Praxis II Chemistry prep

Karen M. Morris

Univ. of Notre Dame

Morris.3@nd.edu

631-6945

The Atom

C6

Carbon12.011

C12

6

C13

6

C14

6

What we observe vs. What we represent

Let’s Practice

1. Explain why atomic weights of elements often differ markedly from whole numbers.

2. Determine the number of protons, electrons and neutrons in each:

U23592

2210 Ni59

28Ne

3. In what way do the isotopes of a given element differ?

1. Explain why one of the following is not a property of an element:a. boiling pointb. densityc. volume

The Periodic TableWhat we observe vs. What we represent

Ds Rg Cn Uut Uuq Uup Uuh Uuo115 116 117 118

Let’s Practice1. Compare the atomic size of row 4 elements to row 5

elements. Describe the general trend as one goes from leftto right across the periodic table.

1. Arrange the following ions in order of increasingsize: Mg2+, Na+, Al3+

1. In the following pairs of ions, identify which ismost likely to exist and explain why you think so.Ca2+ or Ca2- S2+ or S2-

4. Why is the radius of a positive ion smaller thanthe radius of its parent atom?

Chemical Formulae and Equations

Forming Chemical Compounds

Ionic

Covalent

Writing Chemical Equations

Law of Conservation of Matter

What we observe vs. What we represent

Let’s Practice1. The chemical formula of the compound dopamine isC8H11O2N

Would you expect this to be an ionic or covalent compound?Explain…

1. Identify the pure compound as ionic or covalent:HCl CuBr2 F2 Na3PO4

CCl4 AgCl

Let’s Practice1. Balance the following chemical equations

a. H2 + Cl2 HCl

b. Na + H2O NaOH + H2

c. Zn + AgCl ZnCl2 + Ag

d. KClO3 KCl + O2

Observations of Matter1. Representations can be atoms or large quantities of atoms2. Experimentally discovered that all large quantities

represented by one number (discovered by Avogadro) and measured as 6.022 x 1023 atoms

3. This is DEFINED as “1 mole” of matter 4. You measure this mass quantity in grams per mole.

1 mole of C contains 6.022 X 1023 atoms and masses 12.011 g1 mole of H contains 6.022 X 1023 atoms and masses 1.008 g1 mole of U contains 6.022 X 1023 atoms and masses 238.029 g

Compounds

1 mole of carbon dioxide, CO2, contains 6.022 x 1023 molecules of CO2

and masses the sum of 1 Carbon plus 2 Oxygen: (12.011 g) + 2(15.99 g) = 43.99 g

1 mole of calcium carbonate, CaCO3, contains _________ molecules of CaCO3

and masses the sum of __________

A.K.A. as a “molar mass” (mass of 1 mole)

Compounds

We can manipulate the molar mass of CO2 to determine the mass percent of each element in the sample.

This idea is the basis of quantitative analysis of materials.

Molar Mass CO2 = (12.011 g) + 2(15.99 g) = 43.99 g

Mass % of C in the sample = [(12.011)/(43.99)] * 100 = 27.3%Mass % of O in the sample = [2(15.99)/(43.99)]* 100 = 72.7%

or 100% - 27.3% = 72.7% (the easy way ;-)

Let’s Practice

H2O H2 + O2

1. Something is “wrong” with this equation. Fix it.2. After fixing it, determine the mass of each compound.3. How many moles of hydrogen and oxygen do we get

if we break down 3 moles of water?4. How many grams of hydrogen to we get if we break

down 3 moles of water?5. What is the mass percent of hydrogen in water?

Let’s Practice

What is the mass percent of carbon, C, in dopamine?

The chemical formula of the compound dopamine isC8H11O2N

What is the mass of 0.15 moles of dopamine?

What is the molar mass of dopamine?

How many moles of C are in 0.15 moles of dopamine?

How many grams of C are in 0.15 moles of dopamine?

Let’s Practicea) H2 + Cl2 2 HCl

a) Na + 2 H2O 2 NaOH + H2

a) Zn + 2 AgCl ZnCl2 + 2 Ag

a) 2 KClO3 2 KCl + 3 O2

Physical Properties of MatterWhat we OBSERVE vs. what we REPRESENT1. Ice cube melting2. Melted puddle of water evaporating

Phases of matter1. Solid2. Liquid3. Gas

Let’s Practice

1. Draw representations of solid, liquid and gas at the atomiclevel. How are your drawings different? How the same?

1. What happens to a gas volume when it is compressed? What happens to a liquid volume when it is compressed?What happens to a solid volume when it is compressed?

1. What happens to a gas volume when it is heated?What happens to a liquid volume when it is heated?What happens to a solid volume when it is heated?

Chemical ChangesWhat we OBSERVE vs. what we REPRESENT1. Observed:

a. Sometimes NOTHING!! Need instrumentationto measureb. Sometimes solid is formed, bubbles (gas), heat,lightc. Reactant materials will not easily return.

2. Represented:a. Physical rearrangement of atomsb. Balance in number of atoms in reactantsand products.

Let’s Practicea) H2 (g) + Cl2 (g) 2 HCl (g)

a) Na (s) + 2 H2O (l) 2 NaOH (aq) + H2

(g)

a) Zn (s) + 2 AgCl (aq) ZnCl2 (aq) + 2 Ag (s)

a) 2 KClO3 (s) 2 KCl (s) + 3 O2 (g)

General Types of ReactionsReaction Types:

What we OBSERVE vs. what we REPRESENT

Precipitation:MgCl2 + 2 NaOH Mg(OH)2 + 2 NaCl2 clear and colorless solutions poured together and a white solid appears

Acid-Base Neutralization:HCl + NaOH H2 O + NaCla very acidic and a very basic solution poured together

to form a clear and colorless neutral solution(salt + water)