Unit 1: Section 3: Bonding Flashcards
Give the ionic formula for Ammonium
(NH)4+
Give the ionic formula for Carbonate
(CO3)2-
Give the ionic formula for Hydroxide
OH-
Give the ionic formula for Nitrate
(NO3)-
Give the ionic formula for Sulfate
(SO4)2-
What holds positive and negative ions together
Electrostatic attraction
What are ionic crystals (structurally)
Giant lattices of ions
Explain the properties and behaviour of ionic compounds
Can conduct electricity when they are molten or dissolved but not when they’re solid (ions are not free to move in a solidified form)
Have high melting points because they are held together by strong electrostatic forces
Tend to dissolve in water - part of water molecules are negatively charged and some parts are positively charged, the water molecules pull the ions from the lattice and causes it to dissolve
What is a molecule
When two or more atoms bond together
What is a single covalent bond
When two atoms share a pair of electrons
What is a giant covalent structure
Type of crystal structure
Huge network of covalently bonded atoms (macromolecular structure)
Which element forms giant covalent structures and why
Carbon
It can form four strong covalent bonds
Explain the structure of an example of a giant covalent structure
Graphite
C atoms are arranged in sheets of flat hexagons covalently bonded with three bonds each
The fourth outer electron of each carbon atom is delocalised
What are the forces that hold together the bonds in a giant covalent structure
van der Waals forces
Explain the properties of graphite
Weak bonds between layers - sheets can slide over each other making it slippery
Delocalised electron means that it can conduct electricity
Layers are far apart compared to length of the covalent bonds so it has low density
Strong covalent bonds so high melting point
Insoluble - covalent bonds are too strong to break
Explain the structure of diamond
Each C atom is bonded to 4 other C atoms
Atoms arrange themselves in a tetrahedral shape (crystal lattice structure)
Explain the properties of diamond
High melting point - sublimes over 3800K
Hard
Vibrations can travel through the stiff lattice so it’s a good conductor of heat
Can’t conduct electricity
Won’t dissolve in any solvent
What are the unshared electrons in a covalent bond called
Lone pairs
What is a charge cloud
The region of negative charge surrounding an atomic nucleus where you have a big chance of finding an electron
Explain Valence Shell Electron Pair Repulsion Theory
Charge clouds repel each other more in lone pairs then bonding pairs
So bond angles are reduced because bonding pairs are pushed together by lone-pair repulsion
What is a co-ordinate bond
One of the atom provides both of the electrons
What is a dative covalent bond
A co-ordinate bond
Give an example of a dative covalent bond
Ammonium
When do dative bonds form
When one atom has a lone pair of electrons, and the other doesn’t have any electrons available to share
When drawing the shapes of molecules what does the shape of the bonds tell you about the direction
Wedged line (shaded in) - shows a bond is pointing towards you
Broken line - shows a bond is pointing away from you
Line - shows bonds aren’t pointing towards or away from you
How do you find the number of electron pairs
Find the central atom
Look at its group number to find out the number of outer electrons
Add one electron to every atom the central atom is bonded to
Add up all the electrons and divide it by 2 to find the number of electron pairs
Describe the shape of a compound that has a central atom with two electron pairs
Bond angle of 180 degrees
Pairs of bonding electrons want to be as far away from each other as possible
Describe the shape of a compound that has a central atom with three electron pairs
If there are three bonding pairs of electrons the bonding angle will be 120 degrees - called trigonal planar
If there is two bonding pairs of electrons and one lone pair the bonding angle will be a little less than 120 degrees
Describe the shape of a compound that has a central atom with four electron pairs
If there are four bonding pairs then the bonding angle will be 109.5 degrees - shape is called tetrahedral
If there are three bonding pairs the repulsion between the lone pairs and bonding pairs will be greater than the bonding pairs and bonding pairs - the bonding angle is 107 degrees - this is called trigonal pyramidal
If there are two bonding pairs of electrons the two lone pairs will squash the bond angle even further and the bond angle will be around 104.5 degrees (shape is “bent”)
Describe the shape of a compound that has a central atom with five electron pairs
Repulsion between bonding pairs cause three atoms to have a bond angle of 120 degrees and the other two to have a bond angle of 90 degrees - shape is trigonal pyramidal
If there are four bonding pairs and one lone pair of electrons
the lone pair would be positioned where the bonding pair would’ve been in a trigonal planar shape - called seezaw
If there are three bonding pairs and two lone pairs the molecule will be T-shaped
What does ‘expand the octet’ mean
Central atom can have more than eight bonding electron in their outer shells
Describe the shape of a compound that has a central atom with six electron pairs
