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HSC Chemistry Module 2: Acidic Environment

HSC Module 2: Acidic Environment

Acids   

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An acid is a substance which in solution produces hydrogen ions, H+, also called the H3O+ hydronium ion. Acids have pH less than 7 Common substance: Vinegar (acetic acid), Vitamin C (ascorbic acid), lemon juice (citric acid), fizzy drinks (carbonic acids) and rain water, aspirin, battery acid (sulfuric acid) Examples: hydrochloric acid (HCl), sulfuric acid (H2SO4), phosphoric acid (H3PO4) and carbonic acid (H2CO3) They can react with many carbonates to produce a salt, water and carbon dioxide gas. o acid + carbonate  carbon dioxide + water + salt

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They react with bases to form water and salts. o acid + base  water + salt

Common Properties of Acids:      

Acids have a sour taste Acids sting or burn the skin Acids only behave as an acid when water is around In solutions, acids conduct electricity Acids react with metals Acids turn blue litmus red

Bases  

A base is a substance which either contains the oxide O2- or hydroxide ion OH- or which in solution produces the hydroxide ion. Bases have pH greater than 7

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HSC Chemistry Module 2: Acidic Environment

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A soluble base is called an alkali Common substance: oven cleaner, baking soda (sodium hydrogen carbonate), caustic soda (sodium hydroxide), soap, oven cleaners, antacid Examples: Sodium Hydroxide (NaOH), potassium hydroxide (KOH), Ammonium hydroxide (NH4OH)

Common Properties of alkalis/bases are:     

Alkalis have a soapy feel Alkalis have a bitter taste Does not react with metals In solution, alkalis are good conductors of electricity Alkalis turn red litmus blue

Neutral  

pH of 7 Sea water (NaCl solution), water, pure alcohol, sodium chloride, sugar

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 are substances that change colour depending on the concentration of H3O+ hydronium ions to determine whether a solution is acidic or basic. By isolating pigments contained in some flowers and plants, an indicator could be developed to test whether substances were acidic, neutral or basic Most indicators produce 2 different colours, one when acidic, and one when basic. Litmus, phenolphthalein, methyl orange and bromothymol blue are common indicators that change colour over a pH range. Each indicator has its own specific range of pH over which it changes colour Indicators need to be used in combination to determine the exact pH of a substance, as indicators are usually very limited in their pH range. Indicators can be natural or synthetic: o Litmus: a dye extracted from lichen that changes colour around the neutral range (4.5-8.5) o Phenolphthalein: Synthetic indicator, that changes colour in basic range (8.210) o Methyl Orange: Synthetic indicator, that changes colour in acidic range (3.14.4) o Bromothymol Blue: Synthetic indicator, that changes colour in neutral range (6.0-7.6)

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HSC Chemistry Module 2: Acidic Environment

Identify data and choose resources to gather information about the colour changes of a range of indicators

Identify and describe some everyday uses of indicators including the testing of soil acidity/basicity: Testing soil pH  

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Some plants will only grow within narrow pH ranges, so the pH of soil needs to be regularly testedsome plants grow best in acidic or basic conditions Since soil is dark in colour, a white inert solid, barium sulphate, is mixed with the soil before the indicator is added so that the colour of the indicator can easily be determined. If the soil is found to be too acidic/basic it can be neutralised by adding a weaker acid/base.

Testing pH of pools   

Pool water must be near neutral to avoid health problems A few drops of indicator or pH paper is soaked in indicator to measure the pH Water should be near neutral to avoid skin and eye irritations

Monitoring pH of chemical Waste  

Wastes that are produced from laboratories tend to be highly acidic pH of wastes must be neutralised before safely disposed, measured by indicators

Solve problems by applying information about the colour changes of indicators to classify substances as acidic, neutral or basic

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HSC Chemistry Module 2: Acidic Environment

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A drain cleaner, composer of mainly sodium hydroxide (NaOH), was tested using litmus and phenolphthalein indicator. Litmus turned BLUE and phenolphthalein turned dark PINK. Since litmus is blue, it is definitely basic. As phenolphthalein turned a dark pink, it must be strongly basic. Vinegar, ethanoic acid (CH3COOH), was tested using methyl orange, and turned RED. Hence, it is an acid.

