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## Group Activity: Reaction Prediction

### Introduction

You have already learned about chemical formulas for elements and compounds. You have also learned how to balance chemical equations. Finally, you have some familiarity with the basic reaction types listed below:

• Synthesis or Combination
• Decomposition
• Single Replacement
• Double Replacement
• Precipitation
• Acid-base
• Combustion

In this activity you will review these reaction types and study some examples. After some instruction into the details of how the reactions work you will practice predicting products of reactions. In this way you will come to a deeper understanding of the classification of chemical reactions.

### Synthesis

In a synthesis reaction two reactants combine to form one product:

A + B → C

Some example synthesis reactions involve only non-metals. These reactions are hard to predict. This is because there are often multiple possible combinations. Still, a few synthesis reactions involving non-metals are frequently used and easy to memorize. Consider the following reactions to be some you should know by heart:

2H2(g) + O2(g) → 2H2O(l)

3H2(g) + N2(g) → 2NH3(g)

H2(g) + Cl2(g) → 2HCl(l)
(F, Br, or I may substitute for Cl).

Reactions between metals and non-metals are much easier to predict. As you know, the elements in the periodic table are arranged in groups with similar characteristics. Elements in group 1 (Li, Na, K, etc.) are written as pure elements using their atomic symbol. When any element in this group forms an ion by reacting with a non-metal it always has a +1 charge: Li+1, Na+1, K+1. Memorize these predictable charges.

The predictability of the charges adopted by elements when they react is what makes predicting the products of the reaction between metals and non-metals relatively simple. For example, Mg becomes Mg+2 in compounds and O becomes O–2:

2Mg(s) + O2(g) → 2MgO(s)
 Group No. Expected Charge in Compounds Type of Element 1 +1 Metal 2 +2 Metal 13 +3 Metal 15 –3 Non-Metal 16 –2 Non-Metal 17 –1 Non-Metal

For reactions between a metal and a non-metal, here is how it works:

1. Identify the elements and their groups.
2. Predict the charge each element will have as a single atom, even if the formula of the neutral element is molecular.
3. Combine the ions formed by each element into a neutral ionic compound.
4. Complete the chemical equation by writing in the product you just predicted.
5. Balance the chemical equation.

For example, consider a reaction between potassium and chlorine:

K(s) + Cl2(g) → ______
Potassium (K) becomes K+1 in compounds.
Chlorine (Cl2) becomes Cl–1 in compounds.
The correct compound is: KCl.
Complete the equation:
K(s) + Cl2(g) → KCl(s)
Balance the equation:
2K(s) + Cl2(g) → 2KCl(s)

Note that ionic compounds are always solids in these reactions.

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### Decomposition

In a decomposition reaction one reactant forms two or more products:

C → A + B

Some decomposition reactions are easy to predict. They simply take a compound and break it down into the component elements. They are simply the reverse of synthesis reactions as discussed above. Here are some examples:

2NaCl(l) → 2Na(s) + Cl2(g)

Ca3N2(l) → 3Ca(s) + N2(g)

K2S(l) → 2K(s) + S(s)

Predicting the products of the decomposition of simple ionic compounds of two elements requires only that you write the correct formulas of both elements as products before you balance the chemical equation. As you do so, remember that the subscripts in the formulas of the compound being decomposed depend on the charges of the ions in ionic compounds, not on the molecular formula of the component elements. Sodium chloride (NaCl) does not form Na + Cl because the correct formula for chlorine is Cl2, not Cl.

Other decomposition reactions follow a specific pattern based on the polyatomic ion that is part of the compound being broken down. Compounds containing the carbonate ion (CO32–) break down to form an oxide compound and carbon dioxide. Compounds containing the chlorate ion (ClO3) break down to form a chloride and oxygen gas. For example:

Carbonates:
CaCO3(s) → CaO(s) + CO2(g)

Chlorates:
2NaClO3(l) → 2NaCl(s) + 3O2(g)

Predicting the products for these decomposition reactions requires that you substitute other metals in for Na or Ca as used in the examples.

### Single Replacement

In a single replacement reaction one reactant is an element, and the other is a compound. In the reaction the pure element becomes an ion and combines with an ion from the compound. One of the elements in the compound in turn becomes a pure element (as neutral atoms).

A + BC → AC + B

One common type of single replacement reaction involves a solid metal (with neutral atoms) that reacts with an aqueous solution of an ionic compound which includes a metal ion. The metal ion gains electrons to become neutral. The electrons come from the neutral metal atoms, which lose electrons to become an ion. Note that there is always another ion which is unchanged by the reaction. In the first reaction below, the nitrate ion (NO3) switches from being part of a compound with the silver ion to being part of a compound with the copper ion.

Cu(s) + 2AgNO3(aq) → Cu(NO3)2( aq) + 2Ag(s)

3Zn(s) + 2FeBr3(aq) → 3ZnBr2(aq) + 2Fe(s)

Br2(l) + 2NaI(s) → 2NaBr(aq) + I2(g)

Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

To predict the products of a single replacement reaction, several things are required:

1. Identify the atom that changes from being an ion into a neutral atom: write it as an element as one of the products. Non-metal elements replace other non-metals while metals and replace other metals and hydrogen ions.
2. Identify the unchanged ion (in bold in the examples).
3. Identify the atom that changes from neutral into an ion: predict its charge when it becomes an ion.
4. By balancing charges between them, write an ionic compound formula for these two ions: write this formula as a product.
5. Balance the chemical equation.

