What is Reaction types & balancing on the Chemistry Regents?

Reaction types & balancing is a Chemical Reactions topic on the Chemistry Regents. RegentsLab has 36 practice questions on it, each with a full worked solution.

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Reaction types & balancing: Chemistry Regents Practice

Practice Reaction types & balancing for the Chemistry Regents with real-style questions and full worked solutions. Miss one? Read the solution — the exam reuses the same ideas every year.

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Question 49 · 2 pts

When the equation $H_2 + O_2 \rightarrow H_2O$ is balanced using the smallest whole-number coefficients, what is the coefficient in front of $H_2O$?

(A) 1
(B) 3
(C) 4
(D) 2 ✓

Worked solution

$O_2 \text{ provides 2 O atoms, so use } 2\,H_2O$ balance oxygen first

$2\,H_2O \text{ needs 4 H, so use } 2\,H_2$

$2\,H_2 + O_2 \rightarrow 2\,H_2O$

Answer: 2

Atoms of each element are conserved in a chemical reaction; only coefficients (not subscripts) are changed to make the atom count equal on both sides.

Question 50 · 2 pts

Which type of chemical reaction is represented by the balanced equation $2\,Al(s) + 3\,CuSO_4(aq) \rightarrow Al_2(SO_4)_3(aq) + 3\,Cu(s)$?

(A) Single replacement ✓
(B) Decomposition
(C) Synthesis (combination)
(D) Double replacement

Worked solution

$\text{Reactants: one free element } (Al) + \text{one compound } (CuSO_4)$

$Al \text{ takes the place of } Cu, \text{ freeing } Cu \text{ and forming } Al_2(SO_4)_3$ one element replaces another

$\Rightarrow \text{single replacement}$

Answer: Single replacement

In a single replacement reaction a free element replaces a less active element in a compound, producing a new element and a new compound.

Question 22 · 2 pts

Given the equation: $2H_2(g) + O_2(g) \rightarrow 2H_2O(g)$. This reaction is classified as a

(A) double replacement reaction
(B) single replacement reaction
(C) synthesis reaction ✓
(D) decomposition reaction

Worked solution

$\text{Two reactants combine to form one product}$ Pattern: A + B \rightarrow AB

$H_2 + O_2 \rightarrow H_2O \; \text{fits the synthesis pattern}$

Answer: synthesis reaction

A synthesis (combination) reaction occurs when two or more substances combine to form a single product; here hydrogen gas and oxygen gas combine to form water.

Question 38 · 2 pts

Which type of chemical reaction is represented by the equation $2Na(s) + Cl_2(g) \rightarrow 2NaCl(s)$?

(A) Decomposition
(B) Single replacement
(C) Synthesis ✓
(D) Double replacement

Worked solution

$\text{Two reactants combine to form one product}$ Na + Cl2 combine to form NaCl

$\text{This pattern (A + B} \rightarrow \text{AB) defines a synthesis (combination) reaction}$

Answer: Synthesis

In a synthesis reaction, two or more substances combine to form a single product, exactly as shown when sodium metal and chlorine gas combine to form sodium chloride. A common error is calling this single replacement, but no element is displacing another from a compound.

Question 55 · 4 pts · constructed response

Balance the following equation using the smallest whole-number coefficients, and identify the type of chemical reaction it represents: $\_\_Al(s) + \_\_O_2(g) \rightarrow \_\_Al_2O_3(s)$

Model answer: The balanced equation is $4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)$. This is a synthesis (combination) reaction.

Worked solution

$4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)$ Balance Al and O atoms on both sides

$\text{Two reactants combine to form one product, so this is a synthesis reaction.}$

Answer: The balanced equation is $4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)$. This is a synthesis (combination) reaction.

Balancing requires equal numbers of each type of atom on both sides of the equation. Because two elements combine to form a single compound, this reaction is classified as synthesis (combination).

Question 74 · 4 pts · constructed response

Balance the following equation using the smallest whole-number coefficients, and identify the type of chemical reaction it represents: $\_\_Mg + \_\_O_2 \rightarrow \_\_MgO$

Model answer: The balanced equation is $2Mg + O_2 \rightarrow 2MgO$, and it is a synthesis (combination) reaction.

