1. Chemical Reactions
Chapter 7
MAKE INTO TWO TESTS!!!!!!!!!!!

2. 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
Describing Reactions 7.1

3. 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.
Chemical Equations

4. Using Equations to Represent Reactions
Reactants  Products
To 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
Using Equations to Represent Reactions

5. 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

6. 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
Conservation of Mass

7. 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

8. Balancing Equations N2H4 + O2  N2 + H2O
If 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 balanced
In 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)
Balancing Equations

9. 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
Steps to Balancing Equations

10. 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

11. Math Practice H2O2  H2O + O2 Mg + HCl  H2 + MgCl2
Balance the following chemical equations:
H2O2  H2O + O2
Mg + HCl  H2 + MgCl2
Math Practice

12. 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 Chemicals

13. 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 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.
Counting Moles

14. 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 sulfur
The mass of one mole of substance is called a molar mass
The 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 grams C
Molar Mass

15. “Molar mass” is the same as “formula mass”
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 CO2
1 mol CO g CO2
“Molar mass” is the same as “formula mass”
Mole Mass Conversions

16. 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 CO mol CO mol CO2
44g CO2
You can also convert from moles back to grams
2 mol CO2 44g CO g CO2
1 mol CO2
Mole-Mass Conversions

17. Types of Reactions
7.2

18. 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
Classifying Reactions

19. 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
Synthesis & Decomposition

20. Here is another example of a synthesis reaction

21. Another view of a decomposition reaction

22. 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
With precipitation
Single and Double Replacment

23. Single Replacement Reactions

24. 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.
Combustion

25. Quick Lab
Identifying a type of Reaction pg 203

26. 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 ions
Ca  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)

27. 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 RIG
Oxidation Is
Loss
Reduction
Gain
Oxidation-Reduction (Redox)

28. State the type, classify and balance the following reactions:
_Pb(NO3)2 + _HCl  _PbCl2 + _HNO3
_C2H6 + _O2  _CO2 + _HOH
_Ca + _HCl  _CaCl2 + _H2
_Hg + O2  HgO
_SO2 + _O2  _SO3 DR
Combustion SR
Redox
Synthesis
Synthesis
Do you see a Redox reaction?
Mixed Practice

29. Energy Changes in Reactions
7.3

30. 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
Chemical Bonds and Energy

31. 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)
Breaking & Forming Bonds

32. 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
Endothermic and Exothermic Reactions

33. What point on each graph represents the highest energy?
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

34. Reaction rates
7.4

35. 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= speed
Reaction 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
Reaction Rates

36. 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 rate
Surface 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)
Temperature and Surface Area

37. 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 rate
Gas 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
Stirring and Concentration

38. They can be used to speed up or slow down reactions
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
Catalysts
Graph shows how a catalyst can lower the amount of energy needed to cause a reaction