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Hydrazine, N$_2$H$_4$, can be used as a rocket fuel and is stored as a liquid. It reacts exothermically with oxygen to give only gaseous products.
The enthalpy change of a reaction such as that between hydrazine and oxygen may be calculated by using standard enthalpy changes of formation.
(a) Define the term *standard enthalpy change of formation*, $\Delta H_f^\circ$.
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(b) Hydrazine reacts with oxygen according to the following equation.
N$_2$H$_4$(l) + O$_2$(g) $\rightarrow$ N$_2$(g) + 2H$_2$O(g)
(i) Use the data in the table to calculate the standard enthalpy change of this reaction.
[Table_1]
$\Delta H_r^\circ = $ .......................................... kJ mol$^{-1}$
(ii) Although the above reaction is highly exothermic, hydrazine does not burn spontaneously in oxygen. Suggest a reason for this.
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(iii) Suggest why using hydrazine as a rocket fuel could be regarded as being 'environmentally friendly'.
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(c) The bonding in hydrazine is similar to that in ammonia.
(i) Showing outer-shell electrons only, draw a 'dot-and-cross' diagram of an ammonia molecule.
(ii) Draw a diagram to show the three-dimensional shape of an ammonia molecule.
(iii) Draw a diagram to show the shape of a hydrazine molecule. Show clearly which atom is joined to which and show clearly the value of *one* bond angle. [4]
(d) Deduce the oxidation state of nitrogen in hydrazine.
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542
550
550
The alkali metals are a series of six elements in Group I of the Periodic Table. The first ionisation energy of these elements shows a marked trend as the Group is descended.
(a) Define the term first ionisation energy.
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(b) (i) State and explain the trend in first ionisation energy as Group I is descended.
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(ii) Suggest how this trend helps to explain the increase in the reactivity of the elements as the Group is descended.
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(c) In a redox reaction, 0.83 g of lithium reacted with water to form 0.50 dm3 of aqueous lithium hydroxide.
$$2\text{Li}(s) + 2\text{H}_2\text{O}(l) \rightarrow 2\text{LiOH}(aq) + \text{H}_2(g)$$
(i) Calculate the amount, in moles, of lithium that reacted.
(ii) Calculate the volume of hydrogen produced at room temperature and pressure.
(iii) Calculate the concentration, in mol dm-3, of the LiOH(aq) formed.
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(d) When heated in chlorine, all of the alkali metals react to form the corresponding chloride.
Describe what you see when sodium is heated in chlorine and write a balanced equation for the reaction.
description
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equation
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513
538
567
This question refers to the elements shown in the section of the Periodic Table below.
(a) From this list of elements, identify in each case one element that has the property described. Give the symbol of the element.
(i) an element that sinks in cold water and reacts readily with it
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(ii) an element that forms an oxide that is a reducing agent
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(iii) the element that has the largest first ionisation energy
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(iv) the metal in Period 3 (Na to Ar) that has the smallest cation
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(v) the element which has a giant molecular structure and forms an oxide which also has a giant molecular structure
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(vi) the element in Period 3 (Na to Ar) with the greatest electrical conductivity
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(b) From the section of the Periodic Table above, identify two elements whose hydrides form hydrogen bonds between their molecules.
............. and .............
(c) Use the elements in Period 3 (Na to Ar) in the section of the Periodic Table opposite to identify the oxide(s) referred to below.
In each case, give the formula of the oxide(s).
(i) an oxide which has no reaction with water
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(ii) two acidic oxides formed by the same element
............. and .............
(iii) an oxide which dissolves readily in water to give a strongly alkaline solution
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(iv) an oxide which is amphoteric
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567
598
574
Organic reactions involve substances which may be
atoms, molecules, ions or free radicals.
We also apply the terms
electrophilic, nucleophilic, addition, elimination and substitution
to organic reactions.
Consider the following reactions.
$$\text{CH}_4 + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{HCl} \text{ reaction 1}$$
$$\text{CH}_3\text{CH}_2\text{OH} \rightarrow \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O} \text{ reaction 2}$$
$$\text{CH}_3\text{I} + \text{OH}^- \rightarrow \text{CH}_3\text{OH} + \text{I}^- \text{ reaction 3}$$
$$\text{CH}_3\text{COCH}_3 + \text{HCN} \rightarrow \text{CH}_3\text{C(OH)(CN)CH}_3 \text{ reaction 4}$$
(a) Using the terms mentioned above, state as clearly as you can the nature of each of the following reactions.
reaction 1 .................................................................
reaction 2 .................................................................
[2]
(b) By considering the four reactions above, suggest a formula for each of the following substances.
In each case, state which reaction you are considering.
(i) one substance that is an addition product
reaction....... addition product
(ii) one substance that is a leaving group
reaction....... leaving group
(iii) one substance that behaves as an electrophile
reaction....... electrophile
[3]
(c) What is meant by the term nucleophile?
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[1]
(d) Reactions 3 and 4 involve nucleophiles.
For each reaction, give the formula of the nucleophile.
reaction 3
reaction 4
[2]
(e) One characteristic reaction of ethene is its ability to decolourise bromine.
$$\text{CH}_2=\text{CH}_2 + \text{Br}_2 \rightarrow \text{BrCH}_2\text{CH}_2\text{Br}$$
In this reaction, ethene behaves as a nucleophile.
Suggest an explanation for how ethene can behave in this way.
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[1]
513
507
532
Lactic acid, 2-hydroxypropanoic acid, $CH_3CH(OH)CO_2H$, occurs naturally in sour milk and in our muscles when we take hard exercise. Lactic acid is chiral and shows stereoisomerism.
(a) Draw fully displayed structures of the two optical isomers of lactic acid. Indicate with an asterisk (*) the chiral carbon atom in the lactic acid molecule. [3]
(b) Lactic acid may be synthesised from ethanol by the following route.
$CH_3CH_2OH \xrightarrow{\text{step 1}} CH_3CHO \xrightarrow{\text{step 2}} CH_3CH(OH)CN \xrightarrow{\text{step 3}} CH_3CH(OH)CO_2H$
Give the reagent(s) and essential condition(s) for each step.
| reagent(s) | condition(s) |
|------------|--------------|
| step 1 | |
| step 2 | |
| step 3 | |
[6]
During exercise, lactic acid is produced in our muscles from pyruvic acid, $CH_3COCO_2H$. This reaction occurs in the presence of the enzyme lactic acid dehydrogenase.
(c) (i) What type of chemical compound is the enzyme lactic acid dehydrogenase?
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(ii) How would you detect a small quantity of pyruvic acid in a sample of lactic acid?
State the reagent(s) you would use and what would be seen in your test.
reagent(s) ...............................................................
observation ............................................................
(iii) How would you detect a small quantity of lactic acid in a sample of pyruvic acid?
State the reagent(s) you would use and what would be seen in your test.
reagent(s) ...............................................................
observation ............................................................
(iv) What chemical reagent would be used to convert pyruvic acid into lactic acid?
$CH_3COCO_2H \rightarrow CH_3CH(OH)CO_2H$
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[6]
523
566
570
