Video Transcript
The rocket propellants hydrogen peroxide H₂O₂ and hydrazine N₂H₄ both contain single covalent bonds between atoms of the same element. Ammonia reacts with hydrogen peroxide to form hydrazine and one other product. The bond energies of selected bonds are shown in the table. a) Give a balanced chemical equation for the formation of hydrazine from ammonia and hydrogen peroxide.
If we look carefully back at the question, we’re actually given all the information we need. We’re told that the reactants are ammonia and hydrogen peroxide. Ammonia is NH₃ and hydrogen peroxide is H₂O₂. So now, we just have to work out the products. We know that the main product is hydrazine. But what else is formed? We’re actually told in this question that this reaction forms hydrazine and one other product. Looking at the atoms that we have in our reactants, possible products could be hydrogen gas, oxygen gas, water, or nitrogen gas.
Nitrogen gas is unlikely. But hydrogen, oxygen, and water are all common byproducts of reactions. If we were to select hydrogen as our one other product, we’d have no oxygen atoms in our products. So this is the incorrect answer. So actually, the most likely byproduct of this reaction is water. Now, the question asks us for a balanced chemical equation. So we need to count the atoms of each element.
We have one atom of nitrogen on the reactant side, but two on the product side. So let’s start by adding a two in front of the ammonia. Now, the nitrogen is balanced. Next, if we count the hydrogen atoms, we have six from the ammonia and two from the hydrogen peroxide, giving us eight in total on the left. And on the right, we have four from the hydrazine and two from the water, giving us a total of six on the right. This means that we need to add two extra hydrogen atoms to the product side.
An easy way to do that is to add another equivalent of water. Now both the nitrogens and the hydrogens are balanced. So let’s check the oxygen. We can see that we have two oxygens on the reactant side and two on the product’s, which means that everything is now balanced. So, this is our final answer.
What is the energy released by the reaction of ammonia and hydrogen peroxide per mole of hydrazine produced?
The energy released can be calculated by taking the energy of reactants minus the energy of the products. To calculate the energy of the reactants and the energy of the products, what we need to do is work out the sum of the bond energy of all the reactants and the sum of the bond energies of all of the products. So let’s remind ourselves of the reaction equation we came up with in the first part of this question.
So here we have two ammonia molecules plus one hydrogen peroxide molecule reacting to form one molecule of hydrazine and two molecules of water. Drawing the structures of these will help us to answer our question. By drawing out the structures of our reactants and products, it makes it easier to count how many bonds of each type we have. Let’s start by working out the energy of the bonds in the reactants.
Ammonia contains 3NH bonds. But don’t forget the stoichiometry of this equation. We have two ammonia molecules. Now, let’s look at the hydrogen peroxide. We have two OH single bonds and one oxygen–oxygen single bond. This is everything on the reactant side. So let’s move to the product. In hydrazine, we have 4NH bonds and one nitrogen–nitrogen single bond. In water, we have 2OH bonds. But we have two equivalents of water. So that gives us 4OH bonds.
Now, we can use the table given to us to fill in the numbers. Now, we just need to put all of these numbers into a calculator and work out the values. We’re left with 3376 kilojoules per mole for the reactants minus 3547 kilojoules per mole for the products which works out as minus 171 kilojoules per mole. Because our answer is negative, this tells us that the reaction is exothermic. We’re asked for the energy released by this reaction. So we can give our answer simply as 171 kilojoules per mole.
When used as a rocket propellant, hydrazine thermally decomposes into its constituent elements in their standard state. Calculate the energy released per mole of hydrazine reacted.
Again, let’s start by writing out the reaction equation. We know that hydrazine is N₂H₄. We’re told that it decomposes into its constituent elements in their standard state. The two elements that we have are nitrogen and hydrogen. You should know that they exist in their standard state as N₂ gas and H₂ gas. So now we can calculate the energy released in this reaction. We’re going to do a similar calculation to the last question, where we work out the bond energies in all the reactants and take away the bond energies in all the products.
