Investigating The Alkanes

Physical Properties
Boiling Points

• What are the boiling points of methane, pentane and hexane? 

Methane has a boiling point of -161 degrees C, pentane 36 degrees C and hexane 68 degrees C. 

• Which is the first straight chain alkane to be a liquid at room temperature and pressure? 

Pentane. Methane and butane are both gases at room temperature and pressure. 

• In terms of intermolecular forces, explain why the boiling points of the alkanes increase with increasing molecular mass. 

Alkanes have Van der Waal's forces between them. With increasing molecular mass, there is also increasing surface area of straight chain alkanes, resulting in a stronger attraction between alkane molecules. This means more energy is required to break these bonds for a change of state. 

• What is the effect of branching on the boiling points of the alkanes?

Branching tends to lower the boiling point since it decreases the surface area. 

Solubility In Water

Measure out 2-3 cm3 of hexane into a test tube and add about twice this volume of water. Shake, then stand the test tube in a rack. 

• Does the hexane dissolve in water? In terms of intermolecular forces, explain why the two liquids behave in this way. 

The hexane doesn't dissolve in water but forms an immiscible layer instead. This is due to a difference of polarity between the two liquids. Polar solvents dissolve polar compounds best and non-polar solvents dissolve non-polar compounds. While hexane is non-polar, water is polar - so the two liquids do not mix. 

• Is hexane more or less dense than water? How do you know? 

Hexane is less dense than water as it forms the top layer of the two immiscible layers. 

Chemical Properties 

Reaction With Some Common Reagents

Add about 2cm3 of hexane to 2 cm3 of the reagent in a test tube, shake and look for any signs of a chemical reaction having occurred. Reagents: sodium hydroxide solution, bromine water, potassium manganate (VII) solution, and concentrated sulphuric acid. 

• Does hexane appear to react with any of these substances?

Hexane reacts with none of the substances - they all form immiscible layers. All these substances are aqueous solutions, meaning they are polar and hexane, a non-polar substance, won't react with them. 

• The alkanes were once more commonly called 'paraffins'. Why was this name used? 

Paraffin - means little reactivity. The alkanes don't react. 

• In which substance is bromine more soluble - hexane or water? Why? 

Bromine is more soluble in hexane - they are both non-polar and will dissolve into each other.
If it's bromine water, then water molecules are attached to the bromine molecules, and it'll more readily dissolve in water. 

• In which substance is potassium manganate (VII) more soluble - hexane or water? Why? 

Potassium manganate is more soluble in water - they are both polar and will dissolve into each other. 

Combustion Of Alkanes

Fill a test tube with methane from the gas tap. Stopper the tube and stand it in a test-tube rack. Light a split, unstopper the tube and apply the lighted splint to the mouth of the tube. 

• Write a balanced equation for the reaction that occurs.

CH4 + 2 O2 ----------> CO2 + 2 H2O

Using a pipette, place 3 drops of hexane on a watch glass. Light a long splint and use this to light the hexane. 

• Write a balanced equation for the reaction that occurs. 

2 C6H14 + 19 O2 ----------> 12 CO2 + 14 H2O

• Which burns with the sootier flame? Explain why hexane should burn with a sootier flame. 

Hexane - because it has a higher percentage of carbon than methane. 

Put a small piece of paraffin wax on a watch glass and attempt to ignite it with a lighted splint. 

• Can the wax be easily ignited? 

No. 

• Why is the wax harder to ignite than methane even though they both contain alkanes?

Paraffin is a much longer alkane chain and is therefore harder to ignite. 

• Why does a candle have a wick? 

The wick creates a mechanism called capillary action, in which the wick draws the molten wax to the flame, transported the liquid wax as fuel. When the fuel reaches the flame it then vaporizes and burns. 

Cracking Paraffin Oil

1. Put 3-4 cm depth of mineral wool into a boiling tube and add enough paraffin oil to thoroughly soak the mineral wool. 
2. Put several pieces of broken porcelain pot in the boiling tube. 
3. Set up the apparatus as shown in the diagram. 
4. Gently heat the porcelain pieces, allowing the first bubbles of gas to escape. 
5. Then heat more strongly, occasionally warming the oil. 
6. Collect four or five tubes of gas

• Why are the first bubbles of gas not collected?

These first bubbles are displaced air which was in the delivery tube before the experiment began. 

• Why is the porcelain heated strongly before the oil is warmed? 

The porcelain acts as a catalyst and needs to be heated strongly before the oil vaporizes and makes contact with it. The catalyst needs to be activated by the heat. If the catalyst pieces are colder than the boiling point of the paraffin, the oil will condense on them and not react. 

• Why is the porcelain broken into small pieces?

To create a greater surface area. 

• Add a few drops of bromine water to one of the test tubes containing gas, quickly stopper and shake. What happens? Write an equation for the reaction that takes place. 

The bromine water is decolorized. 

C2H4 + Br2 + H2O ----------> C2H4BrOH + HBr

• Try to light the gas in one of the the test tubes with a lighted splint. What happens? Write an equation for the reaction that occurs. 

Water vapour is readily produced. 

