chemistry - the acidic environment

Curtis Moore Year 12 Chemistry

The Acidic Environment1. Indicators were identified with the observation that the colour of some flowers depends on soil composition

“Classify common substances as acidic, basic or neutral”

All of the substances we use are acidic, basic, or neutral. This classification is usually based on the concentration of hydrogen ions produced when the substance is dissolved in water

Acids:

Common Acids:o Hydrochloric acid (HCL), sulphuric acid (H2SO4) and Nitric acid (HNO3)o Vinegar, lemon juice, aspirin

Bases

Strong bases complete ionize in aqueous solution to give OH- and a cation Weak acids do not completely ionize in solution but exist in equilibrium. Common bases:

o Sodium Hydroxide (NaOH)barium hydroxide (Ba(OH)2), potassium oxide (K20) Iron III oxide (Fe2O3), copper hydroxide (Cu(OH)2) and Ammonia(NH3)

o Cleaning agents ammonia , sodium carbonate, oven cleaners

“Identify that indicators such as litmus, phenolphthalein, methyl orange and bromothymol blue can be used to determine the acidic or basic nature of a material over a range, and that the range is identified by change in indicator colour”

Indicators: substances that change depending on the concentration of hydronium (H30+) ions in a solution to determine whether a solution is acidic or basic.

“Identify and describe some everyday uses of indicators including the testing of soil acidity/basicity”

Testing Soil pH:

o Some plants only grow within narrow pH ranges, so the pH of the soil needs to be

regularly tested.

o A neutral white powder (barium sulfate) is sprinkled over the damp soil; a few drops of

indicator(s) are placed on top.

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o The white powder allows the colour change to be seen clearly.

Testing pH of Pools:

o Pool water must be near neutral to avoid health problems.

o A few drops of indicator is placed in a sample of the pool water; alternatively, pH paper,

already soaked in indicator can be used.

Effluent Testing :

o Acidity is a useful way to assess the levels of certain types of pollution from industry.

o Industry technicians and government authorities use indicators to monitor the pH of

water and natural waterways

o Indicators are used to measure the pH, and substances added to neutralise it.

2. While we usually think of the air around us as neutral, the atmosphere naturally contains acidic oxides of carbon, nitrogen and sulfur. The concentrations of these acidic oxides have been increasing since the Industrial Revolution:

“Identify oxides of non-metals which acts acids and describes the conditions under which they act as acids”

Acidic Oxides: one which reacts with water to form acids or one which reacts with bases to form salts. Oxides are compounds that contain OXYGEN. They also react with WATER to form acids. React with BASES to form salts.

Some non-metal oxides that act as acids are: CO2 (carbon dioxide), SO2 (sulfur dioxide),

NO2 (nitrogen dioxide) and P2O5 (phosphorus pentoxide).

When they are in solution, they become acids:

CO2 (g) + H2O (l) H2CO3 (aq) (carbonic acid)

SO2 (g) + H2O (l) H2SO3 (aq) (sulfurous acid)

2NO2 (g) + H2O (l) HNO3 (aq) + HNO2 (aq) (nitric and nitrous acid)

P2O5 (g) + H2O (l) 2H3PO4 (aq) (phosphoric acid)

Basic Oxides: one which reacts with acids to form salts, but does not react with alkali solutions.

Metal oxides behave as BASES.

E.G. Some metal oxides that act as bases are: K2O (potassium oxide), Na2O (sodium oxide) and

MgO (magnesium oxide).

In solution, tend to form basic hydroxides:

K2O (s) + H2O (l) 2KOH (aq) (potassium hydroxide)

Na2O (s) + H2O (l) 2NaOH (aq) (sodium hydroxide)

MgO (s) + H2O (l) Mg(OH)2 (aq) (magnesium hydroxide)

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o AMPHOTERIC OXIDES:

Amphoteric oxides are oxides that can act as BOTH acids and bases.

Their behaviour depends on the reaction they are put in.

The only elements that combine to form amphoteric oxides are beryllium, aluminium,

zinc, tin and lead.

