Download Describing Chemical Reactions Power point

Chemical Reactions
•A chemical reaction is the process by which one or
more substances (reactants) are changed into one
or more different substances (products).
•According to the law of conservation of mass, the
total mass of reactants must equal the total mass
of products for any given chemical reaction.
•Chemical reactions are described by chemical
equations.
•A chemical equation represents, with symbols and
formulas, the identities and relative molecular or
molar amounts of the reactants and products in a
chemical reactions.
•Example: the following chemical equation shows
that the reactant ammonium dichromate yields the
products nitrogen, chromium(III) oxide, and water.
(NH 4 )2 Cr2O7 (s)  N 2 (g)  Cr2O3 (s)  4H 2O(g)
Indications of a Chemical Reaction
•The following changes usually indicate a chemical
reaction has occurred.
•Evolution of Heat and Light – a change in matter
that release energy as both heat and light is strong
evidence a reaction has taken place.
•Some reactions only release heat or light.
•The release of heat or light by itself is not
always a sign of a reaction because some
physical processes also release either heat
or light.
•Production of Gas – the evolution of gas bubbles
when two substances are mixed is often evidence
of a reaction.
•Formation of a Precipitate – if a solid appears after
two solutions are mixed, a reaction has likely
occurred.
•A solid that is produced as a result of a reaction in
solution and that separates from the solution is
known as a precipitate.
•Color Change – a change in color is often an
indication of a reaction.
Characteristics of Chemical Equations
•The equation must represent known facts. All
reactants and products must be identified.
•The equation must contain the correct formulas
for the reactants and products.
•The law of conservation of mass must be satisfied.
Law of Conservation of Mass:
The total mass of substances
does not change during a
chemical reaction.
reactant 1
reactant 2
+
total mass
calcium oxide
CaO
56.08 g
product
=
total mass
+
carbon dioxide
+
CO2
CaCO3
44.00 g
100.08 g
+
calcium carbonate
•A coefficient is added where necessary to equalize
numbers of atoms.
•A coefficient is a small whole number that appears
in front of a formula in a equation.
•Placing a coefficient in front of a formula specifies
the number of moles of the substance (if no
coefficient is written, assume 1).
2H2O (l) → 2H2 (g) + O2 (g)
coefficient 2 moles H2 1 mole O2
•The first step in writing a chemical equation is to
identify the facts to be represented.
•A word equation is where the reactants and
products are represented by words.
•A word equation is qualitative.
Example:
methane + oxygen → carbon dioxide + water
•The next step in writing a correct chemical equation
is to replace the names of the reactants and
products with appropriate symbols and formulas.
•A formula equation (also qualitative) represents the
reactants and products of a chemical reaction by
their symbols or formulas.
Example:
CH4 (g) + O2 (g) → CO2 (g) + H2O (g) (not balanced)
methane + oxygen → carbon dioxide + water
•To complete the process of writing a correct
equation, the law of conservation of mass must be
taken into account.
•The relative amounts of reactants and products
represented in the equation must be adjusted so
that the numbers and types of atoms are the same
on both sides of the equation.
•The process is called balancing an equation and is
carried out by inserting coefficients.
Example:
CH4 (g) + O2 (g) → CO2 (g) + H2O (g) (not balanced)
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g) (balanced)
coefficient
Balancing Chemical Equations
•When balancing a chemical equation you may add
coefficients in front of the compounds to balance
the equation but you may NOT change the
subscripts.
•Changing the subscripts changes the compound.
Steps to Balancing Chemical Equations
•There are four basic steps to balancing a chemical
equation.
•Write the correct formulas for the reactants and
products. DO NOT TRY TO BALANCE IT YET! You must
write the correct formulas first.
•And most importantly, once you write them correctly
DO NOT CHANGE THE FORMULAS!
Aluminum sulfate and calcium hydroxide react to produce aluminum
hydroxide and calcium sulfate.
Al2(SO4)3 +
Ca(OH)2
→
Al(OH)3 +
CaSO4
•Find the number of atoms of each element on the
left side (reactant).
•Compare those against the number of atoms of the
same element on the right side (product).
•You may find a t-chart helpful.
Reactant
2 Al
3S
14 O
1 Ca
2H
Al2(SO4)3 +
Ca(OH)2
→
Al(OH)3 +
CaSO4
Product
1 Al
1S
7O
1 Ca
3H
•Determine where to place coefficients in front of
formulas so that the left side has the same number
of atoms as the right side for EACH element to
balance the equation.
Reactant
2 Al
3S
14 O
18
1 Ca
3
2H
6
Al2(SO4)3 + 3 Ca(OH)2
→ 2 Al(OH)3 + 3 CaSO4
Product
2 Al
1
31SS
10
18
7 OO
31Ca
Ca
63HH
•Check your answer to see if:
•The number of atoms on both sides of the equation
are now balanced.
•The coefficients are in the lowest possible whole
number ratios (reduced).
Reactant
Product
2 Al
3S
18 O
3 Ca
6H
Al2(SO4)3 + 3 Ca(OH)2
→ 2 Al(OH)3 + 3 CaSO4
2 Al
3S
18 O
3 Ca
6H
Helpful Hints
•Take one element at a time, working left to right
except for H and O. Save H for next to last and O
until last.
•IF everything balances except for O, and there is
not way to balance O with a whole number, double
all the coefficients and try again. (Because O is
diatomic as an element.
•Polyatomic ions that exist on both sides of the
equation should be balanced as independent units
Symbols Used in Chemical Equations
(s) or (cr)
solid or crystal
(l)
liquid
(g)
gas
(aq)
∆
→
in aqueous solution (dissolved in water)
change in
“produces” or “yields” indicating result
of reaction