Tims Chem Summary Continued

8/6/2019 Tims Chem Summary Continued

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1. Indicators were identified with the observation that thecolour of some flowers depends on soil composition.

Classify common substances as acidic, basic or neutral

Acids

• Have a sour taste

• Sting/burn the skin

• Conduct electricity well

• Turn blue litmus red

Base

• Have a bitter taste

• Have a soapy feel

• Conduct electricity well

• Turns red litmus blue

An alkali is a soluble base. A neutral solution doesn’t have acidic/basic

properties

Acid Neutral Base

• Vinegar (acetic acid)solution

• Vitamin C (ascorbicacid) in orange juice

• Citric Acid andTartaric Acid used in

liquid foodflavourings

• Sulfuric acid in carbatteries

• Demineralised waterfor car radiators

• Table salt solution

• Cloudy ammonia• Baking Soda

(SodiumHydrogenCarbonate)

• Drain cleaners(Sodiumhydroxide)



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

IndicatorColour in lower

pHColour in transition

pHColour in higher

pHpH range

Methyl orange Red Orange Yellow 3.1 - 4.4

Bromothymolblue

Yellow Green Blue 6.0 - 7.6

Litmus Red Purple Blue 5.5 - 8.0

Phenolphthalein Colourless Colourless Pink8.3 -10.0

IndicatorAn indicator is a substance that in solution changes colour depending on the pH of the solution. There are many different indicators, and the range of pH over whichthese indicators change colour varies. Litmus is the most common and isextracted from lichens. The indicator changes colour in reaction with the pH of asubstance, indicating acidity or basicity dependant on the range of the indicator.Universal indicator is a mixture of several indicators and works over the wholerange.

Identify and describe some everyday uses of indicators includingthe testing of soil acidity/basicity

• Testing soil acidity or alkalinity of soils - Soil acidity or alkalinity is

very important for some species of plants – e.g. azaleas and camellias

need acid soils while most annual flowers and vegetables need alkaline

soils. To test for soil pH, a soil sample is mixed with a solution of universal

indicator. BaSO4 (a white insoluble solid) is sprinkled onto the sample, to

provide a white background against which the indicator colour can be seeneasily. The colour observed is matched against a pH chart to determine

pH.

• Monitoring aquariums – Aqueous indicator solutions or pH paper strips

can be used to test the pH of aquarium water, as some animals (e.g. fish)

are sensitive to the pH of the water in which they live.

• Testing home swimming pools – A sample of water from a pool can be

taken, and tested by using an aqueous indicator solution, or pH paper.

These would have to be compared to a colour chart to determine the pH. A

pH around 7.4 is optimum for lessened irritation to the eyes and skin.



Perform a first-hand investigation to prepare and test a natural

indicator

Prac – Red Cabbage indicator

Aim: to investigate the colour changes of an indicator extracted from redcabbage

Method:

1. A handful of shredded red cabbage was boiled in water over a bunsen

burner for about 5 minutes

2. The resulting liquid solution was poured into a filter paper/filter funnel

apparatus and collected in a beaker3. 20 mL of 1M HCl solution was poured into a measuring cylinder

4. 10 mL was poured into a small test tube which was labelled ‘0’

5. The remaining 10 mL was poured into a beaker and diluted to 100 mL

6. 20 mL of the resulting solution was poured back into the measuring

cylinder, and steps 4 – 5 were repeated 5 times, each test tube being

labelled a successive integer higher

7. Steps 3-6 were repeated starting with 1M NaOH solution and labelling

from 14 down

8. A few drops of red cabbage indicator were added to each test tube and

observations recorded

Results:

Red (0-1) -> (2) -> Purple (3-4) -> Clear (5-6) -> Purple (7-9) -> Blue (10-

11) -> Green (12-13) -> Yellow (14)

There are 6 discrete colour stages for this indicator. This suggests multiple

molecules within the red cabbage indicator solution acting to produce colour

changes. Each molecule has a specific colour when a proton is added or taken

away. The molecules in this case are anthocyanins, and there are about 15

different ones in red cabbage indicator.

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

http://www.hscguide.net/wiki/TheAcidicEnvironment1sub5/edit






Still to complete

Solve problems by applying information about the colour changes of indicators to classify some household substances as acidic, neutralor basic

Still to complete

2. While we usually think of the air around us as neutral, theatmosphere 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 act as acids and describe the

conditions under which they act as acids.

