Transcript Chemical Reactions - JH Rose

Chemical Combinations
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In a reaction, one or more substances reacts
or changes to form new substances.
Chemical reactions are written in shorthand
as
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Reactants  Products
“” is read as yields or produces
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In a reaction (rxn), bonds between atoms
are broken and then formed again.
Atoms are NOT created or destroyed, just
rearranged.
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Part of Dalton’s Atomic Theory – chemical
reactions occur when atoms are joined
separated or rearranged.
This satisfies the Law of Conservation of
Mass which says that matter can neither be
created nor destroyed.
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In order for something to be classified as a
reaction, there must be a CHANGE.
Indicators of chemical change
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Formation of a gas/vapor – bubbles
Formation of a precipitate – solid
Color change (not a shade change) – blue to red
Evolution or absorption of heat – gets hot or
cold
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A Chemical Equation is the representation
of a chemical reaction
It shows formulas and states. States are
written after the formula as a subscript.
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Solid (s)
Liquid (l)
Gas (g)
Aqueous-dissolved in water (aq)
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Used for substances that are soluble in water
(Solubility Rules)
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Catalysts are substances that speed up a
reaction but do NOT participate in the
reaction.
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In other words, the catalyst does not change.
Catalysts are NOT part of the chemical
reaction, so they are not written as a
reactant or product. Instead they are
written above the yields sign.
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Ex. H2O2 (aq)
MnO2>
H2O(g) + O2 (g)
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“Bare Bones”
The skeleton equation just contains the formulas
and states for the substances in the reaction.
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It does not contain the relative amount of reactant and
product.
Skeleton Equations tell you QUALITATIVELY what
your reactants and products are.
However, they do not tell you QUANTITATIVELY
how much reactant and product there is.
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Write the formulas and states of the reactants
(what you start with) on the left of the arrow.
Write the formulas and states of the products
(what you end with) on the right of the arrow.
If a catalyst is present, write it above the arrow.
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 is used to represent heat.
elec. is used to represent electricity.
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Hydrochloric acid reacts with calcium metal to form
hydrogen gas and calcium chloride.
A solution of silver(I) nitrate reacts with copper
metal to produce silver metal and copper(II) nitrate.
Propane, C3H8, burns in the presence of oxygen to
produce carbon dioxide and water vapor.
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Balancing a chemical equation tells you the
relative amount of reactant needed and
product made in a chemical reaction.
It makes the equation satisfy the Law of
Conservation of Mass.
Both sides of the equation MUST be Equal.
 Example
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H2O2 (aq)  H2O(g) + O2 (g) becomes
 2 H2O2 (aq)  2 H2O(g) + O2 (g)
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Coefficients (numbers in front) are used to
balance the equation.
You CANNOT change the subscripts of the
substances!!!!
Once a formula is written for a compound in
a reaction, do NOT change it to balance the
equation.
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Determine the correct formulas for all
reactants and products.
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Write the skeleton equation.
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Magnesium combines with oxygen gas to produce
magnesium oxide.
Mg (s) + O2 (g)  MgO (s)
Count the number of atoms of each element
on each side of the yields arrow.
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Reactant – 1 Mg
Product - 1 Mg
2O
1O
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“Balance” elements one at a time using
coefficients.
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2Mg (s) + O2 (g)  2MgO (s)
Check for atom balance by recounting using
coefficients and subscripts for each element.
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No coefficient is assumed to mean a coefficient of 1
(same as in math)
ReactantProduct-
2 Mg
2 Mg
2O
2O
Finally, make sure coefficients are present in
the lowest whole number ratio.
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2:1:2 - cannot be reduced
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Synthesis A + B  AB
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2 reactants, 1 product
Ex:
Decomposition AB  A + B
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1 reactant, 2 products
Ex:
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Single Replacement (Displacement)
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A + BC  AC + B
element + compound yields element +
compound
Ex. :
Double Replacement (Displacement)
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AB + CD  AD + CB
2 compounds produces 2 NEW compounds
Ex. :
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Combustion
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Complete CH + O2  CO2 + H2O
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Hydrocarbon + oxygen gas yields carbon
dioxide + water
Ex. :
Incomplete CH + O2  CO + H2O
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Hydrocarbon + oxygen gas yields carbon
monoxide + water
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1.
Products for simple chemical reactions can
be predicted based on the reactants.
Classify the reaction by type.
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Synthesis, Decomposition, Single
Replacement, Double Replacement, or
Combustion
Use the Reference Chart to match the
reaction type and predict the products.
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Use examples to help you figure out what the
product will be.
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Synthesis
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Decomposition
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Single element will replace “like” element in compound
Use Activity Series
Double Replacement
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Separate the reactant into its parts
Single Replacement
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Combine the reactants
Compounds “swap” partners
Use solubility rules to determine state
Combustion of a Hydrocarbon
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Products are always CO2 and H2O
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In Single Replacement reactions, you must
determine if the single element is reactive
enough to actually replace the element in the
compound.
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Activity series is used to determine relative
reactivity.
The HIGHER up the element is, the more reactive
it is.
Only single elements that are HIGHER on the
activity series can replace an element in a
compound.
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Ex. Sodium, Na, can replace copper, Cu.
Ex. Silver, Ag, cannot replace potassium, K.
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In Double Replacement reactions, you must
determine if the products are actually DIFFERENT
from the reactants.
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When an ionic compound is dissolved in water, aqueous,
it separates into its component ions.
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Ex. NaCl(aq) separates into Na+ and Cl-
If all of the reactants and products are aqueous ionic
compounds, then nothing actually changes.
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Ex. KCl (aq) + NaNO3(aq)  KNO3(aq) + NaCl(aq)
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All compounds are aqueous – no change
In order for a change to occur, a solid, gas, or molecular
compound must be formed.
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Ex. KCl(aq) + AgNO3(aq)  AgCl(s) + KNO3(aq)
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Silver chloride is a solid precipitate - change
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To eliminate the products that don’t change in a
double replacement reaction, you can write a Net
Ionic Equation.
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Net Ionic Equations only contain the substances that
actually change in a reaction.
Steps Na2SO4(aq) + BaCl2(aq)  NaCl(aq) + BaSO4(s)
1.
Write all aqueous ionic compounds as their
component ions.
Na1+(aq) + SO42-(aq) + Ba2+(aq) + Cl1-(aq)  Na1+(aq) + Cl1-(aq) + BaSO4(s)
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Eliminate any ion that is on both sides.
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Na1+(aq) and Cl1-(aq) are the same on both sides. They are called
SPECTATOR ions.
Write the ions/compounds that are remaining.
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Ba2+(aq) + SO42-(aq)  BaSO4(s)