Counting Atoms How many atoms of each element are in the following molecules?How many atoms of each element are in the following molecules? 1. H 2 SO 4 2. Ca(OH) 2 3. NaCl 4. (NH 3 ) 3 P H 2 O 6. 4Al 2 (CO 3 ) 3

 1. 2 hydrogen, 1 sulfur, 4 oxygen  2. 1 calcium, 2 oxygen, 2 hydrogen  3. 1 sodium, 1 chlorine  4. 3 nitrogen, 9 hydrogen, 1 phosphorus, 4 oxygen  5. 6 hydrogen, 3 oxygen  6. 8 aluminum, 12 carbon, 36 oxygen

 A chemical reaction is when you create a new substance with unique physical and chemical properties.

 Chemists use symbols to represent what happens in a chemical reaction.  This representation is called a chemical equation. Na + Cl 2  NaCl

 Reactants  What is going into the equation.  Products  What is coming out of the equation.

 Arrow   “yields” (“creates”, “produces”, “equals”)  Subscript  The little number after the element that tells how many of each element is present.  H 2 OZnCl 2 Al 2 O 3

 Coefficient  The big number written before the elements when balancing the chemical equations.  2HCl3Al 2 S 3  Representation of Gases  (diatomic elements) Gases are always written with a subscript of 2 when they are alone in the equation  Br 2 I 2 N 2 Cl 2 H 2 O 2 F 2

 Example of the parts: 2Al + 3S  Al 2 S 3 Coefficient Reactants Products Yields Subscripts

2Al 2 O 3 Coefficient Subscripts Coefficients will multiply by the subscripts….2x2….2x3

 Due to the law of conservation of mass, “what goes in, must come out” of an equation. Atoms cannot be created or destroyed. The mass of the reactants entering the equation must equal the mass of the products exiting the equation.

Atoms are not created or destroyed during a chemical reaction. Scientists know that there must be the same number of atoms on each side of the equation. To balance the chemical equation, you must add coefficients to the different parts of the equation.

Balancing equations Atoms must be conserved!

Is this equation balanced? NaCl + H 2 O  NaOH + Cl 2 NaCl + H 2 O  NaOH + Cl 2 left side has 1Na, 1Cl, 2H, and 1Oleft side has 1Na, 1Cl, 2H, and 1O right side has 1Na, 1O, 1H, and 2Clright side has 1Na, 1O, 1H, and 2Cl NO!!!!!! It is not balanced!!NO!!!!!! It is not balanced!!

Is this equation balanced? HCl + NaOH  NaCl+ H 2 O HCl + NaOH  NaCl+ H 2 O left side has 2H, 1Cl, 1Na, and 1Oleft side has 2H, 1Cl, 1Na, and 1O right side has 1Na, 1Cl, 2H, and 1Oright side has 1Na, 1Cl, 2H, and 1O Yes!!!!!! It is balanced!!Yes!!!!!! It is balanced!!

Is this equation balanced? Ca + H 2 O  Ca(OH) 2 + H 2 Ca + H 2 O  Ca(OH) 2 + H 2 left side has 1Ca, 2H, 1Oleft side has 1Ca, 2H, 1O right side has 1Ca, 2O, 4Hright side has 1Ca, 2O, 4H No!!!!!! It is not balanced!!No!!!!!! It is not balanced!! How can we make it balance????How can we make it balance????

 We need more H’s, but we can’t add just the H atom, we have to add the entire molecule the H is attached to: Ca + H 2 O  Ca(OH) 2 + H 2 H2O H2O Count the boxes. These are the coefficients: 1 Ca + 2 H 2 O  1 Ca(OH) H 2

JJust like in math, this equation may be written in a simpler way: CCa + 2 H 2 O  Ca(OH) 2 + H 2

What is balanced in a chemical equation? Three things are balanced in a chemical equation: 1. Atoms 2. Mass 3. Charge

For now, you will only be concerned with balancing atoms. Remember, the number of atoms of each element on the reactants side must equal the number of atoms of each element on the products side!For now, you will only be concerned with balancing atoms. Remember, the number of atoms of each element on the reactants side must equal the number of atoms of each element on the products side!

The End Let’s practice! Let’s practice!

Symbols Used in Chemical Equations

Methane Combustion

Writing Word Equations and Formula Equations Sample Problem A Write word and formula equations for the chemical reaction that occurs when solid sodium oxide is added to water at room temperature and forms sodium hydroxide (dissolved in the water). Include symbols for physical states in the formula equation. Then balance the formula equation to give a balanced chemical equation.

Sample Problem A Solution 1. Write the reactants to the left of the arrow and the products to the right of the arrow. sodium oxide + water  sodium hydroxide 2. Write the chemical formulas for each reactant and product in the reaction. This may involve balancing charges (cross-over method). You will need to use your Common Ion Chart. Then substitute the correct formulas for the names of the reactants and products in the chemical equation. Na 2 O + H 2 O  NaOH (not balanced)

Sample Problem A Solution, continued 3. Add symbols for the physical states of the reactants and products and balance the chemical equation. Na 2 O(s) + H 2 O(l)  2NaOH(aq)

Sample Problem B Translate the following chemical equation into a sentence: BaCl 2 (aq) + Na 2 CrO 4 (aq)  BaCrO 4 (s) + 2NaCl(aq) Aqueous solutions of barium chloride and sodium chromate react to produce a precipitate of barium chromate plus sodium chloride in aqueous solution.

Sample Problem C The reaction of zinc with aqueous hydrochloric acid produces a solution of zinc chloride and hydrogen gas. Write a balanced chemical equation for the reaction. 1.Write the word equation. 2.Write the formula equation. 3. Balance the equation. Zn(s) + 2HCl(aq)  ZnCl 2 (aq) + H 2 (g) zinc + hydrochloric acid zinc chloride + hydrogen Zn(s) + HCl(aq) ZnCl 2 (aq) + H 2 (g) (not balanced)

Sample Problem D Solid aluminum carbide, Al 4 C 3, reacts with water to produce methane gas and solid aluminum hydroxide. Write a balanced chemical equation for this reaction. Al 4 C 3 (s) + 12H 2 O(l)  3CH 4 (g) + 4Al(OH) 3 (s)