Transcript Chemical Equations

Chapter 6: Intro to Chemical
Reactions
• Think of some things in our everyday lives that
could possibly be a result of chemicals reacting.
• Chemical Reaction: The process by which the
atoms of one or more substances are re-arranged
to form different substances.
• 1. Also called a “chemical change”
• 2. they often give a visual signal
• 3. they are represented by a chemical equation
• The word “reaction” is commonly abbreviated to
“rxn”
Evidence for a Chemical Reaction
• A temperature change has occurred.
1. Some absorb heat
a. Would “sense” a coolness in temperature
2. Some release heat (like a forest fire)
• A color change is seen
1. a nail placed outside will change color from silver
to orange/brown (rust)
• An odor is emmitted (like “rotten egg” smell)
• Gas bubbles appear
• A solid appears when liquids are mixed together
Sect. 6.2: Representing a chemical
reaction thru “Chemical Equations”
• Chemical reactions (changes) always involve the
rearrangement of the ways in which atoms are
grouped.
1. This “rearrangement” is represented in a
statement called a chemical equation.
• Chemical equation: a statement, using chemical
formulas and an arrow symbol, to represent the
rearrangement of atoms that has taken place
during a chemical reaction.
Chemical Equations
• A chemical equation has two parts: reactants & products
1. reactants: the starting substances in a chemical equation
a. Before the reaction takes place
b. Are located to the left of the arrow ()
2. products: the substances that are formed during the rxn
a. Are located to the right of the arrow ()
3. The reactants and the products are separated by an arrow
, which indicates the direction in which the reaction is taking
place
a. The arrow is read as “yields” or “reacts to produce”
4. If there are multiple reactants or multiple products, they are
separated from each other by + signs:
EX: reactant 1 + reactant 2  product 1 + product 2
Chemical Equations
• Symbols are also used to show the physical states of
the reactants and products:
1. solid
= (s)
2. liquid
= (l)
3. gas
= (g)
4. aqueous = (aq)
a. Indicates that a substance is dissolved in
water
• Symbols are written to the right of the reactant &
product chemical formulas (with parenthesis)
• Remember: H2, N2, O2, F2, Cl2, Br2, and I2 (diatomic)
Chemical Equations
• Chemical equations do not express numerical
equalities (like 1 + 1 = 2)
1. reactants are ingredients that are “used up” as
products are formed.
2. EX:
CH4 + O2  CO2 + H2O
• All chemical equations obey the “law of
conservation of matter/mass”.
1. atoms are neither created or destroyed during a
chemical reaction.
a. All atoms present within the reactants must be
present in the products.
Sect. 6.3: Balancing Chemical
Equations
• When we make sure that all of the atoms present
in the reactants are the same as what is present
in the products, we are going through a process
known as “Balancing the Chemical Equation” for
a chemical reaction.
• Examine the reaction:
CH4 + O2  CO2 + H2O
1. are the number of hydrogen & oxygen atoms
equal on both sides of the equation?
a. Answer: NO!
Balancing Chemical Equations
• In order to “balance” the chemical equation
CH4 + O2  CO2 + H2O, we must place whole
numbers in front of the compounds, called
“coefficients”.
Coefficient: the smallest integers that can be used
to balance a chemical equation.
1. a coefficient, placed in front of one or more of
the reactants or products, will multiply the number
of each of the atom(s) present.
2. coefficients will create the “balance”, so that
there will be the same # of the atoms present in
both the reactants and the products.
Chemical Equations
• EX: CH4 + 2O2  CO2 + 2H2O
1 carbon
1 carbon
4 hydrogen
4 hydrogen
4 oxygen
4 oxygen
1. placing coefficient of 2 in front of the O2 &
H2O now will show an equal number of oxygen,
hydrogen & carbon atoms present on both sides
of the equation.
2. This equation now demonstrates “balance”,
and also obeys the Law of Conservation of Mass.
Balancing Chemical Equations: The
Steps:
• Step 1: Obtain the reactants, products and their physical
states from the written description of the reaction
1. Write appropriate chemical formulas
• Step 2: Write the unbalanced chemical equation (called a
“skeletal equation”). **Go to questions 13-17 pg 171
• Step 3: Balance the skeletal equation.
1. Count the number of atoms of each element within
each compound of the reactant(s) and compare those
numbers to the products.
2. Place the smallest number coefficients in front of the
reactant/product formulas to show balance
3. Always start with the most complex formula
Balancing Chemical Equations: The
Steps:
• Step 4: Check to see if you used the correct
coefficients, by re-adding all atom types of the
reactants & products.
1. make sure the coefficients used are the smallest
integers possible to have balance.
• EX: Solid potassium reacts with water to form
hydrogen gas and aqueous potassium hydroxide.
1. K(s) + H2O(l)  H2(g) + KOH(aq)
a. Count all atoms on both sides of arrow.
Balancing Chemical Equations
•
K(s) + H2O(l)  H2(g) + KOH(aq)
1-K
1-K
2-H