For example, consider a reaction between magnesium and aluminum nitrate:

Mg(s) + Al(NO3)3(aq) → ______ + ______
Aluminum (Al) becomes a neutral atom so it is one of the products, written as Al(s)
Nitrate ion (NO3) is unchanged.
Magnesium (Mg) becomes Mg+2 in compounds.
Mg+2 combines with NO3 to make Mg(NO3)2 so the 2nd product is Mg(NO3)2(aq)
Complete the equation:
Mg(s) + Al(NO3)3(aq) → Mg(NO3)2(aq) + Al(s)
Balance the equation:
3Mg(s) + 2Al(NO3)3(aq) → 3Mg(NO3)2(aq) + 2Al(s)

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### Double Replacement

In a double replacement reaction both reactants are aqueous ionic compounds. The ionic compounds exchange ions to form two new compounds.

AB + CD → AD + CB

There are two types of double replacement reactions that you need to know. One of these is called a precipitation reaction. Some combinations of ions are insoluble in water so when these ions meet, a solid forms and falls to the bottom of the liquid, like precipitation from the sky. The second is an acid-base reaction. Note the states of matter for these examples, which are both precipitation reactions:

CuSO4(aq) + 2NaOH(aq) →
ĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀCu(OH)2(s) + Na2SO4(aq)

2AlCl3(aq) + 3Na2CO3(aq) →
ĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀAl2(CO3)3( s) + 6NaCl(aq)

Some ions mostly have compounds which are insoluble in water while others are always or nearly always soluble, no matter what ion they are with in a compound. Any ion that is Always Soluble will be soluble even if it is combined with an ion labeled as Always Insoluble. Ions soluble with exceptions are only insoluble with the cations shown.

 Always Soluble Soluble with Exceptions (almost) Always Insoluble NH4+1, Li+1, Na+1, K+1, Rb+1, Cs+1 NO3–1, CH3COO–1 Cl–1, Br–1, I–1 SO4–2 F–1, S–2, CO3–2, OH–1, PO4–3 exceptions: Ag+1, Pb+2, Hg2+2 exceptions: Sr+2, Ba+2, Pb+2, Hg2+2

To predict the products of a double replacement precipitation reaction, several things are required:

1. Identify the ions that compose each of the two compounds.
2. Recombine ions, balancing charges to make two new combinations.
3. Write these as the products in the reaction.
4. Balance the chemical equation.
5. The product that is a solid must include one insoluble ion and must not include any of the always soluble ions.

For example, consider a reaction between sodium phosphate and calcium nitrate:

Na3PO4(aq) + Ca(NO3)2(aq) →
ĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀ______ + ______

The ions for Na3PO4 are Na+1 and PO4–3
The ions for Ca(NO3)2 are Ca+2 and NO3–1
The two new compounds are:
Ca+2 combines with PO4–3 to make Ca3(PO4)2(s)
This product is insoluble because PO4–3 ions are always insoluble unless combined with an always soluble ion.
Na+1 combines with NO3–1 to make NaNO3 so the 2nd product is NaNO3(aq)
This is soluble because both Na+1 and NO3–1 are always soluble.
Complete the equation:
Na3PO4(aq) + Ca(NO3)2(aq) →
ĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀCa3(PO4)2(s) + NaNO3(aq)
Balance the equation:
2Na3PO4(aq) + 3Ca(NO3)2(aq) →
ĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀCa3(PO4)2(s) + 6NaNO3(aq)

Another type of double replacement reaction is the acid-base reaction. An acid, which always contains hydrogen ions (H+), reacts with a base, which always includes hydroxide ions (OH), to make a salt and water. The word ‘salt’ here means any ionic compound, not just sodium chloride. Here is an example reaction:

H2SO4(aq) + 2KOH(aq) → 2H2O(l) + K2SO4(aq)

To predict the products of an acid-base reaction, several things are required:

1. Identify the acid, which always includes hydrogen ions (H+). And identify the base, which always includes hydroxide ions (OH).
2. One product is always water. The other product is a neutral ionic compound made from the two ions which were not hydrogen or hydroxide.
3. Write these as the products in the reaction.
4. Balance the chemical equation.
HCl(aq) + Ba(OH)2(aq) →
ĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀ______ + ______

The ions for HCl are H+1 and Cl–1
The ions for Ba(OH)2 are Ba+2 and OH–1
The two new compounds are:
Ba+2 combines with Cl–1 to make BaCl2(aq)
H+1 combines with OH–1 to make H2O
Complete the equation:
HCl(aq) + Ba(OH)2(aq) →
ĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀBaCl2(aq) + H2O(l)
Balance the equation:
2HCl(aq) + Ba(OH)2(aq) →
ĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀĀBaCl2(aq) + 2H2O(l)