The exceptions are the neutral oxides include N2O (dinitrogen oxide), CO (carbon monoxide) and NO (nitric oxide), which do not form acids.

“Analyse the position of these non- metals in the periodic table and outline the relationship between position of elements in the periodic table and acidity/ basicity of oxides.”

The acidic oxides are on the RIGHT side of the periodic table (non-metals).

The basic oxides are on the LEFT side of the periodic table (metals).

The amphoteric oxides are in-between; NOBEL gases have no oxides.

The basic trend in oxide acidity across the rows of the periodic table is:

Basic → Amphoteric → Acidic

The basic trend in oxide activity down the period groups of the periodic table is:

Acidic→ Amphoteric → Basic.

Non -metals are found mostly in the top-right corner of the periodic table, and thus have acidic

oxides

“Define Le Chatelier’s principle”

When a change is made to an equilibrium system, the system moves to counteract the imposed change and restore the system to equilibrium

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“Identify factors which can affect the equilibrium in a reversible reaction”

The following changes to an equilibrium system will have the following effects, with each effect obeying Le Chatelier’s principle by opposing the change:

o Changes in temperature :o If the forward reaction is exothermic, an increase in temperature causes the system

to shift in the reverse direction.o If the forward reaction is endothermic, an increase in temperature causes the

system to shift in the forward direction.o The opposite effects would be observed for a decrease in temperature.

o Changes in concentration :o An increase in the concentration of one or more reactants, or a decrease in the

concentration of one or more products, causes the system to shift in the forward direction.

o A decrease in the concentration of one or more of reactants, or an increase of the concentration of one or more products, causes the system to shift in the reverse direction.

o Changes in pressure (of homogenous gaseous systems):o An increase in pressure causes the system to shift in the direction of fewer gas

molecules.o A decrease in pressure causes the system to shift in the direction of more gas

molecules.o If the number of particles on each side of the reaction is equal, then a change in

pressure has no effect.

“Describe the solubility of carbon dioxide in water under various conditions as an equilibrium process and explain in terms of Le Chatelier’s principle”

o TEMPRETURE o As the forward reaction is exothermic, an INCREASE in temperature causes the

system to move in the reverse direction.o As the forward reaction is exothermic, a DECREASE in temperature causes the

system to move in the forward direction.o CONCENTRATION

o An INCREASE in the concentration of carbon dioxide or a decrease in the concentration of carbon dioxide causes the system to move in the forward direction.

o A DECREASE in the concentration of carbon dioxide or an increase in the concentration of carbonic acid causes the system to move in the reverse direction.

o PRESSURE

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o An INCREASE in pressure causes the system to move in the forward direction.o A DECREASE in pressure causes the system to move in the reverse direction.

o Thus, the solubility of carbon dioxide in water is dependent on temperature, concentration and pressure.

“Identify natural and industrial sources of sulfur dioxide and oxides of nitrogen:”

“Describe, using equations, examples of chemical reactions which release sulfur dioxide and chemical reactions which release oxides of nitrogen”

Oxides Natural Sources Industrial Sources

Sulphur dioxide (SO2)

Geothermal Hot springs Emissions from Volcanoes Bushfires Natural decay of vegetation on

land, wet lands and oceans2H2S (g) + 3O2 (g) 2SO2 (g) + 2H2O (l)

The Burning of fossil fuels which contain sulphur, such as coal and oil

S(s) + O2 (g) SO2(g)

When metals are extracted from sulphides ores.

