lecture 17.1- endothermic vs. exothermic

Thermochemistry is the study of heatchanges that accompany chemicalreactions and phase changes.

As temperature increases, the kineticenergy (motion) of particles increases.

There are two types of energyKinetic Energy and Potential Energy

Potential energy is stored energy.Potential energy is stored in chemicalbonds.

There are two types of energyKinetic Energy and Potential Energy

Energy can be converted between thetwo types but it cannot be created ordestroyed= The law of conservation of energy

Heat (q) is energy that flows from awarmer object to a cooler object.

• When the warmer object loses heat, itstemperature decreases and moleculesmove slower.

• When the cooler object absorbs heat, itstemperature rises and its molecules speed up.

• Heat flows until both objects havethe same temperature.

HEAT vs. TEMPERATURE Energy measures average

kinetic energy

Depends on the Independent of amount of substance sample size

A drop of boiling water on your hand doesn’thurt as much as a pot of boiling water.Same temperature (100ºC), but differentamounts of heat.

Cold and hot

• Cold is the sensation of moleculesslowing down (decreasing temperature)

• Hot is the sensation of moleculesspeeding up (increasing temperature)

Hot and cold are relative. A warm cup can feelvery hot if your hand has been in snow.

In thermochemistry, it is useful todivide the universe into two parts

The system- The thing we are studyingThe surroundings- Everything else

universe = system + surroundings

• Enthalpy (H) is the heat content ofa system at constant pressure.

• Most processes involve a change in enthalpy (ΔH)which can be measured in a lab.

• ΔH is measured in J or kJ

The change in enthalpy for a reaction is called theenthalpy (heat) of reaction (∆Hrxn).

∆Hrxn is the difference between the enthalpy ofproducts and the enthalpy of the reactants.

The total enthalpy change is a sum of all bondsthat are broken or formed during a reaction.

Bond breaking = requires energy

Bond forming = releases energy

A process or reaction is either

EXOTHERMIC or ENDOTHERMICHeat (energy) goes Heat (energy) goesout of the system into the systemΔH is negative ΔH is positiveenergy is given off energy is absorbed

ENDOTHERMIC =Products have higherenergy than reactants

EXOTHERMIC =Products havelower energy thanreactants

Higher energy → lower energy +

energy released!

TNT

You can tell if a reaction isendothermic or exothermic by touch.

Remember- the system determines thesign of ΔH and we are part of thesurroundings

YOUAREHERE

A reaction that feelscold has heat goingfrom your fingerINTO THE SYSTEM= endothermic

A hot reaction has heatcoming OUT OF THESYSTEM = exothermic

Equations for endothermicprocesses have energy as a reactant. sunlight + 6CO2(g) + H2O(l)

C6H12O6(aq) + 6O2(g)

Equations for exothermic processeshave energy as a product.

Propane(g) + O2(g) CO2(g) + H2O(g) + heat