Energetics Flashcards
What is energetics?
the energy changes that take place with the surroundings during a chemical reaction
What is an exothermic reaction?
reactions that produce heat
Give examples of an exothermic reaction?
combustion (burning)
respiration
neutralisation (acid and alkali)
What is an endothermic reaction?
reactions that absorb/require heat to occur
Give examples of an endothermic reaction?
evaporation (heat the water)
ice cube melting
thermal decomposition
Why is an ice cube melting endothermic?
H20 from a solid to a liquid happens from tempratures of 00 to 220
water is melting, absorbing heat
How are exothermic and endothermic reactions represented?
they are represented graphically
energy profile diagrams
What is △H (between products)?
the difference in energy between the products and reactions in a chemical is known as the energy change △H
H products - H reactants
What does kJ stand for?
kilojoules
What does △ stand for?
change
What does H stand for?
enthalpy
Endothermic reactions have a (positive/negative)△H?
positive
Exothermic reactions have a (positive/negative) △H?
negative
Why do exothermic reactions have a negative △H?
Their products have less energy than their reactants
Why do endothermic reactions have a positive △H?
their products have more energy than thier reactants
What is activation energy (EA)?
the minimum amount of energy required between two or more reactants to overcome their mutual repulsion (e.g. two magnets repelling)
What is bond enthalpy (energy)?
the average energy required to break one mole of stated bond
Energy is required to (make/break bonds - it is an (exothermic/endothermic) process?
Energy is required to break bonds - it is an endothermic process
Energy is (given/taken) when bonds are made - it is an (endothermic/exothermic) process
energy is given when bonds are made- it is an exothermic process
Endothermic Profile Diagram
Exothermic Profile Diagram
Is bond making exothermic or endothermic?
exothermic
Is bond breaking endothermic or exothermic?
endothemic
What are the three types of △H?
△H = H products - H reactants (theoretical)
△H = sum bonds broken - bonds made (theoretical)
△H = mc△T/n (experimental)
What is enthalpy?
energy change for a reaction
Is a negative energy change exothermic or endothermic? Why?
exothermic
the energy given out when new bonds are made is greater than the energy taken to break the original bonds
Is a positive energy change exothermic or endothermic? Why?
endothermic
the energy taken in to break the original bonds is greater than the energy given out when new bonds are made
Exothermic reaction △H is negative/positive?
negative
Endothermic reaction △H is negative/positive?
positive
In terms of products and reactants why would an endothermic reaction be positive?
their products have more energy than their reactants
In terms of products and reactants, why are exothermic reactions negative?
their products have less energy than their reactants
How do you measure enthalpy changes for reactions experimentaly?
by burning a substance, measuring the temprature change it causes, then calclutaing the enthalpy from the formula
What is the equation for speed?
speed = distance/time
What does -1 mean?
per something
e.g. 30 m/s or 30 ms-1
What is the equation for a-1
1/a
What do all combustion experiments have in common?
they are all exothermic and therefore have a negative Q ( heat released)
How do you carry out an energy of different types of alcohol combustion experiment?
a known mass of fuel is burned in a spirit burner and the temprature rise of the known mass of water is measured
from this you calculate the energy given out by the fuel in kJ per gram
from your results you will compare the relative values obtained
In the different types of alcohol combustion experiment which is
a) the independant variable
b) the dependant variable
c) the continuos variable
a) the type of alcohol
b) temprature of water
c) mass difference of burner
If the temprature of 4.18 kJ per Kg per K, what does this mean?
it takes 4.18 Joules of energy to raise one Kg of water by one Kelvin. one Kelvin is equivalent to 0C
How is the mass of fuel burned calculated?
measure the mass of the burner before and after the experiment and then calculate the difference
In the differnet types of alcohol combustion experiment what could be sources of error?
scale error
human error
heat is lost to the surroundings (out of our control)
can’t burn all alcohol
Why do differnet fuels have different energies of combustion?
some react better with oxygen
What does equilibrium mean?
balance
in a chemical equilibrium, the concentration of reactants and products do not change
What is static equilibrium?
once equilibrium is reached there is not more movement
What is dynamic equilibrium?
when the rate (speed) of the fowards reaction is equal to the rate of the reverse, so the net change is 0
What two things does a dynamic equilibrium need?
a closed system (closed experiment, no ‘matter’ can be exhanged with the surroundings, occurs in a sealed container)
reversible
Is a phase change a reaction?