C2H4 + 3 O2 ----------> 2 CO2 + 2 H2O

• Name the type of reaction carried out in the main experiment. Why are reactions of this sort important in the petrochemical industry? 

Thermal cracking. Long chain alkanes can be broken down to shorter chain alkanes and alkenes which are much more useful to the petrochemical industry. 

• Given that the molecular formula of paraffin oil is C20H42, suggest an equation for the reaction that has occurred in this experiment. 

C20H42 ----------> C8H18 + C8H16 + C4H8

Organic Chemistry - Solubility

As members of each homologous series have the same functional group, they're expected to have similar properties, but also to have some sort of trend in these properties with the increasing carbon number.

There are two factors to consider when determining the solubility of an organic compound in water:

• The length of the hydrocarbon chain

Since this part of the molecule is non-polar, it does not facilitate the solubility of the molecule in water and so solubility will decrease as the chain length increases.

As chain lengths increase, the hydrocarbon "tails" of the molecules start to get in the way. By forcing themselves between water molecules, they break the relatively strong hydrogen bonds between water molecules without replacing them with anything as good. This makes the process energetically less profitable, and so solubility decreases. 

• The nature of the functional group

Solubility is determined by the extent to which this part of the molecule is able to interact with water (for example by forming hydrogen bonds). 

Considering these two factors, the lower members of the following homologous series are quite soluble in water:

Alcohols, Aldehydes, Ketones and Carboxylic Acids

These homologous series can't form hydrogen bonds with themselves, but can form hydrogen bonds with water molecules. 

There will also be dispersion forces and dipole-dipole attractions between the organic molecules and water molecules. Forming these attractions releases energy which helps to supply the energy needed to seperate the water molecules and organic molecules from each other before they can mix together. 

Halogenoalkanes are not soluble in water as, despite their polarity, they're unable to form hydrogen bonds with water. 

7.2.2 I can deduce the extent of a reaction from the magnitude of Kc

What does the word magnitude mean?

Magnitude is the size of something.

Explain why the three reactions below do not have units for Kc?

The three reactions do not have a unit for Kc because the sum of the number of moles of the products equals the sum of the number of moles of the reactants. 

Deduce the extent of the reaction if Kc is
a. Significantly larger than 1

The reaction is considered to go almost to completion (very high conversion of reactants into products).

b. Between 0.01 and 100

Both the reactants and products are present and are in significant amounts.

c. Extremely small

The reaction hardly proceeds (very low conversion of reactants into products).

7.2.1 I can deduce the equilibrium constant Kc for homogenous reactions

What can change the value of the equilibrium constant? 

The only thing that changes the value of the equilibrium constant for a reaction is the temperature.

The reaction must be at ________ for the value of the equilibrium constant to be calculated. 

Equilibrium - when the concentrations used in the equation are the equilibrium concentrations for all reactants and products.

Define the term homogeneous 

A mixture which has uniform composition and properties throughout.

The Units Of Kc

aA + bB <----------> cC + dD

If (c+d) - (a+b) = 0     then there are no units

If (c+d) - (a+b) = 1     then Kc -----> mol dm^-3

If (c+d) - (a+b) = -1     then Kc -----> mol^-1 dm^3

7.1.1 Outline the characteristics of chemical and physical systems in a state of equilibrium

Dynamic Equilibrium

Physical Systems

When a liquid has reached its boiling point a significant number of particles will have enough energy to escape from the liquid state and form vapour by evaporating.

At the same time, some of these vapour molecules will collide with the surface of the liquid, lose energy and become liquid by condensing.

There will come a time when the rate of evaporation is equal to the rate of condensation and at this point there's no net change in the amounts of liquid and gas present. The system has reached equilibrium.

Chemical Systems

In a chemical system we are dealing with reversible reactions. There's the forward reaction (reactants to products), backward reactions (products to reactants) and the reverse reaction.

When a chemical system reaches equilibrium the reverse reaction equal the backward reaction and there's no net change observed even though both reactions are still occurring. The concentrations of both reactants and products remain constant over time and this is referred to as the equilibrium mixture.

Characteristics Of The Equilibrium State

At equilibrium state the rate of the forward reaction is equal to the rate of the backward reaction.

Feature of equilibrium state:

• Equilibrium is dynamic

The reaction hasn't stopped but both forward and backward reactions are still occurring.

• Equilibrium is achieved in a closed system

A closed system prevents exchange of matter with the surroundings, so equilibrium is achieved where both reactants and products can react and recombine with each other.

• The concentrations of reactants and products remain constant at equilibrium

They are being produced and destroyed at an equal rate.

• At equilibrium there's no change in macroscopic properties

This refers to observable properties such as colour and density. These don't change as they depend on the concentrations of the components of the mixture.

• Equilibrium can be reached from either direction

The same equilibrium mixture will result under the same conditions, no matter whether the reactions is started with all reactants, all products, or a mixture of both.

Even though the concentrations of reactant and product are constant and equilibrium, this doesn't imply that they're are equal. Most commonly there will be a higher concentration of either reactant or product in the equilibrium mixture, depending both on the reaction and on the conditions. 

