Activity 25: Conservation of Mass

Read B-64

• Challenge:

How does the total mass of the reactants compare to the total mass of the products in a chemical reaction?

Procedure

• Follow procedure on pgs. B-65 to B-66

Data/Evidence:

• Student Sheet 25.1

Which system is a closed system, and which is an open system?

• Open System: the contents of the jar can mix with the air surrounding the outside of the jar.

• Closed System: the chemical reaction is contained inside of the jar - all reactants and products stay in the jar.

• Today, it is not an absolutely closed system - some heat and light will be able to escape.

Can of Soda

• When it is sealed, it is a closed system.

• When it is opened and the gases are released it is an open system with the air around the can.

What type of system is a person - open or closed?

• Open System - we take in food, oxygen, and water, from outside our bodies and excrete waste, carbon dioxide, and other substances.

• In reality, there are very few absolutely closed systems in our universe.

• Our planet, for example, is an open system in that it exchanges mass with the solar system and galaxy.

What is mass?

• How much matter is in something.

• The greater the mass, the more of the substance you have.

Chemical Reactions• Signs of a Chemical Reaction:

– Change in Color of Products– Production of Gas– Emission of Light or Heat– Appearance of a New Solid (Precipitate) or

Liquid.

• These changes in physical appearance or characteristics of the reactants are a way to visually detect a chemical change.

• Change at the atomic level is not visible directly.

Demonstration: Open System

• 1 mL of copper chloride solution and 1 aluminum washer in the bottle.

• What do you think will happen to the mass of the system - the bottle and reactants - as the reaction occurs?

• Will it increase, decrease, or stay the same?

Exothermic Reaction

• Releases energy as the reactants interact chemically.

• This is felt as heat.

• Other exothermic reactions:– Combustion (Burning) and Rusting (Oxidation).

• Do not grip your reaction chambers while conducting the reactions in closed systems.

What evidence have you observed that indicates a chemical reaction occurred?

• Formation of a new solid

• Formation of bubbles

• Change in color of the solution

• Heat generated

• What happened to the mass?

You will also…

• Conduct a second reaction with 1 mL 100,000 ppm copper chloride solution and 4 mLs of sodium hydrogen phosphate solution.

• The reaction will be complete in approximately two minutes.

Lab: Closed System• Use the balances carefully - be accurate!• Fix the lid onto the jar before beginning the

reaction.• Place the reaction chamber on the table - do

not keep it on the balance.• Do not let the substances mix before you

have done the initial measurement of mass. • Press your thumbs against the lids while

inverting the container to start the reactions. • Record detailed observations - how do you

know a reaction occurred?

Discussion

• Record change-in-mass calculations

• What trend do you see in the data?

• What is this telling us about what is happening to the mass of the reactants as compared to the mass of the products?

• What could explain why the mass may have changed?

• How can we reduce error in this activity?

Law of Conservation of Mass

• Mass cannot be created or destroyed in a chemical reaction, but it can change form.

• Mass stays the same in an open system and a closed system, but in an open system, the gas can escape.

• Toxic substances that will have negative effects on humans, animals, or the environment, will still be present after a chemical reaction.

Analysis Question #2• The Law of Conservation of Mass goes

against our perception of what we experience every day.

• When we observe food cooking or wood burning, it appears that mass is lost.

• This is because one of the products formed in these reactions is a gas we cannot see.

• If we could collect all of the gas and water produced when cooking or burning, we would find that mass is conserved.