module 3.2 physical chemistry Flashcards
a chemical reaction occurs when there is a…
- colour change
- effervescence
- precipitate formed
- pH change
- temp change/ energy change/ enthalpy change
what is enthalpy
the thermal energy stored in a chemical system.
we can measure the change in this by seeing if the temp of a reaction changes
what is enthalpy change and what is it measured in
- the difference in enthalpies is the energy change
- measured using triangle H
triangle H can be positive or negative; what does it mean when there is a positive enthalpy
an endothermic reaction has occurred
triangle H can be positive or negative; what does it mean when there is a negative enthalpy
an exothermic reaction has occurred
how can enthalpy change be determined
by measuring the energy transfer between the system and surroundings
____ change is from the system to surroundings
exothermic
____ change is from the surrounding to system
endothermic
what happens during an exothermic reaction
-chemical system loses energy
-surroundings gain energy
-temp of surroundings increase
- triangle H is negative
(bonds lose energy= energy is released from bonds in reactants)-energy stored in the bonds in the products is less than the energy stored in the reactants
what happens during an endothermic reaction
-chemical system gains energy
-surroundings lose energy
-temp of surroundings decrease
- triangle H is positive
(energy is absorbed from surrounding particles
reactants ahve a certain amount of stored energy this means…
reacting molecules only react if they collide with enough energy for bonds between reactants to break- this is activation energy
examples of exothermic reactions
combustion, respiration, neutralisation, condensing, freezing
examples of endothermic reactions
photosynthesis, melting, boiling, thermal decomposition
what happens when a catalyst is added
activation energy is lowered
for any reaction to happen…
- bonds must be broken in reactants
- new bonds must be made in products
breaking bonds require energy which is an ____ process
endothermic
making bonds release energy which is an ____ process
exothermic
if more energy is required to break bonds than is released making bonds, the reaction is…
endothermic
if more energy is released in making bonds than it is needed to break bonds, the reaction is…
exothermic
what is standard pressure at
100kPa (one atmosphere)
what is the value for standard concentration
1 mol dm^-1
what is the definition of standard state
the physical state of a substance under standard conditions
what is standard temperature
298K (25 degrees C)
What is standard enthalpy change of a reaction
standard enthalpy change of a reaction is the enthalpy change that accompanies a reaction in molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states- when molar quantity doubles so does the standard enthalpy change.
what is the standard enthalpy change of formation
the standard enthalpy change of formation is the enthalpy change that takes place when 1 mol of compound is formed from its elements under standard conditions, with all reactants and products in standard states
what are rules when calculating standard enthalpy change of formation
- don’t add a balancing number in front of the product formed
- in the equation, elements are in their elemental form which make up the compound, there is 0 enthalpy for an element (no breaking/making)
how many mols of atoms is 1/ a mol of 02
1 mol of oxygen atoms
what is enthalpy change of combustion (triangle c H tube sign)
the standard enthalpy change of combustion is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen standard conditions , with all reactants and products in their standard states
what has a lower enthalpy change: complete combustion or incomplete combustion?
incomplete combustion
what is enthalpy change of neutralisation (triangle neut H (tube)
the standard enthalpy change of neutralisation is the energy change that accompanies the reaction of an acid by a base to form 1 mol of H2O(l) under standard conditions, with all reactants and products in their standard states
HOW do you measure enthalpy change experimentally
see how much a reaction heats up or cools down water and then use the equation
what’s the equation to measure enthalpy change
Q=mctriangleT
what does Q stand for
Q=heat lost or gained from a reaction measured in joules (J)
what does m stand for
m= mass of object heated/cooled (g)
what does triangleT stand for
triangleT= change in temperature (degrees C)
what does c stand for
c= specific heat capacity with quantity of energy in joules (J), needed to change the temp of one gram (g) of a substance by one degree Celsius.
what are the 3 steps of calculating enthalpy change?
- work out the heat change for a reaction (q)
2.convert the heat change to kJ - use the equation: triangeH=-q/n
triangeH= enthalpy change
is the q positive or negative when the temp goes up
positive
if the temp goes up, the reaction is exothermic, so is the triangeH positive or negative
negative
when calculating enthalpy change using Q=mctriangleT and triangleH=-q/n what must you always do to ensure the value isn’t too big
when u get ur answer for Q=mctriangleT divide by 1000 to get from J to kJ as in the equation triangleH=-q/n, -q is always in kJ
give 4 reasons why experimental values for the enthalpy of combustion of fuel are often less exothermic than the data book values
- heat loss to the surroundings other than water. this includes the beaker but mainly the air surrounding the flame.
