Modeling Chemical Reactions - 3

Modeling Chemical Reactions - 3

Multiple Choice

Diego is performing an experiment using the equipment shown in the diagram. He places filings of zinc metal into a clear colorless mixture of hydrochloric acid and water and then seals the flask with a stopper as shown. He observes that the liquid in the flask bubbles and becomes hot. As time progresses, the zinc metal disappears, and the water level in the cylinder moves down as it fills with gas. Eventually, the bubbles stop forming and the flask contains a colorless liquid, while the cylinder is nearly empty of water. Which of the following would demonstrate a change in properties and disprove Diego’s hypothesis that only a physical change in the water has occurred?

• When the flask cools, the liquid is clear and colorless.

• The gas in the cylinder is flammable and can explode.

• The gas in the cylinder appears colorless.

• The weight of the flask contents is different after the experiment than before the experiment.

​Explanation

​A flammable gas is a product with new chemical properties. The absence of a color change does demonstrate an absence of a chemical change, so A and C are not correct.

Multiple Select

Based on Diego’s experiment, which evidence supports that a chemical reaction has taken place. Note that hydrogen chloride (HCl), which dissolves to produce hydrochloric acid, is a nonflammable gas.

• the gas formed is flammable

• the solution before and after is colorless

• heat is produced

• the solutio in the flask leaves behind a white substance when the liquid evaporates

​Explanation

​* If hydrochloric acid was simply changing to a gas state, which is a physical change, then it would produce a nonflammable gas. The production of a flammable gas demonstrates that a new substance has formed.

​​* Generally, the production of heat and/or light suggests a chemical reaction has taken place.

​* The production of a white substance suggests a new substance was formed.

Multiple Choice

Chlorine bleach and ammonia are two common ingredients in home cleaning products. However, these products should never be mixed together while cleaning, because they can produce dangerous chemicals as a result of their reactions together. One of those reactions involves ammonia and bleach forming the dangerous product chloramine and the harmless product sodium hydroxide. The molecules are pictured here.  In these models, chlorine atoms are medium gray, nitrogen atoms are dark gray, sodium atoms have horizontal bands, oxygen atoms have vertical bands, and hydrogen atoms are light gray.  In this reaction, chloramine is a gas at room temperature. To determine if it follows the law of conservation of mass, the reaction should be carried out in a _____________.

• closed system

• open system

• dark room

• room with plenty of light

​Explanation

​Masses of reactants and products are equal in a closed system, because if one of the products (or reactants) is a gas, it has nowhere to escape and therefore its mass will still be accurately measured.

​

​Strategy for when you are short on time: The question is usually at the end of the paragraph. When short on time, skip to the question and see if you even need the information in the paragraph to answer the question. To answer this question, all you really needed to know was the law of conservation of mass.

Open Ended

A baker buys one dozen fresh eggs. The eggs do not smell. He places them in the back of his refrigerator.Six weeks later, the baker removes the eggs from the refrigerator. The eggs have a very strong and unpleasant smell.Has a physical or chemical change occurred? Use evidence to explain your answer.

​Sample

​The change in smell is a sign that a chemical change has occurred in the eggs. In a chemical reaction, a new product is formed. When a new product is formed, it will have different properties than its reactants. Since the eggs’ odor was different after six weeks, it indicates that a new product was formed that is different from the reactants. This is evidence of a chemical reaction.

Multiple Select

A camper decides to roast a marshmallow. Before roasting the marshmallow, the camper observes that it smells like sugar. After holding the marshmallow over the campfire for 30 seconds, the camper observes that the marshmallow smells like nuts.Which statements explain the marshmallow’s change in smell? Choose the two that best apply.

• The heat energy of the fire burned the sugar in the marshmallow, producing a new, and different compound.

• A camper nearby opened a can of nuts. This caused the change in smell.

• The change in the marshmallow's odor is evidence that a chemical reaction occurred.

• The change in the marshmallow's odor is evidence of a physical change.

​Explanation

​The heat energy of the fire burned the sugar in the marshmallow, producing a new, and different compound. Chemical reactions can be signalled by a change in odor.