Ch. 2 Chemical reactions

1. Ch. 2Chemical reactions Sec.2 – chemical formulas & equations

2. Chemical formulas

3. Chemical formulas • Shortened way to represent a substance • Use chemical symbols & numbers • Show the type & number of atoms in a substance

4. Subscripts • Number written to lower right of element • Tell how many atoms of each element are in the substance

5. How many atoms of each element? • 1 carbon, 1 oxygen • 1 carbon, 2 oxygen • 1 nitrogen, 4 hydrogen • 6 carbon, 12 hydrogen, 6 oxygen • CO • CO2 • NH4 • C6H12O6

6. Writing formulas for covalent compounds

7. Writing formulas for covalent compounds • Made up of 2 nonmetals • Common prefixes used in covalent compound names • Mono – 1 • Di – 2 • Tri – 3 • Tetra - 4 • Penta – 5

8. Carbon dioxide Write chemical symbol of 1st element. Add a subscript if there is a prefix. Write chemical symbol of 2nd element. Add a subscript if there is a prefix. CO2

9. Now you try…write a formula for each covalent compound • CCl4 • H2O • N2O4 • Carbon tetrachloride • Dihydrogen monoxide • Dinitrogen tetroxide

10. Writing formulas for ionic compounds

11. Remember… • Ionic compounds are made up of a metal & a nonmetal chemically bonded together. • The METALLIC element is always named 1st in the compound name. • Sodium chloride – sodium is the metal & • chlorine is the nonmetal

12. Steps for writing ionic compound formulas • Write the chemical symbol for the metallic ion first. • Write the chemical symbol for the nonmetallic ion. • Make sure the ions’ charges equal zero. • Metallic ions always have a positive charge. • Nonmetallic ions always have a negative charge. • An ion’s charge is the same as the number of electrons its group will gain or lose to become stable. • Add subscripts to the chemical symbols to make the compound’s charge equal zero.

13. Sodiumchloride • Na • Cl • Chemical formula: NaCl • Sodium’s chemical symbol is Na. • Chlorine’s chemical symbol is Cl. • Sodium is in group 1. It’s charge is 1+. • Chlorine is in group 17. It’s charge is 1-. • (+1) + (-1) = 0. No subscripts are needed.

14. If the ions’ charges do not equal zero… • Find the LCM of the ions’ charges. • Add subscripts where needed to equal the LCM. • Aluminum oxide Al – group 13 – charge 3+ O – group 16 – charge 2- LCM of 3 & 2 is 6 Subscript for Al is 2 (2 x 3=6) Subscript for O is 3 (3 x 2=6) 4. Formula: Al2O3

15. Now You try….. • KCl • MgCl2 • NaF • Al2S3 • Potassium chloride • Magnesium chloride • Sodium fluoride • Aluminum sulfide

16. Chemical equations

17. Chemical equations • Shortcut way to describe a chemical reaction • Use symbols to show the relationship between the reactants & products • Can be understood by scientists worldwide

18. reactants • Substances or molecules that participate in a chemical reaction • The starting substances in a chemical reaction • Are ALWAYS written on the LEFT side of the arrow in a chemical equation

19. Identify the reactants in these equations • Na + Cl2 • 2 H2O • Zn + 2 HCl • Na + Cl2 2 NaCl • 2 H2O 2 H2 + O2 • Zn +2 HCl ZnCl2 + H2

20. products • The substance that forms in a chemical reaction • The ending substance(s) in a chemical reaction • Are ALWAYS written on the RIGHT side of the arrow in a chemical equation

21. Identify the products in these equations • 2 NaCl • 2 H2 + O2 • ZnCl2 + H2 • Na + Cl2 2 NaCl • 2 H2O 2 H2 + O2 • Zn + 2 HCl ZnCl2 + H2

22. Accuracy is important in chemical equations • Incorrect symbols or formulas do not correctly describe a chemical reaction. • Small mistakes will make a HUGE difference!

23. Balancing chemical equations

24. Law of conservation of mass • Mass is neither created nor destroyed in an ordinary chemical or physical change. • Chemical equations must show the same numbers & kinds of atoms on each side of the arrow.

25. Subscripts vs. coefficients subscripts coefficients Written to the left of the chemical symbol Written the same size as the chemical symbol Apply to every atom in molecule, including subscripts • Written to the lower right of a chemical symbol • Written smaller than chemical symbol • Only apply to single atoms

26. Subscript or coefficient? • Subscript • Coefficient • Subscript • Both! • O2 • 2 CO • H2O2 • 2 CO2

27. Calculating the number of atoms • A coefficient applies to every atom & every subscript in a molecule. • If there is a subscript, multiply the coefficient by the subscript to find the number of atoms. • Coefficients in an equation are “stopped” by the plus sign or the arrow.

28. 2 H2 + O2 2 H2O • Reactant side has 4 hydrogen atoms & 2 oxygen atoms. The coefficient 2 applies only to the hydrogen. • Product side also has 4 hydrogen atoms & 2 oxygen atoms. The coefficient 2 applies to everything here. There is no plus sign or arrow to stop the coefficient.

29. Calculate the total number of atoms • 10 atoms • 72 atoms • 2 atoms • 8 atoms • 2 HCO3 • 3 C6H12O6 • NaF • 4 CaO

30. Steps for balancing chemical equations • Set up a T-chart for the reactants & products. • reactant product

31. Ca + O2CaO • Write the chemical symbol & number of atoms for each side. • reactantproduct • Ca – 1 Ca – 1 • O – 2 O – 1 • Compare to see if the equation is balanced. • This equation is not balanced.

32. Ca + O2CaO • Find the LCM for each element that is • unbalanced. • reactant product • Ca – 1 Ca – 1 • O – 2 O – 1 LCM = 2

33. Ca + O22CaO • Add coefficients to balance the elements. • NEVER CHANGE A SUBSCRIPT!! • Recount the number of atoms for each • element. Repeat steps 4 - 6 as needed. • reactant product • LCM = 2Ca – 1 Ca – 12 O – 2 O – 12

34. 2 Ca+ O22CaO • reactant product • LCM = 2Ca – 1 2Ca – 12 • O – 2 O – 12 LCM = 2 • The equation is now balanced.