CEM1000W Notes
Semester 1
IT
University of Cape Town
,Introduction
11/02
Chemistry is the study of matter and it's properties; changes that matter undergoes and the energy associated
with those changes.
States of Matter
◦ Solids - fixed shape, with particles in organised 3D array, lying next to each other.
◦ Liquids - varied shape (conforms to container, to the extent of the volume of the liquid), with close particles
moving around each other. Has an upper surface.
◦ Gases - varied shape (fills the container), with far apart particles that move randomly. Has no surface.
Properties & Changes of Matter
the characteristics that define unique identities of substances
Physical properties and change: Chemical properties and change:
◦ No change in composition, substance alters ◦ Change in composition - conversion into
its form only a whole new substance
◦ Doesn't require a reaction to show ◦ Shown as in interacts with/changes into
◦ Properties eg. Melting point, electrical, colour, other substances
I
conductivity, density ◦ Properties eg. Flammability,
◦ Change eg. Ice melting (only the form corrosiveness, reactivity with acids
changes, the substance is still water) ◦ Change eg. Decomposition of water with
electricity (the final sample is no longer
water)
Extensive properties are properties dependent on the amount of substance, eg. Mass and Volume
Intensive properties are ones independent on the amount of substance, eg. Density and Temperature
The Scientific Approach
understanding nature through objective, testable discoveries; explains & predicts phenomena Observable laws,
determined when
1. Observations: trends repeat with
‣ The facts that we want to explain no exceptions
‣ Quantitative observations (data) are most useful - these can reveal 'natural laws'
2. Hypothesis
‣ Tentative proposal intended to explain observations
‣ Revised after initial experiments, must be testable
3. Experiment
‣ Procedural steps to test the hypothesis
‣ Controlled - tests a single variable at a time
‣ Results must be reproducible
4. Model
‣ Theory based on experimental results
‣ Conceptual model is refined by ongoing experimentation
Substance: Matter with fixed composition
Atom: The smallest piece of matter that retains its identity during a chemical reaction.
Elements: A substance made up of one type of atom, that cannot be broken down by chemical means.
Molecule: A group of two or more atoms chemically bonded together.
Compound: A group of two or more different elements chemically bonded together.
‣ Many consist of molecules
‣ Can be broken down into simpler substances
‣ Elements have fixed ratios by mass
Mixture: A group of two or more different substances that are physically combined.
◦ Mixtures are not substances, ∴ the components can vary in their parts by mass
◦ Can be separated into components physically, not chemically
◦ Can be homogeneous (same phase), heterogeneous (different phase), or solid alloys (2 metals).
,Measurements
11/02
Density (g.cm-3) = mass(g)/volume (cm-3)
Kelvin = 0 degrees + 273.15 K
Uncertainty
• The rightmost digit is assumed uncertain
• Remaining digits are known and must be carried through the problem accurately
Significant Figures
1. All non-zero numbers are significant.
2. Zeros between 2 non-zero digits are significant.
3. Leading zeros are NOT significant - they are place holders.
4. If there is a decimal place, trailing zeros are significant. If there is no decimal place, they are
not.
5. Convert big numbers to SN first; then apply rules to the digits before the 10 only.
6. These rules do not apply to exact numbers (that weren't measured).
In calculations with
◦ Only multiplication and divide: use the no. of SF as the given measurements with the least
no. of SF.
◦ Only addition and subtractions: use the no. of decimal places as the measurement with the
least no. of decimal places.
◦ Both: use the no. of SF as the given measurements with the least no. of SF.
Rounding Off
1. Removed digit > 5, preceding increases by 1
2. Removed digit < 5, preceding is unchanged
3. Removed digit = 5, preceding is increased by 1 (if odd) or unchanged (if even).
4. Do not round off before final answer.
Precision: the proximity of several measurements to each other
Accuracy: the proximity of a measurement to the true value
Systemic error: produces values all higher or all lower than the actual value
Random error: produces some values higher and some lower than the actual value
, Atoms and Molecules
13/02
Chemists work in 3 different worlds
◦ Macroscopic - visible to the naked eye
◦ Atomic - atoms & molecules, visualised and thought of
◦ Symbolic - representing materials by symbols
Atomic mass number (p+n)
X
The Atom
AI
Contribute to atomic mass
• Nucleus that holds the protons & neutrons
• Cloud of electrons Determine the identity of element
Isotopes: Atoms of an element with the same atomic number but a different l Atomic number
(p)
atomic mass
Atomic mass: The average mass of the isotopes of the element
Most elements occur in chemical combinations with other elements, but a few are free:
◦ Diatomic molecules - O2, N2, S2 - are diatomic in their most elemental form
◦ Noble gases occur in air as independent atoms
◦ Carbon occurs in vast, nearly pure coal deposits
◦ Some metals (Cu, Ag, Au, Pt)
Ionic Compounds
Between one metal and one non-metal.
◦ Cations (positive, formed by metals)
◦ Anions (negative, formed by non-metals)
Ionic bonding: Transferring electrons from one element to another to form an ionic compound
Naming Ionic Compounds
◦ Cation + Anion (-ide)
◦ Eg. Sodium Chloride
Covalent Compounds
Usually between two non-metals.
Covalent bonding: The sharing of electrons between atoms to form a covalent compound.
Note: Some metals have several oxidation states, so the correct one will be given to you in Roman numerals
Learn:
Monoatomic Ions
Polyatomic Ions