- incomplete combustion of FUEL. there may have been some incomplete combustion with carbon monoxide and carbon being produced instead of CO2. you would see carbon as a black layer of soot on the beaker.
- evaporation of FUEL from the wick. the burner must be weighed as soon as possible after extinguishing the flame. otherwise some fuel may have evaporated from the wick. Spirit burners usually have a cover to reduce this error.
- Non-standard conditions. The data book value is a standard value. The conditions for this experiment are unlikely identical to standard conditions.
what’s average bond enthalpy
the energy required to break one mol of a specified type of bond in a gaseous molecule
how can you tell something is exothermic ( the difference between exo and endo)
- bond breaking absorbs energy which is endothermic
- bond making releases energy which is exothermic
- for exothermic reactions, more energy is released than absorbed.
what are the two main reasons why its important to control rate of reaction
due to safety and cost ( important to produce a product quickly but not too expensive)
what are the 5 factors affecting rate of reaction
- concentration
- concentration, pressure in gasses
- surface area
- catalyst
- temperature
what’s collision theory
- for a reaction to happen, particles must collide, they must collide with enough energy, this is activation energy of a reaction.
- they must collide at the correct orientation
- the more effective collisions there are in a volume of space per second, the faster the rate of reaction will be
how does concentration affect rate of reaction
- the concentration is a measure of how much solute is dissolved.
- the higher the conc. of reactants in solution, the greater the rate of reaction. this is because their are more reactant particles in the solution so there are more frequent successful collisions.
how does concentration, pressure in gasses affect rate of reaction
some chemical reactions involve reactants that are gasses. the greater the pressure of a gas, the closer the reactant particles are. the higher the pressure of reactants that are gasses, the greater the rate of reaction. this is because the reactant particles are closer together and so there are more frequent successful collisions.
how does surface area affect rate of reaction
sa is increased if a solid is broken up into more pieces. when a solid is powder, it has a massive sa. the greater the sa of a solid reactant the greater the rate of reaction. this is because there are more particles on the surface that can react so there are more frequent successful collisions.
how does a catalyst affect rate of reaction
a catalyst is a substance which speeds up a chemical reaction but does not get used up. a catalyst works by providing a different route for the reaction that has a lower activation energy. the amount of catalyst at the end of the reaction was the same as the start.= more likely to have successful collisions
how does temperature affect rate of reaction
the higher the temp, the faster a reaction. this is because particles have more energy (they are more energetic) and so more of the particles have enough energy to react when they collide. the particles are also moving faster so collide more frequently. this means there are more frequent successful collisions.
why do reactions not occur at a steady rate- talk about what happens during reaction
they start off at a certain speed, they then get slower until they stop. as the reaction progresses, the conc of the reactants decreases. This reduces the frequency of collisions between particles and so the reaction starts to slow down over time
how do u calculate mean rate of reaction
draw a line of best fit through the data ( don’t include the area where the line is constant (reaction has stopped).
- make a triangle from t=0 to when the moment the line of best fit goes flat.
- do change in Y over change in x to receive mean rate
how do you calculate rate of reaction at a specific time
- select the time u want to measure rate
- with a ruler draw a tangent
- rate at specific time= change in Y over change in X
how do u calculate initial rate of reaction
-draw a tangent from the very start
-plot results and draw a line of best fit
- make a triangle from T= zero
do change in Y over change in X with a line that goes through t=0
-this is initial rate (the fastest rate of reaction)
with rate of reaction graphs, the formation of product is an upwards curve, what dos the using up of a reactant look like
a downwards sloping curve
what are the 2 types of catalyst
heterogenous catalysts
homogenous catalysts
heterogenous catalysts ??
heterogenous catalysts : this involves the use of a catalyst in a different phase ( state) from the reactants. typical examples include a solid catalyst with the reactants as either liquids or gasses
homogenous catalysts ??
homogenous catalysts: this has the catalyst in the same phase (state) as the reactants. typically everything will be contained in a single liquid phase.
what do catalysts do
provide an alternate route for the reaction, with a lower activation energy, this means more SUCCESSFUL collisions can occur in a vol of space per second so the rate of reaction increases. (the rate and energy of collisions are not affected)
is energy equal in a system
no
talk about why puddles evaporate and energy not being equal in a system
- If we took the temperature of a puddle, this would only be average temp of water molecules in it
- even though the puddle is never at 100 degrees, a few particles will have enough energy to turn into a gas and evaporate
what dos the boltzmann distribution curve represent
used to represent the different energies of molecules in a system
what is along the x and y axid of the boltzmann distribution curve
x= energy y= number of particles with a given energy
at the start of the boltzmann distribution curve why does the line intercept 0
cus no molecules/ particles have 0 energy
why at the end does the line on the boltzmann distribution curve never reach 0
there is no theoretical limit for the energy a particle can have
how does increasing the temperature affect the boltzmann distribution curve
moves the curve to higher energies, shift to right and more molecules have energies greater than Ea.