2ZnS(s) + 3O2(g) 2ZnO (s) + 2SO2(g)

Oxides of Nitrogen (NOX)

Lighting O2(g) + N2 (g) 2 NO(g)

N2O formed by bacteria

Indoor domestic appliances )( gas stoves, gas heaters etc)

Motor vehicles

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“Analyse information from secondary sources to summarise the industrial origins of sulfur dioxide and oxides of nitrogen and evaluate reasons for concern about their release into the environment”

These oxides combine with water to form an acidic solution in the atmosphere creating acid rain. Increase in oxides have led to increase production of photochemical smog, especially in large cities which brings forth respiratory problems of humans and animals, ie. Breathing difficulties, tissue damage

“Assess the evidence which indicates increases in atmospheric concentration of oxides of sulfur and nitrogen”

Evidence of the oxides of sulphur and nitrogen being formed: Oxides o sulphur and nitrogen are mainly found in the atmosphere as a result to a number of natural

process such as: Volcanic action , which produces sulphur dioxide Natural decay of vegetation on land and sea (SO2) Lighting which produces nitric oxide

Due to the industrial revolution and increased human population, there has been an increase of these oxides. manmade sources of the pollutants include:

Extracting metals form sulphides ores (SO2)

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Burning of fossil fuels for car and engines and power plants, which all increase the concentration of sulphur and nitrogen in the atmosphere.

The increase in atmospheric concentrations of these oxides is evident form : The formation of acid rain, which has damaged waterways, forest, farm and buildings over

many years The build up of atmospheric pollution, with nitrogen oxides contributing to the

photochemical smog. High levels of these oxides have led to health issues, often in large cities

“Calculate volumes of gases given masses of some substances in reactions, and calculate masses of substances given gaseous volumes, in reactions involving gases at 0˚C and 100kPa or 25˚C and 100kPa”

“Explain the formation and effects of acid rain”

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Rain is normally slightly acidic (pH between 5 and 6) due to the formation of carbonic acid from carbon dioxide.

Acid rain: Rain that has a higher hydrogen ion concentration than normal (pH less than 5). Other than carbonic acid, the acids generally present in acid rain are sulfuric and nitric acids,

which are formed by sulfur dioxide and oxides of nitrogen respectively.

FORMATION of acid rain:

Sulfur dioxide reacts with rain in the atmosphere forming sulfurous acid:

SO2 (g) + H2O (l) H2SO3 (aq)

Sulfurous acid then reacts with oxygen; this is catalysed by air particles:

2H2SO3 (aq) + O2 (g) 2H2SO4 (aq)

Nitrogen dioxide also reacts with rain, making nitric and nitrous acids:

2NO2 (g) + H2O (l) HNO3 (aq) + HNO2 (aq)

Nitrous acid then reacts with oxygen, again catalysed by air particles:

2HNO2 (aq) + O2 (g) 2HNO3 (aq)

Thus, in industrialised areas, rain can contain relatively high levels of very strong acids;

that is nitric and sulfuric acids.

This reduces the pH to about 3-5, which is grossly unnatural and harmful.

EFFECTS of acid rain:

Environmental damage includes the destruction of forests (such as the pine forests of

Europe and North America), the increasing acidity of lakes, leading to the death of

aquatic life (as has been observed in many lakes in Scandinavia) and the ruining of soils,

making them unable to support vegetation.

Urban and structural damage include the corrosion of marble statues and buildings

(marble contains carbonates which acids readily react with).

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3. Acids occur in many foods, drinks and even within our stomachs:

“Define acids as proton donors and describe the ionisation of acids in water”

When acids dissolve in water, it can donate a proton, ie a Hydrogen Ion (H+) to a water molecule

The proton ion in water solution may be presented as H+ (aq) or H3O+ (aq) ( hydronium) For example

HCl (g) H+ (aq) + Cl־(aq) ( donor)

HCl (g) + H2O (l) H3O+ (aq) + Cl־(aq)

“Identify acids including acetic (ethanoic), citric (2-hydroxypropane-1,2,3-tricarboxylic), hydrochloric and sulfuric acid”

Acetic Acid:

Systematic name: Ethanoic acid

Molecular formula: CH3COOH

It is the acid present in vinegar.

It is classified as a weak acid.

Citric Acid:

2-hydroxypropane-1,2,3-tricarboxylic acid

Molecular formula: C6H8O7

It occurs in citrus fruit.

Hydrochloric Acid:

Molecular formula: HCl

It is a very strong acid.

Sulfuric Acid:

Molecular formula: H2SO4

It is also a strong acid.

