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AP Chemistry Help: Types Of Chemical Reactions

Review real example questions for Types Of Chemical Reactions in AP Chemistry.

Question 1 / 10

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Propane gas undergoes complete combustion in the presence of excess oxygen as shown in the equation below.

$C_3H_8(g) + 5 O_2(g) \rightarrow 3 CO_2(g) + 4 H_2O(g)$

Combustion reactions are a specific subclass of which general reaction type?

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Question 1

Propane gas undergoes complete combustion in the presence of excess oxygen as shown in the equation below.

$C_3H_8(g) + 5 O_2(g) \rightarrow 3 CO_2(g) + 4 H_2O(g)$

Combustion reactions are a specific subclass of which general reaction type?

Acid-base reactions
Precipitation reactions
Oxidation-reduction reactions (correct answer)
Double-displacement reactions

Explanation: Combustion is a type of oxidation-reduction (redox) reaction. In this reaction, the oxidation state of carbon increases from -8/3 in $C_3H_8$ to +4 in $CO_2$ (oxidation), and the oxidation state of oxygen decreases from 0 in $O_2$ to -2 in both $CO_2$ and $H_2O$ (reduction). The transfer of electrons is the defining feature of a redox reaction.

Question 2

Hydrochloric acid reacts with solid zinc carbonate, producing a gas and a salt solution.

$2 HCl(aq) + ZnCO_3(s) \rightarrow ZnCl_2(aq) + H_2O(l) + CO_2(g)$

Which of the following is the most appropriate classification for this reaction?

A precipitation reaction because a solid reactant is consumed.
An oxidation-reduction reaction because a gas is formed.
An acid-base reaction because the carbonate ion accepts protons from the acid. (correct answer)
A synthesis reaction because three products are formed from two reactants.

Explanation: This reaction is best classified as an acid-base reaction. The carbonate ion ( $CO_3^{2-}$ ) from $ZnCO_3$ acts as a base, accepting two protons from the hydrochloric acid ( $HCl$ ) to form carbonic acid ( $H_2CO_3$ ), which is unstable and decomposes into $H_2O(l)$ and $CO_2(g)$ . The formation of a gas does not automatically mean a reaction is redox, and no oxidation states change. This is a gas-forming type of neutralization reaction.

Question 3

The reaction of solid sodium with liquid water is vigorous and produces hydrogen gas and an aqueous solution of sodium hydroxide: $2 Na(s) + 2 H_2O(l) \rightarrow 2 NaOH(aq) + H_2(g)$ . A student claims this is primarily an acid-base reaction because water provides a proton to sodium. Which statement best evaluates this claim?

The claim is valid because water acts as a Brønsted-Lowry acid, and the formation of $OH^-$ indicates a base is present.
The claim is invalid because the reaction is best classified as oxidation-reduction due to changes in the oxidation states of Na and H. (correct answer)
The claim is valid because it is a neutralization reaction that produces a salt and hydrogen gas instead of water.
The claim is invalid because it is a precipitation reaction, evidenced by the formation of a gas from a solid and a liquid.

Explanation: While it is true that water acts as a proton source (an acid), the most significant chemical change is the transfer of electrons. Sodium metal (oxidation state 0) is oxidized to $Na^+$ ions (oxidation state +1). Hydrogen in water (oxidation state +1) is reduced to hydrogen gas (oxidation state 0). Because there are clear changes in oxidation states, the reaction is best classified as oxidation-reduction. The student's claim overlooks this primary characteristic.

Question 4

Consider the two reactions represented by the equations below.

Reaction I: $2 H_2(g) + O_2(g) \rightarrow 2 H_2O(l)$ Reaction II: $Ca(OH)_2(aq) + 2 HCl(aq) \rightarrow CaCl_2(aq) + 2 H_2O(l)$

Which of the following correctly identifies the types of Reaction I and Reaction II?

Reaction I is an acid-base reaction, and Reaction II is an oxidation-reduction reaction.
Reaction I is an oxidation-reduction reaction, and Reaction II is a precipitation reaction.
Reaction I is a precipitation reaction, and Reaction II is an acid-base reaction.
Reaction I is an oxidation-reduction reaction, and Reaction II is an acid-base reaction. (correct answer)

Explanation: In Reaction I, hydrogen's oxidation state goes from 0 to +1, and oxygen's goes from 0 to -2. This is an oxidation-reduction reaction (specifically, a synthesis or combustion reaction). In Reaction II, $Ca(OH)_2$ (a base) reacts with $HCl$ (an acid) in a neutralization reaction to produce a salt and water. This is a classic acid-base reaction. No precipitate is formed in Reaction II, and no proton transfer occurs in Reaction I.

Question 5

Which of the following statements provides the most accurate and fundamental definition of an oxidation-reduction (redox) reaction?

A reaction in which a substance combines with oxygen, often producing heat and light.
A reaction involving the transfer of one or more electrons between chemical species. (correct answer)
A reaction in which a solid, insoluble product forms from the mixing of aqueous solutions.
A reaction that involves the transfer of a proton from one species to another.