Six bonding pairs means the shape will be an octahedral - all bond angles will be 90 degrees
If there are five bonding pairs and one lone pair the molecule forms a square pyramidal shape - bonding angles are still 90 degrees
If there are four bonding electron pairs and two lone pairs then the molecule will be a square planar
How do you work out molecules that are bonded in double bonds
Treat each double bond like an extra single bond so if there is a double bond that mean two single bonds
What is electronegativity
The ability to attract the bonding electrons in a covalent bond
How is electronegativity measured
Measured on the Pauling Scale
A higher number means the element is better able to attract the bonding electrons
What does polar and non-polar bonds mean
Polar - atoms have different electronegativities
Non-polar - atoms have similar or equal negativities
δ - what does this mean
Slightly
What is a dipole
A difference in charge between the two atoms caused by a shift in electron density in the bond
What causes a dipole
The difference of electronegativity between the two atoms causes a dipole
How is a permanent dipole induced
If charge is distributed unevenly over a whole molecule
In a simple molecule how is it decided if that molecule has a permanent dipole
If one bond is polar then that molecule has a permanent dipole
In a complex molecule how is it decided if that molecule has a permanent dipole
It will be dependant on its shape
If the molecule is arranged symmetrically so that the dipoles will cancel each other out and there won’t be a permanent dipole
If the bonds point roughly in the same direction the dipoles won’t cancel each other out and there will be a permanent dipole
What are intermolecular forces
Forces between molecules
Are intermolecular bonds stronger or weaker than covalent, ionic or metallic bonds
They are much weaker
What are the three intermolecular forces
Induced dipole-dipole forces
Permanent dipole-dipole forces
Hydrogen bonding
Which of the three intermolecular forces is the strongest
Hydrogen bonding
What is another way of saying induced dipole-dipole forces
van der Waals forces
Explain Van der Waals forces
Electrons in charge clouds move really quickly
At any particular moment there will be more electrons on one side than the other
At this moment the atom will have a temporary dipole in the opposite direction of the neighbouring atom
This will cause another temporary dipole and this will keep happening
The electrons are constantly moving so dipoles are being constantly created and destroyed over time
What is the effect of Van der Waals forces
For the atoms to be attracted to each other
Do all Van der Waals forces have the same strength
No
Larger molecules will have larger charge clouds
Explain how the shape of the molecule affects the strength of the van der Waals forces
If the bonds are straight rather than branched there molecules can lie closer together and the forces between them will therefore be stronger
Give three physical properties that van der Waals forces can affect
Viscosity
Boiling point
Melting point
Describe the trend of boiling points as you go down the noble gases group
The boiling points increase
Explain permanent dipole-dipole forces
There will be weak electrostatic attraction between the δ+ and the δ- charges on neighbouring molecules
What will happen if you place a negative electrostatically charged rod next to a jet of water
Water is a polar liquid because the molecules contain permanent dipoles
The polar molecules of water will move so the oppositely charged molecule is facing the rod so it doesn’t matter what the charge of the rod is
When does hydrogen bonding happen
When hydrogen is bonded to nitrogen, fluorine or oxygen
Explain hydrogen bonding
Fluorine, nitrogen and oxygen are very electronegative so they draw the bonding electrons away from the hydrogen atom
The bond is so polarised and hydrogen has such a high charge density that the hydrogen atoms form weak bonds with lone pairs of electrons on the fluorine, nitrogen or oxygen atoms of other molecules
Explain the effect that hydrogen bonding has on the properties of substances
High boiling points
High melting points
Which one is more dense: ice or liquid water and explain why
Liquid water is more dense
In ice the molecules are arranged in regular lattice structure where the distance of the molecules is greater than the distance between the molecules in a liquid water
Explain the electrical conductivity, melting point and the solubility of simple covalent compounds
Do not conduct electricity because there are no free ions or electrons
Low melting points because the weak forces between the molecules are easily broken
Some dissolve in water depending on how polarised the molecules are
What is the main factor determining the melting and boiling points
The strength of the induced dipole-dipole forces
unless there is a hydrogen bond
What do metal elements exist as
Giant metallic lattice structures
Describe and explain the melting point, ability to be shaped, conductivity and solubility of metals
Melting point - high, the strong electrostatic attraction between metal ions and electrons require lots of energy to break
Ability to be shaped - no specific ions to break so layers can slide over each other making the metal malleable and ductile
