U3AOS2: chemical reactions and equilibrium Flashcards
collision theory
reactant particles must collide for chemical reaction to occur w/ correct orientation and sufficient energy, so the bonds between the reactant particles can be broken
features of maxwell-boltzmann distribution
peak=kinetic energy of most of the particles in the sample
area under curve=total no of particles
left=lower avg KE, right=higher
define activation energy
the minimum amount of energy required to generate a chemical reaction by breaking bonds between reactant particles
effect of changing concentration on equilibrium
increasing conc of reactant or product will favour reaction where substance is reactant, so removing substance
if substance = removed, equil. will shift in order to replace it
adding or removing solids/liquids will X change equ = must change CONC
describe la chatelier’s principle
a system in equilibrium has the tendency to partially oppose any disruption to re-establish equilibrium and minimise effects of changes
define rate of reaction
the change in concentration of a reactant or product per unit time // speed of reaction
most common unit = M/s
factors affecting rate of reaction
proportion: temperature, catalyst
frequency: surface area, concentration, pressure of gases
two types of catalysts
homogeneous: same state as reactants and products
heterogeneous: different state to reactants and products
effect of using catalysts on RR
-reduce AE of reaction as particles on the surface distort and disrupt bonds w/in molecule
-providing alternative chemical pathway
-increases proportions of particles w sufficient energy
-increase frequency of successful collision PER UNIT TIME
-increase ROR
effect of changing temp of reactants on ROR
-increase temp = increase kinetic energy of reactant particles
-increase proportion of particles w sufficient energy
-increase frequency of successful collisions PER UNIT TIME
-increase ROR
effect of changing pressure of reactant gases on ROR
-decrease volume = increase pressure
-increase volume = decrease pressure
-increase number of particles per unit volume if volume dec + pressure incr
-increase frequency of collision
-increase freq. of successful collisions PER UNIT TIME
-increase ROR
effect of surface area of solid reactants on ROR
-increase surface area = increased no. of particles that are exposed and available for collision, reduces shielding of inner particles
-increase frequency of collisions
-increase frequency of successful collisions PER UNIT TIME
-increase ROR
effect of changing concentration of solution on ROR
-increase conc of solution
-increase no. of particles per unit volume
-increase frequency of collisions
-increase frequency of successful collisions PER UNIT TIME
-increase ROR
how do you change the pressure of gases?
by changing the volume
adding inert gases = NO EFFECT
terminology for equilibrium
forward reaction is favoured = equilibrium is shifted to the right
reverse reaction is favoured = equilibrium is shifted to the left
what is dynamic equilibrium
-when the rate of the fwd reaction equals the rate of the rev reaction - both processes are occurring simultaneously
-no observable change
-concentration of products and reactants remain constant but x always equal
is dynamic equilibrium a closed or open system?
closed system to prevent reactant or products from escaping or being introduced
effect of changing pressure on equilibrium
increase pressure = always favours reaction that produces fewer moles of gas
decrease pressure = favours reaction that produces more moles of gas
effect of changing temp on equilibrium
increase temp = favour endothermic reaction, where enthalpy change = POS bc system wants to reduce temperature
decrease temp = favour exothermic reaction, where enthalpy change = NEG bc system wants to increase temp
effect of adding catalyst on equilibrium
increase rate of both forward and reverse reactions - equilibrium may be reached more quickly, but not favour one or change position of equilibrium
define haber process
the industrial process of the manufacture of ammonia from hydrogen and nitrogen
compromise positions of the haber process
-at 200 atm, lower pressure than preferred b/c higher pressure = increased energy costs + more exp. equip to maintain high pressure
-400-450 deg celcius, lower temp favours exo reaction but too slow of ROR and decrease eq. yield (maximise yield per day, not cycle)
-uses iron catalyst
what might a temperature MB curve show you?
