Rate of Reaction, Energetics of a Reaction, Reversible Reactions & Equilibrium Flashcards
https://quizlet.com/gb/335941381/gcse-rate-of-reaction-flash-cards/ [PRIORITISE THIS MEMORISATION]
Describe the effect on the rate of reaction [the speed at which the reaction takes place] of:
(a) changing the concentration of solutions
a) conc: Increasing concentration, increases the number of SOLUTE particles per unit volume.
add more particles
increases the frequency of collision and the rate of reaction.
(b) changing the pressure of gases
b) pressure of gas: Increasing PRESSURE, increases the number of GAS particles per unit volume.
add MORE PARTICLES
OR reduce volume of container
increases the frequency of collision and the rate of reaction.
(c) changing the surface area of solids
REMEMBER: less particles hidden on inside when particles LESS CLUMPED
Smaller group = more particles react
c) S.A. of solids: Increasing surface area = more particles exposed on the outside (fewer unexposed particles in the middle)
The larger the surface area, the faster the reaction as there are more sides for particles to react with.
increases the frequency of collisions and the rate of reaction.
(d) changing the temperature
d) temp: Increasing temperature, increases the frequency of collisions as the particles are moving faster.
(e) adding or removing a catalyst, including
enzymes
e) catalyst e.g. enzymes: Catalysts speed up rate of reaction w/o being used up or changing at the end of the reaction;
do this by ALLOWING the REACTANT PARTICLES to collide more easily making these collisions MORE SUCCESSFUL.
What does a catalyst do? [syllabus]
a catalyst decreases the activation energy, Ea, of a reaction
Also: catalysts…
Catalysts provide an alternative pathway for the reaction, which has a lower activation energy.
This increases the rate of reaction as more particles collide with enough energy to overcome activation energy.
Enzymes
proteins that act as biological catalysts
How do catalysts differ from enzymes?
Unlike chemical catalysts, they work at specific temp & pH.
At extreme conditions, enzymes become denatured & no longer function.
keywords that must be used in answers
particles,
collide
frequency of collisions
kinetic energy or particles
concentration
temperature
surface area
pressure
catalysts
EXTENDED
Describe collision theory in terms of:
(a) number of particles per unit volume
(b) frequency of collisions between particles
(c) kinetic energy of particles
(d) activation energy, Ea
- Describe practical methods for investigating the
rate of a reaction including change in mass of a
reactant or a product and the formation of a gas - Interpret data, including graphs, from rate of
reaction experiments - Evaluate practical methods for investigating the
rate of a reaction including change in mass of a
reactant or a product and the formation of a gas
Define catalyst
catalyst increases the rate of a
reaction and is unchanged at the end of a reaction
For a chemical reaction to occur, particles must:
collide in the correct orientation
with enough energy for a reaction to take place, called the Activation Energy, Ea
Measuring Rate of Reaction
- Measure:
measure mass of substances
looking at rate at which reactants are used up, or rate at which products are formed
also can be measured:
[note: cotton wool allows gas to escape, not liquid]
- mass of substance (using a mass balance)
- volume of gas (using a gas syringe or inverted measuring cylinder)
Interpreting Graphs - compare 2 graphs
Why does the rate of reaction slow down as the reaction completes?
Reactant particles are being used up, so there are fewer particles colliding, resulting in fewer successful collisions
Why does rate of reaction decrease as more products are formed?
reactants => products. bc more products form => concentration of reactants decreases
Fewer reactant particles available to collide.
Energetics of reaction -
To break bonds, what needs to be absorbed?
To make new products…
Energy is required
To make NEW PRODUCTS, bonds between REACTANT PARTICLES must be broken.
Energy is RELEASED to do what?
