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