Investigating the alkanes

Introduction:
The alkanes were originally called "paraffins", meaning "little reactivity". They can be found in crude oil and natural gas and they are also used as fuels and the starting materials for making a huge variety of organic compounds.

A. Physical Properties
a). Boiling Points

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

Methane	-161 ˚C
Pentane	36 ˚C
Hexane	50    ˚C

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

· 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. So with the 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. 

b). Solubility In Water

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

The hexane is immiscible and does not dissolve in water. This is because there is 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 because the layer of hexane is on top of the water. 

B. Chemical Properties 

a). Reaction With Some Common Reagents

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

Having done the test, this shows that Hexane reacts with none of the substances, concluding that they all form immiscible layers. 

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

Maybe the alkanes are typically used as fuels. 

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

Bromine is more soluble in hexane since they are both non-polar and will dissolve into each other. 

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

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

c). Combustion Of Alkanes

·  Write a balanced equation for the reaction that occurs.

CH₄ + 2 O₂ ----------> CO₂ + 2 H₂O

· 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. 

· 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. 

d). Cracking Paraffin Oil

·   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. 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? 

Long chain alkanes are 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

7.2 The position of equilibrium

7.2.1 –Deduce the equilibrium constant expression (K_r) from the equation for a homogenous reaction.

Answer the following questions:

a.      What can change the value of K_c -Temperature

b.     The reaction must be at equilibrium for the value of K_c to be calculated

c.      Define the term homogeneous:

A substance or material that contains only one kind of compound or one element can be defined as homogeneous. Homogeneous is Latin for "the same kind". So for an equilibrium reaction, the reactants are homogeneous since they are the same state, so the products will be the same state as the reactants.

Topic 6: Kinetics Test Correction

Which one of the following factors does not affect the rate of a chemical reaction?

• The enthalpy change of the reaction
• The nature of the reactants (Nature means Orientation)
• The concentration of the reactants
• The presence of a catalyst

The rates of many reactions approximately double with a temperature increase of 10˚C. This is best attributed to a doubling of the fraction of molecules that have the energy needed for reaction.

Reactions between aqueous solutions of sodium thiosulfate and acid can be followed by timing the appearance of the solid sulphur that is produced. The time required for the appearance of the sulphur would be increased by which of the following changes?

·        Raising the temperature

·        Diluting the solution (this is because of the appearance of the solid sulphur)

·        Adding a catalyst

·        Increasing the concentration of the sodium thiosulfate

Define what is meant by the rate of reaction. Describe how the rate changes with time for the above reaction, and explain your answer using the Collision Theory:

·        Rate decreases as time increases.

·        The more concentrated the reactants, the more collisions there will be per second per unit volume.

·        As reactants get used up, concentration decreases.

·        This is why rate greatest at start rather than at the end.

·        Exothermic reaction therefore heat given out will also speed up reaction rate due to increasing collision frequency. Also kinetic energy of particles > E_a.

7.1: Dynamic equilibrium

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

Definition of equilibrium: At equilibrium, the rate of the forward reaction is equal to the rate of the backward reaction.

Physical System
An example is bromine (placed in a sealed container at room temperature) Since this liquid is volatile, with a boiling point closer to the room temperature.

Diagram shows a layer of bromine gas formed above the liquid bromine. This occurs in both forms as bromine's boiling point is closer to room temperature, some particles have more kinetic energy in order to be in a gas form (evaporation), however when these gas particles collide with the surface of the liquid they lose energy and turn back into liquid (condensation). Therefore Equilibrium is only reached when rate of evaporation is equal to the rate of condensation

Chemical System
Whereas in hydrogen iodide, there are the elements Hydrogen and Iodine. Iodine (released as a purple gas) and hydrogen and hydrogen iodide (colourless gas). Equilibrium has been reached since the rate of reverse reaction is between the Iodine and Hydrogen gas. This is considered an equilibrium mixture

Heating Copper Sulfate (CuSO4) crystals
By heating the blue crystals, the hydrated CuSO4 crystals are converted to anhydrous CuSO4 crystals(turning from blue to white). This is a reversible reaction.

The hydrated (blue) Copper Sulfate crystals

Heating the blue Copper Sulfate

The anhydrous (white) Copper Sulfate

Topic 6: Practice Questions

B -D as as pH can be measured as well

A

D -definition of rate of reaction (the concentration per unit time)

A -B. It is not A because magnesium is excess and the concentration of the hydrochloric acid will affect the reaction (increasing the concentration will speed up the reaction.

D -C because Adding a catalyst: the activation energy decreases as a catalyst offers an alternative route with a different (but lower) activation energy.

D -B because the size of the CaCO3 does not mean the surface area but the actual size. So the surface are is not increased.