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### Combustion

In a combustion reaction a hydrocarbon reacts with oxygen to make carbon dioxide and water:

CxHy + O2(g) → CO2(g) + H2O(g)

Sometimes the hydrocarbon molecule includes one or more oxygen atoms but the reaction is still the same:

CxHyOz + O2(g) → CO2(g) + H2O(g)

There is really no prediction involved in writing combustion reaction equations. The second reactant and both products are always the same. Just memorize that oxygen is a reactant and that carbon dioxide and water are products. Here are a few examples of balanced combustion reaction equations:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

2C4H10(l) + 13O2(g) → 8CO2(g) + 10H2O(g)

C2H6O(l) + 3O2(g) → 2CO2(g) + 3H2O(g)

Some fuels, such as natural gas and propane, are gases. Other fuels are usually kept in liquid form, for example, alcohol, gasoline, and diesel fuel. Although we may write them in a chemical equation as a liquid, they all actually burn in the vapor form. Even such fuels as candle wax, which is a solid at room temperature, actually burn in the vapor phase. In the case of a candle, the heat of the flame melts and then vaporizes the wax before the wax is burned in the flame.

### Exercises

1. Do elements go from neutral to ion or from ion to neutral in synthesis reactions between metals and non-metals? Justify your answer with one example from the text.
2. Why is it necessary for the elements in a synthesis reaction to change their charge in the course of a reaction?
3. Decomposition reaction prediction often requires you to know the correct chemical formula of elements. For the following elements, write their chemical formula including the phase of matter. Remember that some non-metals are molecular as elements.
iron
carbon
aluminum
oxygen
phosphorus
magnesium
bromine
nitrogen
silver
fluorine
4. Give the ionic charge of each element or polyatomic ion in these substances. For example, in NaCl the charge of Na is +1 and the charge of Cl is –1. Some substances may have zero charge.
CaF2
K
Al2O3
N2
MgS
NaNO3
LiBr
GaN
S8
Fe(OH)2
5. What do the terms carbonates and chlorates mean?
6. For single replacement reactions, how do you identify which element will become an ion when the reaction takes place?
7. Using the solubility table from the Double Replacement section of the text above, write the neutral ionic compound formula for five insoluble compounds. You will need to provide your own cations, such as Fe3+ or Al3+.
8. Using the solubility table from the Double Replacement section of the text above, write the neutral ionic compound formula for five soluble compounds using the “always insoluble” ions. You will need to use the always soluble ions.
9. Combustion reactions:
1. What kinds of compounds burn as fuel in combustion reactions? Give some example formulas.
2. What is the second reactant and what are the products of combustion reactions?

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For all of the following exercises use the reactant(s) predict the products of a reaction.
Write a chemical equation and balance it.
Synthesis Reactions
1. Li(l) + O2(g) →
2. H2(g) + F2(g) →
3. Mg(s) + N2(g) →
4. Al(s) + S8(l) →

5. Mg(s) + O2(g) →
6. Na(s) + N2(g) →
7. Ba(s) + I2(g) →
8. Ga(l) + P4(s) →
Decomposition Reactions
1. K2O(s) →
2. CuCl(s) →
3. Ca(ClO3)2(s) →
4. Al2(CO3)3 (s) →

5. H2O(l) →
6. B2O3(s) →
7. Mg3P2(s) →
8. NH3(g) →
Ā Single Replacement Reactions
1. Ca(s) + Sn(NO3)2( aq) →
2. Fe(s) + CuCl2(aq) →
3. AgNO3( aq) + Zn(s) →
4. Cl2(g) + AlBr3(aq) →
5. Al(s) + CuSO4(aq) →
6. Fe(NO3)3(aq) + Zn(s) →
7. F2(g) + MgCl2(s) →
8. Mg(s) + Pb(NO3)4( aq) →

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Double Replacement Precipitation Reactions
1. Ca(NO3)2(aq) + Na2CO3(aq) →
2. KOH(aq) + CuCl2(aq) →
3. AgNO3(aq) + CaCl2(aq) →
4. (NH4)3PO4(aq) + ZnBr2(aq) →
5. Pb(NO3)2(aq) + Na2SO4(aq) →
6. Fe(CH3COO)3(aq) + Li2S(aq) →
7. NaF(aq) + Ba(NO3)2(aq) →
8. KI(aq) + Pb(NO3)2(aq) →
Double Replacement Acid-Base Reactions
1. HNO3(aq) + NaOH(aq) →
2. KOH(aq) + HCl(aq) →
3. H2SO4(aq) + Ca(OH)2(aq) →
4. H3PO4(aq) + Zn(OH)2(aq) →
5. HBr(aq) + LiOH(aq) →
6. HCH3COO(aq) + NaOH(aq) →
Combustion Reactions
1. C2H6(g) + O2(g) →
2. C2H4(g) + O2(g) →
3. C6H14(g) + O2(g) →
4. C3H8O(l) + O2(g) →
5. C2H4O2(l) + O2(g) →
6. C8H18(l) + O2(g) →
Last updated: Dec 06, 2022ĀĀĀĀĀĀ Home