Worked solution

$\_\_Mg + \_\_O_2 \rightarrow \_\_MgO$ Start with unbalanced equation

$2Mg + O_2 \rightarrow 2MgO$ Balance oxygen atoms (2 on each side) then magnesium atoms (2 on each side)

$\text{Reaction type: synthesis (combination)}$ Two substances combine to form a single product

Answer: The balanced equation is $2Mg + O_2 \rightarrow 2MgO$, and it is a synthesis (combination) reaction.

Balancing requires the same number of each type of atom on both sides of the equation; since two reactants (Mg and O2) combine to form one product (MgO), this is classified as a synthesis, or combination, reaction.

Question 79 · 4 pts · constructed response

Balance the following equation using the smallest whole-number coefficients: $\_\_Mg + \_\_O_2 \rightarrow \_\_MgO$. Then identify the type of chemical reaction shown (synthesis, decomposition, single replacement, or double replacement).

Model answer: The balanced equation is $2Mg + O_2 \rightarrow 2MgO$, which is a synthesis (combination) reaction.

Worked solution

$\text{Balance oxygen atoms: } 2Mg + O_2 \rightarrow 2MgO$ Two O atoms on left require two MgO on right

$\text{Check: } Mg = 2 \text{ each side}, \; O = 2 \text{ each side}$

$\text{Two elements combine to form one compound} \Rightarrow \text{synthesis reaction}$

Answer: The balanced equation is $2Mg + O_2 \rightarrow 2MgO$, which is a synthesis (combination) reaction.

Balancing requires equal numbers of each atom on both sides; placing a coefficient of 2 in front of both Mg and MgO balances the equation. Since two reactants (Mg and O2) combine to form a single product (MgO), this is classified as a synthesis, or combination, reaction.

Question 6 · 2 pts

When the equation $\_\_Al + \_\_O_2 \rightarrow \_\_Al_2O_3$ is balanced using the smallest whole-number coefficients, what is the coefficient of $O_2$?

(A) 4
(B) 6
(C) 2
(D) 3 ✓

Worked solution

$4Al + 3O_2 \rightarrow 2Al_2O_3$ balance Al atoms (4 and 4), then O atoms (6 and 6)

$\text{Al: } 4 = 2(2) \checkmark$

$\text{O: } 3(2) = 2(3) \Rightarrow 6 = 6 \checkmark$

Answer: 3

Balancing aluminum and oxygen atoms on both sides gives 4Al + 3O2 → 2Al2O3, so the coefficient of O2 is 3.

Question 30 · 2 pts

Given the equation: $2KClO_3(s) \rightarrow 2KCl(s) + 3O_2(g)$. This reaction is best classified as

(A) single replacement
(B) decomposition ✓
(C) synthesis
(D) double replacement

Worked solution

$\text{One reactant, } KClO_3\text{, breaks down into two simpler products}$

$\text{This pattern (one reactant} \rightarrow \text{multiple products) defines a decomposition reaction}$

Answer: decomposition

In a decomposition reaction a single compound breaks apart into two or more simpler substances. Here, potassium chlorate breaks down into potassium chloride and oxygen gas, matching this pattern exactly.

Question 73 · 4 pts · constructed response

Balance the following equation using the smallest whole-number coefficients, then identify the type of chemical reaction it represents: $\_\_Mg(s) + \_\_O_2(g) \rightarrow \_\_MgO(s)$

Model answer: 2Mg(s) + O2(g) → 2MgO(s); this is a synthesis (combination) reaction

Worked solution

$\text{Balance oxygen atoms: place a coefficient of 2 in front of } MgO$ Two O atoms on the reactant side require two MgO units

$\text{Balance magnesium atoms: place a coefficient of 2 in front of } Mg$

$2Mg(s) + O_2(g) \rightarrow 2MgO(s)$

$\text{Two reactants combine to form one product} \Rightarrow \text{synthesis reaction}$

Answer: 2Mg(s) + O2(g) → 2MgO(s); this is a synthesis (combination) reaction

A balanced equation must have equal numbers of each type of atom on both sides, following the law of conservation of matter; a reaction in which two or more substances combine to form a single product is classified as a synthesis (combination) reaction.

