have their own formulas—new formulas with their own symbols and subscripts. Hence, the formulas and subscripts of the products must be determined from the identities and properties of the new substances—they do NOT simply carry over from the reactants.

Look at a couple of balanced reactions as an example:

2H2 + O2 → 2H2O

6CO2 + 6H2O → C6H12O6 + 6O2

SiCl₄ + 4H₂O → H₄SiO₄ + 4HCl

2Al + 6HCl → 2AlCl3 + 3H2

Na2CO3 + 2HCl → 2NaCl + H2O + CO2

So… 

Determine the correct formulas of the PRODUCTS. For neutral ionic compounds, choose subscripts so that the positive and negative charges balance to zero.

Now, just how do we do that?

The actual process of predicting the products of a chemical reaction is to look at what you have and find ways of rearranging it into new substances. Arguably, it is "one of those things" you grow into, perhaps as much "just getting it" as it is having a rubric to do so. 

BUT! We shall try to come up with a system to predict the products of a chemical reaction, all the same!

Ions and Their Charges

General Chemistry Index

Where are we going with this? This page will assist in developing the ability to describe, classify, and give examples of various kinds of reactions: synthesis (i.e., combination), decomposition, single displacement, double displacement, acid/base, and combustion.

Ions and Their Charges

Metals in compounds will usually give electrons to nonmetals or polyatomic ions with a negative charge. Some metals will give different numbers of electrons depending on the other part of the reaction. 

Parts of a compound that give their electrons away are considered positive ions or cations. The parts of a compound that receive the electrons are negative ions or anions. 

Compounds will form in such a way that the net charge is zero. The valence number on the periodic table will reflect the charges of the elements.

Numerous elements will combine into polyatomic "chunks" and, as such, act like a single thing in a reaction. Listed below are some common (and not so common) polyatomic "chunks."

Polyatomic Ions

The following Google Sheet can be sorted… 

Also, you can "command-F" after clicking one of the cells and search it.

More

Other Ions

In many reactions, the group in which an element is located will provide its charge. The table below is a handy reference.

Common Charges from Periodic Table Column / Family / Group
1	2	3-12	13	14	15	16	17	18
+1	+2	varies	+3	±4	-3	-2	-1	0

The following SORTABLE table presents the elements and their charges in list format.

Various Elements and Their Charges

The following Google Sheet can be sorted… 

Also, you can "command-F" after clicking one of the cells and search it.

Diatomic Elements

These elements bond to themselves in order to be more stable.

Hydrogen H2

Nitrogen N2

Oxygen O2

Fluorine F2

Chlorine Cl2

Bromine Br2

Iodine I2

SORTING THE TABLES:

Note: Sorting functionality is limited due to Google implementation of "sharing view only" functions.
 

Activity Series

General Chemistry Index

Where are we going with this? This page will assist in developing the ability to describe, classify, and give examples of various kinds of reactions: synthesis (i.e., combination), decomposition, single displacement, double displacement, acid/base, and combustion.

Activity Series

One chemical property is reactivity. Not all substances are equally reactive. Reactivity relates to how readily something undergoes a chemical reaction.

Reactivity describes how readily a substance undergoes a chemical reaction. Some substances react very easily, while others are much less likely to react under the same conditions.

There's another characteristic of matter related to how likely something is to react. 

In an imaginary way, you could think that if you throw some (let's say fluorine) into a bucket and then toss two other things—a metal probably—in, which one will react? Whichever one will react is higher on the activity series. While activity and electronegativity are both related to electron behavior, the activity series does not perfectly follow electronegativity, especially for metals.

When predicting products, we sometimes need to answer a simple question:

If two elements are competing for the same place in a compound, which one wins?

That is where the activity series comes in.

So, given two different elements in a potential replacement reaction, which one is more likely to replace the other? Their relative activity can be organized into a ranked list.

What Is an Activity Series?

An activity series is a ranking of elements according to their relative tendency to participate in certain chemical reactions.

Metals near the top of the activity series lose electrons more readily than metals near the bottom.

The activity series does not perfectly follow electronegativity. Both concepts involve electrons, but they describe different properties. When predicting single-replacement reactions, use the activity series, not electronegativity.

Activity Series: The reactivity series is a list of metals ranked in order of decreasing of relative tendency to react. (More)

Using the Activity Series

In a single-replacement reaction, an element can generally replace another element that is below it in the appropriate activity series.

An element cannot replace an element above it.

If the replacement cannot occur, the result is:

NR = No Reaction

For example, if metal A is above metal B:

A + BC → AC + B

A can replace B.

However, if A is below B:

A + BC → NR

The activity series is a useful prediction tool, but real reactions can also be affected by concentration, temperature, surface coatings, and other reaction conditions.

Metal Activity Series

Most active / most easily oxidized

The following list gives an extended version of a metal activity series. In a typical single-replacement reaction, a free metal can replace a metal ion that is below it in the series. A metal generally cannot replace a metal ion that is above it. If replacement cannot occur, the reaction is written as NR (No Reaction).