To make new products…
make bonds (energy RELEASED)
bonds between products particles must be formed to make new products
So OVERALL reaction is exothermic when
LESS energy absorbed to break bonds,
MORE energy released to make bonds
And OVERALL endothermic reaction when
MORE energy absorbed to break bonds,
LESS released to make bonds
Define -
- Endothermic
[ENDO = entering]
📍 involves ABSORBING energy; bond BREAKING
📍 TEMP of surrounding DECREASES ; temp drops
📍 PRODUCTS MORE
📍 ΔH POSITIVE (+)
[how much energy taken in; pos. bc products have more energy than reactants]
- Exothermic
[EXO = exiting]
📍 Involves RELEASING energy; bond MAKING
📍 TEMP of surroundings INCREASES ; temp rise
📍 Reactants MORE THAN products
📍 ΔH NEGATIVE (-)
[how many energy taken in; neg. bc products have less energy than reactants]
Define: Enthalpy [delta H]
change in energy between reactants and products
& has the units kJ/mol [kJ mol^-1]
How to calculate ΔH
(Σ energy of bonds broken) - (Σ energy of bonds made)
(endothermic - exothermic)
Steps:
- Draw all covalent bonds in reactant AND product molecules
- Calc energy of bonds broken
- Calc total energy of bonds made
- Calc ΔH using broken - made
NEGATIVE = EXO
2H2 (g) + O2 (g) => 2H2O (g)
H - H = 436 kJ/mol
O = O ;; 498 kJ/mol
O - H ;; 464 kJ/mol
- Draw
- 436 x 2 bc there’s 2 H - H
+ 1 x 498 bc O=O part
= 1370 kJ - 4 O-H (products side) = 4 x 464 = 1856 kJ total energy
- ΔH = 1370 - 1856 = -486 kJ/mol
486 kJ energy RELEASED so reaction is EXOTHERMIC
Activation energy graph - EXOTHERMIC
POINTS ON IT YOU HAVE TO RECOGNISE
reactants, products, Ea
Activation energy = vertical line STARTING from horizontal line of “reactants” going up to top of curve
Products = the 2nd horizontal line
From reactants’ horizontal line to the products’ line, this is ENERGY RELEASED
delta H negative [downwards line from reactant to product line]
x-axis = direction of reaction, y-axis = energy
ENDOTHERMIC; Activation energy graph
POINTS ON IT YOU HAVE TO RECOGNISE
reactants, products, Ea
Activation energy = vertical line STARTING from horizontal line of “reactants” going up to top of curve
Products = the 2nd horizontal line ;; in ENDO, products HIGHER
From reactants’ horizontal line to the products’ line, this is ENERGY ABSORBED [in ENDO]
delta H POSITIVE ; [UPWARDS line from reactant to product line]
x-axis = direction of reaction, y-axis = energy
Endo or exo - change of state.
📍 ENDO
Solid ➡ liquid ➡ gas
Particles absorb energy to weaken/break bonds
📍 EXO
Gas ➡ liquid ➡ solid
Particles release energy to strengthen/make bonds
exo exp.
& endo
Mg
NH4Cl
How to observe exothermic reaction?
📌 gas [not really observed] - hydrogen
📌 bubbling
📌 temperature rose
Observing an ENDOthermic reaction
📌 temperature lowers
Reversible reactions
- bc not all chemical reactions come to an end when they have run of at least 1 of the reactants
right half-arrow on top BC FORWARD REACTION FIRST
reverse reaction on bottom
⇌
- Some chemical reactions are able to reverse their reaction direction by turning products back into reactants
- can only occur if the product/reactant is not limiting or limited by a limiting factor
hydration of salts
Some chemical reactions can be reversed by adding water or by heating to remove water by evaporation.
