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Chemical Equations and Reactions

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Michelle Durham

on 7 April 2015

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Transcript of Chemical Equations and Reactions

anything written over the yeilds arrow indicates a condition of the reactants:
Chemical Reactions and Equations
Indications of a Chemical Reaction:
1. Produces energy as heat or light
**Not a sole indicator as some physical changes can involve heat or light
2. Produces a gas
3. Forms a Precipitate:
** a solid that is produced as a result of a chemical reaction in a solution
4. Color change
Chemical Reactions:
Process by which one or more substances is changed into one or more different substances
Chemical Equations:
symbols and formulas
reactants and products
relative molar amounts
Characteristics of a chemical equation
Must represent known facts
must contain correct formulas for reactants and products
** remember your diatomic elements!!
Must follow the Law of Conservation of Mass
Coefficient: a small, whole number that appears in front of a formula
2 H O
2
In an equation, this means "react to yeild"
Word vs Formula Equations
Word equations:
Reactants and Products are represents by words.
Only qualitiative
no amounts/ quatities given
Formula Equations:
reactants and products represented using symbols and formulas
is still qualitative
gives no amounts and quantities
Formula equations meet 2 of the 3 requirements for a chem equation: Equation must be balanced!
To complete a chem equation the Law of Conservation of Mass must be addresed --- equation must be balanced, same number of each element in both reactans and products
Word equation:
methane + oxygen = carbon dioxide and water
Formula equation
CH + O CO + H O
4
2 (g)
2
2
(g)
(g)
(g)
** not Balanced
g = gaseous
s = solid
aq = aqueous (dissolved in water)
l = liquid
Some symbols you may encounter in a chem equation:
Reversible reaction
Precipitate formed
Gas formed
reactants are heated
heat
2 atm
pressure
0 degrees
MnO
2
reactants are heated
pressure at which reaction is carried out
pressure at which reaction is carried out is greater than atm pressure
temperature at which reaction is carried out
catalyst required for reaction
Reversible reaction: a chem reaction in which the products re-form the original reactants
PRactice Problems page 268
chemical reactions do NOT tell us:
IF a reaction will actually occur or not
the rate of the reaction

Steps for Balancing a Chemical Reaction:
1. Identify the names of the reactants and products ( if a word equation is used)
** don't forget diatomic molecules
2. Write a formula equation using proper symbols and subscripts!
3. Balance the equation following the Law of Conservation of Mass
** Only change coefficeints - NEVER subscripts
**1st: balance atoms of elements that are combined and only appear once on each side of the equation
**Balance polyatomic ions that appear on both sides as single units
**Balance H and O last
Count atoms to be sure that the equation is balanced!!
Reduce coefficients and get rid of any fractions by multiplying by 2
I can:
List 3 observations that indicate a chemical reaction has occurred
List 3 requirements for a correctly written chemical equation
Write a word equation and a formula equation
balance a formula equation
Write and classify the general equation for the following reaction types: synthesis, decomposition, single replacement, double replacement, and combustion
predict the products of the following types of reactions: synthesis, decomposition, single replacement, double replacement and combustion
Types of Chemical Reactions
1. Synthesis: 2 or more substances combine to form a new substance
A + X AX
metals (and nonmetals) can bond with oxygen to form oxides
They can bond with Sulfur to form sulfides
2Mg + O 2MgO

C + O CO

2
2
2
Halogens combine with group 1 metals in the formulas MX and group 2 metals with the formula MX
2
Active metal oxides react with water to form metal hydroxides: CaO + H O Ca(OH)
2
2
Oxides of nonmetals react with water to form oxyacids: SO + H O H SO
2
2
3
2.
Decomposition: a single compound undergoes a reaction to produce 2 or more simpler substances
AX A + X
**Most decomp reactions require heat or electricity: electrolysis
Decomp of binary compounds: 2H O H + O
2
2
2

Decomp of acids: metal oxides and water
H SO SO + H O

2
2
2
4
3. Single Replacement: one element replaces a similar element in a compound
A + BX AX + B
Displacement of a metal in a compound with another metal
displacement of H in water by a metal
2Na + 2H O NaOH + H

2
2
** the more active the metal, the more likely it will react with water (groups I and II)
Displacement of H in an acid by a metal
Mg +2 HCl H + MgCl
2
2
One halogen can replace another in a compound if it is more reactive
Cl + 2KBr 2KCl + Br
Br + KCl no reaction
2
2
2
4. Double replacement: the ions of two compounds exchange places in aqueous solution to form new compounds
AX + BY AY + BX
Produces:
precipitate or
gas or
molecular compound such as water

2KI + Pb(NO ) PbI + KNO
2 (aq)
3
(aq)
2 (s)
3 (aq)
FeS +2HCl H S + FeCl
(s)
(aq)
2
(g)
2 (aq)
HCl + NaOH NaCl + H O
(aq)
(aq)
(aq)
(l)
2
5. Combustion: a substance combines with oxygen, releasing a large amount of energy in the form of light and heat
**Produces water vapor
C H + 5O 3CO + 4H O
2
2
(g)
(g)
2 (g)
8 (g)
3
Activity Series
The ability of an element to react is called it's activity
An activity series is a list of elements organized by their ability to undergo chemical reactions
**most active elements at the top will replace any element lower on the list
helps predict whether a reaction will occur. see table 3 pg 286
practice problems page 287
2
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