Explanation: The fundamental, defining characteristic of all oxidation-reduction reactions is the transfer of electrons from one species (which is oxidized) to another (which is reduced). While reaction with oxygen (combustion) is a common type of redox reaction, it does not define all of them. Proton transfer defines acid-base reactions, and formation of a solid from solution defines precipitation reactions.

Question 6

When heated, solid potassium chlorate decomposes to form solid potassium chloride and oxygen gas.

$2 KClO_3(s) \rightarrow 2 KCl(s) + 3 O_2(g)$

Which statement best classifies this reaction?

It is an acid-base reaction because potassium chlorate is a salt of a strong base and a strong acid.
It is an oxidation-reduction reaction because the oxidation states of both chlorine and oxygen change. (correct answer)
It is a precipitation reaction because one of the reactants is a solid.
It is not an oxidation-reduction reaction because no aqueous ions are involved in the process.

Explanation: This decomposition is an oxidation-reduction reaction. In $KClO_3$ , the oxidation state of Cl is +5 and O is -2. In the products, the oxidation state of Cl in $KCl$ is -1 (reduction), and the oxidation state of O in $O_2$ is 0 (oxidation). Since the oxidation states of multiple elements change, it is a redox reaction.

Question 7

A strip of zinc metal is placed in aqueous copper(II) sulfate, producing copper metal and aqueous zinc sulfate: $\text{Zn}(s)+\text{CuSO}_4(aq)\rightarrow \text{Cu}(s)+\text{ZnSO}_4(aq)$ . Which reaction type best classifies this reaction?

precipitation
double replacement
single replacement (correct answer)
combustion
synthesis

Explanation: The skill being tested is classifying types of chemical reactions. In this setup, zinc metal displaces copper from copper(II) sulfate solution, forming copper metal and zinc sulfate, which exemplifies a single replacement reaction. The balanced equation demonstrates zinc taking the place of copper in the compound, driven by zinc's higher reactivity in the activity series. This is a redox reaction where zinc is oxidized and copper ions are reduced, confirming the displacement mechanism. A tempting distractor is double replacement, but it is incorrect because double replacement involves swapping ions between two compounds, which is a misconception here as only one element is replacing another. To classify reactions effectively, use the activity series for metals to predict if displacement will occur and check if the pattern matches A + BC → AC + B.

Question 8

Solid potassium chlorate decomposes when heated to form potassium chloride and oxygen gas: $2\text{KClO}_3(s)\rightarrow 2\text{KCl}(s)+3\text{O}_2(g)$ . Which reaction type best classifies this reaction?

decomposition (correct answer)
combustion
single replacement
synthesis
acid‑base

Explanation: The skill being tested is classifying types of chemical reactions. Solid potassium chlorate decomposes upon heating into potassium chloride and oxygen gas, which is a decomposition reaction. The balanced equation depicts a single compound breaking down into simpler substances, requiring heat to initiate. This type of reaction often involves unstable compounds releasing gases or forming stable products. A tempting distractor is combustion, but it is incorrect because combustion requires oxygen as a reactant, which is a misconception as oxygen is a product here. To classify reactions effectively, determine if a single reactant is breaking apart and note any energy input like heat to confirm decomposition.

Question 9

A student neutralizes hydrochloric acid with aqueous sodium hydroxide: $\mathrm{HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)}$ . Which reaction type best classifies this reaction?

precipitation
single replacement
acid‑base (correct answer)
combustion
decomposition

Explanation: The skill being tested is classifying types of chemical reactions based on the patterns of reactants and products. Hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water, which is an acid-base reaction involving neutralization where H+ from the acid combines with OH- from the base. The stimulus describes neutralization, a key indicator of this type, producing a salt and water. This reaction is a specific form of double replacement with pH change implications. A tempting distractor is precipitation, but it is incorrect because no insoluble solid forms, reflecting the misconception of assuming all aqueous reactions produce precipitates. To classify reactions effectively, check if an acid and base are reacting to produce water and a salt, signaling neutralization.

Question 10

Aqueous lead(II) nitrate is mixed with aqueous potassium iodide, producing a yellow solid and aqueous potassium nitrate: $\mathrm{Pb(NO_3)_2(aq) + 2KI(aq) \rightarrow PbI_2(s) + 2KNO_3(aq)}$ . Which classification best describes this reaction?

combustion
precipitation (correct answer)
single replacement
synthesis
acid-base

Explanation: This question tests the ability to identify precipitation reactions. In the reaction Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq), two aqueous solutions mix to form a yellow solid precipitate (lead(II) iodide) and an aqueous solution. The formation of the insoluble solid from two soluble reactants is the defining characteristic of a precipitation reaction. Lead(II) iodide is known for its distinctive bright yellow color, making this a visually striking precipitation reaction. Students might choose double replacement (which this also is) or be distracted by the color and choose a different option, but the formation of a solid precipitate is the key identifying feature. To identify precipitation reactions, look for the formation of an insoluble solid product (indicated by (s)) when aqueous solutions are mixed.