1. Biology Dr. Altstiel Naples American High School
Chemical Reactions
Biology
Dr. Altstiel
Naples American High School

2. Characteristics of Chemical Reactions
Chemical reaction always results in the formation of a new substance.
Two kind of substances involved in a chemical reaction:
Reactant – substance that enters into a chemical reaction.
Product – substance produced by a chemical reaction.
General Description of a Chemical Reaction: Reactants changing into products.

3. Characteristics Continued
Chemical reactions always involve a change in energy.
Energy is either absorbed or released in a chemical reaction.
Energy absorbed when sugar changes into carmel.
Energy is released when gasoline is burned.

5. Nature of Chemical Reactions
Chemical Reaction – process in which the physical properties of the original substances change as new substances with different physical and chemical properties are formed.
Examples:
Burning of gasoline
Rusting of Iron
Baking of Bread

6. Capacity to React
In order for chemical reactions to occur, the reactants must be able to combine with other substances to form products.
What accounts for the ability to undergo a chemical reaction?
Valence Electrons – electrons in the outermost energy level.
Atoms will try to fill their outermost energy level by gaining, losing, or sharing electrons.
Chemical bond formed by gaining or losing electrons = Ionic Bond.
Chemical bond formed by atoms sharing electrons = Covalent Bond.

7. Bonding Capacity
The ease in which an atom will form chemical bonds is known as the bonding capacity of an atom.
Bonding capacity determines the ability to undergo chemical reactions.
The ability to undergo chemical reaction is an important chemical property.

8. Section Review What is a chemical reaction?
What is a reactant? A product?
What is the relationship between the arrangement of electrons in an atom and the atom’s chemical properties?
Cooking an egg until it is hard-boiled involves a chemical reaction. Cutting a piece of paper into a hundred little pieces does not involve a chemical reaction. Explain the differences between the two process.

9. Answers to Section Review Questions
A chemical reaction is a process in which the physical and chemical properties of the original substances change as new substances with different physical and chemical properties are formed.
A reactant is a substance that enters into a chemical reaction. A product is a substance produced by a chemical reaction.
The arrangement of electrons in an atom determines the bonding capacity of an atom. The bonding capacity determines its chemical properties, or its ability to undergo chemical reaction.
Cooking an egg involves a chemical reaction that produces a new substance. The physical act of cutting paper does not produce a new substance.

10. Chemical Equations
Chemical equation – an expression in which symbols and formulas are used to represent a chemical reaction.
Writing a chemical equation:
Must first write the correct chemical symbols or formulas for the reactants and products.
Use a “+” sign to show which substances combine.
Draw an arrow () between reactants and products. Arrow means “yields.” Also shows direction of reaction.
Mg + O > MgO + Energy
Magnesium + Oxygen ----> Magnesium oxide + Energy

11. Conservation of Mass
Atoms cannot be created or destroyed during a chemical reaction.
Number of atoms of each element must be the same before and after the chemical reaction.
That is, the number of atoms remains the same on both sides of the arrow in a chemical reaction.
Change that occurs only involves the rearrangement of atoms.
No mass is lost or gained in the reaction. The total mass of the reactants must equal the total mass of the products. This is known as the Law of Conservation of Mass.

12. Determining Whether an Equation is Balanced
Balanced equation – one in which the number of atoms of each element as a reactant is equal to the number of atoms of that element as a product.
Elements are conserved; not molecules.
Never change subscripts when balancing equations.

13. Sample Problem
Determine whether the following equation is balanced. Assume that all the formulas are written correctly.
2Na + H2O  2NaOH + H2

14. Solution 2Na + H2O  2NaOH + H2
Determine whether atoms are conserved by comparing the number of each kind of atom on each side of the equation.
Atoms Na:  2
Atoms H: 2 
Atoms O:  2
The equation does not balance because there are 2 atoms of H to the left of the yield arrow but 4 atoms of H to the right. Also, there is 1 atom of O to the left but 2 atoms of O to the right.
Note that in an unbalanced equation, “2NaOH” represents 2 formula units of NaOH, each of which contains 1 atom Na, 1 atom O, and 1 atom of H.
A coefficient of 2 for H2O would balance the equation.
2Na + 2H2O  2NaOH + H2

15. Review Questions and Practice Problems 1
What is a balanced equation?
Why must a chemical equation be balanced to properly represent a chemical reaction?
Tell how many atoms of each element are present on each of the following:
5NH3 b. 4Ca(OH)2 c. 3BaSO4
Tell which of the following are balanced equations. Assume that all the formulas are written correctly.
Zn + 2HCl  ZnCl2 + H2
H2SO NaOH  H2O + Na2SO4
Pb(NO3) NaI  PbI2 + NaNO3

16. Steps to Balancing Chemical Equations
Write a word equation for reaction.
Write the correct formula for all reactants and products.
Determine coefficients that make the equation balanced.

17. Sample Problem
Write a balanced equation for the reaction between chlorine and sodium bromide to produce bromine and sodium chloride.
Cl2 + NaBr  Br2 + NaCl

18. Chlorine + sodium bromide  bromine + sodium chloride
Solution
Step 1: Write a word equation for reaction.
Chlorine + sodium bromide  bromine + sodium chloride
Step 2: Write the correct formula for all reactants and products. Chlorine and bromine have been mentioned as elements that are diatomic. Their formulas are Cl2 and Br2. Replacing the names of these substances with their formulas gives:
Cl2 + NaBr  Br2 + NaCl
Step 3: Determine coefficients that make the equation balanced.

19. Solution Step 3: Cl2 + NaBr  Br2 + NaCl
Starting with the first element, chlorine, notice that 2 atoms of chlorine to the left of the arrow does not balance with 1 atom of chlorine to the right. By supplying the coefficient 2 before the NaCl, you make the number of chlorine atoms equal.
Cl2 + NaBr  Br NaCl
Check the number of the next element, sodium. There is 1 atom on the left but 2 atoms on the right. By supplying a coefficient of 2 to NaBr, you will bring the sodium into balance.
Cl NaBr  Br NaCl
Check the number of atoms of the next element, bromine. It has 2 atoms on each side, so it is balanced. The equation is now complete. It is a balanced chemical equation.

20. Practice Problems 2
Study each of the formula equations shown below. Use coefficients to balance those equations that are not balanced.
Na + O2  Na2O
CuO + H2  Cu + H2O
Cu + S  Cu2S
Ba(OH)2 + CO2  BaCO3 + H2O

21. Al + Cl2 ---> AlCl3 2. Ag2SO4 + CaCl2 ---> AgCl + CaSO4
3. LiOH H2SO4 ---> H2O Li2SO4
4. Co(C2H3O2) Al ---> Al(C2H3O2) Co
Al(HCO3) > Al2(CO3) CO H2O
P O2 ---> P4O7
H3PO Ca(OH)2 ---> H2O Ca3(PO4)2
Fe2(CO3)3 ---> Fe2O CO2
(NH4)2SO Pb(N03)2 ---> PbSO NH4NO3
Sn HNO3 ---> Sn(NO3) H2
Ca H2O ---> Ca(OH) H2
Na CuSO4 ---> Na2SO Cu