YEAR 12 ATAR CHEMISTRY- Semester 1 Exam

A primary standard must:

- Be obtainable in a very pure form and have a known formula

- Be stable so that it does not alter during weighing by picking up or losing moisture, or reacting with air (deliquescent, hygroscopic)

- Have reasonably high molecular mass to minimise weighing errors

- React according to known chemical equations

- HCl, H2SO4, NaOH or KOH are not suitable as primary standards as they are not available in a suitably pure form

Give examples of primary standards

- E.g. anhydrous sodium carbonate (Na2CO3) and oxalic acid (H2C2O4·2H2O)

What is a good indicator for a strong acid and a weak base?

methyl orange

What is a good indicator for a strong acid and a strong base?

- bromothymol blue

What is a good indicator for a weak acid and a strong base?

- phenolphthalein

Describe the colour change of bromothymol blue in basic and acidic conditions

- Yellow in acidic conditions and blue in basic conditions, at a pH of 7 in neutral conditions it is green

Describe the colour change of methyl orange in basic and acidic conditions

- Red in acidic conditions and yellow in basic solutions

When does methyl orange indicator change points between?

- Changes colour between pH 3.1 and 4.4, between these values the indicator is orange

Describe the colour change of phenolphthalein in basic and acidic conditions

In acidic solutions it is colourless and in basic solutions it has a pink colour

What is the pH range of phenolphthalein?

- Changes colour over the pH ranges 8.3-10.0

What is the pH range of bromothymol blue?

6.0-7.6

What is uncertainty?

the range of values within which true values lie

What is confidence interval?

the amount of uncertainty associated with a sample estimate

Is pure water a strong or weak electrolyte?

- it is a weak electrolyte

System

the chemical reaction

Surroundings

everything else; or the rest of the universe that the system reacts with

Open chemical system

Can exchange matter and energy between the system and the surroundings

Give some examples of open systems

- E.g. evaporation (physical change), melting of a glacier (physical change), combustion (chemical change), acid + carbonate (chemical change)

Closed Chemical System

- No exchange of matter

- Can exchange energy between the system and the surroundings

Give some examples of closed chemical systems

- All aqueous solution/liquid systems are closed systems e.g. HCl and NaOH and the products

What type of reactions occur in closed systems?

- Reversible reactions occur in closed systems

Isolated system

- No heat exchange of heat from the system or surroundings

- No exchange of matter from the system or surroundings

What is a suitable solution for a salt bridge?

KNO3

When does blue litmus paper turn red?

- under acidic conditions

When does red litmus paper turn blue?

- under basic conditions

What is the equations of a buffer?

HA + H2O <-> A- + H3O+

List three ways the rate of reaction can be determined

1. measuring the change in mass

2. measure the change in temperature

3. measure the change in pressure

In an open system what can be exchanged with the surroundings?

- heat energy and matter

In a closed system what can be exchanged with the surroundings?

- only heat energy

What is equilibrium established in a gas/liquid system?

- when the rate of evaporation is equal to the rate of condensation

When is equilibrium established in a saturated solution?

- equilibrium is established in a saturated solution as the rate of dissolving is equal to the rate of crystallisation

Advantages of the Bronsted Lowry model

- not restricted to aqueous solutions

- reactions between two gases can be an acid-base reaction

Disadvantages of the bronsted Lowry model

- cannot be applied to the reaction between acidic and basic oxides

What are amphiprotic substances

- substances that can donate or accept protons (they can behave as either acids or bases)

What is a polyprotic acid?

acids which can donate more than one hydrogen atoms per molecule of acid

Disadvantage of arrhenius theory?

- restricted to aqueous solutions

Give the ionic equation for a strong acid and give the bronsted-lowry approach for this question

- HA(aq) --> H+(aq) + A-(aq)

- Bronsted-lowry: HA(aq) + H2O(l) --> A-(aq) + H3O+(aq)

NOTE: weak acids would be written with a double arrow as they only partially ionise

What is the solubility of group II metal hydroxides? How does this effect the strength of the bases?

- they tend to have low solubility

- they are still strong bases because to the extent to which these solids dissolve, they fully dissociate

Give 4 examples of strong bases

- KOH (potassium hydroxide)

- Ba(OH)2 (barium hydroxide)

- Na2O (sodium oxide)

- Ca(OH)2 (calcium hydroxide)

Give the equation for an acid base reaction

HA + B- <-> A- + HB

What is the expression for the ionisation of water, Kw

Kw= [H3O+][OH-]

At 25 degrees, Kw=[1 x 10^-7][1 x 10^-7] = 10^-14

When is a solution neutral?

when [H3O+]=[OH-]

is the self ionisation of water endo or exothermic?

- it is endothermic

How are salts formed?

- they are formed of neutralisation reactions

What does the positive ion of the salt come from and where does the negative ion of the salt come from?

- the positive ion comes from the base and the negative ion comes from the acid

anions of monoprotic acids e.g. HCl are...

neutral e.g. Cl

Cations of strong bases are...

neutral e.g. Na from NaOH

anions of weak acids are...

basic

anions of polyprotic acids e.g. H2PO4 are...

acidic

- EXCEPT FOR: the hydrogen carbonate ion

Is the ammonium ion acidic or basic?

