Chemistry - General practical points from EQs

appropriate number of significant figures

The accuracy of the final answer cannot be greater than the least accurate measurement

Risk assessment definition

a judgement of how likely it is that someone might come to harm if a planned action is carried out

standard solution definition

a solution with an accurately known concentration

Why is a larger mass of solid often used to make a standard solution?

Gives a smaller weighing error so it will be a more accurate solution

What type of solid should be used when preparing a standard solution?

• Does not absorb or lose moisture to environment

• Has accurately known Mr

• Very pure

• High Mr to reduce weighing error

Preparing a standard solution method

1. Weigh out an accurate mass of a solid in a clean, dry beaker using a mass balance

2. Add enough deionised water to dissolve solid, stir with glass rod

3. Transfer solution with rinsing to a 250cm³ volumetric flask using a funnel

4. Rinse beaker and glass rod with deionised water into flask

5. Make up solution to the mark with distilled water until bottom of meniscus is on mark

6. Stopper the flask and invert

Why does the volumetric flask need to be inverted when a making a standard solution?

To ensure solution is homogenous

instrument uncertainty definition

an estimate of a measurement that gives the range of values within which the true value is thought to lie

Why are measuring cylinders not appropriate for titration?

Have a limited accuracy so not appropriate when need volumes to 1dp

concordant definition

titre values which are within 0.1cm³ of each other

what type of titrations use methyl orange indicator?

• strong acid-strong base

• strong acid-weak base

methyl orange colour in acid

red

methyl orange colour in alkali

yellow

what type of titrations use phenolphthalein indicator?

• Strong acid-strong base

• Weak acid-strong base

colour of phenolphthalein in acid

colourless

colour of phenolphthalein in alkali

pink

Why is deionised water squirted down the sides of a conical flask during titration?

Some reactant may be on walls on conical flask without reacting so this makes sure that all reactants end up in solution and react

back titration

used to determine the unknown concentration of a solution which has been formed by an insoluble solid being dissolved in a known excess of acid.

Reasons why experimental value of enthalpy of combustion be less exothermic than in data book

• Heat losses to surroundings

• Loss of flue from wick/burner, by evaporation

• Loss of water by evaporation

• Incomplete combustion of fuel

• Heat used to raise temperature of calorimeter instead of water

• Not under standard conditions

Improvements to method of determining enthalpy of combustion experimentally

• Use draught shield to prevent heat loss to surroundings

• Use lid on calorimeter

• Reduce distance between flame and calorimeter

• Insulate calorimeter and spirit burner

• burn in supply of pure oxygen

Control variables when measuring how rate of reaction changes with temperature with disappearing cross (RP3)

• volumes of reactants

• concentrations of reactants

• same swirling

• same person observing disappearance of cross

• same light level

why is silver nitrate solution acidified before testing for halide ions?

removes any other ions present which may react with silver ions to form a precipitate and interfere with the test

why is barium chloride solution acidified before testing for sulfate ions?

removes any carbonate ions present as they would also form a white precipitate with barium ions

reflux definition

the continuous boiling and condensing of a reaction mixture which presents evaporation of any reactants

Why does water go in at the bottom of the condenser?

to ensure that it completely fills with water

why are anti-bumping granules used?

to prevent formation of large bubbles

immiscible liquids definition

liquids that do not mix and form two layers

Why can an organic liquid and an aqueous solution be seperated using a tap funnel?

because they are immiscible

Why are deionised water/aqueous solutions added to crude organic liquids before separating them?

to remove soluble impurities

densities of layers in separating funnel

more dense liquid will be the bottom layer

Why are drying agents (eg. anhydrous magnesium sulfate) added to purified organic liquids after seperation?

to absorb water

decant definition

carefully pour a liquid from one container to another in order to leave any solid in the bottom of the original container

RP5 Distillation of a product from a reactions - theoretical reasons for yield less than 100%

• side reactions occur

• incomplete reaction

• impure reagents

RP5 Distillation of a product from a reactions - practical reasons for yield less than 100%

• loss of product during purification steps (eg. in transfers between apparatus)

• product loss during distillation

Why are tertiary alcohols resistant to oxidation by acidified potassium dichromate (VI)?

