SCH4U - Unit Two Thermochemistry

heat

thermal energy in transfer

thermal energy

total of all energy in particles, not heat

chemical system

a group of reactants and products being studied, typically a reaction

chemical surroundings

all matter around the system, can absorb or release thermal energy

exothermic

releases heat

endothermic

absorbs heat

closed vs. isolated system

only energy can move between the system and the surroundings

enthalpy

total energy change of a system (delta H, kJ)

molar enthalpy

the energy change of one mole of a substance, (delta H of, kJ/mol)

thermochemical equations

show the enthalpy changes, endothermic is the reactants, and exothermic is the products

potential energy diagram

a diagram that shows the changes in potential energy

calorimetry limitations

not useful when reactions are too slow, temperature changes are too small to measure, and when reactions occur too fast and dangerous

hess law

the overall enthalpy in a reaction will be the sum of all enthalpy changes in the steps taken

rules for enthalpy changes

if chemical reaction is reversed, you must reverse the sign of delta H,

if coefficient is multiplied, all others and enthalpy change value must be done by same number

formation equations

equations that show the production of a compound from its elements

standard enthalpy of formation

the change when one mol of a compound is formed from its elements, all of which are in standard states (e.g O2(g))

remember P4(s) and S8(s)

at SATP

standard state enthalpy

standard enthalpy of formation of an element already in its standard state is zero (kJ/mol)

equation for standard enthalpy of formation

n x deltaH (products) - n x deltaH (reactants)

collision theory

for a reaction to occur, the particles must collide, they must collide with the appropriate orientation, and with sufficient energy.

activation energy

minimum amount of energy a reactant must have for a collision to be effective

transition state

a high-energy intermediate state of the reactants during a chemical reaction that must be achieved for the reaction to proceed

rates of reaction

dependent on concentration, as reactants are used up the reaction will get slower and slower

concentration increase

collision frequency increases, effectiveness does not

surface area increase

collision frequency increases, effectiveness does not

temperature increase

both collision frequency and effectiveness increases

higher temperature means faster moving particles, and there is more kinetic energy (collision theory)

presence of a catalyst

collision frequency does not increase, but effectiveness does

rate law

equation that connects speed of reaction with concentration, determined empirically

order

the exponent held to concentration value

elementary step

a step involving entity collisions, either one, two, or three

rate determining step

the slowest step in a reaction mechanism, larger activation energy

reaction intermediate

forms in one elementary step and is consumed in another

requirements to satisfy mechanism

elementary steps must add to overall balanced, mechanism must agree with rate law of overall (slow step)

1st law of thermodynamics

energy may change forms, but the total energy stays the same

2nd law of thermodynamics

heat will flow from hotter to colder objects

standard state elements formation

any element in its standard state, as a standard enthalpy of formation of 0

matter

anything that occupies space and has mass

energy

the potential/capacity to produce change