Adding water
Anhydrous copper sulphate + water → Hydrated copper sulphate
CuSO4 + 5H2O → CuSO4.5H2O
Removing water
Hydrated copper sulphate → Anhydrous copper sulphate + water
CuSO4.5H2O → CuSO4 + 5H2O
2 must know examples
- Copper sulfate [JUST SAY COLOUR HERE W/ NOTHING ELSE]
CuSO4.5H2O ⇌ CuSO4 + 5H2O
^ BLUE ^ WHITE
- Cobalt chloride
CoCl2 + 6H2O ⇌ CoCl2.6H2O
^ BLUE ^ PINK
Test for water
in presence of water, these change colour
add water to anhydrous copper (II) sulfate it will turn blue,
or add anhydrous cobalt (II) chloride and it will turn pink
Concept of Equilibrium
Reversible reactions occur in both the forward and backward directions
therefore…
reversible reaction can reach equilibrium in a closed system
so dynamic equilibrium:
DEFINITION
1st mark: the rate of the forward reaction and reverse reaction is equal
+ requiring a closed system where no matter (atoms) can escape or be introduced + no observable change
2nd mark: the concentration of the reactant/s and products remain constant
(given there is no other change to the system such as temperature and pressure)
Equilibrium is dynamic
meaning
the molecules on the left and right of the equation are changing into each other
by chemical reactions constantly and at the same rate
An example of a reaction reaching equilibrium is the reaction between H2 and N2 in the Haber process:
At the start of the reaction, only nitrogen and hydrogen are present
This means that the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highest
As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease
So, the rate of the forward reaction will decrease
However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase
Ammonia will decompose to reform hydrogen and nitrogen
In a closed system, the two reactions are interlinked and none of the gases can escape
So, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached:
Le Chatelier’s Principle
any change will make it return to equilibrium
when a change is made to a system at equilibrium, the system will oppose the change by minimising the effect of the change
[to oppose change, either forward/reverse reaction will be favoured]
if the forward reaction is favoured, more product more,
the system will be returned to equilibrium by increasing the rate of the reverse reaction
The position of equilibrium is said to shift to the right when the forward reaction is favoured
This means that there is an increase in the amount of products formed
position of equilibrium is said to shift to the left when the reverse reaction is favoured
So, there is an increase in the amount of reactants formed
if one reaction forward is exo, then reverse it’s
ENDO
CHANGE in temp - effect on equilibrium
Depends on enthalpy of reaction.
Increase in temp ->
increase in temp -> FAVOURS ENDO
decrease in temp -> favours EXO.
E.g.
1) I2 + H2 ⇌ 2HI
enthalpy = -10.4 kj/mol
exo forward, endo back
2) N2 + O2 ⇌ 2NO
enthalpy = 92 kj/mol
endo forward,
exo back
Concentration
Increase reactant(s) favours products - FORWARD reaction
Increase product(s) favours reactant - REVERSE reaction
e.g.
H2 + 3N2 ⇌ 2NH3
Haber process
Pressure [GASES ONLY]
Increase pressure favours the side with less moles
decrease pressure favours the side with MORE moles
e.g.
2NO2 ⇌ N2O4
2 moles, vs. 1 mole
so here increase pressure -> less moles
decrease pressure -> more moles
Catalyst - addition does what
Equilibrium position DOES NOT change
A catalyst makes system reach equilibrium faster & then speeds up both forward & reverse reactions equally
If the temperature is raised:
heat energy removed, reactants increase
yield from the endothermic reaction increases
The yield from the exothermic reaction decreases
therefore
lowered temp -
heat energy added
right
concentration of product increases
reactants decrease
// yield from the endothermic reaction decreases
The yield from the exothermic reaction increases
at 400 % of methanol in equilibrium mixture is lower than at 300. Suggest explanation [2]
reverse reaction is endothermic
increase in temperature favours the endothermic reaction which is the back reaction
When the equilibrium mixture is heated it becomes a darker brown colour. Is the reverse reaction endothermic or exothermic? Give a reason for your choice.
endothermic
favoured by high temperatures
Good form to use for exam ques
📍 Increase/decrease in ❓ favours ❓ exo/endo because…
📍 The reaction will go in the forward/backward direction. Equilibrium position shifts to left/right.
📍 The concentration of the products, ❓, increase/decrease & reactants’ concentration increase/decreases
📍 Colour goes from what to what - e.g. brown to colourless
Why does decrease in temp favour exo?
Can’t say goes brown - was already a bit brown, JUST “DARKER BROWN”
there is less heat, which, according to Le Chatelier’s Principle, will cause the reaction to proceed to the right to reduce the effect of the change in temperature.
state meaning of term REDOX
reduction and oxidation occur
why is this an addition reaction
only one product is formed
IMPORTANT:
Explain what is meant by equilibrium [2]
rates equal
concentrations do not change
[+ 2 mark format of “endothermic” “favoured by high temperatures”]
Pressure decreased.
Shifts? Why?
Left
more moles
Describe what you’d observe
reactant [dark brown] -> product [yellow]
more BROWN LIQUID
[look at state symbols]
Haber Process
what kind of reaction? what is made?
