HL Chemistry Paper 1b

Measuring variables (split this flashcard)

• mass

  • balance

• volume

  • volumetric flasks

  • pipettes

• time

  • stop watch

• temperature

  • thermometer

  • temperature probe

• length

  • ruler

• pH

  • pH meter

  • indicator

  • pH paper

• electric current

  • ammeter

  • multimeter

• electric potential difference

  • voltmeter

  • multimeter

How to prepare a standard solution

1. Starting from a solid

Calculate concentration and volume using a volumetric flask

Add the solid and then water on top of it to make the total volume match your target volume

2. Already had a solution

Dilute it to make a standard solution of a lower concentration

Add water until you get to the final volume

Why make a standard solution?

• titration

• calorimetry

solutions where you are confident in the concentration

Dilution

Start with a more concentrated solution

Measure how much to use and use a pipette

Water on top of it until you get to the final volume
C₁V₁=C₂V₂

Since the moles stay the same throughout

Colorimetry or spectrophotometry

Measuring observance of light

Concentration of solution and absorbance of sample graph

• Positive linear relationship

• Now you can work backwards to figure out an unknown concentration or d-d splitting

Key equipment:
- colorimeter: measures absorbance/transmittance of specific wavelengths

• spectrophotometer: measures absorbance/transmittance of all wavelengths

Cuvettes: must be help carefully on the sides, very clean

Use the same cuvette for all trials

standardize with water

Purpose of colorimetry or spectrophotometry

• determine unknown concentrations/amounts (using line of best fit of concentration of solution and absorbance by sample graph)

• determine d-d splitting

Drying to constant mass

Key equipment:

• evaporation dish/crucible

• bunsen burner/hot plate/oven

• digital balance

make sure to remove all the water

heating —> cooling —> weighing

Purpose of drying to constant mass

To ensure that a product/sample has been fully dried (all water removed). Repeating cycles of heating, cooling and weighing until the mass stays constant ensures all water has been removed.

Distillation and reflux

Oxidation of alcohols:

• aldehydes

Key equipment:

• round bottomed flask

• condenser (tube that is surrounded by cold water)

Reflux

Drop everything back down into the container so that it can keep reacting. Fully oxidize the alcohol until the reaction is done. Adding anti-bumping granules to make boiling smoother.

Distillation

When things evaporate they go off into a different container. Used to collect aldehydes off of the reaction quickly. Separating ethanol out of water.

Separation of mixtures

To exploit differences in physical properties to separate components that have been physically mixed together (no chemical reactions take place).

Techniques of separating mixtures

Solvation:

• Distillation

• Chromatography

• Recrystallisation

Evaporation

Filtration

Separatory Filtration

Filtration

Equipment:

• glass funnel

• filter paper

• filtrate

• solid residue

Separatory Funnel

Purpose of paper or thin layer chromatography

• to determine/identify the components of a mixture

• to assess purity of a sample

How does paper or thin layer chromatography work?

• based on polarity

• calculate Rf

  • should always be less than 1

Calculating R_f values

R_f= distance spot/distance solvent spot

Equipment for chomotography

• chromatography paper or thin-layer silica gel on plastic/glass

• carefully selected solvent or solvent mixture (might take many trials to determine this)

• UV light/dye to help visualize spots

Purpose of calorimetry

Determine Delta H for a reaction by finding the mass

If they say what is the energy released, you do not have to specify the (-) sign direction. (+) energy is abosrbed

(-) energy is released

Systematic weaknesses of calorimetry

• heat loss to surroundings

• incomplete reaction/combustion

Assumption of calorimetry

• specific heat capacity of water

• Density is the same as water volume = mass of liquid

Extrapolation of calorimetry data

• by extrapolating the graph, the temp rise that would have taken place had the reaction been instantaneous can be calculated

• extrapolate the cooling trend line to determine the temperature if you hadn’t lost heat

Acid-base and redox titration equipment

Equipment:

• volumetric flasks (large volumes)

• pipettes (small volumes)

• burettes (for titration)

• conical flask (hold stuff)

• appropriate indicator

Purpose of titration

To find an unknown concentration from our known concentration and the mole ratio. Or find the total moles or mass in a solution, or percent composition.

Method of titration

• usually 1 rough titre before repeating accurate titres

• accurate titres should be within ± 0.1cm³ of each other

• uncertainty on 1 reading is ± 0.05cm³ but subtracting to find change in volume gives ± 0.1cm³

Weaknesses of titration

• overshooting the endpoint

Acid-base vs redox titration

• acid-base (proton transfer reactions) require an indicator (or pH meter) to identify endpoint

• redox (electron transfer reactions) sometimes require an indicator (like starch), but sometimes are self-indicating as species (especially transition metal ions), change color with changes in oxidation state

Acid-base titrations

pH curve/pH meter titrations

Strong strong acid-base titration

Electrochemical cells equipment

• multimeter

• voltmeter

• ammeter

Purpose of electrochemical cells

• determine the voltage and current produced by a primary (voltaic) cell

• determine the voltage required for a non-spontaneous electrolysis reaction

• determine the current being used in electrolysis (related to the amount of product produced)

Systematic issues of electrochemical cells

• electrode potentials are measured under standard conditions. When measuring under other conditions, voltage values will differ.

• Electrode potentials based on the SHE defined as E=0

Equipment for physical and digital molecular modelling

• molymode model kits

• black = carbon

• white = hydrogen

• red = oxygen

• blue = nitrogen

Limitations of physical and digital molecular modelling

• can not show resonance or delocalized electrons

• can not show polarity

• not all atoms are the same size

Melting point determination equipment

• melting point apparatus

• capillary tubes

• thermometer/temperature probe

How melting point determination works

• take a skinny glass tube (capillary tubes)

• collect a bit of solid at the bottom

• watch the solid melt as it is warming up (observe)

Melting point determination purpose

• determine melting point range of a sample

• determine purity of a sample of a compound

• determine identity of a compound

• pure compounds have higher, more narrow melting point ranges

• impurities disrupt

Recrystallization

A way to separate out impurities.

Recrystallization process

• pick a solvent with a lower solubility

• make it hot

• as it cools

• filtrate

Equipment for Recrystallization