Sec.2 – chemical formulas & equations Ch. 2 Chemical reactions Sec.2 – chemical formulas & equations

Chemical formulas

Chemical formulas Shortened way to represent a substance Use chemical symbols & numbers Show the type & number of atoms in a substance

Subscripts Number written to lower right of element Tell how many atoms of each element are in the substance

How many atoms of each element? 1 carbon, 1 oxygen 1 carbon, 2 oxygen 1 nitrogen, 4 hydrogen 6 carbon, 12 hydrogen, 6 oxygen CO CO2 NH4 C6H12O6

Writing formulas for covalent compounds

Writing formulas for covalent compounds Made up of 2 nonmetals Common prefixes used in covalent compound names Mono – 1 Di – 2 Tri – 3 Tetra - 4 Penta – 5

Carbon dioxide Write chemical symbol of 1st element. Add a subscript if there is a prefix. Write chemical symbol of 2nd element. Add a subscript if there is a prefix. CO2

Now you try…write a formula for each covalent compound Carbon tetrachloride Dihydrogen monoxide Dinitrogen tetroxide CCl4 H2O N2O4

Writing formulas for ionic compounds

Remember… Ionic compounds are made up of a metal & a nonmetal chemically bonded together. The METALLIC element is always named 1st in the compound name. Sodium chloride – sodium is the metal & chlorine is the nonmetal

Steps for writing ionic compound formulas Write the chemical symbol for the metallic ion first. Write the chemical symbol for the nonmetallic ion. Make sure the ions’ charges equal zero. Metallic ions always have a positive charge. Nonmetallic ions always have a negative charge. An ion’s charge is the same as the number of electrons its group will gain or lose to become stable. Add subscripts to the chemical symbols to make the compound’s charge equal zero.

Sodium chloride Sodium’s chemical symbol is Na. Na Chlorine’s chemical symbol is Cl. Sodium is in group 1. It’s charge is 1+. Chlorine is in group 17. It’s charge is 1-. (+1) + (-1) = 0. No subscripts are needed. Na Cl Chemical formula: NaCl

If the ions’ charges do not equal zero… Aluminum oxide Find the LCM of the ions’ charges. Add subscripts where needed to equal the LCM. Al – group 13 – charge 3+ O – group 16 – charge 2- LCM of 3 & 2 is 6 Subscript for Al is 2 (2 x 3=6) Subscript for O is 3 (3 x 2=6) 4. Formula: Al2O3

Now You try….. Potassium chloride Magnesium chloride Sodium fluoride Aluminum sulfide KCl MgCl2 NaF Al2S3

Chemical equations

Chemical equations Shortcut way to describe a chemical reaction Use symbols to show the relationship between the reactants & products Can be understood by scientists worldwide

reactants Substances or molecules that participate in a chemical reaction The starting substances in a chemical reaction Are ALWAYS written on the LEFT side of the arrow in a chemical equation

Identify the reactants in these equations Na + Cl2 2 NaCl 2 H2O 2 H2 + O2 Zn + 2 HCl ZnCl2 + H2 Na + Cl2 2 H2O Zn + 2 HCl

products The substance that forms in a chemical reaction The ending substance(s) in a chemical reaction Are ALWAYS written on the RIGHT side of the arrow in a chemical equation

Identify the products in these equations Na + Cl2 2 NaCl 2 H2O 2 H2 + O2 Zn + 2 HCl ZnCl2 + H2 2 NaCl 2 H2 + O2 ZnCl2 + H2

Accuracy is important in chemical equations Incorrect symbols or formulas do not correctly describe a chemical reaction. Small mistakes will make a HUGE difference!

Balancing chemical equations

Law of conservation of mass Mass is neither created nor destroyed in an ordinary chemical or physical change. Chemical equations must show the same numbers & kinds of atoms on each side of the arrow.

Subscripts vs. coefficients Written to the lower right of a chemical symbol Written smaller than chemical symbol Only apply to single atoms Written to the left of the chemical symbol Written the same size as the chemical symbol Apply to every atom in molecule, including subscripts

Subscript or coefficient? H2O2 2 CO2 Subscript Coefficient Both!

Calculating the number of atoms A coefficient applies to every atom & every subscript in a molecule. If there is a subscript, multiply the coefficient by the subscript to find the number of atoms. Coefficients in an equation are “stopped” by the plus sign or the arrow.

2 H2 + O2 2 H2O Reactant side has 4 hydrogen atoms & 2 oxygen atoms. The coefficient 2 applies only to the hydrogen. Product side also has 4 hydrogen atoms & 2 oxygen atoms. The coefficient 2 applies to everything here. There is no plus sign or arrow to stop the coefficient.

Calculate the total number of atoms 2 HCO3 3 C6H12O6 NaF 4 CaO

Steps for balancing chemical equations Set up a T-chart for the reactants & products. reactant product

Ca + O2 CaO Write the chemical symbol & number of atoms for each side. reactant product Ca – 1 Ca – 1 O – 2 O – 1 Compare to see if the equation is balanced. This equation is not balanced.

Ca + O2 CaO Find the LCM for each element that is unbalanced. reactant product Ca – 1 Ca – 1 O – 2 O – 1 LCM = 2

Ca + O2 2 CaO Add coefficients to balance the elements. NEVER CHANGE A SUBSCRIPT!! Recount the number of atoms for each element. Repeat steps 4 - 6 as needed. reactant product LCM = 2 Ca – 1 Ca – 1 2 O – 2 O – 1 2

2 Ca + O2 2 CaO reactant product LCM = 2 Ca – 1 2 Ca – 1 2 O – 2 O – 1 2 LCM = 2 The equation is now balanced.