No but it can still have a dymanic equilibrium
What is the position of equilibrium (P.O.E)?
whether the reaction is moving left to right or right to left
What three main factors affect the position of equilibrium/dynamic equilibrium?
temprature
concentration of reactants (amounts)
pressure (gases)
What is a system?
a chemical reaction
What is Le Chatelier Principle?
when a change is exerted on a system in a dynamic equilibrium the position of equilibrium moves to the exact opposite
What would happen if you increased the pressure of reactants?
the position of equilibrium would move to the right (more S03)
this is because there are two molecules on the right but three on the left therefore the right has less pressure
What would happen if you increased the temprature?
the position of equilibrium would move to the left
this is because the left is the endothermic direction which absorbs heat therefore the amount of product S03 would decrease
What would happen if you increased the concentration of S02 and 02?
the position of equilibrium would move right
this is because it wants to remove excess reactants which moves to form products
What can reversible reactions do?
go fowards or backwards
In a closed system, a reversible reaction can reach a point of equilibrium. The amount of reactants and products (stays the same/can change). This is becaus the fowards and backward reaction is happening at the (different/same) time (……. equilibrium)
stays the same
same time
dynamic
What do strong acids do when dissolved in water?
split almost completely into ions when dissolved in water
With weak acids the ions are in what with the undissociated molecules in the acids?
equilibrium
Acids are proton (H+) what?
donors
Bases are proton (H+) what?
acceptors
What is a reactant?
a substance present at the start of a chemical reaction
What is a product?
a substance formed in a chemical reaction
When reversible reactions reach equilibrium the foward and reverse actions are (still happening/stopping) but at the (same/different) rate so the concentrations of reactants and products (change/do not change)
still happening
same
do not change
What is a catalyst?
changes the rate of chemical reaction without being changed by the reaction itself
If we remove the products from an equilibrium mixture, what happens?
more reactants are converted into products
If a catalyst is used what happns to the equilibrium?
it is reached must faster because the catalyst speeds up the reactants and products by the same amount
Does a catalyst affect the concentration of reactants and products?
no
What happens if the foward reaction is exothermic and the temprature is increased?
the yield of products is decreased
What happens id the foward reaction is exothermic and the temprature is deacreased?
the yield of products is increased
What will happen if the foward reaction is endothermic and the temprature is increased?
the yield of product will increased
What will happen if the foward reaction is endothermic and the temprature is decreased?
the yield of prduct is decreased
What is the equation for Calorimetry?
reversible reaction; goes fowards and backwards at the same time
-20
CHECK
-103
-95
CHECK ALL OF THESE
-124
-590
An example of an equilibrium problem
In an experiment, 1.56g of propan-1-ol (CH3CH2CH2OH) was completely burned in air. The heat evolve raised the temprature of 0.250 litres of water from 292.1K to 339.4K. Use this data to calculate the energy of combustion per gram of propan-1-ol (the specific heat capacity of water is 4.18 Jg-1 K-1)
In an experiment 1.00g of propanone (CH3COCH3) was completely burned in air. The heat evolved raised the temprature of 150g of water from 18.8 to 64.3 0C. Use this data to calculate the energy of combustion per gram of propanone (the specific heat of water is 4.18 Jg-1K-1)
50cm3 of 1.0 mol dm-3 hydrochloric acid was added to 50 cm3 of 1.0 mol dm-3 sodium hydrochloric solution. The temprature rose by 6.80C. Calculate the energy of neutralisation for this reaction. Assume that the specific heat capacity of the sloution is 4.18 Jg-1K-1
What is reversible reaction?
a chemical reaction which can go both ways
the direction of the reaction depends on the condition of the reactants
What is a catalyst?
a catalyst changes the rate of a chemical reaction without being changed by the reaction itself
In the Haber process what are the three raw materials used in the process?
natural gas
air
water
In the Haber process name a by-product of the process
carbon dioxide
In the Haber process what could be used to remove this by-product from the mixture of gases?
potassium carbonate (alkali)
In the Haber process what is the name of the catalyst used?
iron
The graph shows the yield of ammonia produced at equilibrium under different conditions of temprature and pressure. If the conditions are 200 atmospheres and 4250C, what yield of ammonia is obtained at equilibrium?
