Charles Collingwood draws Lewis structures for ethene and hydrazine, then explains why ethene is nonpolar while hydrazine forms hydrogen bonds. He uses electronegativity values (C 2.5, N 3.0, H 2.1) to show bond polarity and calculates an enthalpy change of −581 kJ using bond enthalpies.
120 degrees
109.5 degrees
C = 2.5; H = 2.1; N = 3.0
−581 kJ
Might be year 11 exam junior exam, section B. Question 7. Draw Lewis electron dot structures for C2H4 and N2H4 showing all valence electrons. Well, C2H4 has got two carbons joined by a double bond and each carbon is connected to two hydrogens. So let's. I'm going to. I've actually picked one of these up earlier on. I'm just going to copy this. We're going to paste it in here. There we go. So this is the ethene Lewis structure. You see, there's two shared pairs of electrons between the two carbons and each carbon is connected to two hydrogens. We also have to do a hydrazine nitrogen joint two another nitrogen and each nitrogen joint nitrogen holding two hydrogens. So I'm just going to insert this here as well so we can look at the difference. Put one in here. We can see that hydrazine. Each nitrogen has got a lone pair of electrons. The nitrogen is joined to the other nitrogen by a single bond and there are two lone pairs, one on each nitrogen. Let's move. For the two marks, the Lewis structures state and explain the HCH bond angle in ethene. Well, ethene, we've got three regions of electron density. So the three regions, the three regions of electron density arrange themselves into a trigonal planar structure trigonal plane. This has got a bond angle bond angle of 120 degrees. And that's why the CC bond is going to be. HCH bond angle is going to be 120 degrees in the HNH bond angle. However, there are four regions of electron density. Perhaps I should have said here ethene, three regions of electron density compared to in hydrazine. Hydrazine there are four regions of electron density. So these arrange into a tetrahedral dense density arranged tetrahedrally. Now, tetrahedral bond angle is 109.5 degrees. So we'd expect this to be the bond angle between the hydrogens. Nitrogen hydrogen to be 109.5 degrees for our 2 marks. Part B. The polarity of a molecule can be explained in terms of electronegativity. Define the term electronegativity. Electronegativity is the attraction of an atom for electrons in a bond. This is the key. It is attraction for electrons. An atom electron attracts them only along a bond. So the power of attraction, if you like the power attraction of an atom for electrons in a bond. Compare the relative polarities of the C H bond and E3 and the NH bond in hydrogen. Well, the relative polarities are given by the difference in electronegativity. So the difference, difference, difference in electronegativity between C and H is much less than that of N and H. We've got electronegativities in the data booklet if we want to look at them. So in other words, therefore the N H bond is much more polar. Put figures on it. Carbon's electronegativity is given in the data booklet as 2.5. What's happened here? I know what's happened here. I'm just going the wrong language. It's 2.5. Hydrogens is 2.1. 2.1. Nitrogen is 3.0. That in brackets. And hydrogen as, again, as we say, is 2.1. So you can see there's a bigger difference between nitrogen and hydrogen than there is between carbon and hydrogen. Well, this is only for one mark, so you don't need to go to that full explanation. Hydrogen is a polar molecule and ethene is nonpolar. Explain why ethene is nonpolar, ethane is non polar. Not just because the bonds have little polarity, but any polarity is cancelled out by the symmetry of the molecule. The overall polarity is the vector sum of the individual dipoles, dipoles. And these cancel out. In the case, in the case of methane, sorry, ethene. In the case of ethene, due to the symmetry of the molecule, These cancel out. The boiling point of hydrogen is much higher than that of ethene. Let's go back and have a look at them, see if we can see why. So this has got a much higher boiling point. We can see that nitrogen, which is electronegative, is attached to hydrogen. This allows the hydrogen to bond to other hydrogen molecules using hydrogen bonding. This can't happen with ethene. So there is extensive intermolecular hydrogen bonding in a sample of hydrazine. This cannot occur with ethene. Perhaps we should say there's extensive intermolecular hydrogen bonding of some hydrogen due to. Due to the hydrogen atoms being attached to electronegative nitrogen atoms. And that gives a reason as well. And we said can't happen with ethene. And there's your two marks. That I believe is the end of the paper. Okay, the last question depends on us working out the equation of the reaction between hydrogen and oxygen using bond enthalpy data. Exactly the same as before. This time we just have to decide. Well, find what the actual atoms are. And hydrazine, N2H4. Go back to the picture of hydrazine. You see there's one NN single bond, four NH single bonds. And that's it. So we run it back forward again. So we add up an n single to 4 nh bonds. And don't forget there's 1 o, o double bond. Add those up and that 2220 kilojoules. Then we look at the products. 1 n triple bond, n for the nitrogen, 4 ohs. That gives minus 2801. So the overall enthalpy change is 2220 minus 2801, which is minus 581 kilojoules.
