Chemistry Le Chatelier's principle

1 chemistry

1.1 effect of change in concentration
1.2 effect of change in temperature
1.3 effect of change in pressure
1.4 effect of change in volume
1.5 effect of adding inert gas
1.6 effect of catalyst

chemistry
effect of change in concentration

changing concentration of chemical shift equilibrium side reduce change in concentration. chemical system attempt partially oppose change affected original state of equilibrium. in turn, rate of reaction, extent, , yield of products altered corresponding impact on system.

this can illustrated equilibrium of carbon monoxide , hydrogen gas, reacting form methanol.

co + 2 h2 ⇌ ch3oh

suppose increase concentration of co in system. using le chatelier s principle, can predict amount of methanol increase, decreasing total change in co. if add species overall reaction, reaction favor side opposing addition of species. likewise, subtraction of species cause reaction fill gap , favor side species reduced. observation supported collision theory. concentration of co increased, frequency of successful collisions of reactant increase also, allowing increase in forward reaction, , generation of product. if desired product not thermodynamically favored, end-product can obtained if continuously removed solution.

effect of change in temperature

the effect of changing temperature in equilibrium can made clear 1) incorporating heat either reactant or product, , 2) assuming increase in temperature increases heat content of system. when reaction exothermic (Δh negative, puts energy out), heat included product, and, when reaction endothermic (Δh positive, takes energy in), heat included reactant. hence, whether increasing or decreasing temperature favor forward or reverse reaction can determined applying same principle concentration changes.

take, example, reversible reaction of nitrogen gas hydrogen gas form ammonia:

n2(g) + 3 h2(g) ⇌ 2 nh3(g)    Δh = -92 kj mol

because reaction exothermic, produces heat:

n2(g) + 3 h2(g) ⇌ 2 nh3(g) + heat

if temperature increased, heat content of system increase, system consume of heat shifting equilibrium left, thereby producing less ammonia. more ammonia produced if reaction run @ lower temperature, lower temperature lowers rate of process, so, in practice (the haber process) temperature set @ compromise value allows ammonia made @ reasonable rate equilibrium concentration not unfavorable.

in exothermic reactions, increase in temperature decreases equilibrium constant, k, whereas, in endothermic reactions, increase in temperature increases k value.

le chatelier s principle applied changes in concentration or pressure can understood having k have constant value. effect of temperature on equilibria, however, involves change in equilibrium constant. dependence of k on temperature determined sign of Δh. theoretical basis of dependence given van t hoff equation.

the value of k changes temperature. in endothermic reaction n2o4(g) ⇌ 2no2(g), equilibrium position can shifted changing temperature. when heat added , temperature increases, reaction shifts right , flask turns reddish brown due increase in no2. when heat removed , temperature decreases, reaction shifts left , flask turns colorless due increase in n2o4. demonstrates le châtelier’s principle because equilibrium shifts in direction consumes energy.


effect of change in pressure

the equilibrium concentrations of products , reactants not directly depend on total pressure of system. may depend on partial pressures of products , reactants, if number of moles of gaseous reactants equal number of moles of gaseous products, pressure has no effect on equilibrium.

changing total pressure adding inert gas @ constant volume not affect equilibrium concentrations (see §effect of adding inert gas below).

changing total pressure changing volume of system changes partial pressures of products , reactants , can affect equilibrium concentrations (see §effect of change in volume below).

effect of change in volume

changing volume of system changes partial pressures of products , reactants , can affect equilibrium concentrations. pressure increase due decrease in volume, side of equilibrium fewer moles more favorable , pressure decrease due increase in volume, side more moles more favorable. there no effect on reaction number of moles of gas same on each side of chemical equation.

considering reaction of nitrogen gas hydrogen gas form ammonia:

n2 + 3 h24 moles ⇌ 2 nh32 moles    Δh = -92kj mol

note number of moles of gas on left-hand side , number of moles of gas on right-hand side. when volume of system changed, partial pressures of gases change. if decrease pressure increasing volume, equilibrium of above reaction shift left, because reactant side has greater number of moles product side. system tries counteract decrease in partial pressure of gas molecules shifting side exerts greater pressure. similarly, if increase pressure decreasing volume, equilibrium shifts right, counteracting pressure increase shifting side fewer moles of gas exert less pressure. if volume increased because there more moles of gas on reactant side, change more significant in denominator of equilibrium constant expression, causing shift in equilibrium.

effect of adding inert gas

an inert gas (or noble gas), such helium, 1 not react other elements or compounds. adding inert gas gas-phase equilibrium @ constant volume not result in shift. because addition of non-reactive gas not change equilibrium equation, inert gas appears on both sides of chemical reaction equation. example, if , b react form c , d, x not participate in reaction:







a


a

+



b


b

+


x


x






↽




−








−




⇀









c


c

+



d


d

+



x


x




{\displaystyle {\ce {{{\mathit {a}}a}+{{\mathit {b}}b}+{\mathit {x}}x<=>{{\mathit {c}}c}+{{\mathit {d}}d}+{{\mathit {x}}x}}}}

. while true total pressure of system increases, total pressure not have effect on equilibrium constant; rather, change in partial pressures cause shift in equilibrium. if, however, volume allowed increase in process, partial pressures of gases decreased resulting in shift towards side greater number of moles of gas.

effect of catalyst

a catalyst increases rate of reaction without being consumed in reaction. use of catalyst not affect position , composition of equilibrium of reaction, because both forward , backward reactions sped same factor.

for example, consider haber process synthesis of ammonia (nh3):

n2 + 3 h2 ⇌ 2 nh3

in above reaction, iron (fe) , molybdenum (mo) function catalysts if present. accelerate reactions, not affect state of equilibrium.