Used to make batteries, fertilizers etc.

“Describe the use of the pH scale in comparing acids and bases”

The pH is a measure of the concentration of hydrogen ions in solution. Low pH means a high level of proton (hydrogen) concentration and thus very acidic and a High pH means a low level of proton concentrations, so it’s basic.

“Describe acids and their solutions with the appropriate use of the terms strong, weak, and concentrated and dilute”

Strong acids: one that disassociate completely or almost completely in water, forming H+

Weak acids: those that partially ionize in water

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Concentrated acids: are those that have a large amount of solute in a given amount of solution. It has a high number of moles per litre

Dilute acids: those that have low amounts of solute ionized in a given amount of solution.

“Identify pH as -log10 [H+] and explain that a change in pH of 1 means a ten-fold change in [H+]”

“Compare the relative strengths of equal concentrations of citric, acetic and hydrochloric acids and explain in terms of the degree of ionisation of their molecules”

Acid Strength Degree of Ionization Citric acid Weak (pH 2) Very low Acetic acid Weak (pH3) Low Hydrochloric acid Strong (pH 1) Very high

Comparison:

Citric and acetic acid are both very weak acids. Acetic acid is a weaker acid with a pH of around 2.9 while critic acids has a pH od around 2.1

Hydrochloric acid is a much stronger acid, around pH1. This means that HCL has a very high degree of ionization, so the forward reaction in: HCL (aq) + H20(l) H3O+ (aq) + Cl- (aq) is much more favoured, producing more ions.

“Describe the difference between a strong and a weak acid in terms of an equilibrium between the intact molecule and its ions”

A strong acid is an acid that releases ALL its H+ ions when in solution.

Its molecules are completely disassociated in solution.

Its ionisation reaction goes to completion:

EG: HCl (g) + H2O (l) H3O+ (aq) + Cl־(aq)

A weak acid is an acid that does NOT completely release all its H+ ions; Some of its molecules

remain intact in solution

Its ionisation reaction with water is a reversible reaction that reaches equilibrium when a

certain number of H+ ions are released:

EG: CH3COOH (s) + H2O (l) H3O+ (aq) + CH3COO־

(aq)

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The terms concentrated and dilute refer ONLY to the amount of acid molecules present in the

solution; they have nothing to do with ionisation.

A CONCENTRATED acid has approximately more than 5 mol/L of solute.

A DILUTE acid has less that 2 mol/L of solute.

Hence, strong acids can be both concentrated and dilute, and weak acids can also be both

concentrated and dilute.

4. Because of the prevalence of acids, they have been used and studied for hundreds of years. Over time the definitions of acids and base have been refined:

“Outline the historical development of ideas about acids including those of:- Lavoisier- Davy- Arrhenius”

Lavoisier:

Proposed that an acid must conatin oxygen Non metals oxides react with water to form acidic solutions

Davy

Redefined an acid as a substance containing hydrogen (rather than oxygen), which could be partially or totally replaced by metals when it reacted together, to produce a salt. He realized this by discovering that HCl had acidic properties

Arrhenius:

Suggested that acids ionized in solution to produce hydrogen ions as the only positive ions in solution

Bases produced hydroxide ions as the only negative ion in solution Completely ionized acids were strong acids, weak id they partially ionized Arrhenius proposed these ideas only when the acid was in an aqueous state

“Outline the Brönsted-Lowry theory of acids and bases”

It states that:

An acid is a PROTON donor (is gives protons to a base)

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A base is a PROTON acceptor (is accepts protons from an acid)

Brönsted-Lowry definition they can be solids, gases, cations, anions, non-water

solutions; even water can be an acid, or a base.

EG: Reacting hydrogen chloride gas and ammonia:

HCl (g) + NH3 (g) NH4Cl (s)

If one looks closer, a proton donation and acceptance has occurred:

HCl + NH3 NH4+ + Cl־

HCl has donated a proton to NH3 OR NH3 has accepted a proton from HCl.

