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THEORY BOOKLETNAME:
HSCCHEMISTRY
ACIDIC ENVIRONMENT 2(Le Chatelier’s Principle)
1300 008 008
HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
matrixeducation.com.auLevel 8, Strathfield Plaza, 11 The Boulevarde, STRATHFIELD, NSW 2135
1. Factors affecting the rate of reaction
Nature of the reactants
Every reaction has its own rate and its own activation energy. In general,
reactions that take place between ions in aqueous solution are extremely
rapid, almost instantaneous.
The rate of a reaction is defined as the change in concentration of reactants
or products over time. In a reaction with a slow rate, the reactants react to
form products over a long period of time.
Activation energies for these reactions are very low because usually
no covalent bonds need to be broken. Define the term ‘activation energy’.
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As we might expect, reactions between covalent molecules, whether in
aqueous solution or not, are much slower. Many of these reactions require
anywhere from 15 minutes to 24 hours or more for most of the starting
compounds to be converted to the products. There are, of course, reactions
that take a good deal longer, but such reactions are seldom useful.
Concentration
Consider a reaction A + B C + D.
In most cases, a reaction rate increases when the concentration of either or
both reactants (A or B) is increased.
For many reactions, though by no means all, there is a ………………….1
relationship between concentration and rate; that is, when the
concentration of a reactant is doubled, the rate also doubles. All of this is
easily understandable on the basis of the collision theory.
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HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
If we double the concentration of A, there are twice as many molecules of A
in the same volume. The molecules of B in that volume now collide with
twice as many A molecules per second as before. Since the reaction rate
depends on the number of collisions per second, the rate is doubled.
Rate of reaction between gaseous H2 and I2 increases with increasing
concentration:
Source: Tro, N. J., Introductory Chemistry, Prentice-Hall Inc, 2003.
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HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
Temperature
In virtually all cases, reaction rate increases with increasing temperature. An
approximate rule for many reactions is that:
As the temperature goes up by 10oC, the rate of reaction doubles.
This rule is far from exact, but it is not far from the truth in many cases. This
is quite a large effect and says, for example, that if we run a reaction at
90 °C instead of at room temperature (20 °C), the reaction will go about 128
times faster.
Increasing temperature provides more energy that the reactants need to
change into products. The reaction of the mixture in the diagram c) below is
therefore the fastest as the reaction mixture is at the highest temperature.
Source: Tro, N. J., Introductory Chemistry, Prentice-Hall Inc, 2003.
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HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
If it takes 20 hours to convert 100g of A to product C at 20 °C, then it would
take only about 10 minutes at 90 °C. Temperature is therefore a powerful
tool that lets us increase the rates of reactions that are inconveniently slow.
It also lets us decrease the rates of reactions that are inconveniently fast. In
some cases we choose to run reactions at low temperatures because
explosions would result (the reaction would run out of control) at room
temperature.
Presence of a Catalyst
Any substance that ………………….2 the rate of a reaction without itself
being used up is called a catalyst.
Many catalysts are known. Some increase the rate of only one reaction
while others can affect several reactions. Although we have seen that
reactions can be sped up by increasing the temperature, in some cases they
are still too slow even at the highest temperatures we can conveniently
reach.
In other cases it is not feasible to increase the temperature, because other,
unwanted reactions would also be sped up. In such cases a catalyst, if we
can find the right one for a given reaction, can prove very valuable.
Many important industrial processes rely on catalysts and virtually all
reactions that take place inside living organisms are catalysed
by…………………….3
How do catalysts increase the rate of reaction?
Catalysts work by allowing the reaction to take a different pathway, one
with lower activation energy.
Without the catalyst, the reactants would have to get over the energy hill.
The catalyst provides a lower hill. As we have seen, lower activation energy
means a higher reaction rate.
2 increases3 enzymesCopyright © Matrix Education 2001-2009 Results ABOVE Expectations! Page 5 of 30
HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
An analogy:
Consider the following analogy which can be used to explain the effect of
catalyst.
There are two ways in which a ball can be pushed over a hill.
