Chem 1a Flashcards
Relative mass of electron
1/1836
Isotope
Have the same chemical properties as their proton and electron no. are the same. Chemical properties depend only on electron structure.
Mass Spectrometer
Instrument used to measure the relative mass of isotopes.
(TF) Stage One: Ionisation
Electron Impact (Electron Gun)
· High energy electrons from an electron gun
· Fired at the sample and knock off an electron
· X(g) → X+(g) + e-
(TOF) Stage Two: Acceleration
· Cations are accelerated by electric field to a constant kinetic energy
· Kinetic energy, KE = 1/2mv2, hence v = √(2KE/m)
· Rate at which ions are accelerated depends on mass
· Ions with a lower mass will travel faster
· Ions with a higher mass will have a lower velocity
(TOF) Stage Three: Ion Drift/Flight Tube
· Ions allowed to drift in a field-free region
· Time taken for ions to reach detector at known distance is measured
· Time depends on mass of ion. Lighter ions reach higher velocities, so low mass ions reach the detector first, high mass ions arrive last
· Mass of ion can be determined from time taken for ion to reach detector
(TOF) Stage Four: Ion Detection
· Cations that reach detector pick up electrons, causing current to flow in ion-current detector
· Greater abundance of ion = bigger current produced
· Detector linked to an amplifier and a recorder, which converts the current into a peak, shown in a mass spectrum
· Height of peak is proportional to size of current generated, so is proportional to abundance of ion
(TOF) Stage Five: Data Analysis
Flight times are analysed and recorded as a plot of abundance v mass/charge (m/z)
Mass spectra graphs show
- m/z for each ion on the x-axis
- relative abundance for each ion on the y-axis
Mass spectra can be used to
- identify unknown elements
- calculate Ar values for known elements
- calculate the Mr values for molecular substances
Mass number
Total number of protons and neutrons in nucleus
Orbital
Sub-units of equal energy which make up sub-levels
Rule of how elements fill up their shells
- Electrons fill lowest energy levels first.
- Each orbital has maximum 2 electrons.
- 2 electrons in same orbital have opposite spin.
- Orbitals of same energy fill singly before pairing up.
First ionization energy
Energy required to remove 1 mole of e- from 1 mole of gaseous atoms as shown by the following equation,
X(g) –> X+(g) + e–
Factors affecting how strongly the outer electron is held in an atom.
· Nuclear charge
· Distance between nucleus and electron i.e. the radius of the particle.
· Shielding - the more full shells there are, the greater they interfere with attraction.
General trend for period 3
- Ionisation Energy increases across period
- Number of protons increase
- Shielding is constant / atomic radius decreases
- More energy is required to remove electron
Exceptions to general trend for period 3, group 3
Al (group 3)
• IE decreases
• Electron is removed from higher energy p sub-level
• Less energy required to remove electron
Exceptions to general trend for period 3, group 6
S (group 6)
• IE decreases
• Pair of electrons in p-orbital
• Extra repulsion = less energy required to remove electron
Ar
Ar = average mass of one atom of an element x 12/mass of one atom of carbon–12
Mr
Mr = average mass of one ‘particle’ of a substance x 12/mass of one atom of carbon–12
Mole
Amount of a substance in g, which contains the same no. of particles as there are atoms in 12g of carbon–12.
Assumptions of gases for ideal gas equation
· Identical particles in continuous random motion.
· Particles have no appreciable volume themselves.
· Collisions between particles are perfectly elastic i.e. there is no loss of (kinetic) energy on collision.
· No forces between the particles.
Empirical formula
EF represents the simplest ratio of atoms of each element in a compound.
Molecular formula
MF represents the actual number of atoms of each element in one molecule of a substance.
Solute
Solid being dissolved
Solvent
Liquid doing the dissolving
Titration
An acid or alkaline solution is accurately added to a fixed volume of the other until neutralization just occurs. This gives the volumes of both the reacting solutions..
Electron impact ionisation
Sample is bombarded with high energy electrons which repel electrons present on the sample, causing it to lose one and gain a positive charge.
Electrospray ionisation
High voltage sample is applied to sample in polar solvent, causing the sample molecule to gain a proton forming MH+
Why must molecules be ionized when measuring their mass in a TOF mass spec?
Ions will interact and be accelerated by an electric field, and will generate a current upon hitting detector.
Apart from the relative mass of each isotope, what else would need to be known for the Ar of an element to be calculated?
Relative abundance/peak height of each element
During TOF, state what is adjusted so that each of the isotopes of an element can be detected in turn.