3-H
1-O
1-O
2. Add coefficients to balance. Most times, this will
take several steps to accomplish.
2K(s) + 2H2O(l)  H2(g) + 2KOH(aq)
2-K
2-K
4-H

4-H
2-O
2-O
Balancing Chemical Equations
• EX: Solid manganese (IV) oxide is reacted with solid carbon
to produce solid manganese and carbon dioxide gas.
•
MnO2(s) + C(s)  Mn(s) + CO2(g)
1-Mn
1-Mn
2-O

2-O
1-C
1-C
2. This skeletal equation is already balanced. Do Not Do
The Following:
2MnO2(s) + 2C(s)  2Mn(s) + 2CO2(g)
**though it is balanced, it is not the smallest possible
integers. In this case, the smallest integer is “1”.
Balancing Chemical Equations
• EX: In solution, lead (II) nitrate reacts with potassium
chromate to produce solid lead (II) chromate and aqueous
potassium nitrate.
1. Write out skeletal (unbalanced) equation for the
reaction:
Pb(NO3)2(aq) + K2CrO4(aq)  PbCrO4(s) + KNO3(aq)
2. Balance equation. In this case, you can save time by
counting the unchanged polyatomic ions as a unit:
1-Pb
1-Pb
2-NO3
1-NO3

2-K
1-K
1-CrO4
1-CrO4
Balancing Chemical Equations
3. Add coefficients to balance equation:
Pb(NO3)2 (aq) + K2CrO4(aq)  PbCrO4(s) + 2KNO3(aq)
4. Check your work:
1-Pb
1-Pb
2-NO3
2-NO3

2-K
2-K
1-CrO4
1-CrO4
5. Equation is balanced…. Your work is done!
Balancing Chemical Equations from
Word Problems
• The steps:
1. Identify from the word problem which
substances are reactants and which are products.
a. Words like react, decomposes, breaks down,
“is burned”, and others will follow the reactants.
b. Words like “to produce”, forms, yields,
“converted to” will be just before the products.
2. Write out the chemical formulas for each of
the substances, indicating their physical states:
(s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous
Balancing Chemical Equations from
Word Problems
3. Place reactants to left, then an arrow, and
products to the right, producing an unbalanced
equation.
a. Separate multiple reactants/products with +
sign.
4. Balance the equation using coefficients.
Balancing Chemical Equations from
Word Problems
• Example: Solid iron (II) oxide when heated
together with solid carbon will be converted to
molten iron metal and carbon dioxide gas. Write
the balanced chemical equation showing this
process.
1. Reactants: solid iron (II) oxide & solid carbon
Products: molten iron metal
carbon dioxide gas
2. Unbalanced: FeO(s) + C(s)  Fe(l) + CO2(g)
Balancing Chemical Equations from
Word Problems
3. Balance the equation:
2FeO(s) + C(s)  2Fe(l) + CO2(g)