C

a). Measuring the mass of reactants (g) using a top pan balance
The Volume of CO₂ being produced, collected in a gas syringe (cm³) and using a stopwatch to measure time (seconds)
Measuring the pH by the using the pH probe of solution and how long it takes to reach 7 (hitting neutralization)

b). Increase in temperature -This will increase the aerage kinetic energy of particles, resulting an increase of collision frequent. Therefore there is a higher number of particles with average kinetic energy > Ea

 Adding a catalyst -An alternative route is provided with lower Activation energy so more particles has the value of average kinetic energy that is higher than Ea so there is more collisions.

Increase of surface are -since there is more particles, the chances is higher for successful collisions

c).  i) Stay the samesince the MgCO3 is excess and HCl is the limiting factor.

ii). Increase. -Stay the same because the concentration of the reactant stays the same so the volume of the product stays the same.

Collision Theory (3)

6.2.5 Sketch and explain qualitatively the Maxwell–Boltzman energy distribution curve for a fixed amount of gas at different temperatures and its consequences for changes in reaction rate.

This graph shows the rate of reaction depends on the proportion of particles that have values of kinetic energy greater than activation energy. –The red line shows that the kinetic energy of the particle is lower, so the green shaded area has more particles than the blue area at the end of the reaction.

Maxwell-Boltzman Distribution

The distribution of the kinetic energy is shown by the Maxwell-Boltzman Distribution. The speed of the particles’ movement show the amount of kinetic energy they possess. The greater the speed of the particles shows they have more kinetic energy. The total of particles remains constant, despite the temperature increasing the area under the curve. The higher velocity the particles move show the higher average kinetic energy.

6.2.6 Describe the effect of a catalyst on a chemical reaction.

A catalyst “is a substance that increases the rate of reaction without itself undergoing permanent damage” –It provides the reaction an alternative route that has a lower activation energy.

In a reaction with a catalyst, 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 catalysts.

Without a catalyst: there are less particles that has the value of kinetic energy greater than the activation energy. Therefore, the rate of reaction will be slower since there are fewer number of particles that will undergo successful collisions.

With a catalyst: there are more particles that have the value of kinetic energy than the activation energy. [activation energy is lower] So the rate of reaction will be faster as a greater number of particles will be able to undergo successful collisions.

6.2: Collision (2)

6.2.4 –Predict and explain, using the collision theory, the qualitative effects of particle size, temperature, concentration and pressure on the rate of a reaction.

1.      What was the independent variable?

The surface area of the marble chips

2.      What was the dependent variable?

The time it takes for the marble chips to react in order to produce 100cm³ of carbon dioxide
3.      What variables were controlled?

The temperature of the room remained the same, the volume of acid used (50cm³) and the mass of the calcium carbonate (around 6.1 grams)
4.      Using collision theory, explain the following the shape of graphs at the start of the reaction.

The shape of the graphs shows that the reaction is an increasing slope.

5.      What does the gradient of the graph at any one point represent?

The gradient shows the volume of carbon dioxide produced over time in seconds.
6.      What are the units for the gradient of the graph?

The unit is cm³ over seconds.
7.      Discuss the reasons for the differences in the shape of the graphs.

The graph shows that the rate of reaction for the marble chip (in powder form) increases since the surface area is more. So the more surface area there is, the higher chances for the particles to collide. It also shows how the smallest-sized chips reacts the slowest. The slow rate of reaction is followed by the medium-sized chips and the large-sized chips. However, it appears the rate of reaction for small-sized chips is faster than the rest.  Decreasing the size of the particle increases the rate of reaction since it has more surface area. There could be some error since it could be that the number of marble chips are not the same in order to keep the mass the same.

6.2: Collision Theory

6.2.1 –Describe the kinetic theory in terms of the movement of particles whose average energy is proportional to temperature in kelvins

The average energy calculated using each particle is proportional to the temperature in Kelvin. Particles move randomly in a substance as they contain kinetic energy. For example, increasing the temperature is would increase the average kinetic energy, so the particles have more energy. However at 0 Kelvin (absolute zero/-273°C) particles do not have kinetic energy at all.

6.2.2 –Define the term activation energy E_a

Definition of activation energy Ea

The minimum value of kinetic energy which particles must have before they are able to react

6.2.3 –Describe the collision theory

Three factors that affect the rate of reaction

1. Collision frequency:	If the frequency of collision is higher, it is more likely that the probability of collision would be more successful.
2. Number of particle with E≥Ea:	If the particles need to overcome repulsion between molecules or to break bonds in the reactants, they must have a minimum amount of energy, in order for collision to take place. For a successful reaction, the kinetic energy value of the particles must be greater than the activation energy [E ≥ Ea] will successfully collide.
3. Appropriate collision geometry or orientation:	Particles must have the right collision geometry in order for the collision to be successful, so the reaction takes place.

6.1: Rate of Reaction

6.1.1 –Define the term rate of reaction

This can be expressed as “change in concentration per unit time or mol dm^-3 s^-1   or the definition is a measure of amount of reactants being converted into products per unit amount of time

6.1.2 –Describe suitable experimental procedures for measuring rates of reactions

6.1.3 –Analyze data from rate experiments

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