CHECK
33
The graph shows the yield of ammonia produced at equilibrium under different conditions of temprature and pressure. In practice, under the conitions of 200 atmospheres and 425oC, the yield obtained in the plant under these conditions is 15%. Suggest why this is so.
the system has not yet reached equilibrium
What would be the effect on the yield of ammonia of increasing the pressure? (ammonia is produced at equilibrium under conditions of temprature and pressure)
increasing the pressure increases the yield of ammonia
Suggest why the Haber process is not operated at even highe pressures.
increasing the pressure of gases is expensive
operating at a higher pressure means pipework is more expensive to resist explosion
What would be the effect of the yield of ammonia made by the Haber process by decreasing the temprature?
lowering the temprature increases the yield
Suggest why the Haber process is not operated at even lower temprature
lowering the temprature slows down the rate of reactions
Use the graph data to explain how ammonia can be removed frim the mixture of gases at atmospheric pressure
cool gases below -330C, ammonia liquifies and can be removed
The boiling point of these gases atre higher at higher pressure. Use your knowledge of particles to explain this
at higher pressure particles are closer together
stronger forces between particles have to be broken down
Some of the ammonia is distributed as liquid ammonia by road using tankers. Suggest an advantage of distributing ammoniaa as a liquid rather than as a gas. Use your knowledge of particles in your asnwer
In a liquid the oarticles are much closer together
More ammonia can be contained in a given volume
The air contains noble gases such as argon. What happens to these noble gases during the process?
Noble gases are not reacted and, as unreacted gases are re-cycled, their concentrations will build up
Describe the conditions used in the Haber process to increase the rate of reaction
Describe and explain the conditions used in the manufacture of ammonia to increase the yield
What is the Haber process used to do?
to make NH3 (l)
ammonia (alkali)
What can ammonia be used for?
fertilisers
Why do you need a compromise temprature in the Haber process?
the foward reaction is exothermic - the yield of product at equilibrium is increased at lower tempratures
if the temprature is too low the rate of reaction will be too low making the process uneconomical
a compromise is chosen - low enough to get a good yield of ammonia but high enough to obtain a reasonable rate of reaction
What catalyst is used in the Haber process reaction and what affect does it have?
iron filings - Fe
it does not affect the position of equilibrium but does affect the rate of reaction
ammonia is produced in a shorter time, reducing the cost of the process
iron is a cheap catalyst
How is the ammonia taken from the Haber process and why?
in its cool state it is tapped off as it is formed otherwise the equilibrium would move to the left
How much ammonia is produced from the Haber process?
yield = 18%
What is the pressure of the Haber process?
200-250 atmospheres
(we’re under 1 atmosphere)
What is the tenprture of a Haber process?
4500C
What is the Haber process reaction?
What is a molecule?
a molecule is a collection of two or more atoms held together by chemical bonds - the fundamental unit of compounds
How many molecules are on each side of the Haber process?
left - 1+3 = 4 molecules of gas
right - 2 molecules of gas
What will happen if the pressure is increased in the Haber process?
an increase in pressure will make the position of equilibrium move to the reaction which produces the smallest number of molecules
in the Haber process this is the forward reaction
more ammonia is produced
Why is the pressure only so high?
there is a limit to the pressure which can be used industrially
high pressures require very storng and expensive quiptment
Explain the choice of high pressure for the Haber process?
a compromise is chosen
high enough to get a good yielf of ammonia
low enough that it wouldn’t add to much to the costs of the process
is burning petrol in a car exothermic or endothermic?
exothermic
is respiration in living cells exothermic or endothermic?
exothermic
is heating water to make it boil exothermic or endothermic?
endothermic
is thermal decomposition of limestone into limes and CO2 exothermic or endothermic?
endothermic
is the reaction of magnesium with oxygen exothermic or endothermic?
exothermic
when heated continuously, pink cobalt chloride cyrstals can be changed into blue crystals
CoCl2 • 6H2O (pink) ⇌ CoCL2 • 2H2O + 4H2O (blue)
the symbol ⇌ tells you that the reaction is reversible
the pink cobalt chloride cyrstals have water trapped inside their crystal structure
blue cobalt chloride crystals have less water trapped insdie their crystals
how can you tell that the reaction to produce blue crystals is endothermic?
moving to oppose change pink to blue
when heated continuously, pink cobalt chloride cyrstals can be changed into blue crystals
CoCl2 • 6H2O (pink) ⇌ CoCL2 • 2H2O + 4H2O (blue)
the symbol ⇌ tells you that the reaction is reversible
the pink cobalt chloride cyrstals have water trapped inside their crystal structure
blue cobalt chloride crystals have less water trapped insdie their crystals
how could you change the blue crystals to pink cyrstals?
