UNIT 5 - Energetics and Thermochemistry - Reactivity 1.1 - 1.4
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what is heat?
a form of energy transfer that occurs as a result of temp difference
what is temperature?
the average kinetic energy of the particles
what is heat capacity?
the heat needed to change the temp of an object by 1 K
what is an open system?
system in which energy can be exchanged with the surroundings
what is a closed system?
a system in which energy can be exchanged with the surroundings but matter cannot
what is an isolated system?
a system in which neither matter nor energy can be exchanged with the surroundings
what is enthalpy?
the chemical potential energy of a system
when does the enthalpy of a system increase?
when heat is added to a system from the surroundings
when does the enthalpy of a system decrease?
when a system gives out heat
what are exothermic reactions?
reactions that give heat to the surroundings, dH is neg, reactants have more stored potential energy than products (most combustion rxns and all neutralization reactions)
what are endothermic reactions?
reactions that absorb heat from the surroundings, dH is pos, products have more stored potential energy than reactants
during reactions in solutions, what kind of reaction leads to a decrease in temperature of the aqueous solvent?
endothermic because the reaction absorbs heat from the solvent
during exothermic reactions in solutions, where is heat transferred to first?
the solvent, increasing its temperature
what is the direction for change in enthalpy?
in the direction of lower stored or potential energy
why are most combustion reactions exothermic?
because the energy needed to break the bonds in the reactants is less than the energy produced as the bonds form in the products
what does standard enthalpy change imply?
pressure of 100kPa, concentration of 1mol/dm^3, all substances are in their standard states
why will the smaller object of two objects made of the same material experience a larger temp increase when the same amount of heat energy is added to both?
because it has less particles for the heat energy to be shared amongst
what does the increase in temp when an object is heated depend on?
mass of the object, the heat added, and the nature of the susbstance
how can heat absorbed by water be calculated from experiments for combustion reactions using a metal calorimeter?
by its temperature and change in mass
how do you calculate dH combustion given that all heat from the reaction is absorbed by the water?
use change in enthalpy of combustion= -Q/n, then sub Q=mcdT (use units of temp given), then substitute info for water for Q and the moles for what is being burned
for combustion calorimetry, what are some factors that would leave to errors in calculating dH of combustion?
not all heat produced was absorbed by the water, some heat absorbed by copper calorimetry, some lost to surroundings, incomplete combustion, experiment not being performed under standard conditions
why are solution calorimeters made fom insulator?
to maximize the amount of heat transferred between the reaction and the water in the system
in solution calorimetry, why must we extrapolate back using the line of best fit for the cooling of the system to when the second reactant was added to the water in the system?
because heat is lost from the system as soon as the temp rises above the surrounding temperature
what is bond enthalpy?
the energy needed to break one mole of bonds in gaseous molecules under standard conditions