Conductivity - delocalised electrons can pass kinetic energy to each other making metals good thermal conductors, they can also conduct electricity because there is a sea of delocalised electrons
Solubility - insolube because of the strength of the metallic bonds
Why do simple covalent compounds have low boiling and melting points
Because you don’t have to break the bonds you just have to break the weak intermolecular forces between the bonds
Do giant covalent structures and metallic bonds dissolve in water
No
What does the solubility of a substance depend on
If the substance is polar or charged then it is soluble in water because water is a polar substance
If you have an unknown compound and you want to predict the structure what can you do
Test the properties and deduce from that the type of structure
What is enthalpy change
The heat energy transferred in a reaction at constant pressure
What is the notation for enthalpy change
ΔH
What are the units of enthalpy change
kj mol^-1
What are the standard conditions
100kPa or 1 atm of pressure
At what temperature are enthalpy changes measured at
298 K (25 degrees celcius)
What type of value will the enthalpy change be if the reaction is exothermic
Negative because the energy is given out to the surroudings
What type of value will the enthalpy change be if the reaction is endothermic
Positive because it’s taking in energy from the surroundings
What is bond enthalpy
The energy needed to break a bond
Is breaking bonds endothermic or exothermic
Need energy to break bonds so bond breaking is endothermic
What is the enthalpy change
The change in heat energy of a substance at constant pressure
What is mean bond enthalpies
The average energy needed to break a certain type of bond over a range of compounds
How do you calculate the enthalpy change
Enthalpy change = Total energy absorbed - Total energy released
How do you find the total energy released by the reaction
Find the total energy needed to form all the new bonds
What are the three different types of entahlpy change
Standard enthalpy change of formation
Standard enthalpy change of combustion
Standard entahlpy change of reaction
Explain standard enthalpy change of formation
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
Explain standard enthalpy change of combustion
The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in their standard states
Explain standard entahlpy change of reaction
The enthalpy change when a reaction occurs in molarquantities shown in the chemical equation, under standard conditions with all reactants and products in their standard states
What are the three things you need to know when measuring the enthalpy change in a lab
Number of moles of stuff that is reacting
Change in temperature
How much stuff you’re heating
What are experiments that measure heat given out by reaction called
Calorimetry experiments
Describe the apparatus needed in when measuring enthalpy changes in a lab
Polystyrene beaker
Solution inside beaker
Thermometer inside solution
Explain how you measure enthalpy change for reactions that happen in solution
Use the thermometer to measure the temperature at regular intervals
Use a polystryene beaker to insulate the reaction
Describe the apparatus needed to measure the enthalpy change of combustion
Stirrer
Thermometer
Water
Combustion chamber
Fuel (reactant)
Air tube
Calorimeter
Explain how you measure the enthalpy change of combustion
As the fuel burns it heats the water and you can measure the temperature change and find out how much heat the water has absorbed if you know the mass of the water and the specific capacity of the water
Explain why the method for working out the enthalpy change of combustion may give inaccurate results
Some heat may be lost to surroundings and there might be some incomplete combustion
Why is finding the starting value of the water and subtracting it from the final value not a good method of finding out the temperature change when measuring enthalpy change of combustion
Because some heat is lost from the calorimeter to the surroundings
How do calculate the temperature change when measuring enthalpy change of combustion
Record temperature at regular intervals (begin a couple of minutes before you start the reaction)
Plot a graph (temp against time)
Plot two lines of best fit one for the results before the reaction and one for after
The distance between the two lines at the time the reaction started is the accurate temperature change for the reaction
How do calculate the standard enthalpy change of combustion
Calculate the amount of heat lost
Calculate the number of moles of fuel (number of moles = mass/Mr)
Use the equation - heat change/number of moles of fuel burned - to calculate
What is standard enthalpies of combustion measured in
kj mol^-1
What is the enthalpy of neutralisation
The energy change when 1 mole of water is formed by the reation of an acid and an alkali
How do you find the enthalpy of neutralisation
Add a known volume of acid to an insulated container
Then add known volume of alkali and record the temperature at regular intervals
Stir solution to make sure it’s evenly heated
Work out the heat energy given out by reaction using the heatc change/number of moles equation