The proportion of reactant and product in the equilibrium mixture is referred to as its equilibrium position. Reactions where the mixture contains more products are said to 'lie to the right', and reactions with more reactants are said to 'lie to the left'. 

Kinetics Heinemann Questions

Pg. 126 SL Chemistry Heinemann

1. The reaction between calcium carbonate and hydrochloric acid, carried out in an open flask, can be represented by the following equation:

CaCO2(s) + 2HCl(aq) ----------> CaCl2(aq) + H2O(l) + CO2(g)

Which of the measurements below could be used to measure the rate of reaction?

I   The mass of the flask and contents

II  The pH of the reaction mixture

III The volume of carbon dioxide produced 

Answer: I, II, III 

The pH of the reaction mixture could be used to measure the rate of reaction because the mixture will go from acidic to neutral, and the rate at which this happens can be measured. 

2. For a given reaction, why does the rate of reaction increase when the concentration of the reactants are increased? 

The frequency of the molecular collisions increases.

3. Based on the definition for the rate of reaction, which units are used for a rate?

mol dm-3 time -1

4. Excess magnesium was added to a beaker of aqueous hydrochloric acid on a balance. A graph of the mass of the beaker and contents was plotted against time (line 1)

What change in the experiment could give line 2?

I   The same mass of magnesium but in smaller pieces 

II  The same volume of a more concentrated solution of hydrochloric acid

III A lower temperature 

Answer: II only

5. The rate of a reaction between two gases increases when the temperature is increased and a catalyst is added. Which statements are both correct for the effect of these change son the reaction?

Increasing the temperature - Activation energy doesn't change

Adding a catalyst - Activation energy decreases 

6. Consider the reaction between solid CaCO3 and aqueous HCl. The reaction will be speeded up by an increase in which of the following conditions?

I   Concentration of HCl

II  Size of the CaCO3 particles

III Temperature 

Answer: I and III only

An increase in the size of the CaCO3 particles will slow down the reaction as there will be a smaller surface area of one of the reactants. 

7. Which of the following is important in determining whether a reaction occurs. 

I   Energy of the molecules

II  Orientation of the molecules 

Answer: Both I and II 

11. When excess lumps of magnesium carbonate are added to dilute hydrochloric acid the following reaction takes place:

MgCO3(s) + 2HCl(aq) ----------> MgCl2(aq) + CO2(g) + H2)(l)

(a) Outline two ways in which the rate of this reaction could be studied. 

The volume of carbon dioxide produced could be measured consistently per unit of time by using a gas syringe or an inverted measuring cylinder filled with water. A stop clock would be used to keep track of the time. 

The change of mass of the reactants could be measured consistently per unite of time as the reactions carries out by placing the reactants in a conical flask on a top pan balance. A stop clock is used to record the time. 

(b) State and explain three ways in which the rate of reaction could be increased. 

Increasing the temperature increases the frequency of collisions and the energy of the reacting molecules. 

Increasing the concentration of HCl will increase the frequency of collisions as there would be more reactant HCl particles. 

Using small pieces of solid magnesium carbonate will increase the surface area and result in an increase in the frequency of collisions as reacting particles will have more area to collide with each other. 

(c) State and explain whether the total volume of carbon dioxide produced would increase, decrease or stay the same if: 

(i) More lumps of magnesium carbonate were used.

The volume of carbon dioxide would stay the same as the same volume of HCl is used and it acts as a limiting factor.

(ii) The experiments were carried out at a higher temperature.

The volume of carbon dioxide would stay the same as the amount of reactants are used.

Kinetics Objectives 6.2.5 to 6.2.7

6.2.5 Sketch and explain qualitatively the Maxwell-Boltzmann energy distribution curve for a fixed amount of gas at different temperature and its consequences for changes in reaction rate

The Maxwell-Boltzmann Distribution Curve

The fact that particles in a gas at a particular temperature show a range of values of kinetic energy is express by the Maxwell-Boltzmann distribution curve.

This shows the number of particles that have a particular value of kinetic energy (or the probability of that value occurring) plotted against the values for kinetic energy. The area under the curve represents the total number of particles in the sample.

6.2.6 Describe the effect of a catalyst on a chemical reaction

A catalyst is a substance that increases the rate of a chemical reaction without itself undergoing permanent change.

Most catalysts work by providing an alternate route for the reaction, which has lower activation energy.

This means that without increasing the temperature, a larger number of particles has the value of kinetic energy greater than the activation energy and so will be able to undergo successful collisions.

6.2.7 Sketch and explain Maxwell-Boltzmann curves for reactions with and without catalyst

Without a catalyst there are a smaller number of particles that has the value of kinetic energy greater than the activation energy. This means that a fewer number of particles will able to undergo successful collisions and the rate of reaction will be slower.

With a catalyst the activation energy is lowered and there is therefore a greater number of particles that has the value of kinetic energy than the activation energy. This means that a greater number of particles will be able to undergo successful collisions and the rate of reaction will be faster.


This can be seen in the reaction of aluminium and iodine catalyzed using water, shown in this video: