Intermolecular forces Past Paper PDF

Intermolecular forces 1. Explain Know the difference between intra and intermolecular forces. 2. Explain how London dispersion intermolecular forces are formed. 3. Be able to predict a shape and type of intermolecular force from the chemical formula of a compound. Pages from the textbook Pag...

Intermolecular forces 1. Explain Know the difference between intra and intermolecular forces. 2. Explain how London dispersion intermolecular forces are formed. 3. Be able to predict a shape and type of intermolecular force from the chemical formula of a compound. Pages from the textbook Pages from the revision guide  Intermolecular forces Intermolecular forces are the forces between molecules. They determine the compounds melting and boiling point Hydrogen Dipole-Dipole bonding forces London dispersion forces Temporary dipole–induced dipole interactions (also called London dispersion forces)  How London dispersion forces are formed 1 These are two xenon atoms. Their electrons are moving constantly.  2 At any one instant the electron cloud may be distributed unequally.  δ = delta 2 This causes partial charges, δ+ and δ–, to develop. An Temporary dipole forms.  3 The Temporary dipole on one atom induces a dipole on a neighboring atom.  4 There is an electrostatic force of attraction between the δ– on one atom and the δ+ on the other atom. These are temporary dipole– induced dipole attractions. They are present between all molecules but they are weak. For a substance to boil, its intermolecular forces must be broken.  Boiling point = -161.5 °C Boiling point Methane Pentane = -36.1 °C Boiling point Propane = Octane Boiling point -42 °C = -125 °C Exam Question: Above are four molecules. i) List the molecules in order of increasing boiling point. ii) Use ideas about London forces to explain why the boiling points are different. Fluorine (F2) As you go down a group: Yellow gas -There are more electrons in each element Chlorine -This means there are stronger (Cl2) London dispersion Forces Green gas -This means more energy is required to separate the Bromine intermolecular forces (Br2) -Boiling point increases Orange liquid Iodine (I2) Grey solid  27 October 2024 Question 1 London forces are also known as: A Temporary dipole-Induced dipole B Induced dipole-Induced dipole C Induced dipole-Induced dipole Induced dipole-dipole D  27 October 2024 Question 2 Permanent-dipole-dipole bonds (A), temporary-dipole-induced dipole (B), Hydrogen bonding (C) From weakest to strongest, what is the order of types of intermolecular bonds? A A, B, C B A, C, B C B, A, C C, B, A D  27 October 2024 Question 3 What happens as you increase the chain length of an alkane? A Boiling point increases B Boiling point decreases C Boiling point stays the same D The boiling point varies  London forces Get ready to mark your answers  27 October 2024 Question 1 London forces are also known as: A Temporary dipole-Induced dipole B Induced dipole-Induced dipole C Induced dipole-Induced dipole Induced dipole-dipole D  27 October 2024 Question 2 Permanent-dipole-dipole bonds (A), temporary-dipole-induced dipole (B), Hydrogen bonding (C) From weakest to strongest, what is the order of types of intermolecular bonds? A A, B, C B A, C, B C B, A, C C, B, A D  27 October 2024 Question 3 What happens as you increase the chain length of an alkane? A Boiling point increases B Boiling point decreases C Boiling point stays the same D The boiling point varies  H-Cl F-F Hydrogen chloride and fluorine are two molecules with similar relative formula masses. But, very different boiling points. This is due to their intermolecular forces Fluorine has London forces between molecules Hydrogen chloride has London forces and permanent- dipole-dipole attractions Permanent-dipole-dipole attractions arise due to In most covalent compounds, the bonding is polar covalent. The shared electrons are attracted more to one nucleus in the molecule than the other. The atom with the higher electronegativity will attract the electrons more strongly. This gives the atom a slight negative charge. The other atom in the molecule will have a slight positive charge.  Electronegativity It will depend on three things : 1. Size of nuclear charge 2. Shielding 3. Atomic radius H He 2.1 - Li Be B C N O F Ne 1.0 1.5 2.0 2.5 3.0 3.5 4.0 - Na Mg Al Si P S Cl Ar 0.9 1.2 1.5 1.8 2.1 2.5 3.0 -  Electronegativity is the ability of an atom to attract bonding electrons in a covalent bond. It is measured using the Pauling scale. The higher the value the more electronegative the element.  Take a molymod kit Carbon (C) Hydrogen (H) Oxygen (O) atoms atoms atoms Nitrogen (N) Chlorine (Cl) Metal atoms atoms ion Make these compounds Are they polar or not? How can you tell? Tip: Look at where the polar charges are. If the polarity of a side or end of a molecule is different then the molecule is polar. One of these is polar and one isn’t. Can you identify which and say why? Identify δ+ and δ- on these molecules. Does the molecule have a permanent dipole? Write the electronegativity values next to each atom, which molecules will have a bigger permanent dipole? Note: C-H bonds are counted as being non-polar. Carbon dioxide Water Hydrogen fluoride Nitrogen trifluoride Fluoromethane Boron Trifluoride Dichloromethane Propanone Ammonia Permanent dipole–permanent dipole attractions form between the molecules.  27 October 2024 Question 1 Which of these is the definition of electronegativity? The ability of an atom to attract bonding electrons A in a covalent bond The ability of an atom to push electrons away from B the nucleus C The ability of an atom to share electrons equally The ability of an atom to form a stable covalent D bond  27 October 2024 Question 2 Which of these molecules does not have a permanent dipole B A C D  27 October 2024 Question 3 Which of these is the most electronegative element? A F B N C O D Cl  27 October 2024 Question 4 Which of these bonds has the biggest difference in electronegativity? A H-Cl B C=O C H-F D O-H Hydrogen bonding Must be: 1. Lone pair of electrons. 2. Hydrogen bonded to N, O or F … between a lone pair of electrons on an oxygen atom and the δ+ charge on a hydrogen atom. … each water molecule can form more than one hydrogen bond. Label the partial charges, lone pairs and show the hydrogen bonds between the 3 sets of molecules Water forms 2 hydrogen bonds as it has 2 lone pairs, ammonia only forms 1 Nitrogen, oxygen and fluorine are very electronegative and small enough for hydrogen bonding to occur.  Which is the odd one out? Ethanol Ethanal Ethanoic acid Boiling point Boiling point Boiling point = 78oC = 20 oC = 118oC This is why you can separate it using distillation Revise: a) What is electronegativity? b) What changes as you go across the periodic table and what effect does this have on electronegativity? c) When is a bond ionic rather than covalent? d) When is a bond non-polar? e) What is a polar covalent bond? f) Draw diagrams of a water and a carbon dioxide molecule, explain why one is polar and the other is not. g) Why is water able to dissolve sodium chloride i) Explain what electronegativity is and how it is derived ii) Explain how differences in electronegativity give rise to a permanent dipole. iii) Explain how a molecule is polar. Create: A summary sheet for revision on intermolecular forces. Use your notes and pages of the BTEC textbook Revise: a) Describe how London dispersion for is formed ( see previous week presentation)? a) What does a molecule need to form a hydrogen bond? b) How is a hydrogen bond shown in a diagram? c) What is the difference between ice and liquid water? d) Why does water have a high melting and boiling point? f) What hydrogen bonding is and what types of bonds give rise to hydrogen bonding. i) Examples of molecules which are capable of hydrogen bonding ii) The effect hydrogen bonding has on boiling point. iii) The unusual properties water has due to hydrogen bonding. When ice freezes the water forms a giant lattice structure It is less dense than liquid water. Which is why it floats.