The Nature of Chemical Reactions

1. The Nature of Chemical Reactions

2. Chemical Reaction • A well defined example of a chemical change • One or more substances are changed into a new substance • Ex. • Grow • Ripen • Decay • burn

3. Chemical Reaction You know a chemical reaction has taken place because you will see changes • Ex. • Change in color • Production of gas • Precipitate

4. Chemical Equation • A representation of a chemical reaction that uses symbols to show the relationship between the substances being reacted and the substances being produced

5. Reactants • Substances you start with that go into a reaction • Products • The new substance(s) that are formed from the reaction • REACTANTS ----------- PRODUCTS • Ex. 2Na + Cl2 -------->2NaCl • Sodium plus Chlorine yields Sodium Chloride

6. THE PRODUCTS AND REACTANTS CONTAIN THE SAME TYPES OF ATOMS… THEY ARE JUST RE-ARRANGED… MASS IS ALWAYS CONSERVED

7. Conservation of mass & energy • In a chemical reaction, matter and energy cannot be created or destroyed… therefore mass and energy are conserved

8. Exothermic reactions • A chemical reaction in which heat is released to the surroundings • Following an exothermic reaction, the temperature of surroundings rises because energy is released

9. Exothermic Reaction

10. Endothermic Reactions • Chemical reaction that requires heat • More energy is needed to break the bonds in the reactants than is given off by forming the product • Following an endothermic reaction the temperature of the surrounding will decrease because energy is needed

11. Endothermic Reaction

12. Energy in Reactions • Activation Energy - The total amount of energy needed for a reaction to take place. • Catalyst / Enzyme – Reduces the total amount of activation energy needed. ** Speeds up the reaction • Inhibitor – Increases the total amount of activation energy needed. ** Slows down the reaction.

13. ACTIVATION ENERGY

14. Activation Energy with a Catalyst

15. Activation Energy with an Inhibitor

16. Reaction Types

17. Synthesis Reactions • Synthesis Reactions • A reaction where two or more substances combine to form a new compound (combined substances) • They have this general form A + B -------- AB • Example 2Na + Cl2 -------- 2NaCl

18. Synthesis Reaction +

19. Decomposition Reaction • Decomposition Reactions • A reaction in which a single compound breaks down to form two or more simpler substances (breaks substances apart) • They have this general form AB -------- A + B • Example 2H2O -------- 2H2 + O2 • Electrolysis – when electric current is used to produce this type of reaction

20. Decomposition Reaction

21. Combustion Reaction • Combustion Reactions • A reaction that uses oxygen and carbon as reactants and at least one product of the reaction always contains oxygen and/or carbon. • Water and carbon dioxide are common products of these reactions • Example 2CO + O2 -------- 2CO2

22. Single Displacement Reaction • Single displacement Reactions • A reaction in which one element takes the place of another element in the compound • They have this general form AX + B -------- BX + A • Example 3CuCl2 + 2Al -------- 2AlCl3 + 3Cu

23. Single Displacement Reaction

24. Double Displacement Reaction • Double Displacement Reactions • A reaction in which a gas, a solid precipitate, or a molecular compound forms from the apparent exchange of atoms or ions between two compounds • They have this general form AX + BY -------- AY + BX • Example Pb(NO3)2 + K2CrO4 -------- PbCrO4 + 2KNO3

25. Double Displacement Reaction

26. Two or more elements or compounds combine to make a more complex substance A + B → AB AB → A + B Compounds break down into simpler substances Occurs when one element replaces another one in a compound AB + C → AC + B AB + CD → AC + BD Occurs when different atoms in two different compounds trade places A = Red B = Blue C = Green D = Yellow

27. Balancing Chemical Equations • Purpose: To show the conservation of mass in a chemical reaction • Process: Add COEFFICIENTS to the equation to balance both sides of the equation.

28. Step 1 to Balancing Equations • 1. Check the equation for balance by counting the number of atoms of each element on each side of the equation

29. Example of Step 1 • 1. __NaCl + __BeF2 --> __NaF + __BeCl2 Na – 1 Na - 1 Cl – 1 Cl - 2 Be – 1 Be - 1 F – 2 F - 1

30. Step 2 to Balancing Equations • 2. Choose coefficients that balance the equation (NEVER change the subscripts) • Coefficients multiply the number of atoms

31. Example of Step 2 • 1. 2NaCl + __BeF2 --> 2NaF + __BeCl2 Na – 1 2 Na – 1 2 Cl – 1 2 Cl - 2 Be – 1 Be - 1 F – 2 F – 1 2 • ** Chlorine and Fluorine are not balanced…until the coefficient is placed. • Placing a coefficient in changes all atoms in that compound.

32. Balancing Equations Tips: • Practice Balancing Equations • It's Elemental - Balancing Act! (practice quiz)

33. Practice Problems Here are some practice problems. The solutions are in the section below this one. 1. __FeCl3 + __Be3(PO4)2 --> __BeCl2 + __FePO4 2. __AgNO3 + __LiOH --> __AgOH + __LiNO3 3. __CH4 + __O2 --> __CO2 + __H2O 4. __Mg + __Mn2O3 --> __MgO + __Mn

34. Solutions 1. 2 NaCl + 1 BeF2 --> 2 NaF + 1 BeCl2 2. 2 FeCl3 + 1 Be3(PO4)2 --> 3 BeCl2 + 2 FePO4 3. 1 AgNO3 + 1 LiOH --> 1 AgOH + 1 LiNO3 4. 1 CH4+ 2 O2 --> 1 CO2 + 2 H2O 5. 3 Mg + 1 Mn2O3 --> 3 MgO + 2 Mn