1. One way is to simply push them directly over the hill. This is analogous
to an increase in temperature for a chemical reaction.
2. The other way is find a path that goes around the hill. This is analogous
to the role of a catalyst for a chemical reaction.
Source: Tro, N. J., Introductory Chemistry, Prentice-Hall Inc, 2003.
The second method obviously requires less energy to achieve, just try it! The
use of a catalyst therefore increases the rate of reaction whilst improving
energy efficiency of the process.
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HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
By lowering the activation energy using a catalyst, less energy is required for
the reactants to undergo reaction. This effect is shown on a reaction profile
as:
Source: Tro, N. J., Introductory Chemistry, Prentice-Hall Inc, 2003.
The reaction between ethene gas, C2H4, and hydrogen gas, H2, to give
ethane gas, C2H6:
C2H4(g) + H2(g) C2H6(g)
goes so slowly without a catalyst that is not practical even if we increase the
temperature to any reasonable level.
If however the mixture of gases is shaken with finely divided solid platinum,
the reaction takes place at a convenient rate. The C2H4 molecules and the
H2 molecules meet each other on the surface of the platinum where the
proper bonds can be broken and the reaction can proceed.
Each catalyst increases the rate of reaction by providing a surface on which
the reactants can meet. In this way bonds within a molecule are weakened,
or interactions between molecules strengthened, facilitating ease of reaction.
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HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
The sucrose molecule fits perfectly within a pocket on the surface of the
sucrase enzyme. Once held in place, the enzyme weakens the bond
between glucose and fructose, thus lowering the activation energy of the
reaction.
Source: http://www.maxanim.com/biochemistry/index.htm
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HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
2. Le Chatelier’s PrincipleStudents learn to
define Le Chatelier’s principle identify factors which can affect the equilibrium in a reversible reaction
When a reaction has reached equilibrium, the forward and reverse reactions
are taking place at the same rate, and the concentrations of all the
components do not change as long as we don’t do anything to the system.
In 1888 Henri Le Chatelier (1850-1936) put forth the statement known as
Le Chatelier’s principle:
If a system at equilibrium is disturbed, then the system adjusts itself so as to minimise the disturbance.
Addition of Reaction Components
Suppose that this reaction has reached equilibrium:
CH3COOH + C2H5OH CH3COOC2H5 + H2O
Acetic acid Ethanol Ethyl acetate
This means that the reaction flask contains all these substances and that the
concentrations no longer change. We now disturb the system by adding
some acetic acid from outside.
The result is that the concentration of acetic acid suddenly increases, which
increases the rate of the forward reaction. As a consequence, the
concentrations of products (ethyl acetate and water) begin to increase. At
the time the concentrations of reactants are decreasing.
Now, an increase in the concentrations of products causes the rate of the
reverse reaction to increase, but the rate of the forward reaction is
decreasing, so eventually the two rates will be equal again and a new
equilibrium is established.
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When that happens, the concentrations once again remain constant, but
they are not the same as they were before. The concentrations of ethyl
acetate and water are higher now, and the concentration of ethyl alcohol
lower. The concentration of acetic acid is higher because we added some,
but it is less than it was immediately after we made the addition.
This always happens when we added more of any component to system
equilibrium. The addition constitutes a stress; the system relieves the stress
by increasing the concentrations of the components on the other side. We
say that the equilibrium shifts towards the right:
Adding CH3COOH + C2H5OH CH3COOC2H5 + H2O CH3COOH
The addition of acetic acid causes the reaction to move toward the right:
more CH3COOC2H5 and H2O are formed and some of the CH3COOH and
C2H5OH are used up. The same thing happens if we add C2H5OH.
On the other hand, if we add H2O or CH3COOC2H5, the reaction shifts to the
left:
CH3COOH + C2H5OH CH3COOC2H5 + H2O AddingCH3COOC2H5
We can summarise by saying that the addition of any component causes
the equilibrium to shift to the opposite side.
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HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
Removal of a Reaction Component
It is not always as easy to remove a component from a reaction mixture as it
is to add one, but there are often ways to do it.
The removal of a component, or even a decrease in its concentration, lowers
the corresponding reaction rate and changes the position of the equilibrium.
