Chemical Energetics, Rates of Reaction, Equlibria and Nitrogen Flashcards
What does an exothermic reaction do
Produce energy so the temperature of the reaction mixture and surroundings goes up
What reactions are exothermic
all combustion
What does an endothermic reaction need
An energy input
How is an energy input created for endothermic reactions
By heating the reaction mixture
Give an example of heating the reaction mixture for endothermic
Thermal decomposition of carbonates
What type of reaction is neutralisation
exothermic
What type of reaction is respiration
exothermic
What type of reaction is photosynthesis
endothermic
Endothermic reactions have…
A negative ^H value
Exothermic reactions have…
A positive ^H value
Where does the energy produced during a reaction come from
the potential energy stored in the bonds
reactions involve a combination of
bond breaking and bond making
Breaking bonds requires
energy to be put in (so is ENDOTHERMIC)
making bonds gives out
energy (so is EXOTHERMIC)
exothermic reaction and bonds energy
more energy is released when product bonds form, than us needed to break bonds in reactants
endothermic reaction and bonds energy
more energy is needed to break bonds in reactants than is released when product bonds form
what is the activation energy
the minimum energy required by particles if they are going to react on collision.
what is required for activation energy
an energy input is needed in all reactions to overcome the activation energy barrier
how will a reaction be exothermic?
more heat energy is given out making bonds than is required to break bonds
how will a reaction be endothermic?
less energy is given out making bonds than is required to break bonds
what is the collision theory
a chemical reaction can only occur between particles when they collide. particles may be atoms, ions or molecukes
what happens if colliding particles have less than the activation energy
they bounce off eachother and no reaction occurs. the activation energy works as a barrier to reactions happening
the faster the particles:
the more energy they have. they are more likely to react when they collide
why are fast moving particles more likely to react when colliding
a higher proportion will have an amount of energy equal to or greater than the activation energy
how to give particles more energy
heat them up / raise temperature
how fast a reaction happens depends on 5 things:
surface area , concentration , pressure, temperature and presence of a catalyst
experiment for particle size if solids
marble chip experiment (CaCO3) using different sized chips
how to ensure marble chip test is fair
ensure temp, total mass if chips, concentration and volume of HCl are kept constant
By having the same mass of smaller particles instead of larger particles:
- surface area of solid is increased
- more H+ ions can colide with CaCO3 particles per second -> more frequent collisions
- rate of reaction is faster
why are catalysts usually used in the form of powders
has the largest surface area so will have the fastest reaction rate
the rate of reaction always decreases with time because:
the marble chips are reacting away (total surface area decreasing) and so is the acid (decrease in conc)
experiment for concentration
iodine clock - time taken for the blue black solution to form is measured. concentration of one of the reactants is varied
how to ensure fair test iodine clock
ensure temp and total volume of solutions kept constant
increasing the concentration of a substance in solution means
- more particles of substance of dm3 of the solution
- particles are closer together
- more frequent collisions
- rate of the reaction increases
what does iodine clock experiment show
rate of reaction increases w concentration
what is rate of reaction to concentration
directly proportional
if the concentration of a reactant is doubled
there is twice as many of the particles present, so twice as many collisions in a given time so rate of reaction will double
in order to increase the pressure
number of particles in a given volume must be increased
increasing pressure means
- particles closer togethter
- more frequent collisions
- rate of reaction increases
experiment for temperature
sulphur cross - amount of time taken for a cross to disappear when a precipitate is formed by the reaction of two solutions. experiment is repeated at different temps
fair test for sulphur cross
same concentrations of solutions, total volume of solution
result of sulphur cross
reaction is faster when temp is higher.
at higher temperatures particles:
- move faster so collide more frequently
- have more energy
- more collisions will have energy greater than or wqual to activation energy
- rate of reaction increases
what is a catalyst
a substance that increases the rate of a reaction and can be recovered chemically unchanged at the end of a reaction
what do catalysts do
lower the activation energy reducing the amount of energy needed for a collision to result in a reaction
what are enzymes
biological catalysts
when will enzymes work
when the temperature is below 40°c otherwise they are denatured
enzymes in yeast used in fermentation
C6H12O6 (aq) -> 2C2H5OH + 2CO2
conditions: yeast, 25-35°c, no O2
what is enzymes in fermentation used for
to make beer and wine and in baking to make bread rise
catalytic converter contains:
alloy of platinum, rhodium and other metals
what structure does the catalytic converter have
honeycomb to give as large of a surface area as possible
why are large surface area and high temperature needed
for the reactions to occur at a fast enough rate to remove most of the pollutants
Nitrogen N2
- diatomic molecule
* has a triple covalent bond which is VERY strong and hard to break
why is nitrogen very unreactive
large amount of energy needed to break bond
the haber process
nitrogen reacts with hydrogen to give ammonia. the reaction doesnt go to completion and us reversible - dynamic equilibrium
what is dynamic equilibrium
reached when the rate of the forward reaction equals the rate of the reverse reaction
how is n2 gas obtained
fractional distillation of air
how is h2 gas obtained
natural gas
Le Chatelier’s Principle
“If a change is imposed on a system in equilibrium the position of the equilibrium will shift in order to minimise the effect of that change”
temperature
• exothermic forward reaction
• low temp gives high yield
HOWEVER
• low temp gives a very slow rate so it would take a long time to produce a sufficient amount of ammonia
COMPROMISE
• intermediate temp of 450°c is used to give reasonable yield and a high rate
pressure
• the smaller the number of moles in a given volume, the lower the pressure
• if we increase the pressure, Le Chatilier states that the system will shift the equilibrium to reduce the pressure and so increase the forward reaction, which produces fewer moles
• High pressure gives high rate of forward reaction, producing NH3
HOWEVER
• high pressure is expensive to remain and can be dangerous
COMPROMISE
• 200 atm used
catalyst
- does not alter the equlibrium position so does not affect yield
- does increase rate of reaction (equilibrium is reached but quicker)
- BUT it speeds up the rate of the forward and backwards equally
- means a lower temp can be used to achieve the same rate of reaction
- finely divided iron catalyst
Properties of ammonia
- alkaline gas, turns pink litmus blue
- very water soluble - NH3 + H20 -> NH4 + OH
reaction doesnt go to completion, so NH3 solution is a weak alkaline - distinctive, pungent smell
- reacts with acids to form salts
uses of ammonia
- making fertilisers
- making nylon and other polymers
- making explosives e.g TNT
- used in household cleaning products
advantages of artificial fertilisers
- contains high proportion of N to aid plant growth
- solid so easily transported
- water soluble so can be applied by spraying
- easily absorbed by plants
disadvantages of artificial fertilisers
- very soluble so easily washed out into lakes and rivers (leading to eutrophication)
- nitrates in water supplies are dangerous, especially to babies
- reduced crop variation and biodiversity as only crops that respond well are grown
- expensive for farmers (so not feasible in developing countries)
advantages of organic fertilisers
- better for the environment, encourages biodiversity
- improves structure of soil
- resulting crops contain no artificial chemicals
- cheaper than artificial
- better for developing countries as they are then not dependent on countries or foreign aid
disadvantages of organic fertilisers
- slower / less effective so less crop produced and food therefore more expensive
- food produced may not appear as pleasing
- more labour intensive i.e collecting and spreading the fertiliser
