The Law of Conservation of Mass 1

Antoine-Laurent de Lavoisier Antoine-Laurant de Lavoisier was an 18th century French scientist and nobleman who is considered the father of modern chemistry. He recognized and named oxygen and hydrogen, was one of the scientists who developed the metric system and through his experiments confirmed that although matter may change its form, the total mass remains constant. Unfortunately, because he was a nobleman he was beheaded during the French Revolution. Antoine-Laurent de Lavoisier The Father of Modern Chemistry

Very Important Information: In the lab, mass is measured in ______ or __________. grams kilograms Mass is measured using either a __________ or a __________. scale balance

So how did Lavoisier prove this? The idea that the total mass of matter remains constant is know as The Law of Conservation of Mass. It can also be stated as: Mass is never created or destroyed. So how did Lavoisier prove this?

Lavoisier’s Instruments By using very precise instruments!

Reactant 1 + Reactant 2 Product According to the Law of Conservation of Mass, the mass of the reactants should equal the mass of the products. Reactant 1 + Reactant 2 Product Which scientist’s results below best supports the Law of Conservation of Mass? Scientist Mass of Reactant 1 Mass of Reactant 2 Mass of Product Scientist A 19 g 22 g 42 g Scientist B 19.4 g 22.4 g 41.7 g Scientist C 19.36 g 22.37 g 41.74 g Scientist D 19.364 g 22.372 g 41.736 g

The Trouble With Gases Many chemical reactions give off a gas. Because of this, the mass of the remaining solid and liquid products is less than the original mass of the reactants. Before Lavoisier, many scientists thought this was proof that mass was destroyed.

The Trouble With Gases Lavoisier understood what was happening, and correctly hypothesized that the difference in mass was equal to the mass of the gas given. So how did he prove it? Lavoisier developed a device to trap the gases that were given off, then he performed many experiments where he showed that the mass of products, including the gas, was equal to the original mass of the reactants.

Understanding Chemical Equations What is a substance? A substance is an element or a compound The substance that originally react, that is to say that they gain, lose or share electrons, are known as the reactants. The new substances that are produced are known as the products. Reactants Products

Solving for Mass When all but one of the masses are known, solving for the mass of the unknown takes the most elementary of algebra. 3.4g + ?g = 4.2g 2.1g + 8.5g = ?g 3.4g + ?g = 12.0g ? = _____ ? = _____ ? = _____ ?g = 4.2g – 3.4 ?g = 12.0g – 3.4g 0.8 g 10.6 g 8.6 g 2.2g + ?g = 5.6g + 2.7g 1.5g + 6.7g = ?g + 3.3g ? = _____ ? = _____ ?g = 5.6g + 2.7g – 2.2g ?g = 1.5g + 6.7g – 3.3g 6.1 g 4.9 g

Hydrogen + Oxygen Water Solving for Mass When this concept is shown with a chemical equation, nothing changes about how it should be solved. Hydrogen + Oxygen Water 16.0 g 128 g ? g 16.0 grams of hydrogen reacts with 128 grams of oxygen. Assuming all of the reactants are used up, how much water will be produced? 16.0 g + 128 g = 144 g

Make sure you place the numbers with the appropriate substance. Solving for Mass When you are solving for masses, the most important thing to pay a attention to is the placement of the given masses. Sloppiness and laziness will make an easy question turn into a wrong answer. 2Na + Cl2 2NaCl If 11.5 grams of sodium are mixed with chlorine gas to make 29.9 grams of sodium chloride, how much chlorine gas was used? 11.5 g ? g 29.9 g Make sure you place the numbers with the appropriate substance. 29.9 g – 11.5 g = 18.4 grams

Understanding Coefficients Coefficients multiply through the entire formula of a compound. 2CO2 = 2C and 4 O 3H2O = 6 H and 3 O 4CaBr2 = 4Ca and 8 Br 5Na2O = 10 Na and 5 O

Understanding Coefficients Equations without coefficients are known as skeleton equations These equations may or may not represent the correct ratio of moles that interact during the reaction shown. Since mass is directly related to the number of particles, if mass does not change, then the number of particles does not change.