In this case, the reaction is shifted toward the side from which the reactant
was removed.
In the case of the acetic acid-ethyl alcohol equilibrium, ethyl acetate has the
lowest boiling point of the four components and can be removed by
distillation. If this is done, the equilibrium shifts to that side:
CH3COOH + C2H5OH CH3COOC2H5 + H2O Removing CH3COOC2H5
The concentrations of CH3COOH and C2H5OH decrease and that of H2O
increases. The effect of removing a component is thus the opposite of
adding one: the removal of a component causes the equilibrium to shift to the side the component was removed from.
No matter what happens to the individual concentrations, the value of the
equilibrium constant remains unchanged.
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HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
Concept Check 3.1
(i) When dinitrogen tetroxide, N2O4, a colourless gas, is enclosed in a vessel a colour will appear indicating the formation of brown nitrogen dioxide, NO2. The intensity of the brown colour indicates the amount of nitrogen dioxide formed, the equilibrium reaction is
N2O4(g) 2NO2(g)
When more N2O4 is added to the equilibrium mixture, the brownish colour becomes darker. Explain what happened.4
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(ii) What happens to the equilibrium reaction
2NOBr(g) 2NO(g) + Br2(g)
When Br2 gas is added to the equilibrium mixture?
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4 The darker colour indicates that more nitrogen dioxide was formed. This happened because the addition of the reactant shifted the equilibrium to the right, forming more products, NO2.Copyright © Matrix Education 2001-2009 Results ABOVE Expectations! Page 12 of 30
HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
Concept Check 3.2
(i) When acid rain containing sulfuric acid attacks a marble statue made of calcium
carbonate, the following equilibrium reaction can be written
CaCO3(s) + H2SO4(aq) CaSO4(s) + CO2(g) + H2O(l)
How does the fact that CO2 is a gas influence the equilibrium?5
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(ii) In a power plant using coal, SO2 is generated from pyrite, FeS2, small amounts of
which are present in the coal, according to the reaction:
4FeS2(s) + 11O2(g) 2Fe2O3(s) + 8SO2(g)
SO2 is much more soluble in water than O2 and reacts with water to form H2SO3.
What is the effect on the equilibrium if the above reaction is performed in a moist
atmosphere?6
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5 The gaseous CO2 diffuses away from the reaction site, thus removing the product from the equilibrium mixture. The rate of forward reaction is greater than the rate of reverse reaction. Therefore the equilibrium shifts to the right.6 Since the reaction is taken in a moist environment, the SO2 which is more soluble in water than O2, will be removed away from the products. Therefore in order to restore its equilibrium, FeS2 and O2 will react to produce more of SO2. This means that the rate of forward reaction has increased and is greater than the rate of reverse reaction. Hence, the equilibrium shifts to the right.Copyright © Matrix Education 2001-2009 Results ABOVE Expectations! Page 13 of 30
HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
Change in Temperature
The effect of a change in temperature on a reaction that has reached
equilibrium depends on whether the reaction is exothermic (gives off heat) or
endothermic (requires heat). Let us look at an exothermic reaction:
2H2(g) + O2(g) 2H2O(l) + 137 000 cal
This equation tells us that 2 moles of H2 react with 1 mole of O2 to give 2
moles of H2O and heat.
If we look upon the heat as a product of this reaction, we can use the same
type of reasoning as we did before.
An increase in temperature means that we are adding heat. Since heat is a
product, its addition pushes the equilibrium to the opposite side. We can
therefore say that, if the reaction is at equilibrium and we increase the
temperature, the reaction goes to the left - the concentrations of H2 and O2
increase and that of H2O decrease. This is true of all exothermic reactions.
For an exothermic reaction
An increase in temperature drives an exothermic reaction toward the
reactants (to the left).
A decrease in temperature drives an exothermic reaction toward the
products (to the right).
For an endothermic reaction:
An increase in temperature drives an endothermic reaction toward
the products.
A decrease in temperature drives an endothermic reaction toward the
reactants.
Remember that a change in temperature changes not only the position of
equilibrium but the value of K as well.