Hence, by Brönsted-Lowry definition, HCl gas is an acid, and NH3 gas is a base.

“Describe the relationship between an acid and its conjugate base and a base and its conjugate acid”

“Identify a range of salts which form acidic, basic or neutral solutions and explain their acidic, neutral or basic nature “

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“Identify conjugate acid/base pairs”

“Identify amphiprotic substances and construct equations to describe their behaviour in acidic and basic solutions”

An amphiprotic substance is one that can act as BOTH an acid and a base.

Their behaviour depends on the environment they are placed in.

EG: The hydrogen carbonate ion (bicarbonate ion) HCO3:is amphiprotic ־

HCO3 ־

(aq) + H3O+ (aq) H2CO3 (aq) + H2O (l)

HCO3 ־

(aq) + OH ־ (aq) CO3

־2 (aq) + H2O (l)

“Identify neutralisation as a proton transfer reaction which is exothermic”

Neutralization reactions consist of an acid reacting with a base to produce a salt + water. This is a proton transfer reaction, which is exothermic

Eg. NaOH (aq) + CH3OOH (aq) NaCH3COO (aq) + H20 (l) in this reaction, a proton from acetic acid and an OH from sodium hydroxide cancels out, providing water. The remainder is a salt, sodium ethanoate.

“Describe the correct technique for conducting titrations and preparation of standard solutions”

Titration is a chemical technique used to experimentally determine the unknown

concentration of a solution through a chemical reaction.

EG: Nitric Acid & Sodium Carbonate:

Example, the titration of a solution of nitric acid (HNO3) of UNKNOWN concentration

with a known solution of alkaline, 0.05 M sodium carbonate (Na2CO3).

The first step in any acid/base titration is to identify the chemical reaction that is going

to occur, by writing a balanced chemical equation. In this case:

2HNO3 (aq) + Na2CO3 (aq) 2NaNO3 (aq) + CO2 (g) + H2O (l)

20 mL of the 0.05 M sodium carbonate was titrated with the nitric acid and the endpoint

was reached after 22.2 mL of nitric was used.

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Looking at the chemical equation, the molar ratio of reaction is 2:1

Hence, 22.2 mL of nitric acid contains TWICE as many moles as 20 mL of sodium

carbonate. Calculating:

n1 = c1 × v1 = 0.05 x 0.02 = 0.001 mol of sodium carbonate.

n2 = 2 × n1 = 0.002 mol of nitric acid.

c2 = n2 / v2 = 0.002 / 0.0222 = 0.09 M

From this titration, the concentration of nitric acid was calculated to be 0.09 M

Method to do a titration:

Volumetric Flask: This is used to prepare and hold standard solutions.

Conical Flask: This is used to hold the reactants during titration. Its shape prevents

the reactants from spilling as they are swirled together.

Burette: The burette is a piece of cylindrical glassware, held vertically, with

volumetric divisions on its full length and a precision tap (stopcock), on the bottom.

It is used to dispense precise amounts of a liquid reagent in titration. Burettes are

extremely precise and accurate to ±0.05 mL.

Pipette: The pipette is a glass tube used to transfer precise volumes of liquid

reagents. Pipettes are usually designed to transfer one measurement of volume,

such as only 25 mL. The reagent is drawn up the pipette using a pipette filler (e.g. a

rubber bulb).

Before titration can begin, all glassware must be RINSED appropriately. The proper rinsing

technique for the different glassware is:

After everything is rinsed appropriately, the glassware is filled.

Usually, the base is placed in the burette, and the acid in the conical flask, but it really

doesn’t matter much. Using the above example again:

Using a funnel, the sodium carbonate is poured into the burette until ABOVE the

zero mark. Hold a white card behind the zero mark, and open the tap slowly until

the meniscus sits JUST on top of the mark. The white card makes the meniscus

clearer.

Using the pipette, a fixed volume of nitric acid (say 25 mL) is drawn from the

volumetric flask and deposited into the conical flask.

A few drops of methyl orange indicator are added to the conical flask, and the solution

turns a clear yellow colour.