-at any temp=range of particle speeds
-most particles don’t have very high or low KE
-x normal distribution = positively skewed
-talk about average KE
an increase in temp generally favours a higher ROR, why?
mainly bc reactant particles have more kinetic energy and so collide with more force, meeting the activation energy required
key constant things at equilibrium:
-amount, in mol and mass, of chemical substances remain constant when eq. is reached
-conc of chemical substances remain constant
-total gas pressure is constant
-temperature is constant
concentration curve features
-flat lines indicate that equilibrium has been reached and conc = constant
-initial steepness/gradient = linked to coefficient
-initially [products] = 0
rate graph features
forward reaction is initially the greatest,
reverse reaction has not begun to starts at 0
reach same point when equilibrium first established
Kc = _____
for aA + bB <-> cC + dD
Kc = [C]c [D]d / [A]a [B]b
Kc = is a constant at equilibrium, can only be affected by TEMPERATURE
Kc represents _____
-the measure of the extent of the reaction ie. tells us if there is more fwd or rev reaction
-larger the Kc value, the greater the proportion of the reactants that is converted to products
no info abt ROR
general Kc values
-more than 10^4 = reaction essentially goes to completion, amount of reactants remaining is negligible
-less than 10^-4 = reaction occurs to a negligible extent only, amount of products formed is neg.
-between two, both fwd and rev reactions are occurring to a moderate extent
effect on Kc is equation is reversed
take the reciprocal
effect on Kc is coefficients are doubled
Kc = squared
effect on Kc is coefficients are halved
Kc = square rooted, power of 1/2
what impacts Kc?
ONLY TEMPERATURE!!
key words for ROR qs
PER UNIT TIME
Qc = _____
same as Kc formula
reaction quotient can be calculated for any mixture of reactants and products at any time during a reaction
effect of dilutions on equilibrium
bc dilution means that conc of all things has decreased,
system will favour reaction that produces greatest mol of particles
why may a reaction not reach the same colour/conc/pressure as previous eq?
bc the system only PARTIALLY OPPOSES to the change, so does not reach the original level of ___ at equilibrium
CO poisoning equations
Hb4(aq) + 4O2(g) <-> Hb4O8(aq), Kc = x
Hb4(aq) + 4CO(g) <-> Hb4(CO)4(aq), Kc = 20 000x
CO poisoning theory
because CO also binds to haemoglobin, and this reaction has a much higher Kc value, then this reaction is more likely to occur. reactions = competing equilibria bc both oxygen and CO compete for the same substance
how to treat CO poisoning?
supply high conc of pure oxygen via mask
1. [O2] increased for first reaction, so less [Hb4] present in blood
2. rev reaction favoured in second eq to partially oppose the less [Hb4], decomposing the carboxyhaemoglobin complexes and producing more [Hb4] to react with O2
how to calculate percentage yield
% yield = actual yield/theoretical yield x 100%
how to increase rate of reaction?
higher concentration/pressure
higher temperatures
higher surface area of solids
using a catalyst
how to increase yield at equilibrium
change pressure depending on equation
change temp depending on exo or endo
addition of excess reactant
removal of products
difference between rate and yield qs
rate = 5 FACTORS and MB distribution
yield = LCP and equilibrium
based on colour changing qualitative observations, is that sort of reaction reproducible?
yes. simply identifying the change in colour and recognising increase or decrease in intensity of colour can be easily identified. so not subject to bias bc unlikely to differ btw experimenters
the value of Kc for a reaction at a temp may be quite high, but the rate of reaction low. why?
Kc is not an indicator of the rate of reaction, only to the extent of which the reaction proceeds. may be other factors affecting
how does pressure affect concentrations?
increase pressure = increase conc
decrease pressure = decrease conc
why may a reaction no longer occur? the volume of the products may be constant
one of the reactants, the limiting reagents, has been fully consumed. the reaction can no longer proceed and has gone to completion
describe a situation where carbon monoxide poisoning may occur
incomplete combustion of a fuel due to a lack of oxygen or ventilation
how does adding water, which is a reactant/product in a reaction, affect equilibrium and Kc?
value of Kc does not change = only changes with temp
adding water = dilutions! decrease concentration of all species present, so favours reaction where greatest number of particles/moles produced
a reaction occurs. why may the concentration of the product reach a constant level?
one of the reactants has been fully consumed.
limiting reagent used up, always a reagent in excess.
reaction therefore can produce no more
health and safety considerations of haber process
- pressurised system: need to monitor seals, valves, pipes and pumps to ensure no leaks -> use sensors + high quality materials
- dangerous if inhaled: must have a well ventilated area, use fume cupboard in school settings, gas masks in industry
- toxic and corrosive: appropriate safety gear must be used, lab coats and rubber gloves and safety goggles
compromise conditions of haber process
-500 deg celcius temp
-250 atmospheres pressure
-excess of reactants are not used, molar ratio sustained
-removal of products - gases pumped into condenser, expand and cool
-four catalyst beds used
equilibrium is not actually achieved bc insufficient time
intensity colour: how might it change?
