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Chemical ReactionsChapter 7
Describing Reactions 7.1
What type of change is happening in the picture?
When charcoal burns, it changes into other substances while producing heat and light.
Burning is a chemical change
Chemical Equations
One way to describe a change of state is to describe what is present before and after the change.
In a chemical reaction, the substances that undergo change are called reactants.
The new substances formed as a result of that change are called products. ◦In the picture, the reactants are the carbon in
the charcoal, and the oxygen in the air.◦The product is CO2 gas.
Using Equations to Represent Reactions
Reactants ProductsTo describe burning of charcoal:
Carbon + Oxygen Carbon Dioxide
C + O2 CO2
A chemical equation is a representation of a chemical reaction in which the reactants and products are expressed as formulas
Conservation of Mass
What happens to the products in a chemical reaction?
During a chemical reaction, the mass of the products is always equal to the mass of the reactants.
This is called the Law of Conservation of Mass, that mass is neither created nor destroyed.
This law was established by Antoine Lavoisier
Conservation of Mass
Equation reads “one carbon atom reacts with one molecule of oxygen and forms one molecule of carbon dioxide”.
If you have 6 C atoms, they will react with 6 O2 molecules to form 6 CO2 molecules
The equation has the same number of atoms on each side of the equation
Balancing Equations
Some chemical reactions are powerful enough to propel a space craft. ◦Rocket fuels contain a compound called
hydrazine, N2H4 ◦When hydrazine burns in the presence of
oxygen, the reaction produces nitrogen, water vapor, and heat.
◦You can describe this reaction by writing a chemical equation:
◦N2H4 + O2 N2 + H2O
Balancing Equations
N2H4 + O2 N2 + H2OIf we count the atoms on both sides, we will see
that the # of atoms on the left are not equal to the # of atoms on the ride side of the equation.
This equation is NOT balancedIn order to show that mass is conserved during a
reaction, a chemical equation must be balanced.You balance a chemical equation by changing the
coefficients (# in front of the formula)
Steps to Balancing Equations
No coefficient- is an understood “1”
First, count the number of atoms of each element on each side of equation
Starting with metals, change coefficients in front of formulas until balanced.
Try◦ Na + H2O NaOH + H2
HCl + CaCO3 CaCl2 + CO2 + H2O
MATH PRACTICE
Write a balanced equation for the reaction between copper and oxygen to produce copper (II) oxide, CuO.◦Steps:
Write equation with reactants on the left and products on the right
◦Cu + O2 CuO Balance atoms
◦2Cu + O2 2CuO
Math Practice
Balance the following chemical equations:
◦H2O2 H2O + O2
◦Mg + HCl H2 + MgCl2
Counting Chemicals
How many shoes do you have? Shoes are counted in pairs. How many eggs are in a dozen? Bottle
rockets in a gross? Pair, dozen, gross are all UNITs we use to
count.How do chemists count particles?DEMO
Counting Moles
Chemists need to be able to count atoms or molecules.
These units are too small to be counted, so chemists have another way to count them.
Chemists use a unit called the mole (No, not that kind of mole!)
A mole is an amount that contains 6.02 x 1023 particles of that substance ◦Aka “Avogadro’s #’◦Particles: atoms, molecules or ions◦Ex: 1 mole of Fe (Iron) contains 6.02 x 1023 atoms of
iron.
Molar Mass
Does a dozen eggs weigh the same as a dozen oranges?
Demo◦A mole of carbon has a different mass than a mole
of sulfurThe mass of one mole of substance is called a
molar massThe molar mass for chemicals is the same as
the atomic mass.◦Ex: Carbon’s mass is 12 amu or 12 grams per one
mole of carbon 12 grams C or 1 mole C
1 mole C 12 grams C
Mole Mass Conversions
For a compound, add the atomic masses of its components atoms◦EX: CO2
◦ 1 atom of Carbon x 12 g◦+ 2 atoms of Oxygen x 16 g
44 grams of CO2 per one mole of CO2
You can use this molar mass to convert moles of a substance to mass and vice versa.
44.0 g CO2 or 1 mol CO21 mol CO2 44.0 g CO2
“Molar mass” is the same as
“formula mass”
Mole-Mass Conversions
Suppose you have 55 g of CO2. You can use the g/mol conversion factor to calculate how many moles:◦55.0 g CO2 1 mol CO2 1.25 mol CO2
44g CO2
◦You can also convert from moles back to grams
◦2 mol CO2 44g CO2 88.0 g CO2
1 mol CO2
Types of Reactions7.2
Classifying Reactions
How do you classify matter?◦(solid, liquid, gas – remember?)