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HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
Concept Check 3.3(i) The conversion of nitrogen dioxide to dinitrogen tetraoxide is an exothermic reaction:
2NO2(g) N2O4(g) + 57 kJ mol-1
NO2 is a brown gas while N2O4 is colourless.To observe this equilibrium we can contain this reaction mixture within a sealed test tube. See figure below.
Source: Petrucci, Harwood & Herring, General Chemistry: Principles and Modern Applications, 8th Ed, Prentice-Hall, 2002.
When the test tube containing the reaction mixture is taken from an ice-water mix at 0 °C (left) and warmed in a cup containing hot water at 50 °C (right) the brown colour is darker; hence, the NO2 concentration is higher than at 0 °C. Why?
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HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
(ii) In a reaction A B, the amount of B has increased when the temperature of the reaction was raised. Was this an exothermic or an endothermic reaction? Explain.
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Molecular Workbench Simulation
Chemical Equilibrium and Le Chatelier's Principle
http://molo.concord.org/database/activities/193.html
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HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
4. Carbon Dioxide and CarbonatesStudents learn to
describe the solubility of carbon dioxide in water under various conditions as an equilibrium process and explain in terms of Le Chatelier’s principle
What is a Carbon dioxide?
Carbon dioxide (CO2) is a c……………….., non-toxic gas with a “faintly
pungent” odor.
The carbon dioxide molecule is l……………. with double bonds between the
carbon and oxygen atoms.
Draw the structure of the CO2 molecule
Physical and Chemical Properties
Gaseous CO2 is readily condensed to the liquid state by cooling and
compression and upon further cooling the liquid freezes to a white solid
(called “dry ice”).
The substance does not melt upon warming, but sublimes at 78.5 °C. Solid
CO2 leaves no trace when it evaporates (except for the water that
condenses on its surface). It is very convenient for use in cooling ice cream
(it is often used in Baskin Robins Ice cream shops), reactive chemicals, and
many other things.
At high temperatures (above 1700 °C) carbon dioxide decomposes to give
carbon monoxide and oxygen.
2CO2(g) 2CO(g) + O2(g)
This is an endothermic reaction.
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HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
The atmosphere contains only 0.0325% carbon dioxide by volume, but
atmospheric carbon dioxide plays an important role in photosynthesis and
the cycling of carbon and oxygen through the environment. You have
learned this in The Identification and Production of Materials.
On the other hand, life is impossible in an atmosphere that contains too
much carbon dioxide. A CO2 concentration of 1% by volume in air can
cause headaches, 10% can cause severe distress and over 30% causes
unconsciousness and death.
Some uses of Carbon Dioxide
Carbon dioxide is inert, does not support combustion and is denser than air,
allowing it to blanket a fire, it is therefore an excellent …………………….
…………………………7 agent. Liquid carbon dioxide (held in tanks at
elevated pressure) is used in fire – extinguishing systems in airplanes, ships,
chemical plants and other industrial installations. Hand – held extinguishers
also often contain carbon dioxide under pressure. When the pressure is
released by opening a valve, the liquid carbon dioxide escapes and
immediately evaporates. Expansion of the gas causes a dramatic drop in
temperature. As a result, the CO2 freezes and forms a blanket of CO2
“snow”.
………………………….8 in production of organic chemicals.
Recovery of oil from depleted wells.
Refrigeration and food preservation.
………………………….9 carbonation
7 fire extinguishing8 Raw material9 BeverageCopyright © Matrix Education 2001-2009 Results ABOVE Expectations! Page 18 of 30
HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
Solubility of Carbon Dioxide in water
Carbon dioxide dissolves readily in water and reacts to form
………………..10 acid, H2CO3. This acid cannot be isolated, and carbon
dioxide is usually shown reacting directly with water to form hydrogen ions
and hydrogen carbonate ions:
CO2(g) CO2(aq) Equation 1CO2(aq) + H2O(l) H+
(aq) + HCO3(aq) Equation 2
In pure water a saturated CO2 solution at 1 atm pressure and 25 °C contains
about 0.034 mol/L of CO2 gas, most of which is simply dissolved in the
water. Only approximately one out of every 400 CO2 molecules in solution
reacts with a water molecule to give carbonic acid (H2CO3). This acid cannot
be isolated but does exist in small concentrations in aqueous CO2 solutions.
