# AP Chemistry: 4.1 Introduction for Reactions – Exam Style questions with Answer- MCQ

Questions

$$2H_{2(g)}+O_{2(g)}\rightarrow 2H_{2}O(g)$$

When $$H_{2(g)}$$ and $$O_{2(g)}$$ are mixed together in a rigid reaction vessel at 25$$^{\circ }C$$, no reaction occurs. When the mixture is sparked, however, the gases react vigorously according to the equation above, releasing heat. Which of the following statements correctly explains why the spark is needed for the reaction to occur when the gases are originally at 25$$^{\circ }C$$?
(A) The reaction is not thermodynamically favorable at 25$$^{\circ }C$$.
(B) $$\bigtriangleup H_{\circ }$$ for the reaction has a large positive value at 25$$^{\circ }C$$.
(C) $$\bigtriangleup S_{\circ }$$ for the reaction has a large negative value at 25$$^{\circ }C$$.
(D) The reaction has a large activation energy at 25$$^{\circ }C$$.

Ans: D

The reaction described is the combustion of hydrogen gas ($$H_{2(g)}$$) with oxygen gas ($$O_{2(g)}$$) to form water vapor ($$H_{2}O(g)$$).

When $$H_{2(g)}$$ and $$O_{2(g)}$$ are mixed together at 25°C in a rigid reaction vessel, no reaction occurs. This suggests that the reaction does not occur spontaneously under those conditions, even though the reactants are present.

To initiate the reaction, a spark is needed. The spark provides the activation energy required for the reaction to proceed. Without the spark, the reactant molecules do not have sufficient energy to overcome the activation energy barrier and react.

Therefore, the correct statement to explain why the spark is needed for the reaction to occur at 25°C is: (D) The reaction has a large activation energy at 25°C.

### Question

$$Na_2CO_3(aq)+2HCl(aq)→2NaCl(aq)+H_2O(l)+CO_2(g)$$

A student combined two colorless aqueous solutions. One of the solutions contained  $$Na_2CO_3$$
as the solute, and the other contained HCl . The chemical reaction that took place is represented by the equation above. What experimental result would be evidence that a chemical reaction took place when the solutions were combined?
A Bubbles formed when the two solutions were combined.
B The total volume of the mixture is close to the sum of the initial volumes.
C The resulting solution is colorless.
D The resulting solution conducts electricity.

Ans: A

A gas $$CO_2$$ was produced from the chemical reaction between $$Na_2CO_3$$ and $$HCl$$. Its production will be evident by the formation of bubbles as the reaction proceeds. The formation of a gas is possible evidence that a chemical change has occurred.

### Question

A chemistry teacher carried out several demonstrations, and students recorded their observations. For one of the demonstrations, a student concluded that a physical change took place, but not a chemical change. Which of the following observations could the student have made of the results of the demonstration?
A Two colorless solutions were combined, and the resulting solution was pink.
B When a solid was added to a liquid, sparks were produced.
C One piece of solid substance was changed into small pieces.
D When two solutions were combined, a precipitate formed.

Ans: C
Changes in phase, size, and shape are examples of physical changes. There is no evidence of a substance being transformed into a new substance.

### Question

A student was given two clear liquids; a colorless liquid and a dark-blue liquid. The student was asked to combine the liquids in a beaker and record observations. Which of the following results, if true, would provide the best evidence that a chemical change took place when the liquids were combined?
A The resulting mixture was cloudy.
B The total volume of the mixture was equal to the sum of the initial volumes.
C The resulting liquid was light blue.
D The liquids formed two separate layers in the beaker.

Ans: A

The formation of a solid (precipitate) can result in the development of cloudiness, and the solid may eventually be visible at the bottom of the beaker. The formation of a precipitate is possible evidence that a chemical change might have occurred.

### Question

$$M^{+}$$ is an unknown metal cation with a +1 charge. A student dissolves the chloride of the unknown  metal, MCI, in enough water to make 100.0 mL of    solution. The student then mixes the solution with   excess $$AgN0_3$$ solution, causing AgCl to precipitate.   The student collects the precipitate by filtration, dries it, and records the data shown below. (The molar mass of AgCl is 143 g/mol.)

Mass of unknown chloride, MCI                        0.74 g

Mass of filter paper                                            0.80 g

Mass of filter paper plus AgCl precipitate          2.23 g

What is the identity of the metal chloride?

(A) NaCl

(B)KCl

(C)CuCl

(D)LiCl