First, let’s ensure that our reaction is balanced. We can see that we have four hydrogens on the reactant side and two in the product’s. So we know it’s not balanced. By adding another equivalent of hydrogen gas, we now have a fully balanced equation and we can begin our calculation. For the reactants, we have 4NH single bonds and one nitrogen–nitrogen single bond. In the products, we have one nitrogen–nitrogen triple bond and two equivalents of a hydrogen–hydrogen single bond.
From this, we can substitute in the values from the table and calculate our energy value. So this gives us the bond energies for both the reactants and the products. And now, we just need to do the subtraction, which gives us minus 95 kilojoules per mole. Again, the negative symbol just denotes that this is an exothermic reaction, which we would expect from a rocket propellant. So we can simply give our answer as 95 kilojoules per mole.
Whereas hydrazine thermally decomposes into its constituent elements, hydrogen peroxide converts to steam and oxygen when heated. The compounds are useful as rocket propellants because their decomposition reactions increase the temperature of the surroundings, T, and the number of gas molecules in the system, N. Compare the changes in T and N when a mole of each compound thermally decomposes.
We’re given quite a lot of information in this question. But we can try to simplify what it is that it’s asking. We know from the previous question that hydrazine decomposes into N₂ and two equivalents of H₂, releasing 95 kilojoules per mole in energy. This energy is what heats up the surroundings and increases T. What we need to do is compare this to the increasing T when hydrogen peroxide decomposes instead.
We’re told in the question that hydrogen peroxide converts to steam which is water and oxygen. We need to make sure that this equation is balanced. In the product, we have two hydrogen atoms which is the same as the reactant side. But we have three oxygen atoms and only two on the reactant side. An easy way to balance this is to put half in front of the oxygen. So we’re using half an equivalent of an oxygen molecule. You could simply times everything by two and have two hydrogen peroxides and two waters with one oxygen. Either is correct.
Now, let’s work out the energy released in this reaction. So again, we’re going to calculate the energy of the reactants minus the energy of the products. So our calculation is going to look something like this: with 2OH bonds and one oxygen–oxygen single bond in the reactants and 2OH bonds and half an O–O double bond in the products. We can substitute values for the bond energies in from the table. And eventually, we get the answer of 105 kilojoules per mole. And it’s negative because it’s exothermic.
So we can see that the decomposition of hydrogen peroxide releases slightly more energy than the decomposition of hydrazine. We can say that because the hydrazine releases less energy when it decomposes than hydrogen peroxide, it will increase the temperature of the surroundings less than when hydrogen peroxide decomposes. So this is the first part of our answer.
We’ve compared the changes in T and now we need to look at N. Remember that N is the number of gas molecules in our system. So starting with hydrazine, let’s look at the number of gas molecules on the product side and the reactant side. On the reactant side, we have one molecule of hydrazine. This decomposes to form one molecule of nitrogen gas and two equivalents of hydrogen gas. So in total, we’re converting one molecule of gas to three molecules of gas.
Now, let’s look at hydrogen peroxide. Again, we have one molecule of gas on the reactant side. But this time, we form one molecule of water and half an equivalent of oxygen, giving us a total of one and a half molecules of gas. Let’s consolidate all of this information so that we can write our answer.
In this table, we can easily compare the difference in the changes of T and N between the two molecules. Let’s focus on hydrazine. The decomposition of hydrazine produces less energy than that of hydrogen peroxide. This results in a smaller increase in the temperature of the surroundings. Hydrazine also converts one molecule of gas into three molecules of product. This creates a far larger change in the number of gas molecules compared to hydrogen peroxide. You could also consider what effects that would have on pressure. But that’s beyond this question.
So let’s summarize this in a sentence: hydrazine produces a smaller increase in T and a larger increase in N. So this is our answer.
