Transcript Unit 3
Unit 4: Stoichiometry –
Calculations with Chemical Formulas and Equations
Reactions and Chemical Equations Atomic and Molecular Weights Moles Stoichiometric Calculations Limiting Reactants Percent Composition and Empirical Formulas
Chemical Reactions
When chemicals are mixed together, one of two things can happen:
Mixture forms
chemicals retain their own physical and chemical properties
individual components can be separated
Chemical reaction occurs
Chemical Reactions
Chemical reaction :
a process in which chemicals are mixed under certain conditions and transformed into new chemicals
atoms of the chemicals rearrange and combine to form new substances
new substances formed often have different physical and chemical properties 2 Na (s) + Cl 2 (g) 2 NaCl (s)
Types of Chemical Reactions
Combustion Reaction
a rapid chemical reaction in which heat is evolved
Requires oxygen as a co-reactant
Usually produces a flame
Burning charcoal: C (s) + O 2 (g)
CO 2 (g)
Burning propane: C 3 H 8 (g) + 5 O 2 (g)
3CO 2 (g) + 4 H 2 O(l)
Types of Chemical Reactions
Combination Reactions
two or more substances react to form one product A + B
C
Producing ammonia: N 2 (g) + 3 H 2 (g)
2 NH 3 (g)
Acid rain: SO 3 (g) + H 2 O (l)
H 2 SO 4 (aq)
Types of Chemical Reactions
Decomposition Reactions
chemical reactions in which one substance reacts to produce two or more other substances C
A + B
Air bags: 2 NaN 3 (s)
2 Na (s) + 3 N 2 (g)
Signs of Chemical Reactions
Visible Signs
formation of a solid, liquid or gas
color change
Signs of Chemical Reactions
Other indications of a chemical reaction:
Energy is released as heat or light
Energy is absorbed from the environment
Chemical analysis
reveals changes in molecular formula, chemical properties, etc.
Chemical Equations
Chemical equation: during the reaction a representation of a chemical reaction that uses the chemical formulas for the compounds used and formed Reactants Products General format for all chemical equations
Chemical Equations
Reactants
the starting substances in a chemical reaction
Products
the new chemicals that are formed as a result of a chemical reaction CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) reactants products
Chemical Equations
Chemical equations provide information about:
formulas for reactants and products
relative number of molecules (or moles) of each reactant or product
the physical state of each reactant or product
(s) = solid
(l) = liquid
(g) = gas (aq) = aqueous (dissolved in water) 2 NaHCO 3 (s) + H 2 SO 4 (aq) Na 2 SO 4 (aq) + 2 H 2 O (l) + 2 CO 2 (g)
Chemical Equations
Aqueous reactions (aq)
water as a solvent
dissolved in water to do the reaction
Water does reaction
solvent only
NOT participate in the can be recovered unchanged AgNO 3 (aq) + NaCl (aq) AgCl (s) + NaNO 3 (aq)
Chemical Reactions
Chemical reactions occur on a molecular or atomic level.
H 2 + O 2
H 2 O H H O O What happened to the other oxygen atom???
Law of Conservation of Matter
Atoms cannot be created or destroyed in an ordinary chemical reaction.
H H + O O H H The reaction to form water requires 2 molecules of H 2 for every molecule of O 2 .
2 H 2 + O 2
2 H 2 O
Balanced Chemical Equations
Chemical equations must be balanced:
They must follow the Law of Conservation of Matter (Mass).
They must account for every atom involved in a chemical reaction.
Balancing Chemical Equations
To balance a chemical equation:
Write the correct formulas for reactants and products. H 2 + O 2
H 2 O
Count the number of atoms of each type on both sides of the equation.
H 2 + O 2
H 2 O 2 H 2 O 2 1 H O
Balancing Equations
If the number of any element differs from side to side, add coefficients to make both sides equal.
H 2 + O 2
H 2 O 2 2 H O 2 1 H O H 2 + O 2 2 H 2 + O 2
2 H 2 O
2 H 2 O
Rules for Balancing Equations
NEVER change a formula or subscript in a formula.
H 2 + O 2
H 2 O H 2 + O 2
H 2 O 2 2H 2 + O 2
2 H 2 O
Rules for Balancing Equations
Don’t insert coefficients inside a formula.
H 2 + O 2
H 2 O H 2 + O 2
H 2 2 O 2 H 2 + O 2
2 H 2 O
Rules for Balancing Equations
Don’t add extra formulas to the equation.
H 2 + O 2
H 2 O H 2 + O 2
H 2 O + O 2 H 2 + O 2
2 H 2 O
Suggestions for Balancing Equations
Work with one element at a time.
Start with an element that is present in only one compound on each side if possible.
Balancing one element may unbalance another element.
You many need to change the coefficients again!!!
Balance diatomic elements last.
Balancing Equations
Examples:
Balance the following equations: Ca + N 2
Ca 3 N 2
Ba + O 2
BaO
P + H 2 Fe + O 2
PH 3 Fe 2 O 3 CaCO 3 + C Ca + NH 3
CaC CaH 2 2 + CO 2 + Ca 3 N 2 Ba(OH) (OH) 3 2 + Al(NO 3 ) 3
Ba(NO 3 ) 2 + Al