Chem Lab 102 Final Review

Error Analysis:

• More precision and replication = reduces uncertainty

• Measurement: our attempt to determine AND communicate the “true” value ~ (best estimate) ± (uncertainty)(units) ~ average and standard deviation

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• Uncertainty: cumulative effect of experimental fluctuations on the data’s precisions

• Error: experimental fluctuations that distort the closeness of the measurements to the “true value”

• Uncertainty is about how spread out or imprecise a measurement could be, while error is about how far a measurement is from the true value.

Error Analysis: Precision versus accuracy in measurements

• Precision: the reproducibility of the measured value using the same experimental conditions

• Accuracy: Closeness of the measured value to the “true” value

Error Analysis: Difference between random error (or uncertainty) and systematic error; in general, how they arise, how they affect measurements and successive calculations, and how they may be mitigated and/or identified in experimental procedure

Random Error: Fluctuations that arise from uncontrolled, and often uncontrolled variables

• Equal probability of being ± “true”

• Detectable by statistics ~ reduced by replication

• Examples: Slight fluctuations when reading a meniscus in a buret, Human reaction time when starting/stopping a stopwatch, Small variations in balance readings due to air currents

Systematic Error: Fluctuations that arise from an artifacts or flaw in the experiment (reproducible)

• Usually affects the result in one direction

• Detectable through experimentation ~ Often difficult to detect

• Examples: A balance that is not calibrated and always reads +0.05 g too high, A thermometer that is miscalibrated and consistently reads 2°C too low, Consistently misreading the meniscus from the wrong angle (parallax error in one direction)

Error Analysis: Equal probability of measurement being too high or too low—affects precision

Precision is limited by uncertainty/random error usually resulting from random fluctuations and is described by relative uncertainty

Error Analysis: Measurements repeatedly skewed in one direction across trials—affects accuracy

Accuracy is limited by error usually resulting from systematic fluctuations and is described by relative error (or percent error)

Error Analysis: Calculation of relative error and relative uncertainty

• Relative error: measures how far a measured value is from the accepted (true) value, showing accuracy as a fraction or precent

• Relative uncertainty: measures the size of the uncertainty compared to the measured value, showing precision as a fraction or precent

• (value)/(reference) x 100 = %

• Difference: relative error compares to true value, relative compares to measured value

Error Analysis: Labflow

• Systematic: faulty equipment or miscalibration and ONE direction VS. different directions

• Systematic: accuracy VS Random: precision

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• Percent Error: (|measured - true| / true) x100

• Standard deviation:

Error Analysis: Possible sources of random or systematic error in the 102L experiments you performed (THR, COL, RK1-2, EQB, ST1-3)

Thermochemistry: Manual

Thermochemistry: the study of the heat energy involved in chemical reactions and changes of physical state

• Bond FORMATION: exothermic VS. Bond FORMING: endothermic

• First law of thermodynamics: total energy of universe must remain constant

• Units of thermochemistry: Joules / Kilojoules

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Calorimetry: (units = Kelvin): using temperature change of a liquid (gaining/losing heat from reaction) to determine the heat change of the reaction

• q_cal = constant x change in temp

• Delta H = q_rxn = - (q_soln + q_cal)

Thermochemistry: Exothermic vs Endothermic reactions

System releases heat energy, surrounding temp increase = exothermic = - enthalpy

System absorbs heat energy from surrounding, surrounding temp decrease = endothermic

• Exothermic: more reactants AND Endothermic: more products

Thermochemistry: Calculation of heat change q using temperature change and specific heat, heat formation, delta Hf, standard heat formation delta*Hf, heat of reaction delta H*rxn

Heat change (q): q_soln=mc(delta)T ~ (mass)(specific heat)(T_final-T_initial)

Specific Heat: amount of energy needed to raise 1 gram of a substance by 1C

Heat of reaction (H_rxn): -q_solution - heat lost by solution = gained by reaction

• q_rxn = -(q_soln + q_cal)

Standard heat of formation: Heat change when one mole of a compound forms from its elements in standard states

Reaction enthalpy: H_f (products) - H_f (reactants)

Thermochemistry: Correct format of molecular equations and net ionic equations

• An ionic compound is solid form may be included in a reaction’s net ionic equation if the reaction involves the solid’s dissolution into or precipitation out of its constituent ions

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Net ionic equation: split compounds into ions unless solid or liquid and cancel out spectator ions

Thermochemistry: Hess’ law as a state function, implications for thermodynamics of separate reactions which combine multiple smaller processes (I.e. dissolution of solid NaOH into an aqueous solution of HCl)

State function: enthalpy change depends on the initial and final states, not path taken

• Have to separate into smaller steps to find total heat of a complex process\

Example: NaOH in HCl system:

• NaOH (s) → Na+ (aq) + OH- (aq)

• HCl → H+ (aq) + Cl- (aq)

• H+(aq) + OH- (aq) → H2O

• total heat = sum of all reactions

Thermochemistry: Interpretation of Temp vs. Time calorimetry data graph & trend lines

After mixing point, reaction starts (temp rises = exothermic, temp falls = endothermic)

• trend line drawn before and after mixing to

Thermochemistry: Prelab

• Exothermic = negative enthalpy change (more products)

• Hess's Law states that if a reaction can be written as the sum of two or more other reactions, the enthalpy change (ΔH) for the overall process must be the sum values of the constituent reactions.

• enthalpy is a STATE function

• NaOH: weigh by difference bc NaOH absorbs water from air, affecting mass

• *given values for each ion* then product values - reactant values

Colligative Properties: Proportionality of colligative properties (freezing point depression, boiling point elevation, vapor-pressure lowering, and osmotic pressure) to van’t Hoff factor of solute and molality of solute

Colligative Properties: Calculation of an unknown solute’s molar mass based on its van’t Hoff factor, the known solvent’s given molal-freezing-point-depression constant, and its calculated molality (mols solute/kgs solvent) in solution.

Colligative Properties: Interpretation of a plotted warming or cooling curve to identify freezing point or boiling point

Colligative Properties: Manual

Behavior of a solution (solute and solvent) compared to an individual compound

• Solvent is major component, solute is the minor component

Physical properties depend on QUANTITY of solute particles present (not identity)

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Properties:

• Freezing-point depression

• Boiling-point elevation

• Vapor-pressure lowering

• Osmotic pressure

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Molality (m): number of moles of solute per kg of solvent

• as m increases, the colligative properties effect increases

Van’t Hoff: number of ions/particles a solute dissociates into solution ~ more ions = stronger

• i=1 is a non-electrolyte

Delta T_f = -iK_fm

• T_f = T_{f (solution)} - T_{f (pure)}