Ex2: Intro to Calorimetry

calorimeter

device in which chemical or physical processes occur in isolation. no heat enters or leaves

basic components of a calorimeter

• insulated reaction container

• means of mixing reaction

• device to measure temp

what is q

heat of reaction → heat absorbed/released in rxn

• in this case, using coffee cup cal → q_p which equals ΔH

specific heat

s → heat necessary to raise the temp of one gram of a substance by 1 degree

how to find q given specific heat, mass, and change in temp

q = msΔt → equals to ΔH in this reaction since it is constant pressure

heat capacity

C → s x m → represents total heat capacity of the object (heat required to raise its temp by 1 degree)

calorimeter constant

the heat capacity of the calorimeter → heat required to rise the temp of the calorimeter by 1 degree

how to solve for cal constat using the hot and cold water method

q_gained=-q_lost → q_cal= - q_{hot -} q_cold→ C_cal ΔT = - msΔT_hot - msΔt_cold

heat of neutralization

heat of reaction of a acid/base neutralization

when and why is the heat of neutralization constant for some acid base reactions

constant when the reaction involves only strong electrolytes (fully dissolve) → net rxn is just the union of H + OH → H2O

what is heat of neutralization for rxn with only strong electrolytes

q = -55.90 kJ/mol H₂O formed = ΔH for constant pressure (exothermic)

what is heat of neutralization for rxn with weak electrolytes

will be larger or smaller than -55.90 kJ/mol H₂O → q_neut = m_sol x s_sol x ΔT_sol

• ΔT → inital temp will be the average of the initial acid and base

• can assume specific heat capacity to be the same as water

if the heat of rxn of neutraliziation is equal to -55.90kJ/mol, what do you know about the rxn

strong electrolytes only

if the heat of rxn of neutraliziation is greater than or less than -55.90kJ/mol, what do you know about the rxn

neutralization involves weak electrolyte species

heat of solution

energy change involve in the dissolving process, qrxn

expression relating q_rxn and q_cal and q_soln for heat of solution and what does each represent

0 = q_sol + q_cal + q_rxn

• qsol → heat absorbed/released by the liquid

• qcal → heat absorbed/released by the calorimeter

• qrxn → heat absorbed/realesed by the rxn itself (heat of solution)

is qrxn specific to the system being studied (the specific mass) or constant for all rxns of one type

specific → depends on the mass of the salt being dissolved

𝚫H_sol

molar enthalpy of solution → qrxn/n

what need to be weighed before starting the experiment

clean, dry calorimeter, including the lid and magnetic stir bar

which balance was used throughout this experiment

top loading balance only

part A: heat of dissolution of NaOH steps

• pour DI into cal, stir for a minute and take Ti of water

• quickly pour weighed NaOH in (only move lid, leave thermometer)

• record temperature readings for 25 minutes

what is the enthalpy of solution from part a (NaOH)

exothermic → this is why ice bath was used to cool NaOH back to room temp

part B: determining heat capacity steps

• measure initial temp of cool DI in a beaker

• measure temp of hot DI in cal every min for 5 min

• 6th minute, pour cool DI into hot DI, record immediate temp and temo every min for 10 min

how are the masses of hot and cool water determined during part B

• mass of hot DI → weighted sometime during 5 minute cooling period

• mass of hot DI + cold DI → weighed at the end of the experiment

• mass of cool DI → difference between other 2 masses

how to determine accurate ΔT for calorimeter rxns (Part a and c)

plot Temp vs time; extrapolate the cooling curve back to y axis to find hypothetical T_final. ΔT = Tf- T_i

how to determine accurate ΔT for hot and cold water part of the experiment (B)

T_i of hot DI → extrapolate cooling slope before mixing of warm water to y axis

T_f of hot and cold DI → extrapolate cooling slope after mixing to y axis

part C neutralization of NaOH with HCL steps

• pour HCl into calorimeter, NaOH in grad cyl

• measure temps of acid and base, adjust base to to acid temp +- 0.5^oC

• quickly pour base into acid, record temp every 5 seconds for 1 min then every 30 seconds for 9 more mins

what needs to be done when taking the temp of the acid and the base

wipe down the thermometer, make sure to drip droplets of NaOH back into the grad cyl (it is the LR)