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 Hydronium Ion    

When in aqueous solution, acids dissociate into anions and H+ ions. Then this reaction occurs: H+ + H2O H3O+ Hydronium more stable than H+ ion In water, acids form hydronium ions

Identify oxides of non-metals which act as acids and describe the conditions under which they act as acids 

Oxides are compounds that contain oxygen

Acidic Oxides 

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Acidic oxides are oxides that: o react with water to form acids o react with bases to form salts Non-metal oxides behave as an acid: carbon dioxide, sulfur dioxide, nitrogen dioxide When they are in solution, they become acids:

Neutral Oxides  

Neutral oxides do not form acids Example: Dinitrogen oxide (N2O), carbon monoxide (CO) and nitric oxide (NO)

Basic Oxides 4|Page

HSC Chemistry Module 2: Acidic Environment

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Basic oxides are oxides that: o React with water to form bases o React with acids to form salts Metal oxides behave as bases Example: Potassium oxide (K2O), sodium oxide (Na2O) and magnesium oxide (MgO) In solution, basic oxides tend to form basic hydroxides:

Amphoteric Oxides   

Amphoteric oxides are oxides that can act as both acids and bases Their behaviour depends on the reaction they are put in Elements that combine to form amphoteric oxides are beryllium, aluminium, zinc, tin, lead

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:

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The acidic oxides are on the right side (non-metals) The basic oxides are on the left side (metals) The amphoteric oxides are in between The noble gases have no oxides

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HSC Chemistry Module 2: Acidic Environment

Define Le Chatelier’s Principle  

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Reactions that go to completion, are called one-directional and will not go backwards Many reactions are reversible and two-directional, they can go from left to right (forward reaction), or right to left (reverse reaction and are represented by a 2 directional arrow Reversible reactions don’t go to completion Reversible reactions reach a position of stability (chemical equilibrium) o At this point, there is no longer any change in the concentrations of the reactants or products, reactions has come to a stop o Does not mean all reactants have become products Example: CO2 (g) + H2O (l) H2CO3 (aq) Water and carbon dioxide can react to form carbonic acid, or carbon acid can decompose to form carbon dioxide and water At equilibrium, no reaction is occurring, but all substances are present The point of chemical equilibrium is NOT FIXED. It depends on the conditions of the reaction; this is the basis of Le Chatelier’s Principle.

Le Chatelier’s Principle  

Systems at equilibrium have constant concentrations of reactants and products Le Chatelier’s Principle states:

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A system will always seek the point of equilibrium

Identify factors which can affect the equilibrium in a reversible reaction Change in Concentration 

When the concentration of a particular substance is: o Increased: the equilibrium point will shift towards the opposite side of the equation, this opposes the change as it reduces the concentration of the species by producing more products on the opposite side. o Decreased: the equilibrium point will shift towards the same side of the equation, this opposes the change as it increases the concentration of the species by the opposite reaction

Changes in Gas pressure (or change in volume) 

For a closed system composed completely of gas, if total pressure on the system is:

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HSC Chemistry Module 2: Acidic Environment

o Increased: the equilibrium will favour the side that reduces pressure, (has less moles), thus opposing the change. (pressure is increased, by reducing volume) o Decreased: the equilibrium will favour the side that increased pressure (produces more moles), thus opposing the change. (pressure is decreased, by increasing volume).

