Types of attractive forces Flashcards
(28 cards)
What does polarity mean?
Polarity refers to a separation of charge. Differences in the polarity of molecules (non-polar and polar) are shown in the physical properties such as MP, BP and solubility in different solvents.
What is a non-polar molecule?
A non-polar molecule has no overall charge separation/ the molecular dipole is zero. The molecule may contain polar bonds but the bond dipoles cancel out due to the symmetry of that molecule’s shape.
What is a polar molecule?
In polar molecules, there is a charge separation/there is a molecular dipole. One part of the molecule is slightly negative (due to a high density of the electron cloud) relative to another part that is slightly positive (due to low density of the electron cloud)
Model answer structure when talking about a lewis diagram…
- Draw the lewis structure
- (Electron density number?) …There are ___ groups of electron density around the central atom. For minimum electron repulsion, these groups of electrons move to the corners of an ____ ( the overall shape determined by the number of negative regions around the centre)
- (Electronegativity) … State the number of covalent bonds and whether they are polar or non polar in reference to the electronegativity difference between two named atoms in each covalent bond
- (Lone pairs?) On the central atom, there are no/____ lone pairs of electrons, therefore the actual shape of the molecule is ____
- (Symmetrical?) State whether the spread of electron density is symmetrical or asymmetrical around the named central atom
- (Dipoles?) State whether the bond dipoles of the named polar bonds cancel out/do not cancel out and link this to polarity.
Name the three main intermolecular forces…
Temporary dipole forces of attraction - all molecules have td-td attractive forces because of the movement of electrons
Permanent dipole forces of attraction - polar molecules are molecule with permeant dipoles and they are the only ones that show permanent dipole forces of attraction
Hydrogen bonding (F, O and N) - Caused by a hydrogen (H) atom which is covalently bound to a more electronegative atom
Temporary dipole forces of attraction
Trend
Who shows this intermolecular force?
All molecules exhibit td-td attractive forces between their molecules. These temporary dipoles are due to an uneven distribution of electrons, as these electrons move randomly in their orbitals. These forces are only temporary or instantaneous. The larger the molecule (or atom), the more electrons there are and hence the more polarisable the molecule is (it is more likely to become polarised temporarily). Thus, the strength of the td-td attractive forces increases with molar mass as larger molecules have more electrons. Therefore more heat energy is required to seperate these molecules
Also in atoms with larger electron clouds (larger atoms), the outer electrons are not attracted as strongly towards the nucleus and so the electron cloud is more easily distorted.
The shape of the molecule is also significant. Long chain molecule can develop larger temporary dipoles due to electron movement than short, ‘fat’ (often spherical) molecules. Long thin molecules can lie closer together increasing the area of close contact between the molecules. The greater the area of contact, the greater the attraction and therefore stronger td-td attractions. As a result, more energy is required to seperate the molecules hence higher BP.
Permanent dipole forces of attraction
All molecules show td-td forces of attraction between their molecules. Polar molecules in addition to td-td forces have permanent dipole-permanent dipole forces of attraction between their molecules. The attractive forces between two polar molecules occurs because of an uneven charge distribution due to attraction of electrons in the molecule towards elements of higher electronegativity. The polar molecules have more attraction between the permanent dipoles of the molecule than between the tds of the non-polar atoms or molecules.
Note that if the number of electrons in the molecule are the same and one of the molecules is polar and the other is non-polar, then the pd-pd attractive forces are more significant in determining the boiling points
Hydrogen bonding
Special type of dipole-dipole force of attraction. It is the strongest of the intermolecular forces. For a hydrogen bond to form, there must be a hydrogen atom that is covalently bonded to another atom which is small and highly electronegative, with a lone pair of electrons on it. There are 3 atoms that fir this category - F,O and N e.g. NH3, H2O and HF. The attractive force exists between molecules occurs through the highly positive hydrogen of one molecule and a lone pair of electrons on the F, O or N of the other molecule.
