Chapter 10 Reaction rates and equilibrium Flashcards
rate of chemical reaction?
how fast a react is being used up or how fast a product is being formed
rate= change in conc/ time (units= moldm-3 s-1)
How does rate of reaction change as the reaction progresses?
rate is fastest at start of reaction (each reactant is at highest conc)
rate slows down as reaction proceeds (reactants are used up)
once all reactions are completely used up, rate=0
Why are some collisions effective and others ineffective?
an effective collision only occurs if:
- particles collide with correct orientation
- particles have sufficient energy to overcome activation energy barrier of reaction
rate of reaction: changing concentration?
increased conc= increased rate
–> more particles in the same volume (more crowded)
–> more frequent collisions
–> more effective collisions in the same given period of time
–> rate increases
rate of reaction: changing pressure?
Increased pressure = gas particles become more crowded
–> closer together and collide more frequently, leading to more effective collisions
Two ways of determining rate of reaction for gas producing reactions?
- monitor volume of gas produced at regular time intervals
–> underwater measuring cylinder / gas syringe - monitor loss of mass of reactants using balance
rate of reaction: changing temperature?
higher temp= increased rate
–> more kinetic energy= particles move around faster
–> collision frequency increases= more effective collisions
–> rate increases
What is a catalyst
substance that speeds up the rate of reaction without being used up
–> provides an alternative reaction pathway with lower activation energy
What is a homogeneous catalyst?
one that has the same physical state as reactants
–> catalyst reacts with reactants to form an intermediate and then breaks down to give product
Examples of homogeneous catalysts
making esters with sulfuric acid
C2H5OH + CH3COOH = CH3COOC2H5 + H2O
–> all substances here are in liquid state
OZONE depletion with CL radical as a catalyst
–> 2O3 = 3O2
All substances are in gaseous state
Heterogeneous catalysts
one that has a different physical state from reactants
–>reactant molecules are adsorbed (weakly bonded) onto surface of catalyst
–> product molecule leaves surface by desoprtion
Economic important and sustainability of catalysts
- catalysts increase rate by lowering activation energy
–> reduces temperature needed and energy requirements
–> meaning less electricity or fossil fuel is used
–> cutting down on costs and producing fewer pollutants
Boltzmann distribution
a graphical representation of the spread of molecular energies in gases
–> Ea= activation energy and normally only a small proportion of molecules have enough/ more energy than Ea
Features of a Boltzmann distribution
- no molecules have 0 energy (curve starts at origin)
- area under curve= total number of molecules
- no maximum energy (curve does not meet x-axis at high energy)
x axis= energy/ y axis= number of molecules with a given energy
How does temperature affect boltzmann’s distribution?
At higher temp:
1. more molecules will have greater energy than Ea
–> greater proportion of collisions will lead to reaction
–> collisions will be more frequent as molecules are moving faster
Boltzmann temp graph
At T2 (higher temp), the peak is much lower and shifted to right
on the end of x axis, it is higher than T1 but Ea is still same
How does catalysis affect Boltzmann’s distribution?
Alternative reaction route with a lower Ea
–> greater proportion of molecules have equal/ more than Ea
–> more molecules will cause effective collisions
–> increased rate
Boltzmann catalysis graph
Same trend but lower activation energy so new line is shifted to the left
What is dynamic equilibrium?
when the forward reaction rate is equal to backwards reaction rate
-concentration of reactants and products do not change (ARE NOT EQUAL)
Closed system
isolated from surroundings so temp, pressure and other conditions are unaffected
Le Chatelier’s principle
when a system in equilibrium is subjected to an external change, the system readjusts itself to minimise the change
Concentration and equilibrium
Increasing reactants conc/ decreasing products conc = rate of reaction shifting to the right
–> more product needs to be made to minimise change
Decrease reactant conc/ increase product conc = rate of reaction shifts left
–> more reactants need to be formed
Investigating changes to equilibrium with concentration (theory)
Equilibrium between aqueous chromate ions (CrO4 2-) and dichromate ions (Cr2O7 2-) is sensitive to changes in acid conc
2CrO4 2- (aq) + 2H+(aq) = Cr2O7 2- (aq) + H2O (aq)
chromate is yellow and changes to orange once H+ ions are added as dichromate is orange
What happens when we add H2SO4 to potassium dichromate or we add NaOH
Adding H2S04 turns the solution orange
Adding NaOH turns solution yellow again
