thermochem full unit notes

heat

form of energy flowing BETWEEN two objects at different temperatures

what direction does heat flow?

from HOT to COLD

temperature

AVERAGE kinetic energy of molecules (measured in celsius or kelvin)

thermal energy

TOTAL heat energy in a substance (measured in joules)

first law of thermodynamics

energy can be converted from one form, but CANNOT be created or destroyed

second law of thermodynamics

heat flows from hot objects to cold objects until thermal EQUILIBRIUM

• thermal equilibrium is where amount of heat energy is the same in both objects

• some energy is lost in the process of a reaction as HEAT due to this

chemical potential energy

stored energy in bonds within and between molecules

kinetic energy

motion of ions, atoms, and molecules

• with translational, rotational, and vibrational movement

exothermic reaction

energy is RELEASED

how do exothermic reactions affect the # of potential & kinetic energy

• chemical bonds are BROKEN (less potential)

• molecules COOL and move slower (less kinetic energy)

endothermic reaction

energy is ABSORBED

how do endothermic reactions affect the # of potential & kinetic energy

• chemical bonds FORMED (more potential energy)

• molecules HEAT and move faster (more kinetic energy)

what does a change in temperature indicate, and what kind of relation is it?

change in AVERAGE kinetic energy, direct relation

where does all energy originate from?

the sun

how is some energy released, since no process is 100% efficient?

released as heat

what is conserved in a chemical reaction?

energy

what does a change in potential energy indicate?

formation and breaking of bonds

specific heat capacity

amount of energy required to raise 1g of substance by 1 degree celsius

• every substance has a different heat capacity

what two things occur in chemical reactions?

bond are broken, requiring energy (ENDOTHERMIC)

bonds are formed, releasing energy (EXOTHERMIC)

what happens with reactants in a chemical reaction, and what type of process is it?

bonds break between reactants, ENDOTHERMIC process

what happens with products in a chemical reaction, and what type of process is it?

bonds form between products, EXOTHERMIC process

what determines if overall reaction is endo or exothermic

NET CHANGE in energy, whether it releases or is conserved

enthalpy

total chemical POTENTIAL energy possessed by a substance

how is enthalpy important to a system?

shows capacity to exchange energy w surroundings during a physical/chemical change at CONSTANT pressure

what alters the enthalpy of a reaction?

• state of matter

• temperature

what can we measure in terms of energy? what can we NOT measure?

• can measure CHANGE in enthalpy

• cannot messaure TOTAL enthalpy in a system

how to calculate the change in enthalpy of a reaction?

enthalpy of rxn = enthalpy of product - enthalpy of reactant

what is the change in enthalpy like in a EXOTHERMIC reaction?

• overall change in enthalpy is NEGATIVE

• reactants have a higher enthalpy

what is the change in enthalpy like in a ENDOTHERMIC reaction?

• change in enthalpy is POSITIVE

• enthalpy of products is HIGHER

molar enthalpy

change in energy per mol of substance

why do we use molar enthalpy?

see what substances release more/less energy during a change

• bc we use same amount

how do you write enthalpy as PART of a chemical reaction?

• exothermic: add energy term on product side

• endothermic: add energy term on reactant side

how do you write enthalpy with (change in)H notation OUTSIDE a chemical reaction?

• exothermic: change in H is negative

• endothermic: change in H is positive

what side of an equation has MORE potential energy in an EXOthermic reaction?

the reactants

what side of an equation has MORE potential energy in an ENDOthermic reaction?

the products

describe a potential energy graph for an exothermic reaction

reactants have more, and then it goes down to the products which have less. energy is LOST

describe a potential energy graph for an endothermic reaction

reactants have less, and then it goes UP to the products that have more. energy is GAINED

catalyst

• speed up exothermic reactions, BUT react very slowly

• reduce # of activation energy needed

do catalysts release or absorb any energy, and where should we put it?

• do NOT release/absorb anything

• put it above the arrow to show that it is not a part of the chemical reaction

activation energy

minimum # of energy needed to start a chemical reaction

what is the difference between a regular and catalyzed chemical reaction, and how does it show on a graph?

catalyzed reactions have a faster rate of reaction (steeper curve on a graph), and use less potential energy (peak of reaction is lower)

calorimetry

determining a change in energy in chemical reactions EXPERIMENTALLY

what are two assumptions we make in calorimetry?

• energy is CONSERVED (1st law)

• heat flows hot → cold until THERMAL EQUILIBRIUM reached (2nd law)

what occurs in an exothermic reaction when measuring it experimentally?

heat is RELEASED from chemical reaction (system), solution gains heat, temperature of solution increases

what occurs in an endothermic reaction when measuring it experimentally?

heat is ABSORBED by chemical reaction (system), solution loses heat, temperature of solution decreases

why is water utilized in calorimetric experiments?

high specific heat capacity

what should we assume about any solutions used in calorimetric experiments?

have all the physical properties of water

• 1g/1mL density

• same specific heat capacity

what are styrofoam calorimeters used for?

measuring energy change in AQUEOUS solutions and reactions

what is an important characteristic of styrofoam calorimeters?

insulated, minimizes heat transfer with outside environment

what are metal calorimeters used for?

COMBUSTION reaction

what is an important characteristic of metal calorimeters

made of copper or aluminum, which are good CONDUCTORS

what are bomb calorimeters used for?

EXPLOSIVE reactions

what is an important characteristic of bomb calorimeters

• rxn occurs in oxygen gas SURROUNDED by water

• withstands high pressure

what is the energy input, and how can it be calculated

• IDEAL energy content of fuel

• can be calculated using hess’s second law

what is the energy output, and how can it be calculated

• amount of USABLE energy doing ACTUAL work

• can be calculated using kinetic energy formula, calorimetry, or Hess’s Law

efficiency

ratio of useful energy to input energy