Chemical Reactions

Ch 6

Essential questions part 1What is a chemical reaction?

How can you tell a reaction has taken place?

What happens with energy in a chemical reaction?

Properties

Physical property – a characteristic of a pure substance

that can be observed without changing it into another substance.

Chemical property – a characteristic of a pure substance

that describes its ability to change into a different substance.

Changes

Physical change – a change in a substance that does not

change its identity.Chemical change – a change in which one or more

substances combine or break apart to form new substances.

How does bonding create a chemical change?

chemical changes occur when bonds break and new bonds form.

Chemical reactions involve changes in properties and changes in energy that you can observe.

Chemical reactions cause:

1.Bonds to break2.atoms to rearrange3.New bonds to form

III. Evidence for chemical reactions

The process where one or

more substances change into one

or more DIFFERENT substances.

clues that reactions occurred A gas is formed – bubbling or fizzing A solid is formed – precipitate (little chunks or

cloudiness forms) Color changes Energy (temp.) changes Odor changes Any change that is a result of a new substance

being formed

Changes in Energy Endothermic – reaction that absorbs

energy into the bonds from the environment, gets colder Example: cooking

Exothermic – reaction that releases energy from the bonds to the environment – gets hotter Ex: fire (fast), explosion (fast), rust

(slow)

Energy in Chemical Reactions

Activation Energy: Energy used to start a reaction

Example: friction used to start a match Using a match to light a candle or

bunsen burner

Compounds

Two or more elements that are chemically combined

Types of Compounds: Review

1. Covalent – formed from 2 or more nonmetals that share electrons

2. Ionic – formed from a metal and a nonmetal that transfer electrons to form charged ions

3. Metallic –formed when two metals share loose electrons between many positive metal ions

Describing Chemical Reactions Essential Questions 6.2

How do you describe a chemical reaction?

How is matter conserved during a reaction?

How to represent:

Element – symbolCompound – formula

Reaction – equation

Chemical Formula -short way to write the name of a compound

1. Tells – what kind of elements make up the compound(Look for symbols)

2. Tells – how many atoms of each element there are

3. Subscript – small number written below and after the symbol to tell how many atoms there are of that element. If there is no subscript, then there is 1 atom

Parts of an equationReactants – a substance that undergoes change in a chemical reaction.

Products – a substance formed as a result of a chemical reaction.

Formulas for covalent compounds When naming a covalent compound, a

prefix is always used on the 2nd element to indicate the number of atoms of the element in the compound.di – two tri – three mono – one

Carbon dioxide Carbon monoxide

Dihydrogen monoxide

D. Formulas for ionic compounds The formulas must have a neutral

charge (total charge of zero). Positive and negative charges must be

equal Magnesuim fluoride MgF2

Mg+2 2 F -1

Describing Chemical Reactions

What are chemical equations? Uses symbols and formulas to show

what happens in a chemical reaction

Ex: NaOH + HCl NaCl + H2O

2. Parts of an Equationa. Reactant

(NaOH and HCl)

b. Product(NaCl and H2O)

Traits of Chemical Reactions

2. Parts of an Equation

c. Arrow Means “produces” or “yields”.It tells you a reaction has taken place.

d. Plus Sign Separates 2 or more elements or compounds in the reactant or product.

e. Coefficient Number in front of the element or formula; it tells how

many molecules there are.2HO

Chemical Equations

6.3 Controlling chemical reactions Essential questions

What is the Law of Conservation of Matter/Mass?

How can you control the rate of a reaction?

V. Law of Conservation of Matter:

Matter or energy cannot be created or destroyed, - it only changes form

All atoms you started with in the equation are there at the end of the reaction, just REARRANGED

How to balance an equation: CH4 + O2 CO2 + H2O 1. List each element and the number of atoms

C = 1 C = 1 H = 4 H = 2 O = 2 O = 3

2. Add a COEFFICIENT to balance the atoms on each side

CH4 +2 O2 CO2 + 2 H2O C = 1 C = 1 H = 4 H = 4 O = 4 O = 4

3. RECHECK the number of atoms on each side of the equation.

VII. Rate of Chemical Reactions How fast or slow a chemical reaction occurs depends on:

1. How hard the molecules bump into each other

2. How many times the molecules bump into each other

Things that affect reaction rate:

VIII. Rates of chemical reactions: 1. Surface area – higher surface area= faster

reaction lower surface area = slower reaction 2. Temperature –higher temperature = faster

reaction lower temp.= slower reaction

Things that affect reaction rates 3. Concentration – higher concentrations =

faster reactions lower concent. = slower reaction 4. Catalyst: a substance that affects the rate of

the reaction without being changed by the reaction Ex. Chlorophyll (speeds up reaction)

5. Inhibitors – a material that decreases the rate of a chemical reaction. (slows down reaction