intensity might decrease upon reaction, quickly at first then slow down. but some colour might remain bc of EXCESS/LIMITING REAGENT
molar ratio of reactions. in a 2:1 ratio, 2 mol of the reactant is introduced. will 1 mol of the reactant form?
no. in a reversible reaction, all the reactants are not sued up if both reactions must be occurring. so not exactly ___ mol produced
algebra with Kc value
BE CAREFULLLL
eg. 0.2/(x^2)(0.2) = 1.56
x = sqr root (0.2/0.2 x 1.56)
what might a concentration time graph tell you about the Kc value?
might be relatively high because [products] is higher than [reactants]. so, there is a higher proportion of the products present at equilibrium. opposite for lower Kc value and lower product line in graph
at equilibrium, does eg. 2.0 mol of the reactant all react?
NO. not entirely reacted because both fwd and rev reactions must be occurring. eg 2-x reacting
what does a high Kc value mean?
[products] = higher than [reactants]
what is the effect of doubling volume on a gas concentration?
double volume, half pressure. decrease pressure = decrease concentration. therefore, [gas] = DECREASED
define a chemical reaction
a process in which one or more substances, the reactants, are converted to one or more different substances, the products
what does the peak of an enthalpy change graph represent
the transition state
for an exothermic reaction, what is the relative energy and stability of products vs reactants?
the products have less energy, but are more stable
how can you measure rate of reaction?
measuring change in conc, mass, volume over time eg. using gas syringe for gas loss, mass for gas gain, solid used -> dried and weighed
similarity in open and closed systems
both can exchange heat matter
when do you exclude water from the Kc expression?
when the system is fully aqueous, so the presence of water as a liquid doesn’t really matter. if not aqueous, include it?
how does the rate of the fwd/back reaction go up or down by when temp/pressure is changed
both do up and down if temp and pressure are both increased - increase general rate of reaction. the reaction that goes up the most and then levels out (goes back down again and is above the reaction which is not favoured) is the one that has the greater NO of particles. eg. if increased temp, fwd reaction w 6 particles goes up more than compared to 2 particles, bc more particles colliding and increased ROR
why do adding inert gases have no effect?
-inert gases are unreactive so do not take part in reaction
-not impact ROR or conc of gases, do not change the volume of container so no change to pressure as the reacting gases will still have the same no of particles in same volume
-no change to yield or POE
what does the effect of a catalyst look like on graphs?
enthalpy: reduced peak
conc-time: steeper gradient and plateaus earlier
rate-time: spike up and then levels off bc still at equilibrium
what happens to addition of reactants/removal of products in reactions with very low Kc values?
bc so small Kc values, neither reaction proceeds to completion to any great extent.
change made usually negligible, so at eq., similar concentrations to OG
what must you consider abt low Kc values?
so low that reaction does not go to completion. consider how impact upon addition of things = minimal. greater impact on reactions with larger Kc values.
what is pH impacted by?
only the concentration of H+ ions
in a reversible reaction, is all of the reactant used up to produce the products in same molar ratio?
no, all the reactant is not used up bc the rates of the FWD and REV reactions must equal each other, and there must be a constant conc of all species at equil, so not all reacted or produced
when can the Kc value be correctly calculated from mol rather than concentration?
so, n=cv. volume becomes irrelevant so let n=c.
when K can be calculated from mol, must be same no of moles on each side for units cancel out
if a temp change has not occurred, has Kc changed?
NO
how can you minimise subjective bias eg. by observation?
use other means of measurement eg. measuring mass or concentration of products instead of simply observing. use a detector that determines at what time the solution has turned cloudy
even if an experiment contains many IVs, why may it still be valid?
because only one IV is being changed at one time in each reaction. the other remains constant. still only measuring the effect of one IV on DV
safety apparatus with toxic gases?
-breathing apparatus so you don’t inhale
-set up gas leak detectors
-exhaust extraction system, only use for necessity and then immediately remove
when can you include water in the Kc expression?
when water is not present/taken from initial situation. It was eventually formed during the reaction and hence it is a product ( not a solvent). Hence you need to include it in the Kc expression.
what does yield refer to?
AMOUNT IN MOL. change in pressure for a reaction with equal no. of moles on each side leads to no change in mol. may change conc but not overall yield