Carbon Dioxide in soda water
Why do carbonated soft drinks have a sharp acidic taste?
Coke and other effervescent soft drinks contain carbon dioxide gas
under pressure. As CO2(g) is dissolved in water (see Equation 1), the
concentration of CO2(aq) in equation 2 is ……………………….11
Since the concentration of reactants has ………..…………….12 , the rate
of forward reaction will also increase while the rate of reverse reaction
remains ………….………………….13 Since the rate of forward reaction
is greater than the rate of reverse reaction, the concentration of
products will ……………….14 The concentration of H+ and HCO3
…………….……15
Hence the position of the equilibrium has shifted towards the r….
………….
Therefore the soft drink has a sharp acidic taste (pH = 3.8).
10 carbonic11 increased12 increased13 unchanged14 increase15 increaseCopyright © Matrix Education 2001-2009 Results ABOVE Expectations! Page 19 of 30
HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
Why does a soft drink undergo effervescence when the cap is removed?
CO2(aq) + H2O(l) H+(aq) + HCO3(aq)
When the cap is opened, the system at equilibrium is opened. As
a result CO2(g) in the system escapes.
To restore equilibrium, CO2(g) must be reproduced. Therefore, CO2(aq) is
converted to CO2(g).
This represents a removal of reactants as CO2(aq) is converted to CO2(g).
Therefore, the rate of forward reaction …..…………….16 as the
concentration of the reactant has decreased.
Since the rate of reverse reaction is ………….…….17 than the rate of
forward reaction, the equilibrium position shifts towards the
……………..18
As a result more CO2(aq) is produced to restore equilibrium.
Hence effervescence is observed.
Why does a soft drink taste “flat” when it loses its carbon dioxide gas?
Loss of CO2(g) represents a removal of reactants as CO2(aq) is converted to
CO2(g).
Therefore, the rate of forward reaction …………………19 as the
concentration of the reactant has decreased.
Since the rate of reverse reaction is …….……………..20 than the rate of
forward reaction, the equilibrium position shifts towards the ……………..21
This represents a decrease in concentration of H+ and HCO3 since more
CO2(aq) is being produced to restore equilibrium.
Therefore, a decrease in concentration of H+ means a decrease in acidity
and loss of sharp acidic taste.
16 decreases17 greater18 left19 decreases20 greater21 leftCopyright © Matrix Education 2001-2009 Results ABOVE Expectations! Page 20 of 30
HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
Solubility of carbon dioxide in water under various conditions
Solubility of CO2 in water:
CO2(g) + H2O(l) H2CO3(aq)
Disturbance Effect on equilibrium (right means increased solubility)
Increase in pressure Favours reaction with fewer moles of gas in product,
reducing pressure, therefore shifts right, CO2 dissolves
more readily.
Increase in concentration
of CO2 in system
System aims to reduce concentration of CO2 and thus it
reacts, i.e., shifts equilibrium right
Increase in temperature Favours endothermic reaction, i.e. reverse reaction,
equilibrium shifts left, and CO2 is less soluble
Concept Check 4.1An equilibrium exists between gaseous and dissolved carbon dioxide in water as shown by the following equation:
CO2(g) CO2(aq)
With reference to Le Chatelier’s principle explain the following:
(a) Fizzing occurs when a bottle of a carbonated drink is opened. 2
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HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
(b) It is observed that the fizzing is less if the bottle is kept under refrigeration rather than at room temperature. Deduce whether the dissolving process is exothermic or endothermic. Explain your reasoning. 2
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Concept Check 4.2
Carbon dioxide is poorly soluble. Small amounts of carbon dioxide can be dissolved in water and like nearly all gases it is more soluble in cold water.
