chem132 experiment 11

standard enthalpy of neutralization strong acids and bases vs. organic acids

*strong acids and bases: higher* *organic acids: lower* most organic acids partially dissociate in aqueous solution, energy required to break the bond between the proton and its conjugate base, therefore lowering measured enthalpy change

to calculate heat capacity, we need to make two assumptions:

1. any thermal energy transferred from calorimeter to outside environment is negligible 2. all dilute, aqueous solutions have same density and specific heat capacity as water

first law of thermodynamics; exothermic vs. endothermic rxns; state functions; heat capacity; specific heat capacity; calorimetry; heat; change in enthalpy; adiabatic calorimeter; heat/enthalpy of neutralization for acid-base reactions

1. be familiar with first law of thermodynamics: conservation of energy; distinguish between exothermic and endothermic reactions; be familiar with thermodynamic terms such as state function, heat capacity, and specific heat capacity 2. calorimetry: technique used to measure thermal (heat) energy changes in chemical reactions; measured by temperature changes 3. *heat: thermal energy flowing from an object at higher temperature to one at lower temperature* 4. most common units for discussing energy is Joule (J) 5. *delta H (change in enthalpy): heat of reaction* (reported in kJ/mole of reactants and products) 6. *adiabatic calorimeter: does not allow transfer of heat or matter between a thermodynamic system and its surroundings* 7. in this experiment, measure heat or enthalpy of neutralization when an acid and a base react to form a salt and water in a thermally insulated vessel called a calorimeter

purpose

1. understand Hess's law and the thermodynamic properties that are dependent only on the initial and final states of a system 2. understand the purpose of setting up and calibrating the coffee cup calorimeter 3. determine the calorimetric constants of a coffee cup calorimeter 4. use the calorimetric constant of coffee cup to determine the enthalpy of neutralization of a strong acid with a strong base and compare it to that of a weak acid with a strong base

neutralization's effect on temperature;

8. neutralization rxn liberates heat and will cause *increase in temperature in the solution in the calorimeter* 9. determining heat capacity of the calorimeter or the calorimeter system--->*measuring heat absorbed by the calorimeter* 10. heat capacity of calorimeter: *amount of heat (in J) required to raise temperature of the calorimeter by 1 Kelvin or by 1 degree celsius* 11. temperature of the calorimeter and its contents will be measured *before and after the mixing of the reactants*--to calculate change in enthalpy

The experimental procedure requires that you wash your thermometer and dry it after you measure the temperature of the NaOH solution and before you measure the temperature of the HCl solution. Why?

If the thermometer is not washed and dried, the measurements for heat will be skewed since leftover NaOH would react with HCl

How should the two heats of reaction for the neutralization of NaOH and the two acids compare? Why?

The heat of rxn for the neutralization of NaOH-HCl should be greater than the heat of rxn for the neutralization of NaOH-CH3COOH since HCl is a stronger acid than CH3COOH and, will therefore release more heat

What will be the effect on the enthalpy change determined if the true concentration of the a.) NaOH was 5% lower than that stated on the label b.) HCl were 5% higher than that stated on the label

a.) less NaOH will mean that HCl would have less NaOH to react with, thus producing a smaller enthalpy change (change in heat) b.) enthalpy change would stay the same (no change in heat) since NaOH is the limiting reactant; once NaOH runs out, the leftover HCl has nothing to react with, thus no temperature change would occur

standard enthalpy change delta H related to heat of reaction by this equation

delta H= q/n where n is the number of moles q is mcat

heat of neutralization

enthalpy change that occurs when equivalent amounts of acid and base are mixed together to form 1 mole of water

q= mcAT

heat of reaction q= heat energy gained or lost m= mass of water c= specific heat capacity AT= temperature change of reaction mixture

#5 on post-lab questions on calculating delta H for a reaction

look over this!!!

What would be the effect on the enthalpy change determined if the temperature probe read 1.0 degree celsius lower than the true temperature?

the enthalpy change would be smaller since the heat of the reaction used to determine delta H would be less/ACTUALLY: the enthalpy wouldn't change because the starting and ending points on the temperature probe would both go down one degree (ex: 22-12= 10; 21-11= 10) (CHECK THIS!!)

endothermic

when heat is absorbed, delta H > 0

exothermic

when heat is evolved; delta H < 0