Changes in Temperature 

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Exothermic: produces heat energy, ΔH is NEGATIVE o For exothermic reactions: forward reaction produces heat, reverse reaction absorbs heat If a system is exothermic, and temperature is: o Increased: the equilibrium will shift to the left, as the reverse reaction is endothermic, and this will cool down the system, to oppose the rise in temperature o Decreased: the equilibrium will shift to the right, as the forward reaction, is exothermic, and this will heat the system, to oppose the fall in temperature o These points are opposite for endothermic systems Endothermic: absorbs heat energy ΔH is POSITIVE. o For endothermic reactions: forward reaction absorbs heat, reverse reaction produces heat o Example of endothermic scenario:

Effect of Catalyst 7|Page

HSC Chemistry Module 2: Acidic Environment

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Catalysts increase the rate of reaction, as equilibrium is reached faster Catalysts DO NOT affect the point of equilibrium

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

Reaction between carbon dioxide and water is reversible

Concentration 

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If concentration of carbon dioxide is increased, by pumping more CO2 in the system, the system will dissolve more carbon dioxide to counteract the change, hence equilibrium shifts to the right, to oppose the change If concentration of carbon dioxide is decreased, by removing CO2 from the system, the system will release more carbon dioxide, to oppose this loss, hence equilibrium shifts to the left, to oppose the change.

Pressure 

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If pressure is increased, the equilibrium will favour the side that reduces pressure (forward reaction), it reduces pressure, by dissolving CO2 (solutions take up less volume than gases), shifts right If pressure is decreased, the equilibrium will favour the side that increases pressure (reverse reaction), it increases pressure by changing the carbonic acid to carbon dioxide and water, shifts left

Temperature  

If temperature is increased, the equilibrium will favour the reverse reaction, as it is endothermic, which will oppose the change by cooling the system If temperature is decreased, the equilibrium will favour the forward reaction, as it is exothermic, which will oppose the change by heating the system

Effect of acids and bases

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If an acid is added, the concentration of H+ will increase, so the equilibrium will shift to the left, producing more gas If a base is added, the OH- will react with the H+ ion (creating water) which will shift the equilibrium to the right (since concentration of water increases). This will counteract the change according to Le Chatelier’s principle If enough basic substance is added, the reaction could go to completion.

Identify natural and industrial sources of sulfur dioxide and oxides of nitrogen Sulfur Dioxide (SO2) 8|Page

HSC Chemistry Module 2: Acidic Environment

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Natural Sources: Volcanic gases, bushfires, decomposition of organic matter and sulfur-rich geothermal hot springs releasing gases Industrial Sources: processing and burning of fossil fuels and extracting metal from sulfur-rich ores, such as galena (PbS)

Nitric Oxide (nitrogen monoxide NO)  

Natural sources: the reaction of nitrogen and oxygen in the atmosphere due to high temperatures of lightning Industrial sources: combustion of fossil fuels, both in cars and in power stations. The nitrogen in the air reacts with oxygen in the hot engines. NOT nitrogen in fuel but in atmosphere

Nitrogen Dioxide (NO2)  

Natural Sources: After nitric oxide is produced by lightning, it slowly reacts with oxygen to produce nitrogen dioxide Industrial sources: combustion of fossil fuels, both in cars and power stations. Power stations release large volumes of NO2 into the atmosphere

Nitrous oxide (or dinitrogen monoxide-N2O)  

Natural sources: the action of nitrogen-fixing bacteria on nitrogenous materials in the soil Industrial sources: Agricultural use of nitrogenous fertilisers increases the materials bacteria can act on, increasing levels of nitrous oxides

Describe using equations, examples of chemical reactions which release sulfur dioxide and chemical reactions which release oxides of nitrogen Sulfur Dioxide 

When organic matter decomposes it produces hydrogen sulfide (H2S), which then oxidises to produce sulfur dioxide:

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The burning of sulfur-rich coal and other fossil fuels directly combines sulfur with oxygen:

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The extraction of metals from metal sulfides also releases sulfur dioxide. Smelting of galena (PbS) for lead:

Oxides of nitrogen 9|Page

HSC Chemistry Module 2: Acidic Environment

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Lightning combining nitrogen and oxygen, due to high temperatures