Ammonia is manufactured in an exothermic reaction [forward] called the Haber process
occurs in 5 stages
Stage 1
H2 is obtained from methane
N2 is obtained from the air
They are pumped into the compressor through a pipe
Stage 2
Inside the compressor, the gases are compressed to around 20,000 kPa or 200 atmospheres
Stage 3
The pressurised gases are pumped into a tank containing layers of an iron catalyst at a temperature of 450 °C
Some of the hydrogen and nitrogen react to form ammonia:
eqn
delta H
conditions
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
ΔH = -92 kJ mol^-1
conditions: iron catalyst, high temp, high pressure
conditions of temp and pressure?
Pressure = 200 atmospheres
Temperature = 450 Celsius
Stage 4
Unreacted H2 and N2 and the ammonia product pass into a cooling tank
The ammonia is liquefied and removed to pressurised storage vessels [removed so it doesn’t break down into N2 & H2 again]
Stage 5
The unreacted H2 and N2 gases are recycled back into the system and start over again
Name catalyst B used and state why it is used
iron;
used to speed up the reaction / increase the rate of reaction.
so remember:
nitrogen from the
hydrogen from the
____ => from ?
nitrogen => air
hydrogen => methane (natural gas)
Reaction
is reversible, so ammonia breaks down into nitrogen & hydrogen
compromise conditions
reaction conditions chosen for the Haber process are not ideal in terms of the yield
but do provide balance between product yield, reaction rate and production cost.
+ from the graph
As the pressure increases, the percentage yield increases
As the temperature decreases, the percentage yield increases
so economical, chemical and practical considerations
Explain trend for temp
high temp decreases yield
but low temp gives low rate of reaction
so optimum 450 Celsius is used
V2O5 catalyst will not work below 400 [contact process]
Lower temp will favour EXOTHERMIC reaction, but 350 Celsius is too slow
Higher temp will increase rate of reaction, but YIELD is LOW
450 compromise - provides best rate and yield.
For pressure
Higher pressure will increase yield
BUT 400 atm needs powerful pumps, strong pipes & tanks
200 atm - SAFER & saves MONEY
ECONOMIC Considerations
raw materials are readily available and inexpensive to purify
Dynamic equilibrium
closed system
reversible reaction
Contact Process
Manufactures?
Raw materials?
used to manufacture sulfuric acid H2SO4
raw materials: sulfur, air, and water
OR sulfur dioxide, air and water
what is this used for
Concentrated sulfuric acid is used in car batteries, making fertilisers, soaps and detergents
the symbol equation for the conversion of
sulfur dioxide to sulfur trioxide in the Contact
process
+ eqn for SO2
+ for SO3
+ for H2S2O7 as reactant
2nd;;; 2SO2(g) + O2(g) ⇌ 2SO3(g)
^ compromise
1st: S (l) + O2 (g) -> SO2 (g)
3rd: SO3 + H2SO4 -> H2S2O7
(oleum)
4th: H2S2O7 + H2O -> 2H2SO4
catalyst
vanadium (V) oxide
V2O5
OTHER conditions
450°C,
200kPa /2atm
sources of sulfur dioxide and oxygen
step 1 - S burned in air to produce SO2
source of SO2: burning
sulfur [in air]
S (l) + O2 (g) -> SO2 (g)
or roasting sulfide ores
metal sulfide + oxygen → metal oxide + sulfur dioxide
source of O2: from air
step 2: sulfur trioxide
reaction type
2SO2(g) + O2(g) ⇌ 2SO3(g)
REVERSIBLE reaction. 450, 2 atm, V2O5 catalyst
step 3: sulfuric acid
SO3 + H2SO4 -> H2S2O7
(oleum)
H2S2O7 + H2O -> 2H2SO4
sulfuric trioxide is mixed with [concentrated?] sulfuric acid to produce oleum (FUMING sulfuric acid)
;; oleum is mixed with water to produce sulfuric acid
optimum 450 is used bc
high temp decreases yield, low temp gives low rate of reaction, so optimum 450
catalyst won’t work below 400 Celsius
moderate pressure 2 atm
position of equilibrium is already on right [4 moles] so high pressure is expensive and not needed.