Chemical reactions are also classified into different types:◦Synthesis◦Decomposition◦Single-replacement◦Double-replacement◦Combustion
Synthesis & Decomposition
Synthesis reactions are reactions in which 2 or more substances react to form a single substance◦A + B AB (like a MARRIAGE)◦Ex: 2 Na + Cl2 2 NaCl◦Video
Decomposition reactions are reaction in which one substance is broken down into two or more simpler substances (opposite of synthesis)◦AB A + B (like a DIVORCE)◦Ex: 2H2O 2 H2 + O2
◦ Video
Here is another example of a synthesis reaction
Another view of a decomposition reaction
Single and Double Replacment
Single Replacement is a reaction where 1 element takes the place of another element in a compound◦A + BC B + AC (like a LOVE TRIANGLE)◦Ex: Cu +2Ag(NO3) 2Ag + Cu(NO3)2
◦ Video
Double Replacement two different compounds exchange positive ions and form 2 new compounds◦AB + CD AD + CB (like DO-Si-DO and change
partners)◦Ex: Pb(NO3) + 2KI PbI2 + 2 KNO3
◦ Video
◦ With precipitation
Single Replacement Reactions
Combustion
Combustion reaction one in which a substance reacts rapidly with oxygen, producing heat and light◦Ex: CH4 + 2O2 CO2 + 2 H2O
◦Always react with oxygen and usually produces CO2 , gas and water Products in combustion are ALWAYS carbon dioxide and
water. (although incomplete burning does cause some by-products like carbon monoxide)
Many reactions can be classified by more than one type.
Oxidation-Reduction (Redox)
Oxidation-reduction reaction: a reaction where electrons are transferred from one reactant to another, aka redox
Ex: Calcium reacts with oxygen to produce calcium oxide
2Ca + O2 2CaO◦ The product (CaO is composed of ions where the reactants were
neutral atoms)◦ When calcium reacts with oxygen, each neutral atom loses
electrons to form Ca+2 ionsCa Ca2+ + 2e-
When an element loses electrons during a chemical reaction it is called oxidation
The calcium lost electrons so it was oxidized
Oxidation Can occur without
oxygen
Oxidation-Reduction (Redox)
Reduction is the process where an element gains electrons
Each neutral oxygen atom gains two electrons becoming O2- ion
O + 2e- O2- The oxygen gained electron and has been
reduced◦Oxidation and reduction always occur together◦If one element loses electrons another element
HAS to gain them
OIL RIGOxidati
onIs
LossReducti
onIs
Gain
Mixed Practice
• State the type, classify and balance the following reactions:
1. _Pb(NO3)2 + _HCl _PbCl2 + _HNO3
2. _C2H6 + _O2 _CO2 + _HOH
3. _Ca + _HCl _CaCl2 + _H2
4. _Hg + O2 HgO
5. _SO2 + _O2 _SO3
Do you see a Redox reaction?
DR
Combustion
SR
Synthesis
Synthesis
Redox
Energy Changes in Reactions
7.3
Chemical Bonds and Energy
Where does the heat come from when you light a propane grill?◦C3H8 + 5O2 3CO2 +4H2O + Heat◦This equation shows that the heat released in the
reaction came from the reactants.◦Chemical energy is the energy stored in the
chemical bonds of a substance◦energy changes in chemical reactions are
determined by changes that occur in the chemical bonding
◦Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products
Breaking & Forming Bonds
Breaking chemical bonds requires energy.◦Where could this energy come from when using
a propane grill? Grills have a lighter which produces a spark,
giving enough energy to break the bonds and start the reaction
The formation of chemical bonds releases energy. (resulting in heat and light that you see)
Endothermic and Exothermic Reactions
Physical changes can release or absorb energy◦Exothermic: releases◦Endothermic: absorbing◦During a chemical reaction energy is either
released or absorbed◦A chemical reaction that releases energy to its
surroundings is called an exothermic reaction
◦A chemical reaction that absorbs energy from its surroundings is an endothermic reaction
•As you go from left to right in each graph, what happens to the reactants?• They react to form the products
•What point on each graph represents the highest energy?• The energy is highest at each curve’s peak
•What do the double-headed arrows represent?• The difference in chemical energy between the reactants
and products•Which type of reaction has products with a greater amount of energy that the reactants?• endothermic
Reaction rates7.4
Reaction Rates
A reaction rate is the rate at which reactants change into products over time◦Rate just means a change over time, like distance over
time= speedReaction rates tell you how fast a reaction is
going◦How fast reactants are consumed, products are formed
or energy released/absorbed. Factors that affect reaction rates are:
◦Temperature◦Surface Area◦Concentration◦Stirring ◦Catalysts
Temperature and Surface Area
How does temperature affect reaction rates?◦ Increasing temp causes particles to move faster and
collide, # of collisions increases then rate increases◦Decreasing the temperature will decrease the
reaction rateSurface area is the amount of area exposed
◦An increase in the surface area increases the exposure of reactants to one another allowing more collisions And therefore allowing an increase in the reaction rate (newspapers)
Stirring and Concentration
Stirring increases the reaction rate by increasing the number of collisions between the particles of the reactants.◦ (washing machine vs. soaking)
Concentration is the number of particles in a given volume. (ex sugar in tea)◦The more particles of reactant, the higher the
reaction rateGas concentration changes with pressure
(less room)◦The greater the pressure of the gas, the greater
it’s concentration and the faster it’s reaction rate
Catalysts
A catalyst is a substance that affects the reaction rate without being used up in the reaction.
They can be used to speed up or slow down reactions
Graph shows how a catalyst can lower the amount of energy needed to cause a reaction