(a) Write an equation showing carbon dioxide in water. Explain in terms of Le Chatelier’s Principle why high pressure is used to increase the amount of carbon dioxide which dissolves. 2
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(b) Suggest a reason why more gas dissolves at lower temperature. 1
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HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
Concept Check 4.3 [2005 NEAP Q23]
Use Le Chatelier’s Principle to relate the increase in burning fossil fuels to a possible increase in the acidity of the oceans.22 4
COURSE OUTCOMES: H4, H8, H13MARKING GUIDELINES
CRITERIA MARKS
Relates increase in atmospheric CO2 to increases in dissolved CO2 and reaction with water to produce H+
(aq), through application of Le Chatelier’s Principle.3 ~ 4
Describes increases in acidity due to more dissolving of CO2 or increased production of H+
(aq)
2
Describes Le Chatelier’s Principle using CO2 in some way. 1
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22 As the concentration of CO2 in the atmosphere rises, more of it will dissolve in the oceans. This is a consequence of Le Chatelier’s Principle, which states that when an equilibrium is disturbed the reaction will shift to minimize the changes. In this case the first equilibrium system affected is CO2(g) CO2(aq). Then CO2(aq) + H2O(l) H+
(aq) + HCO3(aq) This increase in concentration of H+ means the acidity has increased.Copyright © Matrix Education 2001-2009 Results ABOVE Expectations! Page 23 of 30
HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
Concept Check 4.4 [2005 CSSA Q24]
Two identical bottles of soda water (carbonated water), one at room temperature (25 °C) and one just out of the refrigerator, had their pH determined using a probe and data logger. The results are tabulated below.
Soda Water pH Temperature (C)
Bottle A 5.21 25
Bottle B 4.63 4
Account for the difference in pH in terms of Le Chatelier’s Principle.23
COURSE OUTCOMES: H2, H8, H14MARKING GUIDELINES
CRITERIA MARKS
Demonstrates a thorough knowledge of Le Chatelier’s Principle with regard to the solubility of carbon dioxide in water
4
Demonstrates a sound knowledge of Le Chatelier’s Principle Outline changes in the CO2/H2CO3 equilibrium
2 ~ 3
Identifies Le Chatelier’s Principle ORIdentifies that at lower temperatures, [H+] increases
1
23 The solubility of CO2(g) in water is an equilibrium reaction : CO2(aq) + H2O(l) H+
(aq) + HCO3(aq) where H is negative.For this equilibrium, if you increase the temperature then the equilibrium will shift to the LHS reducing the solubility of the carbon dioxide. Since the CO2(g) when dissolved, reacts with water to produce H+ ions, the lower the solubility then the higher the pH. Hence, in bottle A at the higher temperature, we would expect a higher pH, indicating less H+ ions in solution as the solubility of the CO2(g) is lower than bottle B which is at a lower temperature and hence, would have more CO2(g) dissolved and hence a lower pH according to Le Chatelier’s Principle.Copyright © Matrix Education 2001-2009 Results ABOVE Expectations! Page 24 of 30
HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
5. First Hand Investigation: Decarbonation of Soda Water
Students identify data, plan and perform a first-hand investigation to
decarbonate soft drink and gather data to measure the mass changes involved and calculate the volume of gas released at 25 °C and 100 kPa
Soda water or carbonated water is a solution of carbon dioxide in water.
Soft drinks are carbonated water with added flavours and sweeteners.
Materials: Weighing equipment accurate to at least the nearest gram.
An unopened small bottle (or can) of soda water with liquid level as low as
possible; 250 mL or 300 mL size is adequate.
Either a source of dry heat such as an electric hotplate or a saucepan in
which the soda water can be gently warmed. A dry towel and a thermometer
(the warming method) OR 1g of table salt per 50 mL of soda water (the
salting method)
Method:1. Weigh the unopened bottle of soda water: ……………..g
2. Slowly remove the cap controlling the release of bubbles so that the soda water
does not foam out the top and you lose soda water. If this happens you will need
to buy another bottle and start all over again. Observe what happens to the
release of bubbles when you retighten the cap.