Lack of H bonded to C which is bonded to the -OH group

Method to produce pure, crystalline product (recrystallisation)

1. Dissolve in minimum volume of hot solvent

2. Filter solution by hot filtration

3. Leave to cool and crystalise

4. Filter under reduced pressure

5. Wash crystals using ice-cold solvent to remove any aqueous impurities

6. Dry between filter paper or in a warm place

When to use a gas syringe to measure gas volume instead of an inverted measuring cylinder under water?

If a gas is soluble in water

Why is measuring gas volume by measuring change in mass not a suitable method for hydrogen?

Hydrogens has a low Mr, so the mass lost would be small and difficult to measure

What must be done to the reaction before measuring the change in reactant or product at various times in the reaction by titration?

Reaction is quenched

quenched reaction meaning

reaction is stopped

methods of quenching a reaction

• rapid cooling

• adding a chemical to remove a reactant not being monitored

• adding a large known volume of water to sample

When is it possible to find the order of a particular reaction by the continuous monitoring method?

When the reaction only involves one reagent

half-life of reaction definition

the time taken for the concentration of one of the reactants to decrease by half

What does it mean if the half life is constant?

reagent is first order

How is initial rate determined from concentration-time graph?

draw a tangent at t=0 and find gradient of tangent

standard conditions for electrochemical cells

• Temperature = 298 K

• Pressure = 100kPa

• Concentrations of solution = 1.0 moldm^-3

When is the EMF of a cell at a maximum?

When no current flows

Why the EMF of a cell at a maximum when no current flows?

No energy is lost due to internal resistance of the cell as the current flows

What is the point of a salt bridge?

completes the circuit, allowing ions to move

Why is potassium nitrate often used as the solution for a salt bridge?

all potassium salts and nitrate salts are soluble, so potassium nitrate does not react to produce precipitates with any of the ions in the half cells

how to identify the positive electrode in a cell?

If the reading on the voltmeter is positive, then the metal connected to the positive terminal on the voltmeter is the positive electrode. If the reading is negative then the metal connected to the negative terminal is the positive electrode

Why is a wire not used instead of a salt bridge?

In a wire the conducting species are electrons, but in a cell the conduction species are ions. So a wire would not complete the circuit as it would not allow ions to move through

Describe how reactions at the electrodes generate an electric current that can power the motor in an external circuit

Electrons are released from the negative electrode and pass through an external circuit to the positive electrode.

methods of measuring pH of solutions

pH meter, pH probe, data logger, pH paper

What must be done to pH meter/probe before it is used to measure pH of a solution?

must be calibrated

Why must a pH meter/probe be calibrated before use?

to ensure pH readings are accurate

How to calibrate pH meter/probe

1. Rinse with deionised water and dry

2. Place into a standard buffer solution, until pH reading stabilises

3. Rinse with deionised water

4. Repeat with another buffer of a different pH

Why is a reaction swirled when carrying out a titration?

To ensure solution is homogenous and pH is uniform throughout

Method to determine melting point

1. Place some solid in melting point tube

2. Place in melting point apparatus and heat slowly

3. Record temperature at which solid starts to melt and temperature at which it finishes melting

4. Repeat and average the temperatures

5. Compare melting point to known values in data book

Stationary phase definition

a solid or liquid held by a solid support used in chromatography

Mobile phase definition

A liquid or gas which moves through the stationary phase in chromatography

Why is it necessary to wear plastic gloves when holding a TLC plate?

to prevent contamination of the plate

Why is the start line for TLC draw in pencil?

the pencil line is insoluble and will not move with the solvent or interfere with results

Why does the TLC plate have to dry in the fume cupboard?

solvent may be toxic or flammable

examples of developing agents for TLC

ninhydrin spray, UV light

why are many organic compounds handled in fume cupboards?

volatile organic compounds are toxic