Chemical Reactions and the Conservation of Mass

Chemical Reactions and Conservation of Mass

Multiple Choice

Which of the following is NOT an example of chemical change?

Multiple Choice

Identify which indicator of chemical change was observed in the situation below:

Raidon saw bubbles formed after putting vinegar in a jar of baking soda.

• Evolution of light or heat.

• Temperature change

• Formation of a gas (bubbling or an odor)

• Color change

• Formation of a precipitate

Multiple Choice

Identify which indicator of chemical change was observed in the situation below:

Solid lead iodide formed after mixing aqueous lead (II) nitrate and potassium iodide.

• Evolution of light or heat.

• Temperature change

• Formation of a gas (bubbling or an odor)

• Color change

• Formation of a precipitate

Multiple Select

In the chemical reaction, "solid lead iodide formed after mixing aqueous lead (II) nitrate and potassium iodide", which is/are the reactant/s?

• lead iodine

• lead (II) nitrate

• potassium iodide

• potassium nitrate

Open Ended

If a scientist started with 100 billion molecules in his experiment, how many molecules will he have in the end?

Word Equations

• Statements that indicate the reactants and products in a chemical reaction.

• Ex. Iron (s) + chlorine (g)  -> iron (III) chloride (s)

• read as:

  “Solid iron and chlorine gas react (combine) to produce solid iron (III) chloride”

Translating Word Equations to Skeleton Equations

• A skeleton equation uses chemical formulas rather than words to identify the reactants and products of a chemical reaction.

• The word equation

  Iron (s) + chlorine (g) -> iron (III) chloride (s)

  
  

• The skeleton equation

  Fe(s) + Cl2(g) -> FeCl3 (s)

• NOTE: A skeleton equation is not yet “balanced” by coefficients

Another example...

–You can read the reaction as:

“6 atoms of solid sodium plus 1 formula unit of solid iron (III) oxide yields 3 formula units of solid sodium oxide and 2 atoms of solid iron”

CONSERVATION OF MASS

During a chemical reaction, atoms are neither created nor destroyed (Conservation of Mass).

Liquid water undergoing electrolysis will decompose into hydrogen gas and oxygen gas.

• H₂O (l) -> H₂ (g) + O₂ (g)

Open Ended

In the electrolysis of water, what is the total mass of the reactant/s?

Open Ended

In the electrolysis of water, what is the total mass of the product/s?

Poll

Did the chemical equation, H₂O (l) -> H₂ (g) + O₂ (g), show conservation of mass?

• Yes

• No

Multiple Choice

What should be the chemical equation for the electrolysis of water to show conservation of mass?

• 2 H₂O (l) -> 2 H₂ (g) + O₂ (g)

• H₂O (l) -> H₂ (g) + O₂ (g)

• H₂ 2 O (l) -> H₂ (g) + O₂ (g)

• H₂O₂ (l) -> H₂ (g) + O₂ (g)

Weigh it Right

Given the balanced chemical equations in the scenarios assigned to your group, compute for the total mass of the reactants and the total mass of the products. After 5 minutes, a representative will present your group work with your answers to the guide questions.

Guide Questions

• Compare the total mass of the reactants and the total mass of the products.

• Do all of the chemical reactions given above conformed to the Law of Conservation of Mass?

• What does the mass of the reactants and products tell us about the Law of Conservation of Mass? Why?

• State the law of conservation of mass.

Challenge Question

From our discussion of gases, we have mentioned that gases’ mass is negligible. With this, why do we still need to include the gases in the chemical equation?