1. Chapter 7 Chemical Reactions
Section 7.1 Describing Reactions
Section 7.2 Types of Reactions

2. Section 7.1 Describing Reactions
Chemical Equations
** In order to understand chemical reactions, you first must be able to describe them.
Chemical equations are a useful way of describing a change to state what is present before and after the change.
Chemical rxns contain reactants and products.
Reactants- the substances that undergo change
Products- the new substances formed as a result of that change

3. Section 7.1 Describing Reactions
Using Equations to Represent Reactions
During a chemical reaction, reactants change into products.
Word equation: Reactants→Products
→ (yields, forms, creates)
Chemical equation-a representation of a chemical reaction in which the reactants and products are expressed as formulas

4. Section 7.1 Describing Reactions
Conservation of Mass
During chemical reactions, the mass of the products is always equal to the mass of the reactants.
Antoine Laviosier ( )-established the law of conservation of mass
Key Concept: The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction.

5. Figure 2

6. Section 7.1 Describing Reactions
Balancing Equations
Key Concept: In order to show that mass is conserved during a reaction, a chemical equation must be balanced.
You can balance a chemical equation by changing the coefficients, the numbers that appear before the formulas.
No number in front of the formulas is understood to be 1.

7. Section 7.1 Describing Reactions
Balancing Equations
Changing the coefficient, changes the amount of that reactant or product represented in the chemical equation.
Two Steps To Balance Equations
Count the number of atoms of each element on each side of the equation.
Change one or more coefficients until the equation is balanced.

8. Section 7.1

9. Section 7.1 Describing Reactions
Counting With Moles
**Chemists need practical units for counting things.
Key Concept: Because chemical reactions often involve large numbers of small particles, chemists use a counting unit called the mole to measure amounts of a substance.
mole (mol)-an amount of a substance that contains approximately 6.02 x 1023 particles of that substance
(Avogadro’s number)

10. Section 7.1 Describing Reactions
Molar Mass
Molar mass-the mass of one mole of a substance
**The molar mass of an element is the same as its atomic mass expressed in grams.
Calculate the molar mass of a compound by adding up the atomic masses of its component atoms, and then expressing the sum in grams.
Ex. (NaCl) 1 atom of Cl is 35 grams and one Na atom is 23 grams, so the molar mass of NaCl would equal to 58 grams; 1 mole of NaCl=58 grams

11. Section 7.1 Describing Reactions
Mole Mass Conversions
Once you know the molar mass of a substance, you can convert moles of that substance into mass, or a mass of that substance into moles.
Use the conversion factor 1 mole of the substance equals to that substance’s molar mass.
Ex. 1 mole of CO2=44.0g of CO2; C=12g and O=16g; 1 atom of C and 2 atoms of O
1C=12g and 2Oxy.=32g ( =44g)

12. Section 7.1 Describing Reactions
Chemical Calculations
Chemical equations can be read as recipes for making new substances.
Key Concept: In chemical reactions, the mass of a reactant or product can be calculated by using a balanced equation and molar masses of the reactants and products.

13. Calculations With Chemical Equations
Figure 8
Calculations With Chemical Equations

14. Section 7.1 Describing Reactions
Converting Mass to Moles
Make sure that you have a balanced equation.**
Step 1: Determine how many moles of substance you are trying to make.
Using Mole Ratios
Step 2: Use the balanced equation to determine what number (moles) of a product is given by a certain number (moles) of a reactant and vice versa.

15. Section 7.1 Describing Reactions
Converting Moles to Mass
Step 3: Convert the number of moles of the substance to grams by using the molar mass of the substance.

16. Section 7.2 Types of Reactions
Classifying Reactions
Reactions are often classified by the type of reactant or the number of reactants and products.
Key Concept: Some general types of chemical reactions are synthesis reactions, decomposition reactions, single-replacement reactions, double-replacement reactions, and combustion reactions.

17. Section 7.2 Types of Reactions
Synthesis Reaction
Def.-a reaction in which two or more substances react to form a single substance
A + B→C
8 Fe + S8 ---> 8 FeS

18. Section 7.2 Types of Reactions
Decomposition Reaction
Def.-a reaction in which a compound breaks down into two or more simpler substances
AB→A + B
2 H2O ---> 2 H2 + O2

19. Section 7.2 Types of Reactions
Single Replacement Reaction
Def.-a reaction in which one element takes the place of another element in a compound
A + BC→B + AC
2 Fe + 6 NaBr --> 2 FeBr3 + 6 Na

20. Section 7.2 Types of Reactions
Double Replacement Reaction
Def.-a reaction where two different compounds exchange positive ions and form two new compounds
AB + CD→AD + CB
Pb(NO3)2 + 2 KI ---> PbI2 + 2 KNO3

21. Section 7.2 Types of Reactions
Combustion Reaction
Def.-a reaction in which a substance reacts rapidly with oxygen, often producing heat and light
CH4 + 2O2→CO2 + 2H2O
Common products of combustion are carbon dioxide and water.

22. Let’s observe a combustion reaction…
What happens when a candle burns?
2 C30H O2 --> 60 CO H2O
We can identify a combustion reaction because the products are ____ and ____.

23. Section 7.2 Types of Reactions
Reactions as Electron Transfers
Key Concept: The discovery of subatomic particles enable scientists to classify certain chemical reactions as transfers of electrons between atoms.
Oxidation-reduction reaction-a reaction in which electrons are transferred from one reactant to another

24. Section 7.2 Types of Reactions
Oxidation
Def.-any process in which an element loses electrons during a chemical reaction
Ex. Ca reacting with O; neutral calcium becomes an ion with a charge of 2+; calcium will lose those 2 electrons to oxygen
Ex. Sodium reacts with chlorine; neutral sodium becomes an ion with a charge of 1+; sodium will lose its 1 electron to chlorine

25. Section 7.2 Types of Reactions
Reduction
Def.-the process in which an element gains electrons during a chemical reaction
Ex. Ca reacting with oxygen; neutral oxygen becomes an ion with a charge of 2-; oxygen will gain 2 electrons from calcium
A reactant is said to be reduced if it gains electrons.

26. Section 7.2 Types of Reactions
Oxidation-Reduction Reactions
Oxidation and reduction always occur together.
When one element loses electrons, another element must gain electrons.
**Oxidation-reduction reactions do not always involve complete transfers of electrons.
Some elements partially lose electrons or partially gain electrons.