Understanding Coefficients To illustrate the Law of Conservation of Mass, we place coefficients in equations to show that mass is not created or destroyed. These equations are called “balanced equations” because they show the same number of elements on each side.

Yes, this equation is balanced. Understanding Coefficients You can determine whether or not equations are balanced using a simple accounting method and the skill you just learned. Is the following equation balanced? 4 Fe + 3 O2  2 Fe2O3 Reactants side Products side ___ Fe ___ 4 4 ___ O ___ 6 6 Yes, this equation is balanced.

No, this equation is not balanced. Understanding Coefficients Is the following equation balanced? Al + O2  Al2O3 Reactants side Products side ___ Al ___ 1 2 ___ O ___ 2 3 No, this equation is not balanced.

Yes, this equation is balanced. Understanding Coefficients You can determine whether or not equations are balanced using a simple accounting method and the skill you just learned. Is the following equation balanced? 2 K + 2 H2O  2 KOH + H2 Reactants side Products side ___ K___ 2 2 ___ H ___ 4 2+2 4 ___ O ___ 2 2 Yes, this equation is balanced.

Al4C3 (s) + H2O (l) → CH4 (g) + Al(OH)3 (s) Understanding Coefficients Is the following equation balanced? Al4C3 (s) + H2O (l) → CH4 (g) + Al(OH)3 (s) Reactants side Products side ___ Al ___ 4 1 ___ C ___ 3 1 ___ H ___ 2 7 ___ O ___ 1 3 No, this equation is not balanced.

No, this equation is not balanced. Understanding Coefficients and Balancing Determining if an equation is balanced or not is a very simple task, but once again carelessness will cause an incorrect answer. Do NOT pretend to understand better than you really do. Make a table and get it RIGHT! Is the following equation balanced? N2 + 3O2  N2O5 Reactants side Products side ___ N ___ 2 2 ___ O ___ 6 5 No, this equation is not balanced.

Yes, this equation is balanced. Understanding Coefficients Is the following equation balanced? For some of you, it will be easier to just ignore the polyatomic ions and focus on the other elements. 2LiI + Pb(NO3)2  2LiNO3 + PbI2 Reactants side Products side ___ Li ___ 2 2 ___ I ___ 2 2 ___ Pb ___ 1 1 Yes, this equation is balanced.

_____ Li _____ _____ N _____ 6 6 3 3 6 6 6 3 2 4 2 4 2 2 2 1 When you are given sets of coefficients, all you need to do is plug them in to see if they are balanced or not. Once again, this is a test of your patience more than anything else. ___Li + ___N2 ___Li3N 6 2 2 3 1 2 6 1 2 3 2 1 A 6, 2, 2 B 3, 1, 2 C 3, 2, 1 D 6, 1, 2 _____ Li _____ _____ N _____ 6 6 3 3 6 6 6 3 2 4 2 4 2 2 2 1

Always check every answer choice! When you are given a molecule to place into the equation, the same principle applies. C6H12O6 H2O2 2H2O 2CO CH4 + 2O2 CO2 + ____ _____ C _____ _____ H _____ _____ O _____ A 2CO B H2O2 C 2H2O D C6H12O6 1 7 1 1 1 1 3 1 4 12 4 4 4 4 2 4 8 4 4 4 4 4 4 Always check every answer choice!

_____ C _____ _____ H _____ _____ O _____ 1 1 4 4 2 When you are given a molecule to place into the equation, the same principle applies. However, in this case there is actually a short cut! CH4 + 2O2 CO2 + ____ _____ C _____ _____ H _____ _____ O _____ A 2CO B H2O2 C 2H2O D C6H12O6 2C + 2O 1 1 2H + 2O 4 4 H’s 4H + 2O 4 2 2 O’s 6C + 12H + 6O Third, determine which answer meets this need. Second, figure out what more you need. First, add up everything that is already there.