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Writing Chemical Equations
• Reactant = substance that undergoes a reaction
• Product = the new substance formed when reactants undergo a chemical change
• Ex: Fe+3 + O-2 Fe2O3 (rust)
reactants products
Equations
• Word Equations: – Ex: vinegar + baking soda sodium acetate
+ water + carbon dioxide• Chemical Equations:
– Ex: HC2H3O2 (aq) + NaHCO3 (s) NaC2H3O2 (aq) + H2O (l) + CO2 (g)
– Shows the state of substance: aq – aqueous, s – solid, l – liquid, g- gas
aqueous = solid dissolved in water.
Balancing Chemical Equations (REVIEW)
• Law of Conservation of Mass = matter is neither created nor destroyed…– Thus, mass of products = mass of reactants– Atoms in a chemical reaction don’t change,
they rearrange!
• The same number of each kind of atom must be on the left side as well as the right!
• Ex: H2CO3(aq) H2O(l) + CO2(g)
H = 2, C =1, O = 3 H = 2, C =1, O =3
A Balancing Act
• To indicate more than one unit of a compound, a coefficient is placed in front
– Ex: 2H2O
H = 4, O = 2• NEVER change the subscripts of a compound, that
would change its identity!!
– H2O is water, H2O2 is hydrogen peroxide
• Rule: start by balancing the elements that appear only once of each side of the equation.– Ex: Balance oxygen last!
H2CO3(aq) H2O(l) + CO2(g)
Writing Equations!
• Step 1: Write word equation– Hydrogen + oxygen water + energy
• Step 2: Replace words with formulas– H2(g) + O2(g) H2O(l) + energy
• Step 3: Balance the atoms on both sides!– 2H2(g) + O2(g) 2H2O(l) + energy
• Step 4: Check it!– H = 4, O = 2 H = 4, O =2
Example
• Write the balanced equation:
– Magnesium chloride + silver nitratemagnesium nitrate + silver chloride
– MgCl2 + AgNO3 Mg(NO3)2 + AgCl
– Now balance it!
– MgCl2 + 2AgNO3 Mg(NO3)2 + 2AgCl
You Try!1. Magnesium metal + water form solid magnesium
hydroxide and hydrogen gasMg(s) + H2O(l) Mg(OH)2(s) + H2(g)Mg(s) + 2H2O(l) Mg(OH)2(s) + H2(g)
• Potassium chlorate breaks down into potassium chloride and oxygen
KClO3 KCl + O2
2KClO3 2KCl + 3O2
3. Solid potassium reacts with liquid water to produce aqueous potassium hydroxide and hydrogen gas.K(s) + H2O(l) KOH(aq) + H2(g)2K(s) + 2H2O(l) 2KOH(aq) + H2(g)
Categories of Reactions
• There are 5 types of reactions:
1. Synthesis/Combination = 2 or more substances combine to form a single product• A + B AB
• Ex: 2Na + Cl2 2NaCl
2. Decomposition = a compound breaks down into 2 or more simpler substances
1. AB A + B
2. Ex: H2CO3 H2O + CO2
• Sodium + Chlorine --- Sodium chloride
3. Single-Replacement = an element that is not combined replaces an element that is part of a compound
-All compounds are aqueous!
A + BC AC + B
Ex: Mg(s) + Zn(NO3)2(aq) Mg(NO3)2(aq) + Zn(s)
• Double-Replacement = different atoms in 2 different compounds replace each other
-All reactants are aqueous
-Products will include a gas, a ppt (precipitate), or H2O
AC + BD AD + BC
Ex: AgNO3 + KCl AgCl + KNO3
5. Combustion = substance rapidly combines with oxygen producing heat and light.
-Reactants contain C,H,O
-Products are CO2 and H2O vapor
• CxHy + O2(g) CO2(g) + H2O(g)
• Ex: 5CH3OH + O2(g) 5CO2(g) + 8H2O(g)
You Try!
• H2SO4 H2 + SO4
– Decomposition
• Fe + O2 FeO
– 2Fe + O2 2FeO
– Synthesis
• Al + Cl2 AlCl3– 2Al + 3Cl2 2AlCl3– Synthesis
• BaCl2 + Na2SO4
BaSO4 + NaCl
– BaCl2 + Na2SO4
BaSO4 + 2NaCl
– Double Replacement
• Mg + CuSO4
Cu + MgSO4
– Single Replacement
First balance, then put into 1 of 5 categories: