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Chapter 14: Experimental Chemistry

Data Gathering

  • Quantitative chemical experiments involve numbers

    • the concentration of an acid in .345M

  • Qualitative chemical experiments do not involve numbers

    • Silver nitrate forms a white precipitate when added to a solution

  • Observations and details of an experiment should be recorded in a notebook.

    • contains complete experiment description, the equipment, chemicals used, and occasionally diagrams

Calculations

  • Calculations can be done via dimensional analysis or memorized equations/laws

Accuracy and Precision

  • Accuracy is the closeness between the measured value and true value

  • Determinate errors can affect accuracy

    • poor technique or incorrectly calibrated tools

  • Precision is the closeness of repeated measurements to each other

  • Indeterminate errors are a measure of precision

    • errors in the last digit of measurement

    • random errors that cannot be eliminated

Significant Figures

  • Significant figures (sig figs) is the precision of a measure number

  • Digits are significant if:

    • digit is not a zero

    • the zero is embedded

    • trailing zero is in a number that has a decimal point

  • digits are not significant if zeros are the left of all nonzero digits

  • Exact numbers involve no uncertainty

    • five plate at the dinner table

    • 4.184 joules in each calorie

  • How to obtain and write correct answers:

    • number with the fewest sig figs in a multiplication or division problem determines the number of sig figs in the answer

    • number with the fewest decimal places in addition or subtraction problems determines the number of decimal places in the answers

Uncertainty

  • Two types of uncertainty: absolute and relative

  • Absolute uncertainty is the uncertainty of the last digit of measurement.

    • for 45.47mL, the last digit is uncertain and the it could be ±.01mL

  • Relative uncertainty of a number is the absolute uncertaintly divided by the number itself

    • (.01mL) / (45.47mL) = 2x10^-4

Rounding

  • Steps of rounding:

    • If the digit just after the kept digits is less than 5, the remaining digits are dropped

      • rounding 6.23499 to three sig figs yield 6.23 because 4 is less than 5

    • if the digit after the kept digit is 5 or more, the last kept digit is increased by one

      • 34.25589 to three sig figs yields 34.3

      • 8.445000 to three sig figs yields 8.45

Graphs

  • Graphs illustrate the relationship between two variables

  • X-axis is typically independent variable

    • experimenter selects independent variable (IV)

    • concentrations of standard solutions that a chemist prepares

  • Y-axis is typically dependent variable

    • dependent variable (DV) is a measured property of the IV.

    • the amount of light that each standard solution absotbs

  • constructing graphs:

    • label x and y axis

    • number the axes

    • plot the data points and draw lines to connect dots

Determination of Physical Properties

Scale Reading

  • Parallax errors are caused by incorrect reading techniques

  • Meniscus of liquid is caused by the surface tension, which causes the curvature

Determination of Mass by Weighing

  • Weight = (gravitational constant)(mass)

  • Samples collected on balances should be collected in appropriate containers, not directly on the balance.

Liquid Volume Measurement

  • Accurate liquid measurements can be made with pipets or burets

  • Glassware with “TD” or “to deliver” is pouring the the volume stated

    • graduated cylinders

  • Glassware with “TC” to “to contain” will not pour the full amount in the container because a film, or excess solution, will remain

    • volumetric flasks

  • Burets are controlled by a valve called the stockpot.

    • should be rinsed with the solution, not distilled water, before filling

Temperature Measurement

  • Thermometers are used for temperature measurement

  • Mercury is used to measure temperature

Determination of Melting and Boiling Points

  • Experiments determining boiling point must be done at 1 atmosphere of pressure for accuracy

  • Closed-end capillary tubes can be used for melting point determination

Determination of Density

  • Density = Mass / Volume

  • density of liquid or solid = grams of material / cubic centimeters of material

  • density of gasses = grams of gas / liters of gas

  • pycnometers can be used to determine liquid density

Determination of Specific Heat

  • a solid, typically metal, is heated to a certain temperature then submerged in water, where the temperature of the water is measured

  • q = (mass)(specific heat)(ΔT)

Sample Manipulations

Heating

  • Bumping is a violent burst of boiling that could splatter hot liquid

    • typically occurs in test tubes

    • occurs because burner flame superheats one portion of liquid

      • hot water baths, steam baths, sand bath, and electrical heating can minimize bumping

    • powders added to very hot liquids can cause bumping

Cooling

  • Crushed ice bath and water is most effective to cool

    • Ice is particularly efficient

  • Heat-resistant lab glassware should be used because glass will shatter at rapid temperature change

Mixing

  • Volumetric flasks should be used to prepare molar concentrations of specific volumes

  • acid should be added to water

    • if water is added to sulfuric acid, it will not mix because of the density, causing high heat and splattering/boiling

Dilution

  • (Cinitial)(Vinitial) = (Cfinal)(Vfinal)

    • where c is concentration

    • v is volume

Gas Collection

  • Pneumatic troughs can collect gas produced in a reaction

Separation Techniques

Precipitation

  • For Ba 2+ (aq) + SO4 2- (aq) → BaSO4 (s)

    • Barium solution should be added slowly with rapid stirring to precipitate sulfate ions as barium sulfate

    • once precipitation is complete, the mixture should be heated to form the smaller crystals to larger ones for easy filtration

Filtration

  • Filtration is used to separate solid particles from a liquid

  • Folded, cone-shaped filter paper on a flask lets liquid pass through while solid is left behind

Centrifugation

  • Small amounts of precipitate can be collected through centrifugation

    • spins at high speeds to compact solid at the bottom

  • Supernatant is the clear liquid remaining after centrifugation

Distillation

  • Distillation separates two liquids with two different boiling points, one low and one high

Chromatography

  • The stationary phase of chromatography has different attractive forces for different parts of the mobile phase

  • Different attractive forces cause the mobile phase to move at different speeds

  • Rf = (distance of color spot)/ (distance of water)

Instrumental Techniques

pH Determination

  • pH meter or pH paper can determine pH

  • Litmus paper simply determines if the solution is acidic or basic

Spectroscopy

  • Spectrophotometer measures the amount of light that a colored solution absorbs

  • Absorbance of a sample is the quantity determined by the spectrophotometer

  • Reagent blank zeroes out the spectrophotometer to get an accurate reading

  • Beer’s Law: Absorbance (A) = abc

    • a is absorptivity

    • b is optical path

    • c is concentration

Experimental Reaction

Synthesis of Gasses

  • By decomposing certain compounds, gasses can be collected

    • KClO3 forms O2 when a catalyst is present

  • Gasses that do not react with water can be collected via pneumatic trough

Synthesis of Insoluble Salts

  • Precipitated in double-replacement reactions or reactions of gasses with soluble substance

    • Ag (aq) + Cl (aq) → AgCl (s)

    • Ba (aq) + SO4 (aq) → BaSO4 (s)

    • Ni (aq) + SO2 (g) + H2O (l) → NiSO3 (s) + 2H (aq)

Preparation of Soluble Salts

  • Can be separated by evaporation, with or without heat

  • Pure salt sample can only be obtained if the solution only contains the ions of that salt

  • Filtration can be used to separate unneeded ions

Qualitative Analysis of Inorganic Ions

  • Qualitative analysis techniques determine if a sample contains a certain ion

  • Sequence of reactions used to analyze a sample is called qualitative analysis scheme, or qual-scheme

  • When a separation is by filtration, the liquid is called the filtrate.

  • When separation is by centrifugation, the liquid is called the supernatant

Chemical Hazards

Highly Flammable Compounds

  • Organic compounds, typically with low molar masses

    • methane, butane, etc.

Explosive Compounds

  • Ethers react to produce explosive peroxides over time

    • Dry picric acid is explosive

    • nitrogen triiodide and nitroglycerin are shock sensitive

Strong Oxidizers

  • Perchloric acid, sulfuric acid, nitric acid, and hydrogen peroxide can cause immediate skin injury

  • Perchloric acid with organic material causes spontaneous combustion

  • White phosphorus burn spontaneously in air with hazardous fumes

Compounds Incompatible with Water

  • Active metals may react explosively with water

    • Li, Na, K, Rb, Cs, Ca, Ba, Sr

Compounds with High Heats of Solution

  • Substances can emit large amounts of heat in water

    • active metals, their solible oxides and hydroxides, and concentrated sulfuric and nitric acids

    • calcium oxide and phosphorus pentoxide

    • mixing concentrated bases with concentrated acids

Compounds with Possible Health Hazards

  • Benzene, chloroform, carbon tetrachloride

  • Chlorinated organic compounds

Chapter 14: Experimental Chemistry

Data Gathering

  • Quantitative chemical experiments involve numbers

    • the concentration of an acid in .345M

  • Qualitative chemical experiments do not involve numbers

    • Silver nitrate forms a white precipitate when added to a solution

  • Observations and details of an experiment should be recorded in a notebook.

    • contains complete experiment description, the equipment, chemicals used, and occasionally diagrams

Calculations

  • Calculations can be done via dimensional analysis or memorized equations/laws

Accuracy and Precision

  • Accuracy is the closeness between the measured value and true value

  • Determinate errors can affect accuracy

    • poor technique or incorrectly calibrated tools

  • Precision is the closeness of repeated measurements to each other

  • Indeterminate errors are a measure of precision

    • errors in the last digit of measurement

    • random errors that cannot be eliminated

Significant Figures

  • Significant figures (sig figs) is the precision of a measure number

  • Digits are significant if:

    • digit is not a zero

    • the zero is embedded

    • trailing zero is in a number that has a decimal point

  • digits are not significant if zeros are the left of all nonzero digits

  • Exact numbers involve no uncertainty

    • five plate at the dinner table

    • 4.184 joules in each calorie

  • How to obtain and write correct answers:

    • number with the fewest sig figs in a multiplication or division problem determines the number of sig figs in the answer

    • number with the fewest decimal places in addition or subtraction problems determines the number of decimal places in the answers

Uncertainty

  • Two types of uncertainty: absolute and relative

  • Absolute uncertainty is the uncertainty of the last digit of measurement.

    • for 45.47mL, the last digit is uncertain and the it could be ±.01mL

  • Relative uncertainty of a number is the absolute uncertaintly divided by the number itself

    • (.01mL) / (45.47mL) = 2x10^-4

Rounding

  • Steps of rounding:

    • If the digit just after the kept digits is less than 5, the remaining digits are dropped

      • rounding 6.23499 to three sig figs yield 6.23 because 4 is less than 5

    • if the digit after the kept digit is 5 or more, the last kept digit is increased by one

      • 34.25589 to three sig figs yields 34.3

      • 8.445000 to three sig figs yields 8.45

Graphs

  • Graphs illustrate the relationship between two variables

  • X-axis is typically independent variable

    • experimenter selects independent variable (IV)

    • concentrations of standard solutions that a chemist prepares

  • Y-axis is typically dependent variable

    • dependent variable (DV) is a measured property of the IV.

    • the amount of light that each standard solution absotbs

  • constructing graphs:

    • label x and y axis

    • number the axes

    • plot the data points and draw lines to connect dots

Determination of Physical Properties

Scale Reading

  • Parallax errors are caused by incorrect reading techniques

  • Meniscus of liquid is caused by the surface tension, which causes the curvature

Determination of Mass by Weighing

  • Weight = (gravitational constant)(mass)

  • Samples collected on balances should be collected in appropriate containers, not directly on the balance.

Liquid Volume Measurement

  • Accurate liquid measurements can be made with pipets or burets

  • Glassware with “TD” or “to deliver” is pouring the the volume stated

    • graduated cylinders

  • Glassware with “TC” to “to contain” will not pour the full amount in the container because a film, or excess solution, will remain

    • volumetric flasks

  • Burets are controlled by a valve called the stockpot.

    • should be rinsed with the solution, not distilled water, before filling

Temperature Measurement

  • Thermometers are used for temperature measurement

  • Mercury is used to measure temperature

Determination of Melting and Boiling Points

  • Experiments determining boiling point must be done at 1 atmosphere of pressure for accuracy

  • Closed-end capillary tubes can be used for melting point determination

Determination of Density

  • Density = Mass / Volume

  • density of liquid or solid = grams of material / cubic centimeters of material

  • density of gasses = grams of gas / liters of gas

  • pycnometers can be used to determine liquid density

Determination of Specific Heat

  • a solid, typically metal, is heated to a certain temperature then submerged in water, where the temperature of the water is measured

  • q = (mass)(specific heat)(ΔT)

Sample Manipulations

Heating

  • Bumping is a violent burst of boiling that could splatter hot liquid

    • typically occurs in test tubes

    • occurs because burner flame superheats one portion of liquid

      • hot water baths, steam baths, sand bath, and electrical heating can minimize bumping

    • powders added to very hot liquids can cause bumping

Cooling

  • Crushed ice bath and water is most effective to cool

    • Ice is particularly efficient

  • Heat-resistant lab glassware should be used because glass will shatter at rapid temperature change

Mixing

  • Volumetric flasks should be used to prepare molar concentrations of specific volumes

  • acid should be added to water

    • if water is added to sulfuric acid, it will not mix because of the density, causing high heat and splattering/boiling

Dilution

  • (Cinitial)(Vinitial) = (Cfinal)(Vfinal)

    • where c is concentration

    • v is volume

Gas Collection

  • Pneumatic troughs can collect gas produced in a reaction

Separation Techniques

Precipitation

  • For Ba 2+ (aq) + SO4 2- (aq) → BaSO4 (s)

    • Barium solution should be added slowly with rapid stirring to precipitate sulfate ions as barium sulfate

    • once precipitation is complete, the mixture should be heated to form the smaller crystals to larger ones for easy filtration

Filtration

  • Filtration is used to separate solid particles from a liquid

  • Folded, cone-shaped filter paper on a flask lets liquid pass through while solid is left behind

Centrifugation

  • Small amounts of precipitate can be collected through centrifugation

    • spins at high speeds to compact solid at the bottom

  • Supernatant is the clear liquid remaining after centrifugation

Distillation

  • Distillation separates two liquids with two different boiling points, one low and one high

Chromatography

  • The stationary phase of chromatography has different attractive forces for different parts of the mobile phase

  • Different attractive forces cause the mobile phase to move at different speeds

  • Rf = (distance of color spot)/ (distance of water)

Instrumental Techniques

pH Determination

  • pH meter or pH paper can determine pH

  • Litmus paper simply determines if the solution is acidic or basic

Spectroscopy

  • Spectrophotometer measures the amount of light that a colored solution absorbs

  • Absorbance of a sample is the quantity determined by the spectrophotometer

  • Reagent blank zeroes out the spectrophotometer to get an accurate reading

  • Beer’s Law: Absorbance (A) = abc

    • a is absorptivity

    • b is optical path

    • c is concentration

Experimental Reaction

Synthesis of Gasses

  • By decomposing certain compounds, gasses can be collected

    • KClO3 forms O2 when a catalyst is present

  • Gasses that do not react with water can be collected via pneumatic trough

Synthesis of Insoluble Salts

  • Precipitated in double-replacement reactions or reactions of gasses with soluble substance

    • Ag (aq) + Cl (aq) → AgCl (s)

    • Ba (aq) + SO4 (aq) → BaSO4 (s)

    • Ni (aq) + SO2 (g) + H2O (l) → NiSO3 (s) + 2H (aq)

Preparation of Soluble Salts

  • Can be separated by evaporation, with or without heat

  • Pure salt sample can only be obtained if the solution only contains the ions of that salt

  • Filtration can be used to separate unneeded ions

Qualitative Analysis of Inorganic Ions

  • Qualitative analysis techniques determine if a sample contains a certain ion

  • Sequence of reactions used to analyze a sample is called qualitative analysis scheme, or qual-scheme

  • When a separation is by filtration, the liquid is called the filtrate.

  • When separation is by centrifugation, the liquid is called the supernatant

Chemical Hazards

Highly Flammable Compounds

  • Organic compounds, typically with low molar masses

    • methane, butane, etc.

Explosive Compounds

  • Ethers react to produce explosive peroxides over time

    • Dry picric acid is explosive

    • nitrogen triiodide and nitroglycerin are shock sensitive

Strong Oxidizers

  • Perchloric acid, sulfuric acid, nitric acid, and hydrogen peroxide can cause immediate skin injury

  • Perchloric acid with organic material causes spontaneous combustion

  • White phosphorus burn spontaneously in air with hazardous fumes

Compounds Incompatible with Water

  • Active metals may react explosively with water

    • Li, Na, K, Rb, Cs, Ca, Ba, Sr

Compounds with High Heats of Solution

  • Substances can emit large amounts of heat in water

    • active metals, their solible oxides and hydroxides, and concentrated sulfuric and nitric acids

    • calcium oxide and phosphorus pentoxide

    • mixing concentrated bases with concentrated acids

Compounds with Possible Health Hazards

  • Benzene, chloroform, carbon tetrachloride

  • Chlorinated organic compounds