Unit 7: Chemical Reactions: Conservation of Mass

What Is a Chemical Reaction?

• A chemical reaction occurs when one or more substances (reactants) are chemically changed into one or more new substances (products)

• Chemical changes involve movement of electrons as well as breaking bonds and forming new bonds

2H2(g) + O2(g) 2H2O(l)

What are the reactants?

What is the product?

Signs of a Chemical Change1. Heat and/or light is given off

2. A gas is produced-often seen as bubbles

3. A precipitate is formed-when 2 solutions are mixed, a solid sometimes forms and separates from the mixture.

4. An UNEXPECTED color change occurs (see picture)

Chemical Reaction Symbols

I Do:

Write a chemical equation for this reaction:

Silver nitrate reacts with sodium chloride to produce sodium nitrate and silver chloride

We Do:• Write a chemical equation for this

reaction: Magnesium metal reacts with hydrochloric acid (HCl) to yield magnesium chloride and hydrogen gas.

*Remember: H, N, O, F, Cl, Br, and I are diatomic. (So for hydrogen gas, write H2).

You Do:

In a combustion reaction, hydrogen gas is added to oxygen gas to produce water.

Hydrogen balloon

Laws of Reactions

Chemical reactions follow 2 main laws. Both laws deal with conservation. What does conservation mean?

Nothing is gained or lost

The Law of Conservation of Mass

The law of conservation of mass states that mass is neither created nor destroyed. The form of matter may change, but the TOTAL amount of mass stays constant.

Conservation of mass

mass (g) of reactants = mass (g) of products

Conservation of Mass

# atoms before reaction = # atoms after reaction

Do this reaction follow the law of conservation of mass?

Would this reaction verify the law of mass?____

Why or why not?________________________

Conservation of Mass***This law only applies in a closed system. If

products can escape (like a gas can if the system is not closed), mass will appear to have been lost. But it was not destroyed. It simply escaped from the system.

I Do:

24.3 grams of copper (II) chloride reacts with 73.0 grams of aluminum. Aluminum chloride and copper are produced. What is the total mass of the products?

1. Write reaction:

 

 

 

2. Make grams reactants = grams products

3. Answer_______

We Do:

23.0 grams of sodium react with an unknown amount of chlorine to form 58.5 grams of sodium chloride. How many grams of chlorine reacted?

1. Write reaction:

 

 

2. Make grams reactants = grams products

3. Answer_______ Video of this reaction

You Do:Data is shown for a reaction between magnesium and sulfur.

Calculate the mass of the unreacted sulfur in this reaction:

1. Write reaction:

 

 

 

2. Make grams reactants = grams products

3. Answer_______

95.5 g