Balancing Chemical Equations By Brian Goldstein

Transcription

1 Balancing Chemical Equations By Brian Goldstein

2 Help... Click Click Click to see the previous page to see the next page to return to the Table of Contents

3 Table Of Contents The Atom Electrons Reactivity Bonding Valence Compounds Exceptions Balancing Quit

4 What s the deal? After using this PowerPoint Tutorial, you will be able to: Describe the atom in terms of its charge Describe the roll of electrons in determining atom charge Determine the charge of an atom given the electron configuration Predict how atoms will bond given their electron configurations Identify polyatomic ions Identify diatomic molecules Determine the number of atoms in both the product and reactant sides of the equation given the subscripts and coefficients Correctly balance chemical equations

5 The Atom An atom is one of the elementary building blocks of all matter. It is composed subatomic particles called protons, neutrons and electrons. An atom is an electrically neutral particle This is because it has equal numbers of negatively charged particles, electrons; and positively charge particles, protons.

6 example One atom of the gas chlorine (Cl) has 17 positive protons and 17 negative electrons: (+17p) + (-17e) = 0 no charge Positive Protons +17 Negative Electrons -17 Neutral Atom 0

7 Electrons Are arranged around the nucleus of the atom in Energy Levels. These represent the most likely location of an electron at any given time. This is based on the amount of energy that an electron has at a given time (yes - electrons can hop from level to level). But, each level can only hold a fixed number of electrons at one time

8 Reactivity Some elements will react (or combine) when they come in close proximity to another element. Combinations of atoms are called compounds. The manner that compounds are formed is dependent upon the number of electrons that an atom has in its outermost energy level; these electrons are called valence electrons.

9 Lazy & Happy Atoms! Atoms are happy if they don t have to react with anything (think of them as lazy ) An atom s goal in life is to be happy - or not to react. An atom will not react if its outermost energy level is full. ex. The noble gasses do not react with anything - this is because they have full outermost energy levels.

10 What if? If the atom s outermost energy level is not full - its goal is to fill it ex. Cl (chlorine) has 7 electrons in its outermost energy level - it wants to gain one more to be happy Na (sodium) on the other hand, has 1 electron in its outermost energy level. It has a choice: gain 7 more electrons or lose 1. Remember that atoms are lazy...

11 Lewis Dot Structures Lewis Dot Structures show the valance configurations of atoms. In other words - they show you how many electrons are in their outermost shells they can make it easy to visualize the way atoms may bond

12 Bond Ionic Bond

13 Bond Ionic Bond Na will lose or give up one electron to be happy. So When we bring Na looking to give 1 electron and Cl seeking to gain 1 electron near each other there is a transfer of electrons called a chemical bond. You just made NaCl - sodium chloride - yes - table salt A compound!

14 Valence & Ions It is easy to predict how atoms may combine by looking at the number of electrons that an atom is likely to lose or gain. Again, this number is called valence. Atoms that have gained or lost electrons are called ions.

15 - Ionic Bonds Ions and Ionic Bonds Ionic bonds form as a result of the attraction between positive and negative ions. There is a transfer of electrons

16 Negative Charge If an atom gains an electron the atom becomes a negatively charged ion; written 1- Likewise, if an atom gains 2 electrons the atom becomes a doubly negative charged ion; written 2-

17 chlorine atoms have: Positive Protons +17 Negative Electrons -17 Neutral Atom 0 If a chlorine atom gains an electron it becomes a negatively charged ion: Positive Protons +17 Negative Electrons -18 Negative Ion 1-

18 oxygen atoms have: Positive Protons +8 Negative Electrons -8 Neutral Atom 0 If an oxygen atom gains 2 electrons it becomes a negatively charged ion: Positive Protons +8 Negative Electrons -10 Negative Ion 2-

19 Positive Charge If an atom loses an electron the atom becomes a positively charged ion; written 1+ Similarly, if an atom loses 2 electrons the atom becomes a doubly positive charged ion; written 2+

20 sodium atoms have: Positive Protons +11 Negative Electrons -11 Neutral Atom 0 If a sodium atom loses an electron it becomes a positively charged ion: Positive Protons +11 Negative Electrons -10 Positive Ion 1+

21 calcium atoms have: Positive Protons +20 Negative Electrons -20 Neutral Atom 0 If a calcium atom loses 2 electrons it becomes a positively charged ion: Positive Protons +20 Negative Electrons -18 Positive Ion 2+

22 Na will lose its lone electron and will become Na + Cl will gain one electron (from sodium in this case) and become Cl -

23 Na will lose its lone electron and will become Na + NaCl, a neutral compound is formed!!! Cl will gain one electron (from sodium in this case) and become Cl -

24 On the periodic table, All of the alkali metals have 1+ charge

25 On the periodic table, All of the alkaline earth metals have 2+ charge

26 On the periodic table, All of the noble gasses have 0 charge

27 On the periodic table, All of the Halogens have a 1- charge

28 On the periodic table, All of group 6A have a 2- charge

29 On the periodic table, All of group 5A have a 3- charge

30 On the periodic table, The charges of these ions are not as regular as the previous 6 groups. Consult a periodic table for the correct charge!