Question 79 · 4 pts · constructed response

Balance the following equation using the smallest whole-number coefficients, then classify the reaction as synthesis, decomposition, single replacement, or double replacement: $\_\_Al + \_\_O_2 \rightarrow \_\_Al_2O_3$

Model answer: 4Al + 3O2 -> 2Al2O3; this is a synthesis (combination) reaction

Worked solution

$4Al + 3O_2 \rightarrow 2Al_2O_3$ Balance Al atoms (4 on each side) and O atoms (6 on each side)

$\text{Two elements combine to form one compound, so this is a synthesis reaction}$

Answer: 4Al + 3O2 -> 2Al2O3; this is a synthesis (combination) reaction

Balancing requires equal numbers of each type of atom on both sides of the equation; when two or more reactants combine to form a single product, the reaction is classified as synthesis (combination).

Question 19 · 2 pts

Which type of chemical reaction is represented by the equation $2H_2 + O_2 \rightarrow 2H_2O$?

(A) Double replacement
(B) Decomposition
(C) Single replacement
(D) Synthesis ✓

Worked solution

$\text{Two or more substances combine to form one product}$ Definition of a synthesis reaction

$H_2 + O_2 \rightarrow H_2O$ Two reactants form a single product

$\text{This matches the pattern } A + B \rightarrow AB$

Answer: Synthesis

A synthesis (combination) reaction occurs when two or more reactants combine to form a single product. Here, hydrogen and oxygen gases combine to form water, fitting the general form A + B → AB, which identifies it as a synthesis reaction.

Question 57 · 4 pts · constructed response

Balance the following equation using the smallest whole-number coefficients, and identify the type of reaction it represents: $\_\_Al(s) + \_\_O_2(g) \rightarrow \_\_Al_2O_3(s)$

Model answer: $4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)$; this is a synthesis (combination) reaction.

Worked solution

$\text{Balance Al atoms and O atoms so both sides match}$

$4Al + 3O_2 \rightarrow 2Al_2O_3$

$\text{Left: 4 Al, 6 O. Right: 4 Al, 6 O , balanced}$

Answer: $4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)$; this is a synthesis (combination) reaction.

Balancing requires that the number of atoms of each element be equal on both sides of the equation. Using coefficients of 4, 3, and 2 balances both aluminum (4) and oxygen (6) atoms. Because two reactants combine to form a single product, this is classified as a synthesis reaction.

Question 67 · 4 pts · constructed response

Balance the following equation using the smallest whole-number coefficients, and identify the type of chemical reaction it represents: $\_\_Fe + \_\_O_2 \rightarrow \_\_Fe_2O_3$

Model answer: $4Fe + 3O_2 \rightarrow 2Fe_2O_3$; this is a synthesis (combination) reaction.

Worked solution

$4Fe + 3O_2 \rightarrow 2Fe_2O_3$ Balance Fe atoms: 4 on each side

$3(2) = 6\text{ O atoms left}; 2(3) = 6\text{ O atoms right}$ Check O atoms are balanced

Answer: $4Fe + 3O_2 \rightarrow 2Fe_2O_3$; this is a synthesis (combination) reaction.

Two or more substances (Fe and O2) combine to form a single product (Fe2O3), which is the definition of a synthesis reaction; the coefficients 4, 3, 2 balance both iron and oxygen atoms.

Question 78 · 4 pts · constructed response

Given the unbalanced equation: $Mg + O_2 \rightarrow MgO$. Balance this equation using the smallest whole-number coefficients and classify the reaction type.

Model answer: Balanced equation: $2Mg + O_2 \rightarrow 2MgO$. This is a synthesis (combination) reaction.