Electric field
During TOF, how does adjusting the electric field enable the isotopes of an element to be separated?
Deflection depends on mass or m/z
How is the Mr of a covalent compound obtained from its mass spectrum?
Highest m/z value
Explain why chromium is placed in the d block in the Periodic Table.
Highest energy electron is in 3d sub-shell
What is used in a mass spectrometer to deflect the positive ions?
Magnetic field
How is an ionic compound formed?
Metal atom reacts w/ non-metal atom, resulting in ion formation. Oppositely charged ions R attracted to each other by electrostatic forces; forming an ionic bond.
What affects the strength of an ionic bond?
- Charge of ion
- Radius of ion
As radius of an ion decreases, how does this affect the strength of an ionic bond?
Causes it to become stronger.
As the charge of an ion increases, how does it affect the strength of an ionic bond?
Cause it to become stronger.
Explain why Calcium Fluoride has a greater melting point than Calcium Chloride.
F- is a smaller ion than Cl-, so has a greater charge density, causing it to have a greater attraction to Ca+ which requires more energy to overcome, causing it to have a higher m.p.
Why are ionic compounds typically soluble in water?
Water molecules have a slight negative charge due to the presence of lone pairs on the oxygen. Cations are attracted to them and form co-ordinate bonds. This attraction is powerful enough to overcome the ionic attractions.
Why are ionic compounds typically insoluble in organic solvents?
Organic substances aren’t charged/polar, or have a very small charge which is not powerful enough to overcome the attractions in ionic compounds.
Why can ionic compounds only conduct electricity when molten?
The electrons are only free to move whilst molten.
Co-ordinate bond
When both electrons in the shared pair come from one of the atoms in a bond.
Why are simple molecular substances typically gaseous at room temperature?
They have weak intermolecular forces between molecules so have low melting and boiling points.
Why are simple molecular gases insoluble in water?
They don’t have a strong enough attraction with water to overcome the hydrogen bonds present.
Why are simple molecular gases soluble in organic solvents?
Both are molecular, and are attracted to each other by van der waals forces.
Why are simple molecular substances non-conductive?
There isn’t sufficient contact between the molecules to allow electrons to move through the whole solid or liquid, even when delocalised.
Metallic bonds
Cations arranged in a regular giant lattice and surrounded by a ‘sea’ of delocalised electrons. Are difficult to overcome, so high mp and bp.
Why do group 1 metals have comparatively lower mp and bp to other metals?
- Inefficiently packed, so can’t form many bonds.
- Have relatively large atoms and large distance between nuclei and electrons.
- Only have one electron in outer energy level.
Why are metals able to conduct electricity?
Delocalised electrons are free to move throughout the structure as a solid and liquid. They can cross grain boundaries.
Why are metals malleable and ductile?
Atoms are able to roll over each other into new positions without breaking the metallic bond. Causes metal to be elastic.
Why are metals so strong?
Have grain boundaries.
Malleable
Can be beaten into sheets
Ductile
Can be stretched into a wire
Electronegativity
Power of an atom to attract electrons in a covalent bond.
Electronegativity increases…
- Across a Period.
- Up a Group.
What causes a molecule to be non-polar?
When all atoms in the molecule have an equal charge density, causing the overall charge to cancel out.
What cause a molecule to have ionic character?
When there is an uneven distribution of electron density between a shared pair of electrons, due to differences in electronegativity between atoms.
Polarising ability
Ability to attract electron density
Polarisability
Has electron density taken away and be polarised
Dipole-dipole forces
Forces resulting in differences in polarity between molecules. Attraction due to opposing charges.
Activation energy
Minimum energy needed on successful collision for particles to react.
How does the peak on a maxwell-boltzmann distribution change when temperature increase?
Curve shifts to the right and lowers as there are more particles with more kinetic energy.
Reaction rate
Change in conc. of product/reactant divided by time
Factors for rate of reaction
Concentration or pressure.
Surface area.
Temperature
Catalysis
Why does an increase in concentration/surface area cause an increase in reaction rate?
Successful collisions are more likely to occur.
Why does an increase in temperature cause an increase in reaction rate?
Means particles are more likely to react upon successful collision as they possess a greater kinetic energy.
Catalyst
Substance which alters chemical reaction but remains chemically unchanged at the end.
How does a catalyst work?
Provide an alternative reaction route with a lower Ea.
Order of a reagent in a reaction
The power to which the concentration term is raised in the rate equation.
Overall order of a particular reaction
The sum of the powers of the concentration terms in the rate equation.