31 Neutral Compounds Most compounds like NaCl are neutral - or - they have no charge. Remember: Na + + Cl - NaCl w/ no charge (+1) + (-1) = 0 Sometimes more than one of the same element can help to create a compound.

32 more To show this, we use a subscript after the element to denote the number of ions of each element in a compound. ex: H 2 O the subscript tells us that there are 2 hydrogen ions in water because there is nothing after the O, we assume that there is only 1 of them

33 Polyatomic Ions Polyatomic ions are groups of atoms, like compounds, but they have a charge remember - most compounds are neutral The list of the most common polyatomic ions is on the next slide

34 Singly Positive Charge NH 4 + Singly Negative Charge C 2 H 3 O - 2 H 2 PO - 4 CN - HSO - 4 ClO - OH - ClO - 2 MnO - 4 ClO - 3 NO - 2 ClO - 4 NO - 3 Doubly Negative Charge CO 3 2- CrO 4 2- Cr 2 O 7 2- O 2 2- HPO 4 2- SO 3 2- SO 4 2- S 2 O 3 2- Triply Negative Charge PO 4 3-

35 Polyatomic ions behave like individual atoms, so if you need to show that there are more than 1 of them you need to include subscripts To do this neatly, the ion is wrapped in parentheses. Cu(NO 3 ) 2 How many atoms of Cu, N and O are there here?

36 C o u n t Cu(NO 3 ) 2 T h e m 1Cu, 2N and 6O

37 What happens if we need two Cu(NO 3 ) 2 compounds? How would we write this?? To show this we add coefficients in front of the compounds to change the number of compounds 2Cu(NO 3 ) 2 Now - how many atoms of Cu, N and O are there here?

38 2Cu(NO 3 ) 2 4N and 2Cu, 12O Count Again!!!

39 - Covalent Bonds How Covalent Bonds Form The force that holds atoms together in a covalent bond is the attraction of each atom s nucleus for the shared pair of electrons.

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41 - Covalent Bonds How Covalent Bonds Form The oxygen atom in water and the nitrogen atom in ammonia are each surrounded by eight electrons as a result of sharing electrons with hydrogen atoms.

42 - Covalent Bonds How Covalent Bonds Form An oxygen molecule contains one double bond, while a carbon dioxide molecule has two double bonds. A nitrogen molecule contains one triple bond.

43 - Covalent Bonds Unequal Sharing of Electrons Fluorine forms a nonpolar bond with another fluorine atom. In hydrogen fluoride, fluorine attracts electrons more strongly than hydrogen does, so the bond formed is polar.

44 - Covalent Bonds Unequal Sharing of Electrons A carbon dioxide molecule is a nonpolar molecule because of its straight-line shape. In contrast, a water molecule is a polar molecule because of its bent shape.

45 If you assume that all compounds are neutral, you can determine the charge of the individual atoms (using the periodic table if necessary) ex: MgCl 2 We know that there are 2 atoms of Cl We also know that each Cl holds a 1- charge so if there are 2 negative charges provided by the Cl s; the Mg must have a 2 + charge because (1 - ) + (1 - ) + (2 + )= 0 Practices

46 In like form - if you know the the charges of the atoms you can determine how many atoms will be needed to create neutral compound. ex: if you know that Ti has a 3 + charge; when combined with Cl - in order to make a neutral compound you must have 3 Cl for every 1 Ti looks like this: TiCl 3 Practice

47 Diatomic Molecules There are 7 elements that are almost always found in pairs in nature.

48 - Describing Chemical Reactions Conservation of Mass The principle of conservation of mass states that in a chemical reaction, the total mass of the reactants must equal the total mass of the products.

49 Or.. Matter cannot be created or destroyed - only changed. (weird I know ) for chemistry this means the number of atoms that you start with, called reactants must equal the number of atoms you finish with, called products. ex: if you want to make water - you must start with 2 hydrogen and 2 oxygen (they are diatomic molecules - remember?) you must end with the same - 4 atoms, 2 of each. H 2 + O 2 But is this correct?? Did we conserve mass?? H 2 O

50 Let s look... + O 2 There are 2 oxygen H 2 There are 2 hydrogen H 2 O Here, there are 2 hydrogen but... only 1 oxygen

51 Oops! To fix this little problem - we need to change the number of compounds that we have to do this we add coefficients in front of the compounds to change the number of compounds (remember?) H 2 + O 2 did we do it?? 2H 2 O

52 Let s look again... O 2 There are 2 oxygen + H 2 There are 2 hydrogen 2H 2 O Here, there are 2 oxygen. So that is the same on both sides. But...Now we have 4 hydrogen!!

53 Oops Still not quite right. We need to go back to fix one more thing the hydrogen! 2H 2 + O 2 2H 2 O I think this is it let s look...

54 Looks Good!! + O 2 There are 2 oxygen 2H 2 There are 4 hydrogen Click here for some... 2H 2 O Now there are 2 oxygen and 4 hydrogen!! Practice

55 Congrats!! That it No more...