3. Reweigh the bottle including the cap: ………………….g
4. Calculate the mass of carbon dioxide lost when the cap was removed.
5. Now choose either the warming method or the salting method.
Copyright © Matrix Education 2001-2009 Results ABOVE Expectations! Page 25 of 30
HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
WARMING METHOD: Uses reduced water solubility of gas with temperature rise.
6. Carefully stand the open bottle on an electric hotplate or in a saucepan of heated
water. Insert the thermometer. Stirring encourages release of gas bubbles.
7. Heat the soda water bottle to about 38C (Human body temperature)
8. Remove the soda water bottle, dry it well with a towel, then reweigh the bottle
and its cap.
Weight of bottle and cap after warming: …………………….g
Calculate the change in weight due to loss of CO2 gas to the air: see calculations
below.
SALTING METHOD: Uses the addition of ions which attract water molecules, thereby
reducing the ability of water to dissolve gases.
6. Weigh about 1g of table salt for each 50 mL of soda water accurately:
…………..g
7. Add the salt very slowly and carefully to the soda water so that the water does
not degas too quickly. This is especially important in the early stages. If soda
water foams over the top of the container you will lose weight and need to start all
over again. Control spraying so that spray does not leave the container and
affect weighing.
8. Reweigh the bottle and cap after salting: ………………….. g
Allowing for the weight of salt added to the soda water, calculate the change in
weight due to loss of CO2 gas to the air:
Calculations To calculate the volume that this mass of CO2 gas would occupy, you need
to convert the mass in g to mole using the equation:
The molar mass of CO2 = 12 + 2 16 = 44g/mole. Therefore, number of
moles of CO2, n = m/44
The molar volume of a gas at 25 °C and 100 kPa pressure is 24.8 L The
volume of gas can be calculated.
Copyright © Matrix Education 2001-2009 Results ABOVE Expectations! Page 26 of 30
HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
For 1g of CO2
at 25C and 100kPa
Conclusions
Compare the volume of gas released with the volume of liquid soda water which contained that amount of gas.24
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What do these volumes illustrate about the distance between CO2 particles in the gas phase compared with the distance between CO2 particles in solution?25
………………………………………………………………………………………
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Explain why bubbles of CO2 gas escape from solution in a bottle when the cap is undone.26
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24 Volume of gas released is a few times greater than the volume of soda water which contained that gas.25 These volumes indicate that CO2 molecules are further apart when in the gas phase than when dissolved in soda water.26 When the cap is undone the pressure of CO2 acting on the water surface decreases. The number of CO2 molecules entering the water is now much less than the number of CO2 molecules leaving the water. Equilibrium shifts to oppose the change by CO2 passing from solution to gas phase.Copyright © Matrix Education 2001-2009 Results ABOVE Expectations! Page 27 of 30
HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
Concept Check 5.1 [2004 NEAP Q25]
As part of your practical work you decarbonated a beverage. A student decarbonated a sample of soda water by opening the bottle it was in and leaving it for a period of time, weighing it at regular intervals. She also used a non-carbonated sample of water as a control, recording its mass at the same intervals.
Mass (g)
Initial After 12 hours 24 hours 36 hours 48 hours 60 hours
Soda water 385.0 382.6 381.1 380.7 380.3 380.0
Plain water 385.0 384.7 384.2 383.7 383.4 383.0
(a) Graph the information shown for each water sample on the same graph.
379
380
381
382
383
384
385
0 12 24 36 48 60 72Time / hours
Mas
s / g
ram
s
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HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
(b) Interpret the trends shown in the graph.27
Marking Guidelines
Criteria MarksInterprets each graph and forms relationship between them 2
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(c) Use the graph to determine the volume of CO2 gas produced at 25oC and 100 kPa. Show your working.28
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27 The plain water graph shows the loss of mass due to evaporation. The soda water graph shows the loss of mass due to water evaporation AND carbon dioxide gas. When no more CO2 remains the line parallels the water graph.28 From the graph, the mass of carbon dioxide gas lost is 3.0g. 1.69LCopyright © Matrix Education 2001-2009 Results ABOVE Expectations! Page 29 of 30
HSC Chemistry Acidic Environment 2[Le Chatelier’s Principle]
ANSWERS
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