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Above reaction takes place in high temperatures of engines and power plants Nitrogen dioxide is formed when nitric oxide reacts with oxygen in the air:

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

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The gases NO and NO2 are collectively referred to as NOx It is difficult to quantitatively state that oxides of sulfur and nitrogen have been increasing because these oxides occur in relatively low concentrations, such as 0.01 ppm Chemical instruments able to measure very low concentrations (SO2) have only been commercially available since the 1970s, so there is no reliable data for these gases before this time Analysis of gas found in ice-core samples excavated from Antarctica shows that the level of N2O has increased by about 10% The increased burning of fossil fuels after the Industrial Revolution did lead to a rise in oxides of sulfur, as the air quality of major industrial cities (London) deteriorated. 1952 “Great Smog of December” killed 4000 people due to the effects of sulfur and nitrogen compounds in the air. Acid rain forms when atmospheric water reacts with NOx and SO2 compounds Increased occurrence of acid rain shows the increase in atmospheric concentrations of SO2 and NOx compounds Damage to forests, buildings and aquatic life has also been observed in industrialised areas, due to acid rain from elevated levels of atmospheric SOx and NOx. Acid rain can affect acidity of lakes which can kill aquatic life. Acid rain can have an effect on limestone, marble and metal. Increasing smog production in big cities is reducing our quality of life, and increasing levels of acid rains are destroying forests in Europe and Northern America.

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 Reasons for Concern Acid Rain: 

Acid rain: sulfur dioxide and nitrogen dioxide are acidic oxides that react with water to form acids

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HSC Chemistry Module 2: Acidic Environment

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These acids combine with rain droplets forming acid rain. Acid rain is very destructive, it corrodes limestone structures and disrupts natural ecosystem by altering pH levels. Nitrogen dioxide also forms acid rain.

Health Problems   

Sulfur dioxide is a severe respiratory irritant and can cause breathing difficulties at concentrations as low as 1ppm. Sulfur dioxide triggers asthma attacks and aggravates emphysema Nitrogen dioxide is also a respiratory irritant, it can begin to destroy tissue and form nitric acid.

Photochemical smog 

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Release of oxides of nitrogen (NOx) leads to the formation of photochemical smog because of the reaction between oxides of nitrogen and atmospheric oxygen in the presence of UV light. o Noxious chemicals formed by photochemical reactions include ozone, and PAN Photochemical smog has a high social cost, as it reduces a citizen’s standard of living, as it exacerbates many respiratory complaints, such as asthma. Ozone has harmful effects at concentrations as low as 0.1 ppm. Ozone is produced by:

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Ozone is poisonous to humans.

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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 100kPa   

A 300mL bottle of soft drink was decarbonated by vigorous shaking, and then releasing the gas by opening the cap, no drink was allowed to spill. The bottle was weighed at the beginning and the end It was assumed that any mass loss was due to carbon dioxide escaping

Results

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HSC Chemistry Module 2: Acidic Environment

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° and 100kPa Aim – To determine the amount of carbon dioxide gas in a bottle of soft drink. Risk Assessment Risk Water Bath – Could burn skin Electrocution – Liquid near electronic balance Slip Over – Soft drink spilled on the floor

Precaution Do not put hands near water bath, place under running water if contact occurs Clean up any spilled soft drink immediately Clean up any spilled soft drink immediately

Equipment 300 mL Bottle of Soft Drink Electronic Balance Water Bath Water Method Weigh unopened bottle on electronic balance and record mass. Place in water bath at 50°C for 30 minutes with lid removed. Re-weigh the opened water bottle, ensuring the lid is also on the balance. Calculate the mass lost and therefore the volume of gas released. Results Initial Mass (g) 342.57 Discussion 12 | P a g e

Final Mass (g) 340.17

Change in Mass (g) 2.40

HSC Chemistry Module 2: Acidic Environment Although we tried to contr...