Lecture: Chemical Change and Chemical Equations

Mole song: https://www.youtube.com/watch?v=PvT51M0ek5c&spfreload=10

1. Chemical reactions break bonds and form new bonds between atoms: ie fire, light, smell

2. Chemical rxns always produce a new substance w/ new

characteristic properties ?

3. In a closed system (?),

chemical rxns produce a change in energy, given off or taken in, but

energy cannot be created or destroyed: why?

Law of Conservation of Energy ? and

Law of Conservation of Mass ?

Energy-Absorbing Reaction Energy-Releasing Reaction

Products

ProductsReactants

Reactants

Go to Section:

A. energy released is called exothermic: ie: hot packs, flames, your metabolism (37 C)

B. energy absorbed is called endothermic: ie ice packs

C. the energy required to start the reaction is called the

activation energy: ie scratching a match for a flame

Activation energy

Activation energy

Graph: note relationship of start/finish of line relative to the y-axis: energy:

4. Chemical equations A. Law of Conservation of Mass: in a closed system, all elements (and their mass) must be present,and their # of atoms, be equal before and after the rxn:(must be balanced)

-Counting Atoms:

NO3: (coefficient of 1)N = 1, O = 3

2NO3: (coefficient of 2)N = 2, O = 6

(NO3)3: (coefficient of ?)N = 3, O = 9

2(NO3)4: (coefficient of ?)N = 8, O = 24

-formula mass: sum of all the atoms’ mass in a chemical formula in units of amu (?):

formula mass for Na is = 22.99 amu (from periodic table)

formula mass for H2O is:

one molecule: H2O H = 2 x 1amu = 2amu O = 1 x 16amu = 16amu

formula mass = 18amu

two molecules: 2H2O H = 4 x 1amu = 4amu O = 2 x 16amu = 32amu

formula mass = 36amu

three molecules: (H2O)3 H = 6 x 1amu = 6amu O = 3 x 16amu = 48amu

formula mass = 54amu

coefficient

-Avogadro’s number is 6.02 x 10 23

it’s used in chemistry to show a

relationship between amu and grams

- mole: a substance with 6.02 x 1023 atoms (if an element) or molecules (if a compound)

so, a mole is a #,

like a dozen is a #:

what # does dozen mean?

what # does mole mean?

which is a stronger hydrochloric acid (HCl) and why:

2 molar HCl or 5 molar HCl ?

-molar mass:

- mass of all the particles of a substance (ie: Na or water) which has

one mole of particles (6.02 x 1023):

- measured unit is grams

- just change the amu unit to gram unit:Na: 22.99 amu =

22.99 grams in one mole

H2O: 18 amu =

18 grams in one mole

The water has the same # of particles as Na (mole),

and less mass

but more elements, why?

- calculating number of particles (atoms or molecules):

-use this formula:

Molar mass = sample mass

Avogadro’s # # particles

Example: How many particles (atoms) are of Na if:

- molar mass of Na = 22.99 g (22.99 amu)

- sample size of Na = 12.9g

22.99 g = 12.9 g

6.02 x 1023 ? # particles

? # particles x 22.99 g = 6.02 x1023 x 12.9 g

? # particles = 6.02 x1023 x 12.9 g

22.99 g

? # particles = 3.39 x 1023

Example:

Solve for mass of the H2O sample if the:

- Molar mass = 18 g (H2 = 2 amu, O = 16 amu)

- # particles = 296 x 1050

18 g = ? mass.

6.02 x 10 23 296 x 10 50

18 g x 296 x 10 50 = ? mass

6.02 x 10 23 1

18g x 296 x 10 50 – 23

6.02

? mass = 8.9 x 10 29 grams (sigfig?)

Balance equations intro video (6’ 44” Khan academy)

http://www.youtube.com/watch?v=RnGu3xO2h74

B. a chemical equation is written w/

reactants left and products right:

reactant + reactant product

H + H + O H2O

C. an arrow indicates a chemical rxn occurred:

a new substance is formed (product)

D. rules for balancing equations: due to L.C.M.

-subscripts tell the # of atoms,

-coefficients tell # “molecules”

-Subscripts do NOT change,

-only coefficients change!!!!!!!!!

-use only whole #’s (no fractions)

-do not split up compounds

-use the least # of atoms (coefficients: LCD)

-start w/ product side, then the simplest which is unbalanced

-use a pencil!!!!!!!

So, balance these equations using the rules:

2Mg + O2 MgO2

2Mg + 2O2 2MgO2

Br2 + KI KBr2 + I2

2Br2 + 2KI 2KBr2 + I2

Zn + HCl ZnCl2 + H

Zn + 2HCl ZnCl2 + 2H

Fe + Cl2 FeCl32Fe +3 Cl2 2FeCl3

HCl + CaCO3 CaCl2 + CO2 + H2O

2HCl + CaCO3 CaCl2 + CO2 + H2O

H2 + N2 NH3

3H2 + N2 2NH3

Cl2 + KBr Br2 + KCl

Cl2 + 2KBr Br2 + 2KCl

CH4 + O2 CO2 + H2O

CH4 + 2O2 CO2 + 2H2O

Balance equations practice video:

http://www.physics-chemistry-interactive-flash-animation.com/chemistry_interactive/basic_chemical_reaction_balance_learning.htm

5. Chemical rxn rate/speed is due to # of atomic collisions, called:

collision theory (like cars colliding).

Based on these four factors:

A. concentration: increase particles per volume: more collisions

B. surface area: increase opportunity w/ greater surface: more collisions

C. temperature: increase energy thus activity: more collisions

D. catalysts: lowers activation energy, less energy needed to start rxns: more collisions

6. Classifying chemical reactions, four categories:

A. synthesis: to put together, form bonds:

two or more substances combine to form one new substance:

A + B AB :

C + O2 CO2

2Na + 2Cl2 2NaCl2 B. decomposition: to break down, break bonds:

complex substance breaks down into two simpler substances (opposite of synthesis):

AB A + B:

H2O H2 + O (Hydrolysis: splitting water)

H2CO3 H2O + CO2

C. single displacement: an uncombined element replaces an element which is part of a compound:

A + BC AC + B:

Zn + 2HCl ZnCl2 + H2

2Na + 2H2O 2NaOH + H2

D. double displacement: different elements in two different compounds replace each other:

AC + BD AD + BC:

AgNO3 + KCl KNO3 + AgCl

MgCO3 + 2HCl MgCl2 + H2CO3

E. combustion:

also known as burning:

when oxygen combines w/ a substance (often a carbon compound) and

one of the products is a release of energy:

-light (light bulb?)

-heat (light bulb ?)

-sound (break sound barrier ?)