Worked solution

$Mg + O_2 \rightarrow MgO$ unbalanced: 2 O atoms on left, 1 O atom on right

$Mg + O_2 \rightarrow 2MgO$ balance oxygen by placing a 2 in front of MgO

$2Mg + O_2 \rightarrow 2MgO$ balance magnesium by placing a 2 in front of Mg

$\text{Two reactants combine to form one product} \Rightarrow \text{synthesis reaction}$

Answer: Balanced equation: $2Mg + O_2 \rightarrow 2MgO$. This is a synthesis (combination) reaction.

Balancing requires equal numbers of each type of atom on both sides of the equation; once balanced, this reaction is classified as a synthesis (combination) reaction because two elements combine to form a single compound.

Question 83 · 4 pts · constructed response

Balance the following equation using the smallest whole-number coefficients, and identify the type of chemical reaction it represents: $Fe_2O_3 + C \rightarrow Fe + CO_2$

Model answer: Balanced equation: $2Fe_2O_3 + 3C \rightarrow 4Fe + 3CO_2$. This is a single replacement reaction.

Worked solution

$2Fe_2O_3 + 3C \rightarrow 4Fe + 3CO_2$ Check: 4 Fe, 6 O on each side; 3 C atoms on each side

$\text{Carbon replaces iron as the element bonded to oxygen, so this is a single replacement reaction}$

Answer: Balanced equation: $2Fe_2O_3 + 3C \rightarrow 4Fe + 3CO_2$. This is a single replacement reaction.

Balancing requires equal numbers of each atom type on both sides; here carbon displaces iron from combination with oxygen, which is the pattern of a single replacement reaction.

Question 46 · 2 pts

Given the balanced equation: $2Na + Cl_2 \rightarrow 2NaCl$. This reaction is classified as a

(A) decomposition reaction
(B) single replacement reaction
(C) double replacement reaction
(D) synthesis reaction ✓

Worked solution

$\text{Two reactants (Na and Cl}_2\text{) combine to form one product (NaCl)}$

$A + B \rightarrow AB \Rightarrow \text{synthesis (combination) reaction}$

Answer: synthesis reaction

When two or more substances combine to form a single new substance, the reaction is classified as synthesis, matching the pattern shown by sodium and chlorine forming sodium chloride.

Question 76 · 4 pts · constructed response

Balance the following equation for the combustion of propane by writing the correct coefficients, and identify the type of chemical reaction it represents: __C3H8 + __O2 -> __CO2 + __H2O

Model answer: The balanced equation is C3H8 + 5O2 -> 3CO2 + 4H2O; this is a combustion reaction.

Worked solution

$\text{Balance carbon: 3 C on each side requires 3 CO}_2$

$\text{Balance hydrogen: 8 H atoms require 4 H}_2\text{O}$

$\text{Balance oxygen: 3(2) + 4(1) = 10 O atoms on product side, requiring 5 O}_2$

$\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}$

Answer: The balanced equation is C3H8 + 5O2 -> 3CO2 + 4H2O; this is a combustion reaction.

Balancing a combustion equation requires matching the number of carbon, hydrogen, and oxygen atoms on both sides of the equation; a hydrocarbon reacting with oxygen to form carbon dioxide and water is classified as a combustion reaction.

Question 51 · 2 pts

When the decomposition reaction $KClO_3 \rightarrow KCl + O_2$ is balanced with the smallest whole-number coefficients, what is the coefficient of $O_2$?

(A) 3 ✓
(B) 2
(C) 1
(D) 6

Worked solution

$\text{Use } 2\,KClO_3 \text{ to get an even number of O atoms: } 6\ \text{O}$ each KClO3 has 3 O

$6\ \text{O atoms} = 3\,O_2$

$2\,KClO_3 \rightarrow 2\,KCl + 3\,O_2$ 2 K, 2 Cl, 6 O on each side

Answer: 3

Decomposition breaks a single compound into simpler substances; coefficients are chosen so every element's atom count is equal on both sides.

Question 52 · 2 pts

When the equation $Pb(NO_3)_2(aq) + KI(aq) \rightarrow PbI_2(s) + KNO_3(aq)$ is balanced with the smallest whole-number coefficients, what is the coefficient of $KI$?