The strength of the hydrogen bond is due to the small size of the hydrogen atom. The bond becomes very polar and the hydrogen atom is almost a bare hydrogen nucleus (nearly H+). This is attracted strongly to a lone pair of electrons. These powerful intermolecular forces require significantly more heat energy to break.
Note that the hydrogen bond forms between molecules not within the molecule.
Properties of polar molecules
Polar molecules are more fragile than ionic solids, most are soluble in polar solvents and some react with water, these molecules do not conduct electricity and they have low MP/BP
Properties of non-polar molecules
Soft, most are soluble in non-polar solvents, do not conduct, low MP/BP
Enthalpy of fusion
The enthalpy of fusion of a substance is the enthalpy change which occurs when one mole of a substance changes from a solid to a liquid (melting). Energy is absorbed in fusion, since energy is required to seperate particles of the substance the value is always positive.
Enthalpy of vaporisation
The standard enthalpy of vaporisation of a substance is the enthalpy change which occurs when one mole of a substance changes from a liquid to a gas (vaporised).Energy is absorbed in vaporisation, since energy is required to seperate the particles of the substance the value is always positive.
Enthalpy of sublimation
The standard enthalpy of sublimation of a substance is the enthalpy change which occurs when one mole of a substance changes from a solid to a gas (sublimes). Energy is absorbed in sublimation, since energy is required to seperate the particles of the substance so the value is always positive.
What is enthalpy?
What are the two types and explain them…
Heat energy. Describes the heat contents of a system and its actual value cannot be measured. An enthalpy change (delta H) however can be measured. The unit of heat energy is joules (J)
For an exothermic reaction, delta H is negative and energy is released into the surroundings (in a questions temperature is shown to rise)
For an endothermic reaction, delta H is positive and heat flows from the surroundings into the system (in a question the temperature is shown to decrease.)
Energy can be converted from one form to another but is conserved overall which is the law of conservation of energy (delta H = delta H (products) - delta H (reactants) )
Bond breaking is …
Bond making is …
Bond breaking is endothermic as you have to put in energy to break bonds. Bond making on the other hand is exothermic.
Explain exothermic reactions
The enthalpy of the reactants is greater than the enthalpy of the products and so energy is LOST from the reactants to the surroundings during the reaction. The change in enthalpy, delta r H is negative.
i.e Exothermic reactions have a lower activation energy
Explain endothermic reactions
The enthalpy of the reactants is less than the enthalpy of the products and so energy is ABSORBED by the reactants from the surroundings during the reaction. The change in enthalpy, delta r H is positive
i.e. Endothermic reactions have a higher activation energy
Standard heat of reaction (enthalpy)
delta r H
Measuring the change per mole of a reaction
Enthalpy of combustion
The standard enthalpy of combustion of a substance is the enthalpy change which occurs when one mole of the substance is completely burnt in oxygen, under standard conditions. Since energy is released in combustion reaction, the delta c H is always negative.
Hydrocarbon + O2 always makes H2O and CO2
Enthalpy of formation
The standard enthalpy of formation of a substance is the enthalpy change when one mole of a substance is formed from its constituent elements under standard conditions.
Hess’s Law
Calculating enthalpy changes using delta f H (enthalpy of formation) , delta c H (enthalpy of combustion) and delta r H.
Entropy
Entropy (s) is the measure of the disorder of a system.
When there is an increase in entropy - more disorder - delta s is positive
When there is a decrease in entropy - less disorder - delta s is negative
The more disorder of a reaction - the higher the chance of a reaction occurring
A reaction will favour the side (reactants or products) that has maximum entropy
Spontaneous change and Gibbs Law
Spontaneous reactions are reactions that have a natural tendency to occur. There are two driving forces for spontaneous changes. The first is enthalpy (force to minimum energy) and the second is entropy (force towards maximum disorder). They are two opposing forces.
Entropy increases when ….
A solid melts to produce a liquid
A substance dissolves
There is an increase in the number of particles (moles)
