AS Chemistry 3.1.3 Bonding - Intermolecular Forces PDF

AS CHEMISTRY 3.1.3 BONDING ELECTRONEGATIVITY Electronegativity is defined as: “The ability of an atom to attract a pair of electrons in a covalent bond” Each element has its own electronegativity value as their electronic...

AS CHEMISTRY 3.1.3 BONDING ELECTRONEGATIVITY Electronegativity is defined as: “The ability of an atom to attract a pair of electrons in a covalent bond” Each element has its own electronegativity value as their electronic structure dictates how good they are at attracting a bonding pair of electrons. THE TREND MOST ELECTRONEGATIVE N O F LEAST ELECTRONEGATIVE The trend shows that electronegativity increases as you move UP and to the RIGHT of the periodic table. Fluorine is the most electronegative element, but nitrogen, oxygen and fluorine are all classed as “highly electronegative”. The explanations for this are the same as for the trends in 1st Ionisation Energy as it is all about the strength of attraction between the nucleus and the outer electron(s). From left to right across a period, nuclear charge increases, but shielding does not. Shielding is lowest in elements at the top of a group. All this leads to fluorine having an electronic structure most effective for attracting electrons towards it. Group 8 do not have electronegativity values as they do not tend to form covalent bonds as they have a full outer energy level. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING BOND POLARITY When two atoms of the same element are covalently bonded together, the electrons are shared equally (50:50). This means the bond is not polar as the charges are H X H balanced. When two different elements are covalently bonded 𝜹+ 𝜹- together, the electrons are not shared equally as one atom is better at attracting them than the other due to the difference in electronegativity. H X Cl unbalanced. The 𝜹 sign is used to show that each end is This means that the bond is polar as the charges are “slightly” charged. Since the shared pair of electrons spend more of their time around the most electronegative element, that end of the bond becomes slightly negative (𝜹-). As they spend less time around the least electronegative element, that end of the bond becomes slightly positive (𝜹+). Bottom line, if two different elements are covalently bonded together, even in much larger molecules, the bond will be polar. HINTS | TIPS | HACKS A polar bond can be referred to as a permanent dipole (as in two “poles”, like a bar magnet) The greater the difference in electronegativity, the more polar the bond is. The more polar a bond is, the weaker it is. INTERMEDIATE CHARACTER When bonds are polar they are ALMOST ionic. The pair of electrons are shared unequally, but they are not transferred. Look at this as a “grey area” between a purely covalent bond and a purely ionic bond. In truth, many ionic substances have some “covalent character”, and many covalent substances have some “ionic character”. In these cases we refer to these substances as having “intermediate character” as they are neither purely covalent, not purely ionic. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING INTERMOLECULAR FORCES (IMFs) This section is all about simple covalent molecules and the forces that act between them….. intermolecular forces. The strength of intermolecular forces between molecules dictate their physical properties such as melting / boiling point and solubility. Generally speaking, simple covalent substances have very low melting / boiling points as all intermolecular forces are considered very weak (compared to covalent, ionic and metallic bonds). There are three types of intermolecular force. INDUCED DIPOLE (Van der Waal’s) FORCES Dipole = one end of the molecule is 𝜹+ and the other end is 𝜹-. Induced = forced to occur. This is the weakest form of intermolecular force. ALL covalent molecules have induced dipole forces between them. However it is the strongest force between non-polar molecules….. H X H A non-polar molecule, the electrons are shared equally. 𝜹+ 𝜹- 𝜹+ 𝜹- 𝜹+ 𝜹- H X H H X H H X H H )))))))))) X H However, at any given moment, the This causes an induced dipole in electrons could, by chance, be the second molecule. unevenly distributed, which causes Now both molecules are an “instantaneous dipole”. temporarily polar and are attracted The 𝜹- end of the molecule repels to each other. These are what are known as the electrons in the second induced dipole or Van der Waal’s molecule. forces. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING HINTS | TIPS | HACKS Induced Dipole forces are the only, and therefore, the strongest IMFs between: - Monatomic elements (e.g. Group 8) - Molecular elements (e.g. N2, O2, the halogens, P4 & S8) - Large, non-polar molecules (e.g. hydrocarbons / alkanes) Induced dipole strength varies. The greater the number of electrons in the atoms / molecule, the stronger the induced dipole forces. The stronger the IMFs, the higher the melting / boiling points. e.g. Ar > Ne > He I2 > Br2 > Cl2 > F2 C3H6 > C2H6 > CH4 Yes, the greater the Mr of the atom / molecule the stronger the induced dipole forces… but this is not the answer. To get the marks you must reference the “number of electrons” in the molecule increases the strength of the induced dipole forces, so therefore more energy is needed to break them (melt / boil the substance). AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING PERMANENT DIPOLE FORCES These forces occur between polar molecules and are stronger than induced dipole forces. I stress “molecules”, here as it must be the molecule as a whole that has a permanent dipole. Induced dipole forces are intermittent / temporary. Permanent dipole forces are….. permanent, as they are caused by a differences in electronegativity, so all molecules are polar, all of the time. e.g. 𝜹+ 𝜹- 𝜹+ 𝜹- 𝜹+ 𝜹- H X Cl )))))))))) H X Cl )))))))))) H X Cl LARGER MOLECULES Spotting a permanent dipole in a diatomic molecule (like above) is easy. So long as the two elements are different you will have a permanent dipole! When it comes to spotting permanent dipole forces in larger molecules, it gets a little more tricky. You are looking for one of two things: 1. Lone Pair(s) If the molecule has a lone pair, it’s a safe bet that it is polar, and therefore the 𝜹-. IMFs between them are permanent dipole as that “end” of the molecule will be e.g. 𝜹- H H 𝜹- 𝜹- P N Cl Cl H C C O H H Cl H H H H AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING 2. Asymmetry Most bonds are polar as they are usually between two different elements. However, the molecule as a whole must be polar. If the elements around the central atom are different, and the molecule is not symmetrical, then the chances are it will have permanent dipole IMFs. You’ll find more examples of this in organic chemistry more than anywhere! e.g. Symmetrical ❌ Polar bonds cancel out so only IMFs are induced dipole. H F 𝜹- 𝜹+ 𝜹- F F O C O C S H F F H H F Asymmetrical ✅ Polar bonds do not cancel out so the molecule is polar overall. H 𝜹+ 𝜹- 𝜹+ H 3C C 𝜹- C O Cl H H 3C H Important! Spotting permanent dipoles will get easier when you learn about shapes of molecules later in this section. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING IMFs & PHYSICAL PROPERTIES The strength of IMFs between covalent molecules has a big impact on their physical properties. MELTING & BOILING POINT 200 150 100 Boiling Point (oC) 50 0 -50 -100 -150 -200 F2 Cl2 Br2 I2 HF HCl HBr HI Halogens Halogen Halides The Halogens These are non-polar so only have very weak induced dipole forces between them. You can see how their boiling points increase as the number of electrons in the molecules increases from F to I. The Halogen (Group 7) Halides HF has a much higher boiling point than the rest as it shows Hydrogen Bonding between the molecules. HCl, HBr and HI all show permanent dipole forces between them. However, note how they increase from HCl to HI. This is because the number of electrons in the molecules increases so they have stronger induced dipole forces (remember ALL molecules have induced dipole forces between them!). You’ll see the same pattern with the Group 5 and Group 6 Halides. The Power of Induced Dipoles Notice how both Br2 and I2 have a higher boiling point than HF, despite them only having induced dipole forces between the molecules and HF shows hydrogen bonding! Don’t underestimate the strength of induced dipole forces between larger molecules. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING SOLUBILITY The solubility of covalent molecules depends on the intermolecular forces between the solute and the solvent. Generally speaking, polar molecules are soluble in polar solvents and non-polar solutes dissolve in non-polar solvents. “Like dissolves like”. In summary: H H H C H O C C H H H H H C C H C H H H H Polar Solvent Non-Polar Solvent (e.g. H2O) (e.g. cyclohexane) Generally Insoluble Non-Polar However, very small Soluble Molecules molecules such as O2 or CO2 are soluble Permanent Dipole Soluble Insoluble Hydrogen Bonding Soluble Insoluble AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING DISSOLVING IN WATER When it comes to polar molecules dissolving in water, we must remember that there are strong Hydrogen Bonding IMFs between water molecules themselves. So, the any molecules that are going to dissolve must be polar enough to disrupt / overcome these so that they can fit in between the water molecules. Below is a representation of how ethanol (C2H5OH) interacts with water molecules when it dissolves. Pay close attention as you may be asked to show this in an exam. H 𝜹- 𝜹+ H H H O 𝜹- ))))))))) H C C O 𝜹+ H H H 𝜹- )))) )))) ) O H H Pay attention to: 1. the bond angle around the O atom in ethanol. 2. the fact that the 𝜹- of one molecule is attracted to the 𝜹+ of the other. 3. The bonds in the molecules “line up”. e.g. You can draw a straight line along the O-H bond in ethanol, along the hydrogen bonding IMF, then along the O-H bond in water. AQA www.chemistrycoach.co.uk © scidekick ltd 2024