1. Chapter 7 Reactions

2. Chemical Changes Substances react and form a new substance. Reactants go in. Products come out. Reactants Products Represented by chemical equations

3. Chemical Equations For the process of burning: Word equation: Carbon + Oxygen → Carbon dioxide Chemical equation: C + O 2 → CO 2

4. Law of Conservation of Mass Mass is neither created nor destroyed in a chemical reaction. It may change form (solid to liquid or gas). If 1 atom of Carbon goes into a reaction, 1 atom of carbon must come out. It can’t be lost or multiplied.

5. Chemical Reactions C + O 2 → CO 2 + → 1 atom of carbon goes in and 1 atom of carbon comes out 2 atoms of oxygen go in and 2 atoms of oxygen come out Mass is neither created nor destroyed.

6. Chemical Reactions N 2 H 4 + O 2 → N 2 + H 2 O +→ + Why is this wrong?

7. Balancing Equations N 2 H 4 + O 2 → N 2 + H 2 O + → + + Why is this wrong? 2 atoms of nitrogen go in and 2 come out 4 atoms of hydrogen go in but only 2 come out 2 atoms of oxygen go in but only 1 comes out Mass appears to be lost and that cannot happen!!

8. Balancing Equations You can balance equations by changing the coefficients. Coefficients are numbers that come in front of the formulas. Never!!!! Change the subscripts in a formula!! Changing subscripts makes it a different formula.

9. Balancing Equations N 2 H 4 + O 2 → N 2 + H 2 O N 2 H 4 + O 2 → N 2 + 2H 2 O The 2 in front of water is a coefficient. Now 2 atoms of nitrogen go in and 2 come out. 4 atoms of hydrogen go in and (2x2=4) 4 come out. 2 atoms of oxygen go in and (2x1+2) 2 come out.

10. Steps for Balancing Equations Count the atoms of each element on each side of the equation. Then change coefficients 1 at a time until the equation is balanced. REMEMBER: You cannot lose or gain atoms!!

11. Practice Balance the following equations: Cu + O 2 → CuO N 2 + H 2 → NH 3 K + Br 2 → KBr Na + Cl → NaCl

12. Practice Answers Cu + O 2 → CuO Cu + O 2 → 2CuO 2Cu + O 2 → 2CuO I add a coefficient of 2 to get 2 oxygen atoms but that also makes me have 2 copper atoms as products. I then have to add a 2 in front of copper. Now 2 copper in and out and 2 oxygen in and out.

13. Practice Answers N 2 + H 2 → NH 3 N 2 + H 2 → 2NH 3 First I add a 2 to the product to end with 2 nitrogen atoms, but now I am ending with 6 hydrogen atoms. N 2 + 3H 2 → 2NH 3 Adding a 3 in front of hydrogen gives me 6 atoms in the beginning as well.

14. Practice Answers K + Br 2 → KBr K + Br 2 → 2KBr First I add a 2 to the product to end with 2 bromine atoms, but now I am also ending with 2 potassium atoms. 2K + Br 2 → 2KBr Adding a 2 in front of potassium fixes this.

15. Practice Answers Na + Cl → NaCl 1 atom of each go in and 1 atom of each come out The equation is already balanced.

16. Moles Because chemical reactions often involve large numbers of small particles, chemists use a counting unit called the mole to measure amounts of a substance.

17. Moles 1 mole = 6.02 X 10 23 particles 1 mole of flowers = 6.02 X 10 23 flowers or 602,000,000,000,000,000,000,000 flowers 1 mole of apples = 6.02 X 10 23 apples 1 mole of oxygen = 6.02 X 10 23 atoms of oxygen

18. Molar Mass For an element, the molar mass is the same as the atomic mass expressed in grams. So for carbon, the atomic mass is 12.0 amu, and the molar mass is 12.0 grams. Elementatomic massmolar mass Oxygen15.999 amu 15.999 grams Nitrogen14.007 amu 14.007 grams

19. Mole-Mass Conversions Once you know the molar mass of a substance, you can convert moles of that substance into mass (or mass into moles). Carbon has a molar mass of 12.0 grams. How many moles are in 5 grams of carbon? 22 g X 1 mol/12.0 g = 1.83 moles or 1.8 moles in significant digits mol is the abbreviation for moles

20. Mole-Mass Conversions How many moles are in 35 grams of oxygen (in significant digits)?

21. Mole-Mass Conversions How many moles are in 35.0 grams of oxygen (in significant digits)? 35.0 g X 1 mol/15.999 g = 2.19 mol O

22. Finding the mass of a compound To find the mass of a compound: CO 2 Find the mass of each element in the compound. C = 12.011 O = 15.999

23. Finding the mass of a compound C = 12.011 g O = 15.999 g Then multiply the mass by the number of atoms of each element. There is only 1 atom of carbon so 12.001 x 1 = 12.001 There are 2 atoms of oxygen (O 2 ) so 15.999 x 2 = 31.998

24. Finding the mass of a compound Then add the masses of each element together. 31.998 g O +12.001 g C 43.999 g CO 2

25. Practice How many moles are in 55.0 grams of CO 2 ? 55.0 g X 1 mol / 43.999 g = 1.25 mol CO 2 How many moles are in 25.0 grams of H 2 O? How many grams are in 2.3 moles of CO 2 ?

26. Practice How many moles are in 25.0 grams of H 2 O? 1.0079 x 2 + 15.999 = 18.015 (mass of H 2 O) 25.0 g x 1 mol/ 18.015 g = 1.39 mol How many grams are in 2.3 moles of CO 2 ? 12.011 + 15.999 x 2 = 44.009 (mass of CO 2 ) 2.3 mol x 44.009 g/1 mol = 101.22 g = 1.0 x 10 2

27. Calculations In chemical reactions, the mass of a reactant or product can be calculated by using a balanced chemical equation and molar masses of the reactants and products.

28. Calculations How much oxygen is required to make 144 grams of water? 2H 2 + O 2 → 2H 2 O First determine how many moles of water you are trying to make. Molar mass of water = 1.0079 (2) +15.999 = 18.015 grams 144 g H 2 O x 1 mol H 2 O/ 18.015 g H 2 O = 7.99 mol H 2 O

29. Calculations 144 g H 2 O = 7.99 mol H 2 O 2H 2 + O 2 → 2H 2 O Now you can use the balanced equation to convert. (7.99 mol H 2 O) x (1 mol O 2 )/(2 mol H 2 O) = 4.00 mol O 2 Finally convert moles back to grams. (4.00 mol O 2 ) x (15.999 g O 2 )/(1 mol O 2 ) = 63.9 g O 2

30. Practice Calculations How much magnesium do you need to make 34.5 grams of Magnesium Oxide? 2Mg + O 2 → 2MgO How much sodium chloride will 22.3 grams of sodium make? 2Na + Cl 2 → 2NaCl

31. Practice Calculations How much magnesium do you need to make 34.5 grams of Magnesium Oxide? 2Mg + O 2 → 2MgO Molar mass of MgO = 24.305 + 15.999 = 40.304 g/ 1 mol MgO 34.5 g MgO x 1 mol MgO/40.304 g MgO x 2 mol Mg/2 mol MgO x 24.305 g Mg/ 1 mol Mg= 20.8 g Mg

32. Practice Calculations How much sodium chloride will 22.3 grams of sodium make? 2Na + Cl 2 → 2NaCl 22.3 g Na x 1 mol Na/ 22.990 g Na x 2 mol NaCl/ 2 mol Na x (22.990 + 35.453=58.443)g NaCl/ 1 mol NaCl = 56.7 g NaCl