Lab 1 & 2 Flashcards
What was the purpose of the first experiment?
strong acid is reacted with strong base in calorimeter and heat capacity of calorimeter is calculated
What is calorimetry?
measurement of the amount of heat evolved or absorbed during a chemical or physical process
How does a cold pack with ammonium nitrate work?
NH4NO3 is dissolved in water – endothermic reaction causing the resulting solution to drop in temperature by 20ºC
What is the reaction tested for a strong acid and strong base?
HCl + NaOH → NaCl + H2O
–> exothermic reaction
What is the surroundings of this reaction?
calorimeter, products of the reaction (water and aqueous salt)
What was the calorimeter consistent of?
test tube, stirrer, and a polyethylene foam insulating jacket
What is the system of this reaction?
strong acid (HCl) and strong base (NaOH)
What is the general reaction of an acid and base?
Net-ionic equation of neutralization where complete ionization occurs:
H+(aq) + OH-(aq) → H2O(l) + heat
What is the molar heat of neutralization (∆Hºneutralization)?
-55.84 kJ / mol
How can the amount of heat absorbed by the surroundings be measured?
measuring the temperature of the surroundings before and after the reaction
How is the final temperature of the reaction taken when the exact instant the reaction ends is unknown?
approximate value by graphing temperature vs time readings and extrapolating back to the instant of mixing: t = 0s –> y-int
Is the strong acid or base the limiting reagent?
acid (HCl)
What must be done to the probe to ensure a proper reading is done?
wipe with damp paper towel
How can you safely increase/decrease temperature of NaOH in the graduated cylinder?
rinse outside with water
Where should the stirrer be with relation to the probe inside the graduated cylinder?
probe is inside the ring on the stirrer
What is potassiumthiocyanate?
KSCN
What is iron(III) nitrate?
Fe(NO3)3
What is the reaction of KSCN and Fe(NO3)3?
[Fe(H2O)6]3+ (aq) + SCN- (aq) –> [Fe(H2O)5(SCN)]2+ (aq) + H2O(l)
or
Fe3+(aq) + SCN-(aq) –> Fe(SCN)2+(aq)
What are the spectator ions of the reaction?
nitrate ion (NO3-), potassium ion (K+)
What is the purpose of this experiment?
to find equilibrium constant of potassiumthiocyanate and iron (III) nitrate and molar absorption coefficient of Fe(SCN)2+
How are the equilibrium concentrations determined?
spectrophotometric analysis
What is the colour expected for Fe(SCN)2+?
intense yellow/orange colour (even at a very low concentration because they absorb visible light
What does the light absorbed amount depend on?
- wavelength of light
- concentration of the absorbing species
- distance the light travels through the sample
What is spectrophotometry?
instrumental technique that examines the interaction between electromagnetic radiation (light) and a sample
– amount of light absorbed by the sample at each wavelength is measured
What is graphed from the spectrophotometry?
absorbance vs concentration linear graph (directly proportional when wavelength where amt of light is not too small or big)
What is the Beer-Lambert Law?
A = εlc
where A is absorbance
ε is molar absorption coefficient in L/mol*cm
l is path length in cm
c is concentration in L/mol
What is path length? What is the value in this experiment?
distance the light passes through the sample (1cm - length of cuvette)
How are path length and amount of light absorbed related?
directly proportional: longer path length = more light absorbed
What is the slope of the absorbance vs concentration graph?
ε x l measured in L/mol
What does it mean for the RMSE to be close to 0?
closer the points are to a perfect match for the equation of the line
What is the dissociation reaction of Fe(NO3)3?
Fe3+(aq) + 3NO3-(aq)
–> [Fe3+] = [Fe(NO3)3]
What is the dissociation reaction of KSCN?
K+(aq) + SCN-(aq)
–> [SCN-] = [KSCN]
How will the Fe3+ + SCN- –> Fe(SCN)2+ reaction proceed?
reaction proceeds to the right (product side) and almost ALL the SCN- will react to mean Fe(SCN)2+
What does it mean for the concentrations if all of the SCN- reacts?
initial concentration of SCN- = equilibrium concentration Fe(SCN)2+ = x
What does the initial concentration refer to?
concentration of the ion after dilution (mixing SCN- and Fe3+ with DI water)
What are the conditions necessary for creating samples of Fe(SCN)2+?
SCN- volume < Fe3+ volume
small volumes (1 mL) of SCN- and Fe3+ (for higher absorbance)
volume of water less than 5mL (for higher absorbance)
Why must SCN- volume < Fe3+ volume?
avoid precipitation of other species
Why are the Fe3+ solutions acidified?
prevent the precipitation of Fe(OH)3
