Homework Help! Part 1!?

Question

Can anyone help me solve these riddles?

1. For the pairs of atoms listed below, state whether they will tend to form ionic bonds, covalent bonds or polar covalent bonds. Explain the reason for your choice.

(a) Potassium and chlorine
(b) Iodine and iodine
(c) Oxygen and hydrogen

3. Using the ‘electron pair repulsion theory’, state and explain the shape of the following compounds and in each case sketcha diagram to show the arrangemant of the atoms in space, labelling the bond angles.
(a) AlCl3
(b) SiH4
(c) PCl5

4. Enthalpy changes for the reactions of carbon and aluminium with oxygen are given in the following equations, where all substances are in their standard states.

C(s) + O2(g) → CO2(g) delta H = -394 kJ mol -1

4Al(s) + 3O2(g) → 2Al2O3(g) delta H = -3352 kJ mol -1

(a) What is the value for the standard enthalpy of combustion of carbon?

(b) What is the value for the standard enthalpy of combustion of aluminium?

Thanks for any advice!

Ok, according to my research, 1a is an Ionic Bond because non metal and metal. 1b is a covalent bond because equal amount of electrons and 1c is polar covalent bond because unequal amount of electrons, can someone confirm or disprove my understanding of this?

Thanks!

Answer 1

1a) Will form ionic bonds as potassium is a metal and chlorine is a non-metal

1b) Will form pure covalent bonds as there is no difference in electronegativity (attraction of electrons)

1c) Will form polar covalent bonds because there is a significant difference in electronegativities between hydrogen and oxygen. (See a data booklet)


3. Too complicated to draw diagrams here. The electron pair repulsion theory is that a lone pair and a lone pair will repel each other more than a lone pair and bonded pair. This in turn will repel more than a bonded pair and a bonded pair. The shape of each molecule can be determined by the number of electron pairs. AlCl3 has four electron pairs around the central atom and will therefore have a tetrahedral shape (this will appear pyramidal as one electron pair is a lone pair). SiH4 also has 4 electron pairs around the central atom and will be tetrahedral. PCl5 has 5 electron pairs and will therefore be trigonal bipyramidal. Bond angles should be available from your notes or a textbook.


4a) The standard enthalpy of combustion is the energy required to completely burn one mole of the substance in its standard state. As this is the equation given above, delta H = -394kJ mol -1

4b)The definition is the same as above but the equation shows 4 moles of aluminium. Hence delta H = (-3352) / 4 = -383kJ mol -1

Answer 2

1.a). Ionic - K ionises an electron and Cl has a high 'Electron Affinity'.
b). Covalent - Same electronegativity so they share electrons.
c). Polar Covalent. - Oxygen has a higher electronegativity and so has a tendency to pull electrons in the covalent bond towards itself.

3 a). Trigonal planar. The three Cl's attach to the Al. Because of 'electron pair repulsion theory' the Cl's will push each other as far apart as possible, without bond breaking. Hence any angle Cl - Al - Cl is 120 degrees.
b) Tetrahedral - The four H's attach directly to Si, and again will repel each other as far a possible giving an angle H - Si - H of approximately 109.47 degrees.
c). Trigonal bipyramidal - Each Cl is directly bonded to P. There is a central trigonal plane were three Cl's repel each other by 120 degrees. The other two Cl's extend orthogonally above and below the plane. giving and angle Cl - P - Cl of 90 degrees.