Unit 3: Reactions and Stoichiometry

Answer: A — The correct answer is D because the change in color is a clear indication that a chemical reaction has occurred, resulting in the formation of a new compound with a different color. Options A, B, and C are incorrect because adding an acid, base, or inert gas to the mixture may not necessarily result in a color change, and do not provide a clear explanation for the observation.

Answer: A — The correct answer, B, is the best description of what occurred because a chemical reaction involves the formation of one or more new substances, which is consistent with the observation of a color change. Option A is incorrect because physical changes do not result in the formation of new substances. Option C is incorrect because the color change is not due to light reflection, but rather the formation of a new substance. Option D is incorrect because nuclear reactions involve changes to the nucleus of an atom and are not typically associated with simple color changes.

Answer: B — The correct answer is A because increasing the temperature will provide more energy to overcome the activation energy barrier, thus increasing the rate of reaction. However, since the reaction is exothermic, increasing the temperature will shift the equilibrium to the left, decreasing the equilibrium constant. This is based on the principle that for exothermic reactions, an increase in temperature favors the reactants, thus decreasing the value of K. Options B, C, and D are incorrect because they inaccurately describe the effects of temperature on the rate of reaction and the equilibrium constant for an exothermic reaction.

Answer: A — The correct answer is C because a catalyst speeds up a chemical reaction by lowering the activation energy, allowing the reaction to reach equilibrium faster. The equilibrium constant (K) remains unchanged because the catalyst affects the rate of the reaction, not the equilibrium position. Option A is incorrect because the catalyst does not change the equilibrium constant. Option B is incorrect because the catalyst affects both the forward and reverse reactions equally. Option D is incorrect because the catalyst does not change the equilibrium position of the reaction.

Answer: C — Option B is correct because a chemical reaction by definition involves the transformation of one or more substances (reactants) into new substances (products). Option A is incorrect because a chemical reaction can result in the formation of multiple products. Option C is incorrect because chemical reactions can occur at a wide range of temperatures. Option D is incorrect because the law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction, so the total mass of the reactants must equal the total mass of the products.