Chemical Equations and Conservation of Mass What are they?

chemical reaction- one or more substances are changed into new substances. Na + Cl NaCl products reactants Reactant – substances that react Product – new substances produced

Evidence of Reactions Bubbling Turns cloudy Temperature change Color change

Aqueous-solid dissolved in water Chemical Equation Expression that describes a chemical reaction using chemical formulas and other symbols Symbol Meaning (cr) Crystalline solid (l) Liquid (g) Gas (aq) Aqueous-solid dissolved in water

Symbols for Chemical Equations Pb(NO3)4 + 4KI  PbI4 + 4KNO3 “Pb(NO3)4” is a formula.  Means produces (or yields) + Means added together Subscripts - Number of a particular atom in a molecule Coefficients – Number of molecules

Father of Modern Chemistry 1743 - 1794 Antoine Lavoisier First Described the “Law of Conservation of Mass”

Conservation of Mass HgO Hg + O2 Antoine Lavoisier found that the mass of the reactants and the products are equal, even when the states of matter change. 0.7g Oxygen became a gas HgO Hg + O2 He started with: 10g of Mercury Oxide (HgO) He ended up with: and 9.3g Mercury… ...But what happened to the O2? 10 g. = 0.7 + 9.3 g. Matter is neither created nor destroyed.

Conservation of atoms-the number of each type of atom on the reactants side of the chemical equation MUST be equal to the number of each type of atom on the products side of the equation. Coefficient-represent the number of units of each substance taking part in the reaction Balanced chemical equation-the same number of atoms of each element on both sides of the equation

Hg (mercury) can exist by itself... This balances the equation! Lavoisier’s Chemical Equation: 2 HgO  2 Hg + O2 Hg (mercury) can exist by itself... but, oxygen will need to bond with another oxygen to make O2 (diatomic) To balance the atoms we need to: Put the coefficient of 2 in front of reactant HgO. Put the coefficient of 2 in front the product Hg. This balances the equation!

N2 H2 NH3 Why or Why Not? Is this balanced? 10 Let’s Count the Atoms: There are 2 nitrogen atoms There are 2 hydrogen atoms 1 nitrogen and 3 hydrogen…

Atoms can only bond in certain ways.. H2 N2 N H2 H N2 H N H H2 H2 H N That’s why we can’t change the subscripts. N2 H2 H N N2 N N2 N H

Here is what it means... Subscripts - Small #’s below an element. Coefficients - Large #’s in front of the formulas. H2 I can’t live without you! 2H2 H H H When balancing equations, we can only change the coefficients!

1 N2 1 H2 1 NH3 We can only change coefficients before the symbols. 13 Now, back to the balancing... 1 N2 1 H2 1 NH3 We can only change coefficients before the symbols. N H N H 1 molecule of nitrogen 1 molecule of hydrogen 1 molecule of ammonia

Do both sides have the same amount of atoms? N2 + H2 NH3 N H 2 1 2 3

Then it is a balanced equation. Do both sides have the same amount of atoms? N2 + H2 NH3 3 2 N H (2) 2 1 (6) (6) 2 3 N2 + 3 H2 2NH3 Then it is a balanced equation.

Four Steps to Balance Equations: 1. Set up your equation. List the metals, nonmetals, oxygen, and hydrogen below equation. 2. Count the number of atoms you have on both sides. 3. Balance by changing the coefficients and recounting. 4. Start the process again if it still does not balance.

H2 + O2  H2O O H 1. Set up your equation. List the elements in this order below the equation: Metals, Nonmetals, Oxygen, and Hydrogen H2 + O2  H2O O Metals Nonmetals Oxygen Hydrogen H

2. Count the number of atoms you have of each on both sides. H2 + O2  H2O 2 O 1 2 H 2

3. Balance by changing the coefficients and recounting. 2 2 H2 + O2  H2O O H 2 1 (2) Need to have at least 2 “O” (4) (4) But it changes the number of “H” How are you going to make “H” add up to 4? Is this balanced? Yes!

Need to have at least 2 “Cl” Changing the Cl changes the “H”? Let’s try another: 2 Mg + HCl  H2 + MgCl2 1 Mg 1 (2) 1 Cl 2 Need to have at least 2 “Cl” Is this balanced? Yes! 2 Changing the Cl changes the “H”? (2) 1 H

Na + HCl  NaCl + H2 Na Cl H 1 2 2 (2) Let’s try another: In this case, we will start with hydrogen since it is the only one unbalanced. (2)

Na + HCl  NaCl + H2 Na Cl H 1 2 2 2 (2) (2) (2) But, changing the hydrogen in HCl affects the number of chlorine atoms. Na + HCl  NaCl + H2 Na Cl H 1 2 2 2 (2) (2) (2)

Na + 2HCl  NaCl + H2 (2) 1 1 (2) (2) 1 (2) 1 (2) 1 2 Changing the chlorine on the product side affects the sodium (Na) on the reactants side. So we must now change sodium as well. Na + 2HCl  NaCl + H2 2 2 (2) 1 Na 1 (2) (2) Cl 1 (2) 1 (2) 1 H 2

Independent Practice