An acidic oxide is one which either reacts with water to form an acid, orreacts with bases to form salts (or both). E.g. carbon dioxide anddiphosphorous pentoxide P 2O5

CO2( g)+H 2O(l ) H 2CO3(aq) 2H +(aq)+CO2−3(aq) (carbonicacid)CO2(aq)+2NaOH (aq) H 2O(l )+2Na+(aq)+CO2−3(aq) (sodiumcarbonate)Or alternatively:H 2CO3(aq)+2NaOH (aq) 2H 2O(l )+2Na+(aq)+CO2−3(aq)

The latter is more correct, as the acidic oxide would react with water toform the acid first. It would depend on the relative concentrations of the oxide and the acid in solution, as it is an equilibrium reaction.However, since both create the same products, this is negligible.And similarly for P 2O5

A basic oxide show basic character, and react with acids to form salts,but not with alkali solutions e.g.

CuO+H 2SO4(aq) CuSO4(aq)+H 2O(l )CuO( s)+H 2O(l ) Cu2+(aq)+2OH −(aq)

Amphoteric oxides are those showing both acidic and basic character,and those that react with neither acids or bases are neutral oxides e.g.NO, CO, N 2O

Analyse the position of these nonmetals in the Periodic Table and outline the relationship between position of elements in the Periodic Table and acidity/basicity of oxides.

identify factors which can affect the equilibrium in a reversible reaction

Reversible reactions occur when products can react to generate reactants. When

a reaction starts, forward reaction generates products from reactants. Backward













reaction then generates products, which form at an increasing rate as product

concentration increases. The equilibrium occurs at the point where formation of

products is equal to the rate of reactant formation, no net change in

concentration.

Factors:

• Concentration species - increasing/decreasing concentration of a

species will cause reaction equilibrium to shift so that it

decreases/increases the species concentration. This because it naturally

results in more/less collisions or more/less decomposition to form

more/less of that chemical. Note that reactions involving solids and liquids

experience little effect, as concentrations remain almost unchanged (note:

this does not include dissolved substances).

• Pressure in a gaseous reaction – an increase/decrease will cause a

increase/decrease in concentration (and vice versa for volume).

Depending on which side of the reaction has more particles, the

equilibrium will shift in that direction in order to reduce number of

particles and thus pressure (or vice versa). Note that increasing reaction

by increasing concentration of gas not involved in reaction e.g. argon hasno effect

• Temperature - If the temperature is lowered, the amount of energy in

the system decreases and the exothermic reaction is favoured since fewer

particles have sufficient energy to form products with a higher potential

energy. And vice versa

• Catalysts - increases speed at which equilibrium is reached, does not

alter equilibrium position as activation energy of both product and

reactants formation is decreased

Notable exceptions:

• When solid or liquid is involved in reaction – the concentration of these

substances stays constant

• The addition of water to an aqueous reaction involving water –

concentration of water does not change significantly, but other substancesbecome more dilute





Factors:

• Temperature – solubility decreases as temperature increases, opposite to

liquids and solids. Increased average kinetic energy of CO2 molecules

means they have greater overall tendency to escape from solution.

Equilibrium shifts to left for all equations until new equilibrium is reached

• Pressure – solubility increases with increased pressure, more carbon

dioxide dissolves to decrease pressure and act against change, equilibrium

shifts to right.

• Dissolution of ions – dissolution of ions displaces carbonic acid ions and

CO2

molecules from hydration shells and causes equilibrium to shift to left

and increase CO2 gas concentration

• pH of water – increased pH means more hydroxide ions, which react with

carbonic acid to neutralise it and produce water, resulting in more CO2

dissolved to produce acid to counteract change. If pH lowered, increased

acidity means increased concentration of H3O+ ions, shifting equilibrium of

(3) and (4) to left to decrease its concentration. This means increased

concentration of the reactants on left, which has a cascade effect shifting

all equilibrium to left and increasing CO2 gas concentration.

Identify natural and industrial sources of sulfur dioxide and oxides of nitrogen.

Sulfur Dioxide (SO2)

Natural

•Volcanos

Industrial

•Burning fossil fuels with sulfur impurities (power plants and car

engines)

•Smelting of some metal ores – these are often sulfides, that

release sulfur when smelted.