So, the reaction is carried out at just above atmospheric pressure because:
High pressures can be dangerous and very expensive equipment is needed
A higher pressure causes the sulfur dioxide to liquify
Explain why reaction speed increases with temperature.
particles have more energy;
move faster;
collide more frequently;
more successful collisions;
(iii) Explain why the time taken to produce a precipitate would increase if the experiments
were repeated at 50°C.
any 3 ;;;
less energy;
particles move slower;
less collisions, fewer successful collisions;
slower rate;
substitution reaction.
hydrogen (atoms) replaced by (atoms) of a different element
Explain why all cell reactions are exothermic and redox [3]
(exothermic because) a cell produces (electrical) energy/electricity
electrons are lost AND gained
Which electrode, zinc or iron, is the negative electrode? Give a reason for your choice.
zinc [1]
[cond] it is the more reactive metal [1]
Suggest two ways of increasing the voltage of this cell.
replace zinc with magnesium
replace iron with copper
use (more) concentrated sulfuric acid
Explain why water behaves as a base in both of these reactions.
because it accepts a proton [2]
accepts hydrogen ion or H+ ONLY [1]
proton and H+
[2]
Why is the rate in experiment D faster than the rate in experiment A? [3]
propanoic (A) or hydrochloric (D)
It (D) has strong (acid) and A has weak acid/(D) stronger/(D) ionises more (1)
It (D) has higher concentration of hydrogen ions (1)
more collisions (in D) (1)
A liquid has a fi xed volume but takes up the shape of the container.
A gas takes up the shape of the container but it does not have a fixed volume. [3]
particles move in all directions/randomly in both liquids and gases (1)
no bonds/very weak forces between particles in gases (1)
molecules can move apart/ separate (to fill entire volume) (1)
Suggest a method of measuring the rate of this reaction.
measure volume of carbon dioxide [1]
time [1]
Vanadium(III) oxide is basic and vandium(IV) oxide is amphoteric.
Describe how you would obtain a sample of vanadium(III) oxide from a mixture of
these two oxides.
add sodium hydroxide(aq) or other named alkali [1]
[cond!!] vanadium(IV) oxide dissolves / reacts [1]
filter (to remove vanadium(III) oxide) [1]
i) Why does the rate of the backward reaction increase with time
products are being formed
i) After some time why does the appearance of the mixture remain unchanged
reaction has come to equilibrium [1]
rates equal or no change in concentration [1]
v) When a few drops of concentrated hydrochloric acid are added to the cloudy
mixture, it changes to a colourless solution. Suggest an explanation
equilibrium to left or favours backward reaction or
equilibrium moves to use up hydrochloric acid
BiOCl used up or BiCl3 formed
Sulfuric acid is made by the Contact process.
(a) Sulfur is burned by spraying droplets of molten sulfur into air.
Suggest and explain an advantage of using this method. [2]
What volume of air contains 1 dm3
of oxygen?
fast(er) reaction;
large(r) surface area;
4.76
Sulfur dioxide is more expensive than air.
What is the advantage of using an excess of air?
moves equilibrium to right;
increases yield (of sulfur trioxide)
) Explain why a higher pressure is not used.
high yield at 2 atm;
Describe how concentrated sulfuric acid is made from sulfur trioxide.
dissolve/react sulfur trioxide in (concentrated) sulfuric acid;
add water to product
Give another example of a reversible reaction.
N2 + 3H2 ⇌ 2NH3
A sealed tube containing the equilibrium mixture is placed in ice-cold water. There is an increase
in the amount of yellow solid in the equilibrium mixture.
What can you deduce about the forward reaction in this equilibrium?
ICl (l) + Cl 2(g) ICl 3(s)
Explain your deduction.
equilibrium goes to RHS OR equilibrium goes to products side;
M2 exothermic reactions are favoured by low temperatures;
M3 the forward reaction is exothermic;
This is carried out in the presence of a catalyst at 450°C and 2 atmospheres pressure.
(i) How is the sulfur dioxide made?
burn sulfur in air
Give another use of sulfur DIOXIDE
preserve food or sterilising
If the temperature is decreased to 300°C, the yield of sulfur trioxide increases.
