Chemical Reactions & Stoichiometry

1. Chemical Reactions & Stoichiometry Mr. Shields Regents Chemistry U11 L01

2. Stoichiometry Before we begin … What exactly does Stoichiometry mean? Stoichiometry (definition) The study of quantitative relationships Between the amounts of reactants used & Products formed in a chemical reaction. For example: CH4 + 2O2 CO2 + 2H20 i.e 1 part + 2 parts = 1 part + 2 parts

3. Now we need to discuss what a chemical reaction is. (We’ll Get into the number stuff later) Chemical Reactions occur all around us every day. Can you think of some? • Rusting metal • Burning wood in your fireplace • Using electricity from batteries • Running your car engine • Thinking / feeling • The browning of paper in sunlight • Cooking • Leaves changing color • Oxygenation of Blood • Food digestion & Growing And these are only a few of many possible examples

4. Chemical equations Let’s consider the rusting of iron. We can state what happens in words. How would we describe this process? Iron reacts with Oxygen to form Rust. How would we describe the burning of Gas in your cars engine? Gasoline reacts with oxygen to form carbon dioxide Water, and heat

5. What is the common thread in these reactions ? One or more substances are being converted into other substances When we write chemical reactions … FOR EXAMPLE: Fe + O2 Fe203 (rust) Those things listed on the left side of the chemical equation are known as … - REACTANTS Substances on the right side of the equation are… - PRODUCTS

6. Reactants are substances we start with and Products are substances we end up with & which have different physical and chemical properties from the reactants For example “Gasoline reacts with oxygen to form carbon dioxide, Water, and heat”? What are the reactants and what are the products? Note that the properties of these substances before reaction are different than the properties of the substances after reaction

7. How do we separate reactants and products in chemical reactions ? Yep … By using an arrow (  ) This arrow is more important than just separating Reactants from Products. What else does it tell us? It tells us the DIRECTION of the reaction path going this way guys  Or … even this way 

8. In our example … “Iron reacts with Oxygen to form rust” How do we write this, not in words, but as a Chemical equation? Exactly … Fe + O2 Fe2O3 What are the reactants And what are the products The arrow is read as “yields” Look at this equation closely. What’s wrong with it?

9. Conservation of Mass In our example “Fe + O2 Fe2O3” we have 1 atom Of iron yielding 2 atoms of iron and 2 atoms of Oxygen yielding 3 atoms of oxygen. What law does this equation violate and what does The law state? The law of conservation of mass: “Matter can neither be created nor destroyed in ordinary chemical reactions”

10. Conservation of Mass Since our example of a chemical reaction Fe + O2 Fe2O3 violates the law of conservation of mass the Equation is said to be “NOT BALANCED” The equation as written tells us WHAT WE NEED or have produced but NOT HOW MUCH we need of each reactant or how much of each Product is produced THIS IS KNOWN AS A “SKELETON EQUATION”

11. Let’s look at this equation a little more closely. Fe + O2 Fe2O3 • But instead of dealing with atoms and molecules • Let’s discuss this equation in terms of MOLES • Do you remember what a mole is ?? • 1 mole of anything = 6.02 x 1023 Mole In terms of MOLES the above equation says: 1 mole of Fe reacts with 1 mole of oxygen to yield 1 mole of Iron (III) oxide.

12. 1 mole Fe + 1 mole O2 1 mole Fe2O3 But we know this can’t be! Why? For one, 1 mole of iron can’t create 2 moles of iron Why are there 2 moles of iron ?? Now Let’s look at this equation from a mass point of view: 1 mole of Fe = 55.85 g 1 mole of O2 = 31.98 g 1 mole of Fe2O3 = 159.64 g Fe + O2  Fe2O3, 55.85g + 31.98g = 159.64g ??? No way!

13. #R atom #P • Fe 2 • O 3 The mass of the Reactants DOES NOT Equal the mass of the Products! Fe + O2 Fe2O3, 55.85g + 31.98g = 159.64g (87.83g) So whether we look at moles or mass this chemical Eqn. doesn’t obey the Law of Conservation of Mass! To do so the # of atoms on the reactant side MUST EQUAL the number of atoms on the product side To satisfy the Law of Conservation Of mass we will need to BALANCE This equation. We need to make the #R atoms & #P Atoms equal

14. Note… Coefficients in balanced chemical equations must always be WHOLE NUMBERS! 4Fe + 3O2 2Fe2O3 And Not fractions 2Fe + 1 ½ O2 Fe2O3 But we can use this as an intermediate step to obtain the above! How? In our next lesson we’ll see how we go about balancing chemical equations!