Chemical Reactions

1. Chemical Reactions Writing Chemical Equations

2. Bonding and Valence Electrons • The number of valence electrons is the key to how an atom reacts with other atoms because valence electrons are involved in bonding. • One easy way to show the number of valence electrons is with an electron dot diagram. • An electron dot diagram consists of the symbol surrounded by dots. Each dot stands for one valence electron.

3. Remember… • When atoms react one of two things happen: • Either the number of valence electrons increases to eight. • Or, all the valence electrons are lost. • You can tell how many valence electrons are in an atom by knowing what group each element is in.

4. Electron Transfer and Bonding • Atoms with five, six, or seven valence electrons don’t have quite enough electrons to total the stable number of eight. • An atom with one, two, or three valence electrons can lose a few and become stable. • In this process, losing and gaining electrons, atoms form bonds with each other.

5. Fluorine As An Example • Fluorine forms molecules made of two fluorine atoms. • Each atom shares one of its 7 valence electrons with the other atom. • By sharing, both atoms have 8 valence electrons.

6. How Many Bonds? • The number of bonds these atoms can form equals the number of valence electrons needed to make a total of eight. • For example, oxygen has six valence electrons, so it can form two covalent bonds. In a water molecule, oxygen forms one covalent bond with each hydrogen atom

7. Look at the diagram of a molecule of oxygen • This time twopairs of electrons are shared between the oxygen atoms, forming a double bond • Double Bond - A chemical bond formed when two atoms share two pairs of electrons with each other.

8. Chemical Equation • A chemical equation is a shorter, easier way to show chemical reactions, using symbols instead of words. • We use chemical equations because they follow a common structure that all chemists understand.

9. Chemical Formulas • Remember – all elements are represented by a symbol. • We use these symbols to represent compounds too…by combining them. • This combination is called a formula. • A formula also shows the ratio of the different atoms that make up that substance by using subscripts. • Subscripts are the very small numbers in a formula that are written beside the element symbol.

10. Chemical Formula cont. • For example: • Propane has the ratio of 3 carbon atoms to 8 hydrogen atoms. • The chemical formula would look like this – C3H8 • If there is no subscript, the number next to the symbol is assumed to be 1. • CO2 • What is the ratio of atoms in Iron oxide (rust) if its formula isFe2O3? • The formula tells you that there are 2 atoms of iron for every 3 atoms of oxygen.

11. Structure of the Equation • A chemical equation summarizes a reaction. • It tells you the substances you start with and end with. • The materials that are on the left are those that you start with and are called the reactants. • The materials that are on the right are those that you end with and are called the products.

12. Structure of an Equation continued… • The formulas for the reactants are written on the left, followed by an arrow and the product on the right. • Example: • Reactant + ReactantProduct • CaO + CO2 CaCO3 Reactant Reactant Product

13. Conservation of Mass • No matter how many reactants and products are involved, all the atoms present at the start of a reaction are present at the end. • Think about what happens with students in school…you have a set in one class then when the bell rings, there is a new set in a different class…there are still the same students and number of students in the building, just in different locations. • The amount of matter involved in a chemical reaction does not change. The total mass of the reactants must equal the total mass of the products. This is called the conservation of mass.

14. Balancing Chemical Equations • Remember, the principle of Conservation of mass indicates that the same number of atoms exist in the products as were present in the reactants. • To balance an equation, begin by looking at the formulas: • H2 + O2 H2O • How many atoms does the hydrogen molecule have? • 2 • How about oxygen? • 2 • How many of each kind of atom are present in one water molecule? • 2 Hydrogen and 1 Oxygen

15. Balancing Chemical Equation for water… • Since we started with 2 Hydrogen and 2 Oxygen reactants, according to the conservation of mass, shouldn’t water have the chemical formula of H2O2? • No, that would be the formula for Hydrogen peroxide (per = 2). • To balance this equation, we must use a coefficient. • A coefficient is a number placed in front of a chemical formula in an equation. It tells you how many atoms or molecules of each reactant and product take part in the reaction.

16. Now let’s finish balancing the chemical equation for water… • Balance the number of oxygen in this chemical equation by writing the coefficient 2 for water. • Now the oxygen is balanced, but there are 2 hydrogen in the reactants and 4 in the product. • How can this be balanced? • Double the number of hydrogen on the left side (reactant) by changing the coefficient for hydrogen to 2. • Now there are 4 hydrogen atoms and 2 oxygen atoms on each side making the equation balanced. This is what we start with: • H2 + O2 H2O • Then we add 2 in front of water: • H2 + O2 2H2O Then we add 2 in front of Hydrogen: • 2H2 + O2 2H2O 4H and 2O 4H and 2O

17. Let’s Practice • Mg + O2 MgO • How many of each atom do we have? • 1Mg 2O 1Mg 1O • How do you balance this? • 2Mg + O22MgO • Now balance these: • C + Cl2 CCl4 = • C + 2Cl2 CCl4 • Al2O3 Al + O2 = • 2Al2O34Al + 3O2