Explain why this lower temperature is not used.
rate too slow or rate not economic
Sulfur trioxide is dissolved in concentrated sulfuric acid. This is added to water to
make more sulfuric acid. Why is sulfur trioxide not added directly to water?
reaction too violent or forms a mist
How could you show that the first reaction is reversible?
add water to yellow powder or to anhydrous salt [1]
it would go green
Sulfurous acid is a reductant. What would you see when acidified potassium
manganate(VII) is added to a solution containing this acid?
change from purple or pink
to colourless
Name a source of sulfur.
fossil fuels
Describe step 2, giving reaction conditions and a chemical equation. Reference to reaction
rate and yield is not required. [5]
M1 vanadium pentoxide/vanadium(
V) oxide/V2O5 (catalyst);
M2 1–5 atmospheres (units required);
M3 450°C (units required);
M4 2SO2 + O2 → 2SO3;
M5 equilibrium/reversible reaction;
Dilute sulfuric acid is a typical acid.
A student adds excess dilute sulfuric acid to a sample of solid copper(II) carbonate in a
test-tube.
(i) Give three observations the student would make.
bubbles / effervescence/ fizzing;
dissolves /disappears / forms a solution;
blue (solution);
Give the names of all products formed.
carbon dioxide and water and copper(II) sulfate;
One source of sulfur dioxide is burning sulfur in air.
Describe how sulfur dioxide can be made from the ore zinc sulfide.
heating/roasting/burning (zinc sulfides)
in air/oxygen
Vanadium(V) oxide is an effi cient catalyst at any temperature in the range 400 to 450 °C.
Scientists are looking for an alternative catalyst which is effi cient at 300°C.
What would be the advantage of using a lower temperature?
position of equilibrium shifts right/yield increases
to save energy
The process does not use a high pressure because of the extra expense.
Suggest two advantages of using a high pressure?
Explain your suggestions.
faster reaction/rate
more collisions per second/higher collision frequency
fewer moles/molecules (of gas) on right
(so) position of equilibrium shifts right/yield increases
When the iron(II) sulfate is heated strongly, further decomposition occurs.
2FeSO4(s) → Fe2O3(s) + SO2(g) + SO3(g)
The gases formed in this reaction react with water and oxygen to form sulfuric acid.
Explain how the sulfuric acid is formed.
M1 Sulfur trioxide reacts with water to make sulfuric acid or equation
M2 sulfur dioxide reacts with oxygen to form sulfur trioxide or equation
Sulfuric acid is a strong acid. Hexanesulfonic acid is also a strong acid. It has similar properties
to sulfuric acid.
(a) Sulfonic acids are made from alkanes and oleum, H2S2O7.
C6H14 + H2S2O7 → C6H13SO3H + H2SO4
(i) Describe how oleum is made from sulfur by the Contact process. Give equations
and reaction conditions [6]
S + O2 SO2
or sulfur burnt / roasted / heated in air to form sulfur dioxide
2SO2 + O2 2SO3
(catalyst) vanadium(V) oxide / vanadium pentoxide
(temperature) 440 to 460o
(dissolve) sulfur trioxide in sulfuric acid (to form oleum)
Explain the terms strong acid and weak acid.
) a strong acid is completely ionised, [1]
a weak acid is partially ionised
explain why the reaction mixture decreases in mass
loss of carbon dioxide gas
enzyme
biological catalyst that speeds up rate of reaction
explain why it is important that the pieces of marble are the same size and shape
pieces have same surface area
same volume/quantity of carbonate
know when reaction stopped how?
no more carbon dioxide
RMBR acids diff, why one is faster than the other is bc
1) FASTER = STRONG(ER) ACID
2) SLOWER = LESS IONISED
hydrochloric acid is a strong acid
ethanoic acid less ionised
economics of haber process require as much NH3 made as quickly as possible
explain how can be done using info [5]
📍 high pressure favours lower volume side / ammonia side + / increases yield
📍 high pressure increases rate
📍 low temperature favours exothermic / increases yield
📍 low temperature gives low rate
📍 catalyst lowers activation energy / increases rate
📍 450 degrees low enough to give economic yield but with catalyst gives fast enough rate
📍
show clearly on grid means
dotted lines coming across
predict temp of soluton in experiment 4 after 1 hour. explain ans
24 degrees (initial temp)
reaction finished
advantage & disadvantage
- of temp readings taken after 1 min
adv: fair test
disadv: temperature still changing