The metal activity series below is arranged from most active to least active:

Most active or most easily oxidized 

Lithium              Li(s) → Li⁺(aq) + e⁻

         Cesium                    Cs(s) → Cs⁺(aq) + e⁻ 

Rubidium                Rb(s) → Rb⁺(aq) + e⁻

Potassium         K( s ) → K 1+ ( aq )   + e –

Barium                 Ba( s ) → Ba 2+ ( aq )   + 2e – 

Strontium                Sr(s) → Sr²⁺(aq) + 2e⁻

Calcium                 Ca( s ) → Ca 2+ ( aq )  + 2e –

Sodium                 Na(s) → Na⁺(aq) + e⁻

Magnesium              Mg( s ) → Mg 2+ ( aq )   + 2e –

          Beryllium                Be(s) → Be²⁺(aq) + 2e⁻ 

Aluminum         Al( s ) → Al 3+ ( aq )   + 3e –

Manganese         Mn (s) → Mn 2+ ( aq )   + 2e –

Zinc                         Zn( s ) → Zn 2+ ( aq )   + 2e –

Chromium         Cr(s) → Cr³⁺(aq) + 3e⁻

Iron                         Fe( s ) → Fe 2+ ( aq )   + 2e –

Cadmium                 Cd(s) → Cd²⁺(aq) + 2e⁻

Cobalt                 Co( s ) → Co 2+ ( aq )   + 2e –  

Nickel                 Ni( s ) → Ni 2+ ( aq )   + 2e –

Tin                         Sn( s ) → Sn 2+ ( aq )   + 2e –

Lead                 Pb( s ) → Pb 2+ ( aq )   + 2e –

Hydrogen         H₂(g) → 2H⁺(aq) + 2e⁻

Bismuth                 Bi(s) → Bi³⁺(aq) + 3e⁻

Copper                 Cu( s ) → Cu 2+ ( aq )   + 2e –

           *Silver                Ag( s ) → Ag + ( aq )   + e – 

*Mercury                Hg( l ) → Hg 2+ ( aq )   + 2e –            *Some sources swap Hg and Ag (e.g. this)

Palladium               Pd(s) → Pd²⁺(aq) + 2e⁻

Platinum                 Pt( s ) → Pt 2+ ( aq )   + 2e –

Gold                 Au( s ) → Au 3+ ( aq )   + 3e –

Least active or most difficult to oxidize

NOTE: Hydrogen is not a metal. It is included as a reference point. Metals above hydrogen can generally replace H⁺ from ordinary dilute acids; metals below hydrogen generally cannot.

Nonmetals 

For nonmetals, electronegativity can provide a general guide to an element's tendency to attract electrons, but electronegativity should not be treated as a universal nonmetal activity series. 

(Most electronegative to least)

Name                        Electronegativity

Fluorine F                            3.98
Oxygen O                              3.44
Chlorine Cl                            3.16

Nitrogen N                            3.04

Bromine Br                           2.96
Iodine I                                  2.66

Sulfur S                              2.58

Selenium Se                          2.55

Carbon C                               2.55
Hydrogen H                       2.20

Phosphorus P                        2.19

The halogens provide the clearest and most useful nonmetal replacement series.

Halogen Activity Series:

F₂ > Cl₂ > Br₂ > I₂

The Big Rule

For single-replacement reactions:

Higher replaces lower within the appropriate activity series.

If the free element is above the element it is attempting to replace in the appropriate metal or halogen activity series, a reaction can occur.

NR — No Reaction

Use the metal activity series for metal replacement and the halogen activity series for halogen replacement.

______________

A few sources…

https://openstax.org/books/chemistry-2e/pages/17-3-electrode-and-cell-potentials?utm_source=chatgpt.com

https://chem.libretexts.org/Courses/Calvin_University/Chem_230%3A_Essential_Inorganic_Chemistry/03%3A_Oxidation_and_Reduction_of_Elements/3.04%3A_Standard_Reduction_Potentials?utm_source=chatgpt.com

https://chem.libretexts.org/Ancillary_Materials/Reference/Reference_Tables/Electrochemistry_Tables/P1%3A_Standard_Reduction_Potentials_by_Element?utm_source=chatgpt.com

https://www.thoughtco.com/activity-series-of-metals-603960

https://courses.lumenlearning.com/cheminter/chapter/chart-activity-series-of-metals/

https://www.sd308.org/cms/lib/IL01906463/Centricity/Domain/2189/Activity%20Series%20Chart.pdf

http://foradorimath.weebly.com/uploads/4/6/3/5/4635110/2a_-_activity_series.pdf

https://en.wikipedia.org/wiki/Reactivity_series#Table

https://www.templateroller.com/template/585124/activity-series-metals-and-non-metals-cheat-sheet.html

Comments:

Different sources sometimes show slightly different activity-series rankings. One important example is lithium. Lithium may appear lower in simplified reactivity charts, especially when visible reactions with water are being considered. However, when metals are compared using standard aqueous electrode potentials, lithium has an exceptionally strong tendency to be oxidized.