- acidic as it hydrolyses in water to form an acidic ion

What do buffers consist of?

- weak acid and its conjugate base in equimolar amounts

Give some examples of common buffer solutions

- CH3COOH/CH3COO-

- HCO3-/H2CO3

- H2PO4-/HPO4 2-

List three ways to make a buffer solution

1. add equimolar amounts of a weak acid and the salt of its conjugate base

2. add weak acid to a strong base

3. add weak base to a strong acid

List the equations involved in maintaining blood pH

HCO3- + H3O+ <-> H2CO3 + H2O (buffer system)

H2CO3 + H2O <-> 2H2O + CO2 (not a buffer system)

What is the general equation of an indicator?

- HIn(aq) + H2O(l) <-> In-(aq) + H3O+(aq)

What is the equivalence point?

- when stoichiometrically equivalent amounts of acid and base have reacted i.e. neither acid or base remain in the conical flask

What is the end point of the titration?

- when the indicator changes colour permanently for the first time to signal the equivalence point has been reached and the titration is to be stopped

What is the volumetric flask used for?

- holds a fixed, accurately known volume of solution

- used to prepare standard solutions or perform dilutions

What is a burette used for?

- accurately delivers a variable volume of solution

What is a pipette used for?

- accurately delivers a fixed volume of solution (aliquot)

Where is the titration carried out?

- carried out in a conical flask

What is a conical flask used for?

- contains an aliquot of the other reagent and indicator

What is a standard solution?

- solution with known concentration

What are two ways in which a primary standard can be prepared?

- primary standard- dissolve a known mass of primary standard in water in a volumetric flask

- secondary standard- use a standard solution in a titration to calculate the concentration of an unknown; this becomes a secondary standard

Give two examples of primary standards

- anhydrous sodium carbonate (Na2CO3)

- oxalic acid dehydrate (H2C2O4.2H2O)

What does anhydrous mean?

- no water present in the compound

What is the endpoint of a titration?

the point where the indicator changes colour

What are concordant results

titres within 0.20ml of each other

What is the volume of one single drop from the burette

0.05ml

What is a titre?

- a variable volume of solution that is added from the burette into the conical flask until the endpoint of titration has been reached

What is an aliquot?

- an accurately measured volume of solution with known concentration

What is a random error?

- an error that follows no regular pattern

What is a systematic error?

- errors that produce a constant bias in a measurement that cannot be eliminated by repeating the measurement

List three uncertainties associated with the pipette, the burette and the volumetric flask in volumetric analysis

- 20mL pipette (+-)0.03mL

- 50mL burette (+-)0.02mL for each reading

- 250mL volumetric flask (+-) 0.3mL

List 2 uncertainties associated with the top loading balance and the analytical loading balance

- 100g capacity top loading balance (+-)0.001g

- 60g capacity anaytical balance (+-) 0.0001g

Give two uncertainties associated with a measuring cylinder or a graduated beaker

- 50mL measuring cylinder (+-)0.3mL

- 50mL graduated beaker (+-) 5mL

List 3 mistakes that could occur during an acid-base titration

- misreading the numbers on a scale

- mistakenly using a pipette of incorrect volume

- spilling a portion of a sample

How can random errors be reduced?

- take multiple measurements of the same quantity then calculating the average

- (in volumetric analysis the average of three concordant titres is used to reduce random error)

How can systematic errors be eliminated or minimised?

- they can be eliminated or minimised through calibration of apparatus and the careful design of the procedure

What is a secondary standard solution?

- a solution whose concentation is found by titration

Explain the titration process

1. a carefully measured volume of a solution (aliquot) is added to a conical flask

2. a variable volume of another solution (titre) is then carefully added from a burette into the conical flask until the reaction between acid + base is complete

3. an indicator colour change signals the end point of titration

Give some examples of systematic errors in titrations

- a faulty balance

- a 20.0mL pipette that delivers 20.2mL

- some of a substance or solution being left in the original container, such as a beaker, when being transferred into a new container

- an unsuitable indicator being used

- the mass of the primary standard being inflated because it has not been dried properly and contains some water

- a person reading a scale on the burette with a constant parallax error

Give some examples of random errors that can occur during titrations

- difficultly in judging the fraction between two 0.1mL scale markings on a burette

- difficulty in judging where the meniscus sits on the line when using a pipette to measure a volume

- inherent uncertainty in the last value after the decimal place in the measurement of the mas of a primary standard on an analytical balance

What is the Humphry Davis theory?

- suggested that the acid properties of a substance were associated with hydrogen and not oxygen

- suggested that acids react with bases to produce salt and water

What is the Arrhenius theory?

- acids are substances that produce hydrogen ions in solution and bases are substances that produce hydroxide ions in solution

What is the bronsted-lowry theory?

- acids are proton donators and bases are proton acceptors identified the hydronium ion as more accurate in aqueous solutions than hydrogen

- introduced the concept of acid-base pairs