3.2 physical changes Flashcards
define enthalpy
the total energy stored in a chemical system
when referring to a reaction, what is a system?
the actual chemical reaction and the bonds and atom involved
when referring to a reaction, what are the surroundings?
they are everything that ISN’T the system - everything around the reaction - aqueous solutions, beakers, air around it etc
define enthalpy change
the heat energy given out or taken in per mole as the reaction progresses
what is meant by standard conditions of a reaction?
1 bar of pressure (1 atm) and 298 K
elements in their standard states which is there physical state in the given conditions above^
define exothermic reaction
a reaction which gives out energy
energy is transferred from the system to the surroundings
define endothermic reaction
energy is taken in
energy is transferred from surroundings to the system
exothermic: increase or decrease in temp?
increase in temp
endothermic: increase or decrease in temp?
decrease in temp
is making bonds exothermic or endothermic?
exothermic because making bonds releases energy
(BEndo-MExo) mexo
is breaking bonds exothermic or endothermic?
endothermic because breaking bonds requires energy (BEndo-MExo) bendo
why do exothermic reactions give out energy?
they release more energy making bonds than they take in breaking bonds
so overall more energy is released
why do endothermic reaction take in energy?
they take in more energy breaking bonds than they release making bonds
so overall more energy is taken in
describe the energy level diagram for an exothermic reaction. is ∆H +ve or -ve?
reactants have higher energy than products - goes down
∆H = NEGATIVE
describe the energy level diagram for an endothermic reaction. is ∆H +ve or -ve?
products have higher energy than reactants - goes up
∆H = POSITIVE
give two equations used to find enthalpy change
enthalpy of reactants- enthalpy of products
energy taken IN - energy given OUT
bonds broken - bonds made
define enthalpy change of neutralisation
give the symbol
enthalpy change when one mole of water is formed from a neutralisation reaction (under standard conditions)
symbol: ∆neutH⊖
define enthalpy change of formation
give the symbol
enthalpy change when one mole of compound is formed from its elements in their standard states (under standard conditions)
symbol: ∆fH⊖
define enthalpy change of combustion
give the symbol
enthalpy change when one mole of substance/fuel reacts/combusts completely with oxygen
symbol: ∆cH⊖
give the equation for heat change
what are they measured in
heat change = mass x specific heat capacity x temp change
Q = mc∆T
Q = J
m - grams
t - ˚C
how do you calculate enthalpy change from using heat change? (3 steps)
- calculate heat change using Q=mc∆T
- convert Q from J to kJ
- find enthalpy change by doing -Q/n
(n=moles)
what is average bond enthalpy?
energy required to break 1 mole of bonds in gaseous molecules
define activation energy
minimum energy required for a reaction to take place
state hess’s law
total enthalpy change is not affected by the route the reaction takes, providing the start and end substances are the same in each route.
what must particles do to react?
they must collide with:
- the right orientation
- and with enough energy
these are successful collisions
name 5 factors affecting rate of reaction
temp concentration pressure - gases surface area catalysts
define rate of reaction
the change in concentration of reactant or product per unit of time
how can you calculate ROR using a progress curve?
at a specific time: draw a tangent and find the gradient
overall: change in products/reactants conc/time
give 3 equations for calculating ROR
reactant lost/t
product made/t
1/t
where t = time
state and describe the effect of increasing concentration on ROR
increased conc = faster ROR
1. increasing conc gives more particles in the same volume so they are closer together
- this means there’s a greater chance of them colliding for frequently and successfully
- more chance they’ll react and overcome activation energy
state and describe the effect of increasing pressure on ROR
increased pressure = faster ROR
1. increased pressure means gas particles are pushed closer together
- this means there will be chance for frequent and successful collisions
what is the purpose of a catalyst?
what do they allow a reaction to do?
- increase ROR without being used up in the reaction
- allows reaction to take a different route with a lower activation energy
define heterogeneous catalyst
a catalyst that is in a different phase from the reactants
define homogeneous catalyst
a catalyst that is in the same phase as the reactants
what is meant by the phase of a substance?
the physical state
how can catalysts be more economical?
- by lowering energy demands for industrial processed
- so there lowering cost of producing such energy
how can catalysts help the environment?
- catalytic converters in cars reduce the pollution released by cars by speeding up reactions in the engines
- also reduce waste and improve atom economy by allowing reactions to take alternative routes
describe the general shape of a Boltzmann distribution curve (BDC)
what is the y and x axis?
bell shape
y axis: no. of particles with given energy
x axis: energy
what does the peak on BDC show?
average energy (most particles have this amount of energy
what is the area under a BDC?
the number of particles
where is the activation energy on a BDC?
further along when the line goes down
what does changing temp do to:
- the overall shape of the BDC
- the average energy (position of peak)
- the area under graph
- the curve is flatter
- the average energy is higher (peak is further right)
- area under graph stays - same no. of particles
what does changing temp do to:
- the activation energy
- the number of particles exceeding Ea
- rate of reaction
- activation stays the same
- more particles exceed activation energy
- increases ROR - more particles with Ea - more successful collisions
what does using a catalyst do to:
- the overall shape of BDC
- activation energy
- number of particles exceeding Ea
- ROR
- shape does not change
- Ea decreases
- more can exceed activation energy
- ROR increases - more particles can exceed Ea = more successful collisions
describe the term dynamic equilibrium
occurs when, in a closed system, the rate of the forwards and backwards reactions are equal and the concentrations of the products and reactants remain constant
give 3 factors affecting the position of equilibrium
- concentration
- pressure
- temperature
what happens to the position of equilibrium when you:
- increase concentration of products?
- increases concentration of reactants?
- why do these shifts occur?
- equilibrium shifts to the reactants (left)
- shifts to the products (right)
- opposes change in concentration by producing more of the opposite substance
what happens to the position of equilibrium when you:
- increase the pressure?
- decrease the pressure?
- why do these shifts occur?
- shifts to the side with fewer gas molecules/less moles
- shifts to the side with more gas molecules/more moles
- counteract changes in pressure - less molecules = lower pressure and more molecules = higher pressure
what happens to the position of equilibrium when you:
- increase temp?
- decrease temp?
- explain why this shifts occur
- shifts in endothermic direction to try and decrease temp
- shifts in the exothermic direction to try to increase the temp
- equilibrium tries to oppose the change in temp
how does a catalyst affect the position equilibrium? explain your answer
catalyst do not position of equilibrium
- catalysts speed up the forward and backwards reactions by the same amount
- so there is no need for the equilibrium to shift as the reaction is still balanced
what is the equilibrium constant and why do we use it?
Kc is a constant that tells us where equilibrium lies using the ratio of reactants to products
what do square brackets around an element mean?
concentration of ______. eg. [Fe] = conc of iron
how do you calculate Kc?
eg. aA + bB ⇋ cC + dD
Kc = [C]^c[D]^d / [A]^a[B]^b
(conc of products / conc of reactants)
what can you deduce about the position of equilibrium when Kc = 1?
equilibrium lies halfway between reactants and products
what can you deduce about the position of equilibrium when Kc «_space;1?
lies to the left - towards the reactants
equilibrium favours reactants
what can you deduce about the position of equilibrium when Kc»_space; 1?
lies to the right - towards the products
equilibrium favours products
why is it important to compromise on a temperature in a reaction?
if temp is too low - ROR is slow
if temp is too high for an exothermic reaction - too much product would be formed = low yield
why is it important to compromise on a pressure in a reaction?
if pressure is too low - reaction will be too slow
- low yield (if products have less moles of gas)
if pressure is too high - expensive, puts stress on equipment a d side reactions may occur