- more molecules posses the activation energies at higher temps, molecules also have a higher average energy/ temperature.
- for a higher temp, the peak is shifted to the right
what is the effcet of adding a catalyst on the boltzmann distribution curve
- in the presence of a catalyst, a greater proportion of molecules exceeds the new lower activation energy, more particles can now get involved in successful collisions.
- a catalyst provides an alternate route with a lower activation energy, a greater proportion now have an energy equal to, or greater than the lower activation energy. on Collison, more molecules will react to form products. the result is an increase in rate of reaction
what are reversible reactions
reactions that can go backwards or forwards
what are equilibrium reactions
if reversible reactions are kept in closed systems, BOTH the forward and backward reactions can happen at the same time
talk about what you see in dynamic equilibrium - talk about conditions
in dynamic equilibrium there is no observable change to the reactant mixture but the reaction hasn’t stopped
- the forwards and backwards reaction of a reversible reaction are occurring at the same rate/speed.
- the conditions must be a closed system so particles cannot get in and out of the reaction mixture, only energy
there is a reversible reaction of a colorless gas and dark blue gas.
- observations at the start of the reaction
- changes that would be seen overtime
- how would a scientist know equilibrium has been established
- colorless gas to a little bit of blue forming
- cahnges to blue (more intense but to a point because its a reversible reaction so will go back to colourless)
- no colour change- it will be one cooler ( a blue ( midwayish))
state Le Chateliers principal
If conditions of an equilibrium reaction change, the position of equilibrium for the reaction will also respond by changing.
If conditions are changed, equilibrium will shift to oppose the change
position of equilibrium- changing the conditions of a reaction at equilibrium will either…
- shift the position of the equilibrium to the right (more product made/higher yield)
- shift the position of the equilibrium to the left (less product made/lower yield)
what are the 4 things that can affect position of equilibrium
- changing conc
- changing temp
- changing pressure
- usage of a catalyst
how does changing concentration affect position of equilibrium
- increasing the concentration of the reactants shifts the equilibrium to the right (more forwards reaction)
- decreasing the conc of the reactants sends the equilibrium to the left (more backwards reaction)
how does changing temperature affect position of equilibrium
- increasing the temp will shift the equilibrium in the direction of the endothermic reaction
- decreasing the temp will shift the equilibrium in the direction of the exothermic reaction
how does changing pressure affect position of equilibrium
- this applies to gas reactions only
- here the rule depends on the number of gas molecules (mols) on each side of the equation
- if pressure is increased, equilibrium is shifted in the direction with fewer gas molecules
- if pressure is decreased, equilibrium shifts in the direction with more gas molecules (moles)
how does a catalyst affect position of equilibrium
a catalyst dos not change the position of equilibrium, it merely speeds up the rates of the forwards and reverse reactions equally. A catalyst will however, increase the rate at which an equilibrium is established.
what does increasing pressure do to the yield and rate of reaction for N2+3H2<==>2NH3
increase yield and rate of reaction
what does increasing temperature do to the yield and rate of reaction for N2+3H2<==>2NH3 (exothermic)
decreases the yield and increases rate of reaction.
what does a catalyst do to the yield and rate of reaction for N2+3H2<==>2NH3 (exothermic)
increases rate of reaction but doesn’t affect yield
when increasing the pressure and temperature, what do u need to be aware of
- cost
- increase in pressure could cause explosions
- safety issues
whats equilibrium constant
-the exact position of equilibrium is calculated using the equilibrium law. For any reaction at equilibrium its possible its possible to write an expression for the equilibrium constant Kc in terms of equilibrium concentration
aA+bB<==> cC +dD
equilibrium law: Kc= [C]^c[D]d/ [A]a[B]b ( products/ reactants)
things to Rember for equilibrium constant
- square brackets[] are shorthand for ‘concentration of’
- a,b,c,d are the balancing numbers of overall equation
- [A][B][C][D] are the equilibrium concentrations of the reactants and products of the equation.
