Gen Chem: Labaratory Techniques – Flashcards

Accuracy refers to a measurement giving the actual value for the sample with a limited deviation.

Ex. Determining the weight of a sample to be 1 gram plus or minus 0.001 gram is an accurate measurement.

Precision is the repeatability of a set of measurements on the same sample.

Ex. MEasuring the same 1 gram sample three times ina row and finding values of 1.001 g, 1.000 g and 1.001 g is a precise series of measurements if this is within the limits of experimental error that the scientists wants to achieve.

Yes, it's possible to be accurate without being precise.

Ex. Three measurements of the 1 gram sample as

.9 g, 1.0g, and 1.1g give an accurate value of 1 g for the sample, but are much less precise than the three samples determined to be 1.001g, 1.000g and 1.001g. Three determinations of the weight of a 1g sample as

1.001g, 1.1001g, and 1.100g may be precise but not accurate.

Accuracy and precision are determined by a combination of the correct laboratory apparatus and using the apparatus properly.

A wide variety of vessels can be used, and it does not necessarily mean that it needs to be made out of glass.

Solvent compatibility with plastic and rubber labware needs to be considered.

You want to make sure that no organic solvents will leach from the plastic into the organic solvents contaminating the experiment.

• test tubes
• beakers
• erlenmeyer flasks
• round bottom flasks
• transfer pipets
• petri dishes
• watch glasses
• bottles
• jars
• vials

• graduated cylinder
• buret
• graduated pipet
• volumetric flask

The pH meter determines the acidity or basicity of a solution by comparing the voltage (electrical potential) produced by the solution compated to the voltage of a known standard solution and uses the difference in voltage (the potential difference) between them to calculate the pH.

There are two categories: standard laboratory balances and analytical balances.

Standard laboratory balances: When weights are in milligram range and require accuracy within +/- 0.5 mg

Analytical balances: used for much more sensitive weighing applications with accuracy to +/- 0.01 mg.

The Centripital force acting on the electron as it revolved around the nucleus was the electrical force between the charges.

L = nh/2pi

h=planck's constant = 6.626 x 10^-34 j s

n= principal quantum number (only a + integer)

Note: The angular moment only changes in discrete amounts with respect to n, because the h, 2 and pi are constant.

E= - RY / n^2

Ry= Rydberg energy = 2.18 x 10 ^-18 J/electron

Note: The energy of the electron changes in discrete amounts with respect to n. There is a negative sign in the above eqn, because there is zero attractive force when the electron and proton are completely separated. Therefore, an electron would have a negative energy as a result of attractive forces.

The energy of an electron is related to its orbital radius.

The electron is at it's lowest energy state at the

ground state level.

When electrons are excited by heat energy (or other) it  rapidly returns to the ground state emitting energy in the form of photos.

E= hc / ?

h= Planck's constant

c= velocity of the light in a vacuum (3.0 x 10^8  m/s)

? = wavelength of the radiation

False

The Different electrons in an atom will be excited to Different energy levels.

Each line on the emission spectrum corresponds to a specific electronic transition.

Each element can have its electrons excited to different distinct energy levels, each elements possesses a unique Atomic Emission Spectrum, which can be used as a fingerprint.

E = H C / ?

= - RY ( 1 / n?i - 1 / n?f)

The energy of the emitted photon corresponds to the precise difference in energy between the high-energy state and the lower-energy final state.

False.

When an electron is excited to a higher energy level, it must absorb energy.

True

The excitation of electrons in a particulat element results in

absorption at specific wavelengths

True

The Bohr Model only offers a the explanation of a hydrogen atom and ions containing only 1 electron, such as He1+ and Li2+

- Does not take into consideration of repulsion of multiple electrons.

Bohr's assumes that electrons follow a circular orbit at a fixed distance from the nucleus, which is no longer valid. Rather electrons are described as being in a state of rapid motion within regions of space around the nucleus, called orbitals.

n= principal quantum number

l = Azimuthal Quantum Number (angular momentum)

ml= magnetic quantum number

ms= Spin quantum number

No two electrons in a given atom can have the same set of 4 quantum numbers.

The energy state is the position and energy of an electron described by the quantum numbers.

1) A quantum number that takes on any positive integer in an atom

2)  The maximum n that can be used to describe the electrons of an element at its ground state corresponds with that element's period (row) in the periodic table

3) The maximum number of electrons in an electron shell n is 2n?

The difference in energy levels of adjacent shells decrease as the distance from the nucleus increases.

ex. The energy difference beween n=3 and n=4 is less than n=2 and n=3

1) Azimuthal Quantum number tells the shape of the orbital and refers to the subshells or sublevels that occur within each principal energy level.

2) The range of l is - to n - 1.

3) l = 0, 1, 2, and 3 are known as the sharp, principal diffiuse and fundamental subshells or s, p, d and f subshells, respectively.

4) The max number of electrons in a subshell is 4l + 2

5) The greater the value l, the greater the energy level of the subshell. Note: However the energies of subshells from different principal energy levels may overlap.

1) It describes the orientation of the orbital space. The magnetic quantum number specifies the particular orbital within a subshell where an electron is highly likely to be found at a given point in time.

2) The possible values of ml are all integers from l to -l including 0.

S subshell (l=0) has only one possible value ml (0) = 1 orbital

p subshell (l=1) has 3 possible ml values (-1, 0, +1) = 3 orbitals

d subshell (l=2) has 5 ml values (-2,-1,0,+1,+2) = 5 orbitals

f (l=3) has 7 ml (-3,-2,-1,-0+1,+2,+3) = 7 orbitals

3) True:

1) The spin of a particle is ins intrinsic angular momentum and is a characteristic of a particle.

2) + 1/2 and -1/2

3) If two electrons are in the same orbital, they must have opposite spins due to Pauli Exclusion Principle.

4) Parallel Spin- electrons in different orbitals (different ml values) with the same ms values.

Paired Spin- Electrons with opposite spins (different ms values) in the same orbital (same ml values).

1) The subshells are filled from lowest to highest energy level according to the Aufbau Principle.

2) n + l

3) The lower n value has a lower energy is the n + l values are the same.

The number of electrons of an atom must be determined to know which subshells are filled.

The Hund's Rule: Applies in subshells that contain more than one oribtal, such as the 2p subshell with its 3 orbitals.

Within a given subshell, orbitals are filled such that there are a maximum number of half-filled orbitals with parallel spins. Electrons prefer empty orbitals to half filled ones because energy must be overcome for two electrons carrying repulsive charges to exist in the same orbital

Valence electrons are found on the outer most shell that are available for chemical bonding.