Gen Chem: Labaratory Techniques

 

 

How is accuracy defined in the lab?

 

Give an example.

 

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.

 

 

Define precision in the lab.

 

Give an example

 

 

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.

 

 

Is it possible to be accurate without being precise or vice versa?

 

Give an example.

 

 

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.

 

 

How is accuracy and precision determined?

 

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

 

What needs to be considered when using laboratory glassware?

 

 

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.

 

 

 

 

Why is it important to check plasticware or rubber before using it in an experiment?

 

 

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

 

 

What are some non-calibrated glassware?

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

 

 

What are some calibrated glassware?

  • graduated cylinder
  • buret
  • graduated pipet
  • volumetric flask

 

What does pH meter determine and how?

 

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.

 

How is weight determined in the lab?

 

What are the two kinds?

 

 

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.

 

 

 

What was the force that The Bohr model assumed that allowed the negative charged electron to travel around the positively charged proton?

 

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

 

What is the Equation for the angular momentum in Bohr’s Model?

 

 

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.

 

How is the energy of an electron found in Bohr’s Model?

 

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.

 

 

 

 

What is the energy of the electron related to?

 

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

 

 

What level is the electron at it’s lower energy state?

 

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

ground state level.

 

When do electrons emit energy in the form of photons?

 

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

 

 

How is electromagnetic energy of the photons determined?

 

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

 

True or False.

 

The different electrons in an atom will be excited to the same energy levels.

 

False

 

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

 

What is a Line Spectrum?

 

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

 

What is the atomic emission spectrum?

 

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.

 

What is the final equation that was determined when the spectrum of hydrogen was calculated?

 

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.

 

True or False

 

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

 

False.

 

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

 

 

True or False

 

The excitation of electrons in a particular element results in absorption at specific wavelengths

 

True

 

The excitation of electrons in a particulat element results in

absorption at specific wavelengths

 

True or false

 

The wavelengths of absorption corresponds directly to the wavelengths of emission since the energy of the difference between the levels remains unchanged.

 

True

 

What does the Bohr model only explain?

 

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.

 

 

 

What is the major difference between Bohr’s Model & Modern quantum mechanical model?

 

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.

 

What is Heisenberg uncertainty principle?

It is impossible to simultaneously determine, with perfect accuracy, the momentum (defined as mass times velocity) and the position of an electron.

 

 

What are the four quantum numbers in the modern atomic theory?

 

n= principal quantum number

l = Azimuthal Quantum Number (angular momentum)

ml= magnetic quantum number

ms= Spin quantum number

 

What does Pauli Exclusion Principle state?

 

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

 

 

What is the energy state in Modern atomic theory?

 

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

 

1) What is the principal quantum number?

 

2) What is the maximum number n and how is it found?

3) what is the Maximum number of electrons in an electron shell?

 

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?

 

 

 

What is the relationship between the energy levels of adjacent shells as the nucleus increases?

 

Give an example.

 

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) What is Azimuthal Quantum Number (l)?

2) What can the range be?

3) What are the 4 subshells?

4) What is the maximum number of electrons that can exists within a subshell?

5) What is the relationship with l and energy of a subshell?

 

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) What is the Magnetic quantum number, ml?

2) What are the possible values of ml?

3) True of False: The shape and energy of each orbital are dependent upon the subshell in which the orbital is found.

 

 

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) What is the spin quantum number, ms?

2) what are the orientations of the spin designated as?

3) What type of spin does electrons in the same orbital have and why?

4) What are parallel spins and what are paired spins?

 

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) When writing the electron configuration of an atom, how are the subshells filled?

 

2) what formula is used to determine to rank subshells by increasing energy?

 

3) What happens if the two subshells possess the same 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.

 

 

What must you determine to know which subshells are filled?

 

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

 

 

What is the Hund’s rule?

 

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

 

 

What are valence electrons?

 

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

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