Chemical Equations & Reactions

Count the atoms Cl2 2Cl 2Cl2 Compounds can also have brackets to indicate the number of atoms. E.g. Ca(OH)2 O H Ca Remember that “OH” is a polyatomic ion The two outside of the brackets represents 2 atoms of hydrogen and two atoms of oxygen.

Try these! a) NaOH b) Be(NO3)2 Na = 1, O = 1, H = 1 c) 3Ca(OH)2 Be = 1, O = 6, N = 2 Ca = 3, O = 6, H = 6

Counting Atoms The symbol of an element represents 1 atom of this element.    Example:   Ca = ______ Ca? Subscript numbers only apply to the number it touches.  N2 = ______N? H2O means 2 H and 1 O CaCl2 means ______ Ca and _____ Cl?

When brackets are used, a number outside of the brackets will apply to all atoms inside: Ca(OH)2 means 1 Ca, 2 O and 2 H Ba3(PO4)2 = ______Ba, _____P, ______O? A coefficient before the symbol represents the number of atoms of that particular element. Example: 3C = _____ C?

Multiplication is used when two numbers are present 2H2O means 4 H and 2 O 3SrCl2 means… _____Sr and _____Cl? 2Mg(NO3)2 means…___Mg, ____N, ____O?

Writing Chemical Equations Products on right side of the arrow Reactants on left side of the arrow hydrogen + oxygen → water Plus sign means “reacts with” Arrow means “produce”

So, let’s consider this chemical equation: H2 + O2 → H2O How many hydrogen and oxygen atoms in the a) reactants? _____ H, _____O? b) products? _____H, _____O? 

What about now? 2H2 + O2 → 2H2O How many hydrogen and oxygen atoms in the a) reactants? _____H, _______O? b) products?  _____H, _____O? What do you notice about the second equation?

Notes and practice – Counting atoms

Writing Chemical Equations Products on right side of the arrow Reactants on left side of the arrow hydrogen + oxygen → water Plus sign means “reacts with” Arrow means “produce”

Balancing Chemical Equations The law of conservation of mass states that the mass of the reactants in a chemical equation must equal the mass of the products. Remember: “atoms cannot be created or destroyed” so we must have the same number of atoms of each type in the reactants and products

Balancing Chemical Equations

So… Here is a balanced equation Mg + O2 → MgO 2 Mg + O2 → 2 MgO - Balanced!

Ex. Hydrogen reacts with oxygen to produce water Types of equations Word equation: All elements are written out in words to describe the reaction Ex. Hydrogen reacts with oxygen to produce water

Types of Equations Skeleton equation: Summarizes the reaction in symbol form. Does not follow the law of conservation of mass. Ex: H2 + O2 → H2O HOFBrINCl!

Types of Equations Balanced Chemical equation: Includes coefficients to represent equal number of atoms on both sides of the equation Ex: 2 H2 + O2 → 2 H2O Coefficient – a number placed in front of an atom or chemical formula.

Tricks for Balancing The ONLY number you can change is the number in front of the formula (coefficient) Balance all atoms other than O and H (leave them for the end!) Balance O, then H Count polyatomic ions together if they are together on both sides. Ex: CaSO4 + 2 NaCl → CaCl2 + Na2SO4 CHECK to make sure ALL atoms are balanced at the end

Notes and Practice Word Equations

Video – Chemical Equations Video – Mr. Anderson Video - Introduction to Balancing Chemical Equations

Web Site Web Site - Phet

Try Some! __Mg + __O2 → __ MgO __SF4 + __H2O → __ SO2 + __ HF __ Al + __ H2SO4 → __ Al2(SO4)3 + __H2 __ Ca(OH)2 + __ H3PO4 → __ Ca3(PO4)2 + __H2O

Balance the following skeleton equation: a) Mg + HCl → MgCl2 + H2 b) Ca + N2 → Ca3N2 c) NH4NO3 → N2O + H2O d) AlCl3 + H2S → Al2S3 + HCl e) C4H10 + O2 → CO2 + H2O f) O2 + C6H12O6 → CO2 + H2O g) NO2 + H2O → HNO3 + NO h) Cr2(SO4)3+ NaOH → Cr(OH)3+ Na2SO4 i) Al4C3 + H2O → CH4 + Al(OH)3

Answers: a) Mg + 2HCl → MgCl2 + H2 b) 3Ca + N2 → Ca3N2 c) NH4NO3 → N2O + 2H2O d) 2AlCl3 + 3H2S → Al2S3 + 6HCl e) 2C4H10 + 13O2 → 8CO2 + 10H2O f) 6O2 + C6H12O6 → 6CO2 + 6H2O g) 3NO2 + H2O → 2HNO3 + NO h) Cr2(SO4)3+ 6NaOH → 2Cr(OH)3+ 3Na2SO4 i) Al4C3 + 12H2O → 3CH4 + 4Al(OH)3