Conservation of mass Balancing Chemical equations

Law of conservation of mass During chemical reactions, THE MASS of the PRODUCTS are ALWAYS equal to THE MASS of the REACTANTS

Chemical equations N2H4 + O2 N2 + 2H2O reactants products How does this equation show that mass is conserved during the combustion of hydrazine? coefficient

Which has a greater mass? Molar mass Which has a greater mass?

MOLEs and Avogadro’s # MOLE = approx. 6.02 x 1023 particles of a substance What? Why? It’s practical – just like a “dozen” of eggs or a “pair” of shoes Chemical rxns almost always involve large numbers of particles Because the math says so (at regular pressure and 0o C)

Molar mass For single elements- For compounds/molecules- Molar mass = Atomic mass expressed in GRAMS Oxygen (O) = 16.0 amu = 16.0 g Carbon (C) = 12.01 amu = 12.01 g For compounds/molecules- Molar mass = SUM of its component atoms’ atomic masses expressed in GRAMS CO2 = 12.01 g x 2(16.0 g) = 44.01 g

Molar masses O2 H20 NaCl

Types of chemical reactions

Put on the safety glasses. Move to the back of the room. Try not to be too astounded. How do you know that a chemical reaction took place? What type of chemical reaction took place? How do you know it was that type of reaction?

All of these reactions produce CO2. 1) CaCO3 + 2 HCl CaCl2 + H2CO3 2) CaCO3 CaO + CO2 3) 2C2H6 + 7O2 4CO2 + 6H2O All of these reactions produce CO2. Are they the same type of reaction? NO!! Double replacement Decomposition Combustion

SYNTHESIS = TWO OR MORE SUBSTANCES REACT TO FORM A SINGLE SUBSTANCE Chemical reactions are defined by the way that reactants and products interact: SYNTHESIS = TWO OR MORE SUBSTANCES REACT TO FORM A SINGLE SUBSTANCE General equation: A + B AB Examples: 2 Na + Cl2 2 NaCl 2 H2 + O2 2 H2O The product of a synthesis is always a compound (more than one element)

Chemical reactions are defined by the way that reactants and products interact: DECOMPOSITION = A single substance breaks into two or more simpler substances General equation: AB A + B Examples: CaCO3 CaO + CO2 2 H2O 2 H2 + O2 A decomposition is the reverse reaction of a synthesis

Chemical reactions are defined by the way that reactants and products interact: SINGLE REPLACEMENT = One element or substance takes the place of another element or substance in a compound General equation: A + BC B + AC Examples: Cu + 2 AgNO3 2 Ag + Cu(NO3)2 2 K + 2 H2O H2 + 2 KOH Copper (Cu) takes the place of Silver (Ag) and Potassium (K) takes the place of one Hydrogen (H)

Chemical reactions are defined by the way that reactants and products interact: DOUBLE REPLACEMENT = Two different compounds exchange positive ions and form two new compounds General equation: AB + CD AD + CB Example: Pb(NO3)2 + 2 KI PbI2 + 2 KNO3 Lead (Pb2+) and Potassium (K+) are the cations. There are one of each in both compounds. There are still one of each in the new compounds, but bound to new anions.

COMBUSTION = A substance reacts with oxygen, Chemical reactions are defined by the way that reactants and products interact: COMBUSTION = A substance reacts with oxygen, often producing CO2, H2O and heat (exothermic) Examples: CH4 + 2 O2 CO2 + 2 H2O 2 H2 + O2 2 H2O CO2 and H2O are not ALWAYS both produced…unless an organic compound is combusting Isn’t this a synthesis reaction?

Identifying chemical reactions Explain why the formation of water can be classified as a synthesis reaction, or a combustion reaction. Describe a single replacement reaction.