Chapter 6: Chemical Reactions Section 1: The Nature of Chemical Reactions

Chemical Reactions Change Substances Definition: chemical reaction – a process in which a substance becomes a new substance. There are often signs that a chemical reaction is taking place: Color Change Gas Production (bubbling) Energy Transfer (getting hot or cold, glowing)

Chemical changes rearrange atoms. Chemical Reactions Chemical changes rearrange atoms. The new substance or substances contain the same atoms. For Example: Our cars use gasoline, which has the chemical formula C8H18. Gasoline burns when it reacts with oxygen, O2. When the gas burns…

C8H18 O2 + H2O CO2 + Chemical Reactions Hydrogen Carbon Oxygen Carbon Hydrogen Oxygen

So, in a chemical reaction…things REACT to PRODUCE other things. Reactants & Products So, in a chemical reaction…things REACT to PRODUCE other things. Definition: reactants – the substances in a chemical reaction that are changed. Definition: products - the NEW substances produced in a chemical reaction.

Lets look at the reaction of gasoline and oxygen again… Energy & Reactions Lets look at the reaction of gasoline and oxygen again… It would be far to dangerous to use gasoline as a fuel if it immediately reacted with oxygen, right? What do we need in order to start the reaction? A SPARK!

Energy must be added to some reactants to start the chemical reaction. Energy & Reactions Energy must be added to some reactants to start the chemical reaction. The added energy breaks the bonds that holds the reactants together. Once the bonds are broken, the reactants rearrange into the products.

In a chemical reaction, mass is never gained or lost. Mass is CONSERVED One of the most important things to remember about a chemical reaction is that… In a chemical reaction, mass is never gained or lost.

Endothermic & Exothermic When a chemical reaction releases energy, it is called exothermic. Definition: exothermic reaction – a chem. reaction that releases energy. Exo. reactions make the air warmer.

Endothermic & Exothermic When a chemical reaction absorbs energy, it is endothermic. Definition: endothermic reaction – a chem. reaction that absorbs energy. Endo reactions make the air colder.

Chapter 6: Chemical Reactions Section 2: Reaction Types

We can classify reactions based on what happens during the reaction. Synthesis Reactions We can classify reactions based on what happens during the reaction. There are 4 different types of reactions: Synthesis Decomposition Single Displacement Double Displacement Lets talk about Synthesis first…

Synthesis Reactions When things combine together in a chemical reaction, we call it “synthesis”. Definition: synthesis reaction – a reaction in which 2 or more substances combine to form a new compound.

A synthesis reaction has the equation: Synthesis reactions A synthesis reaction has the equation: A + B  C A & B represent the reactants. C represents the product.

Decomposition Reactions When a chemical reaction breaks substances apart, it is called “decomposition”. Definition: decomposition reaction – a reaction where a compound breaks down into 2 or more substances.

Decomposition Reactions A decomposition reaction has the equation: C  A + B C represents the reactant compound. The compounds breaks into A and B.

In displacement reactions, substances change places. Single Displacement In displacement reactions, substances change places. Definition: single replacement reaction – a reaction where one element takes place of another.

In displacement reactions, substances change places. Single Displacement In displacement reactions, substances change places. Definition: single replacement reaction – a reaction where one element takes place of another. AX + B  BX + A

DoubleDisplacement Definition: Double Displacement reaction – a reaction where 2 elements trade places. AX + BY  AY + BX Notice that X takes the place of Y and Y takes the place of X.

Balancing Chemical Equations A Beginner’s Tutorial

Whenever a chemical reaction happens, one thing must always be true: Chemical Equations Whenever a chemical reaction happens, one thing must always be true: A chemical reaction never creates or destroys atoms. This is the Law of Conservation of Mass.

Lets look at the reaction that makes Water: H2 + O2  H2O Chemical Equations Lets look at the reaction that makes Water: H2 + O2  H2O Something’s Wrong here… If we look at the left side of the reaction, we find that there are… 2 Hydrogen and 2 Oxygen The right side of the reaction has… 2 Hydrogen and 1 Oxygen

The subscripts can’t be changed, though. Chemical Equations This reaction is NOT correct!! It needs to be “balanced”. To balance it, we can add coefficients to the elements & compounds. H2 + O2  H2O The subscripts can’t be changed, though.

Adding the coefficients added more atoms to the reaction. Chemical Equations: Adding Coefficients (2) H2 + (1) O2  (2) H2O Adding the coefficients added more atoms to the reaction. Now on the left side, we have… 4 Hydrogen and 2 Oxygen And on the right side, we have…

Multiply the Coefficient by the Subscript. Chemical Equations (2) H2 + (1) O2  (2) H2O Multiply the Coefficient by the Subscript. (2) x 2 = 4 H (1) x 2 = 2 O (2) x 2 = 4 H (2) x 1 = 1 O