(A) 1
(B) 3
(C) 4
(D) 2 ✓

Worked solution

$PbI_2 \text{ has 2 I atoms, so use } 2\,KI$ balance iodine

$2\,KI \text{ gives 2 K, so use } 2\,KNO_3 \text{ (also balances N and O)}$

$Pb(NO_3)_2 + 2\,KI \rightarrow PbI_2 + 2\,KNO_3$

Answer: 2

In a double replacement reaction the positive ions of the two compounds exchange partners; coefficients are set so each element balances.

Question 65 · 4 pts · constructed response

Balance the following equation using the smallest whole-number coefficients, and identify the type of reaction it represents: $\_\_Al + \_\_O_2 \rightarrow \_\_Al_2O_3$

Model answer: 4Al + 3O2 -> 2Al2O3; this is a synthesis (combination) reaction

Worked solution

$\text{Balance Al atoms: need 2 Al per } Al_2O_3$

$\text{Balance O atoms: 3 } O_2 \text{ gives 6 O atoms, matching 2 } Al_2O_3 \text{ (6 O atoms)}$

$4Al + 3O_2 \rightarrow 2Al_2O_3$

$\text{Two reactants combine to form one product, so this is a synthesis reaction}$

Answer: 4Al + 3O2 -> 2Al2O3; this is a synthesis (combination) reaction

Balancing requires equal numbers of each type of atom on both sides. Since two elements (Al and O2) combine to form a single compound, the reaction is classified as synthesis (combination).

Question 22 · 2 pts

A single compound $AB$ breaks down to form two simpler substances, $A$ and $B$, according to the equation $AB \rightarrow A + B$. This equation represents which type of reaction?

(A) Synthesis
(B) Double replacement
(C) Single replacement
(D) Decomposition ✓

Worked solution

$\text{One reactant, } AB, \text{ produces two or more products, } A \text{ and } B$

$\text{This pattern, one substance breaking into simpler substances, defines a decomposition reaction}$

Answer: Decomposition

A decomposition reaction has a single reactant that breaks apart into two or more simpler products, matching the general form AB to A plus B. This is the reverse pattern of a synthesis reaction, in which two substances combine to form one product.

Question 42 · 2 pts

When the equation $\_\_Fe + \_\_O_2 \rightarrow \_\_Fe_2O_3$ is balanced using the smallest whole-number coefficients, what is the coefficient of $O_2$?

(A) 3 ✓
(B) 1
(C) 2
(D) 4

Worked solution

$4Fe + 3O_2 \rightarrow 2Fe_2O_3$ balance Fe atoms, then O atoms

$\text{Fe: } 4 = 4 \quad \text{O: } 3(2) = 2(3) = 6$

Answer: 3

Balancing requires equal numbers of each atom on both sides. With 4 Fe atoms and 3 O2 molecules providing 6 O atoms, the product must be 2 Fe2O3, giving the fully balanced equation 4Fe + 3O2 -> 2Fe2O3. The coefficient of O2 in the smallest whole-number balanced equation is 3.

Question 57 · 4 pts · constructed response

Balance the following equation using the smallest whole-number coefficients: $\_\_Al(s) + \_\_O_2(g) \rightarrow \_\_Al_2O_3(s)$

Model answer: $4Al + 3O_2 \rightarrow 2Al_2O_3$

Worked solution

$\text{Balance Al atoms: place a 2 in front of } Al_2O_3 \text{, requiring 4 Al on the left}$

$\text{Balance O atoms: } 2Al_2O_3 \text{ has 6 O atoms, so use } 3O_2 \text{ on the left}$

$4Al + 3O_2 \rightarrow 2Al_2O_3$ Check: 4 Al = 4 Al; 6 O = 6 O

Answer: $4Al + 3O_2 \rightarrow 2Al_2O_3$

Balancing requires equal numbers of each type of atom on both sides of the equation. Adjusting coefficients (never subscripts) for Al and O2 until the aluminum and oxygen atom counts match on both sides gives the smallest whole-number coefficients 4, 3, 2.

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