THE TITRATION:

Finally, the titration can be performed.

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The conical flask is placed on a white tile (to make the solution’s colour clear) under the

burette, which is held in a retort stand.

The tap is slowly opened, and the conical flask is continuously swirled.

When the first colour change is noticed, the tap is immediately closed. A swirl of the

conical flask will likely return the solution back to its original colour.

Very slowly open the tap so that solution flows out in drops, and stop when the endpoint

is reached, as shown by the colour of the indicator:

In the case of the above example, the endpoint is pale pink.

The first titration performed is a rough draft and often overshoots the endpoint. This first

titration is rejected.

Titration is performed multiple times to achieve the accurate results

“Qualitatively describe the effect of buffers with reference to a specific example in a natural system”

5. Esterification is a naturally occurring process which can be performed in the laboratory

“Describe the differences between the alkanol and alkanoic acid functional groups in carbon compounds”

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“Identify the IUPAC nomenclature for describing the esters produced by reactions of straight-chained alkanoic acids from C1 to C8 and straight-chained primary alkanols from C1 to C8”

Step 1: name the alky group, which comes from the alcohol ( methyl , ethyl, propyl, butyl etc)

Step 2: name the alkanoate part ( methanonate, ethanoate, propanoate etc)

“Explain the difference in melting point and boiling point caused by straight-chained alkanoic acid and straight-chained primary alkanol structures”

Alkanols have high melting and boiling points due to the polarity and potential for hydrogen bonding present within the molecules.

Similarly, the polarity of an alkanoic acid is one reason why the melting and boiling points of an alkanoic acid are high, a property increased due to the greater molar mass of alkanoic acids relative to alkanols. When combined with the potential for two occurrences of hydrogen bonding to occur on the -COOH chain (and thus much strong intermolecular forces, driving up the melting and boiling point), this means that the melting and boiling points of alkanoic acids are higher than that of alkanols.

Remember- Alkanoic acids have higher melting and boiling points because the -COOH chain increases intermolecular hydrogen bonding.

“Identify esterification as the reaction between an acid and an alkanol and describe, using equations, examples of esterification”

An ester is formed when an alkanol and an alkanoic acid join: Alkanol + Alkanoic Acid Ester + water

Example:• Ethanol + Ethanoic acid Ethyl ethanoate + water

• HCOOH + CH3CH2OH HCOOCH2CH3 + H2O

• The underlined atoms eat the ones that in equation join together to form water

“Describe the purpose of using acid in esterification for catalysis”

In the process of esterification, concentrated sulfuric acid is used as a catalyst. Acting as a strong dehydrating agent, concentrated sulfuric acid can rapidly remove any water produced during the reaction.

C2H5OH(l) + CH3COOH(l) C4H8O2 (l) + H2O(l)

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According to Le Chatelier’s principle, a system will work to minimise any disturbance to its equilibrium. As such, if water is being removed, then the equilibrium will shift to favour the forward reaction in order to replace the lost water, increasing the amount of ester produced. As such, concentrated sulfuric acid can be used to increase the yield of ester.

Remember- Acid is used during esterification not only to lower the activation energy required for the reaction to take place, but also to increase the output of products.

“Explain the need for refluxing during esterification:”

Heating the reaction flask has 2 main benefits:

The higher the temperature, the faster the rate of reaction;

equilibrium can be reached much faster than if it was left at room

temperature.

Also, esterification is an endothermic reaction; increasing the heat of

the flask encourages the forward reaction, creating more ester.

However, the ester, alkanol and alkanoic acid are all highly volatile

substances; any open heating will cause the reactants as well as products to evaporate

away.

A refluxing apparatus is basically a condenser placed vertically onto a boiling flask; it cools

any vapours that boil off so that they drip back into the flask.

Refluxing allows the mixture to react at high temperatures without fear that the reactants or

products will evaporate away.

“Outline some examples of the occurrence, production and uses of esters”

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chemistry - the acidic environment

Documents

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oxides of nonmetals

nonmetal oxides

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