Oxides of Nitrogen (NOx – as several are possible)

Natural

•Lightning

Industrial

•High temperature combustion of any fuel (e.g. in power stations,

car engines, high temperature domestic gas heaters)

Describe, using equations, examples of chemical reactions which release sulfur dioxide and chemical reactions which

release oxides of nitrogen.

assess the evidence which indicates increases in atmospheric concentration of

oxides of sulfur and nitrogen

Nitrogen dioxide and sulfur dioxide are washed out by rain, so there is no

significant buildup in atmosphere. Nitrous oxide however, has steadily increased

by about 15%, from measurements made over the last century. There are

problems associated with collecting evidence for sulfur and nitrogen oxides,

namely:

• Concentrations of both are very low, below 0.1ppm, and only recently

(since about the 1950’s) are instruments accurate enough to reliably

measure the levels, so trends before this period could be invalid

• Sulfur dioxide and nitrogen dioxide form sulfate and nitrate ions which are

changed chemically as they move around the hydrosphere, so measuring

traces of these compounds is difficult

Most evidence comes from observed occurrences such as acid rain. There appearsto be an increase from data but it is inconclusive due to lack of long-term trendsand inaccuracies of earlier measurements.

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

Equal numbers of molecules of different gases occupy the same volume in equal







isothermal and isobaric conditions. At 0˚C and 100 kPa, gases occupy 22.71 L permole and 24.79 L/mol at 25˚C.

explain the formation and effects of acid rain

Sulfur dioxide and nitrogen dioxide gases released dissolve in water to form

sulfuric acid and nitric acid which is washed out of the atmosphere by rain,

forming wet deposition acid rain.

Reaction with hydroxyl radicals:

SO2( g )+2OH H 2SO4(aq)

(OR)2SO2( g )+O2( g ) 2SO3( g )SO3( g )+ H 2O(l ) H 2SO4(aq)

2 NO2( g )+ H 2O(l ) HNO2(aq)+ HNO3(aq)

2 HNO2(aq)+O2( g ) – (catalysed by impurities)−

2 HNO3(aq)(OR)

4 NO2( g )+2 H 2O(l )+O2( g ) 4 HNO3(aq)

Effects due to low pH precipitation include:

• Corrosion and tarnishing of metal and bridges, soiling and surface erosion

of marble and stone structures

CaCO3( s)+2 H +(aq) Ca2+(aq)+CO2( g )+ H 2O(l )

• Crown dieback in trees

• Leeching of leaf nutrients

•

Killing of leaf tissue



• Leeching of Ca2+ and Mg 2+ ions from soil as they are mobilised

due to decreased pH, reducing soil fertility

• Inhibits microbial activity

• Increased acidity of lakes, killing aquatic life e.g. snails can only tolerate

up to pH 6.0

• Mobilisation of Al 3+ ions in soil due to reduced pH. This flows into

lakes and precipitates out, clogging fish gills and suffocating them

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

See ‘describe the solubility of carbon dioxide in water under various conditions as anequilibrium process and explain in terms of Le Chatelier’s principle’

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.

Industrial origins of sulfur dioxide and oxides of nitrogen can be found under the

point ‘Identify natural and industrial sources of sulfur dioxide and oxides of nitrogen.’

Sulfur Dioxide

• Respiratory effects from gaseous SO2 – high levels can cause

breathing difficulty for people with asthma, and in the long-term can cause

respiratory illness.

• Respiratory effects from sulfur particles – sulfate reacts with other

chemicals in the air to form tiny sulfate particles. These can gather in the

lungs and are associated with respiratory disease, difficulty in breathing

and premature death.

• Acid Rain - as detailed above – plant, water and buildings damaged

Oxides of Nitrogen

http://www.hscguide.net/wiki/TheAcidicEnvironment2sub6

http://www.hscguide.net/wiki/TheAcidicEnvironment2sub6



• Ground level ozone - Nitrates and other compounds can react to form

ozone at ground levels, which can cause breathing difficulties in asthma

sufferers, the elderly and children.

•

Acid Rain - as detailed above – plant, water and buildings damaged• Global warming - Nitrous oxide is a greenhouse gas – accumulates in the

atmosphere and with other greenhouse gases leads to a gradual increase

in the earth’s temperature over time – climate change

• Particles – Nitrates can react to form nitric acid vapour and other

particles, which can cause negative effects on the respiratory system –

including damage to lung tissues and the worsening of diseases such as

emphysema.

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Tims Chem Summary Continued