cool down (heat goes foward)
use an ice bath
when heated continuously, pink cobalt chloride cyrstals can be changed into blue crystals
CoCl2 • 6H2O (pink) ⇌ CoCL2 • 2H2O + 4H2O (blue)
the symbol ⇌ tells you that the reaction is reversible
the pink cobalt chloride cyrstals have water trapped inside their crystal structure
blue cobalt chloride crystals have less water trapped insdie their crystals
what temperature would you observe when this is done?
increase because its exothermic
when heated continuously, pink cobalt chloride cyrstals can be changed into blue crystals
CoCl2 • 6H2O (pink) ⇌ CoCL2 • 2H2O + 4H2O (blue)
the symbol ⇌ tells you that the reaction is reversible
the pink cobalt chloride cyrstals have water trapped inside their crystal structure
blue cobalt chloride crystals have less water trapped insdie their crystals
suggest how the colour changes of these crystals could be used
thermometers in a room
many fuels ontain sulphur as an impurity
state the gas that forms when sulphur burns in air and describe the environmental effect of this gas in the atmosphere
sulphur dioxide is formed
when mixed with air particles an help form acid raid which in turn damages buildings, wildlife and water resources
chose the best conditions for the Haber process and explain why they would produce a high yield of ammonia and why they are used in industry
2000atm and 4500oC
200atm and 450oC
20atm and 450C
200atm and 450oC
450oC is a compromise of temperature
it is best in this reaction to use an extremely low temperature to increase the yield of product but this is uneconomical as the rate of reaction is too low so you must have a compromise of 450oC
200atm is a relatively high pressure to increase the yield of ammonia but if it is any higher it is too expensive and there is a danger of explosions
ONE MORE MARK CHECK
brown nitrogen dioxide, NO2, exists in equilibrium with colourless dinitrogen tetroxide, N2O4
2NO2 (g) ⇌ N2O4(g)
ΔH = -57.2 kJ mol-1
the pressure is increased. when equilibrium is restored, the appearance of the mixture of gases will be:
a) colourless
b) unchanged
c) paler brown
d) darker brown
c) paler brown
the reaction moved foward with the increase in pressure
brown nitrogen dioxide, NO2, exists in equilibrium with colourless dinitrogen tetroxide, N2O4
2NO2 (g) ⇌ N2O4 (g)
ΔH = -57.2 kJ mol-1
the temperature is increased. when equilibrium is restored, the appearance of the mixture of gases will be:
a) colourless
b) unchanged
c) paler brown
d) darker brown
d) darker brown
the reaction moved backwards
CHECK BACKWARDS SHIZ
in the catalytic converter of a car engine’s exhauset system, the following reaction occurs:
2NO(g) + 2CO(g) ⇌ N2(g) + 2CO2(g)
ΔH = -745 kJ mol-1
the temperture in a catalytic converter is high
state the effect, if any, on the position of equilibrium if the temperature is lowered
give a reason for your answer
effect = increase in exothermic direction
reason = endothermic reactions absorb heat so if the temperature is lowered then temperature needs to be released (exothermic) to restore equilibrium
in the catalytic converter of a car engine’s exhauset system, the following reaction occurs:
2NO(g) + 2CO(g) ⇌ N2(g) + 2CO2(g)
ΔH = -745 kJ mol-1
the temperture in a catalytic converter is high
the gases from the engine are not cooled before entering the converter
explain why this is so
higher temperature increases the rate of reaction and so it is economical for the system to have a lower temperature
higher temperature = particles on left have activation energy
in the catalytic converter of a car engine’s exhauset system, the following reaction occurs:
2NO(g) + 2CO(g) ⇌ N2(g) + 2CO2(g)
ΔH = -745 kJ mol-1
the temperture in a catalytic converter is high
state the effect, if any, of the position of equilibrium if the pressure on the reacting gases is increased
give a reason for your answer
effect = increase in product
reason = there are less atoms on the left once pressure in increased so the position of equilibrium shifts to the right to restore equilibrium
the reaction of heated magnesium with steam is faster than the reaction of magnesium with cold water. this is mainly because
a) in cold water, the water molecules do not collide as frequently with magnesium
b) the coating of oxide on magnesium decomposed when it is heated
c) the fraction of particles with enegry greater than the activation energy is higher in the recation with steam
d) the recation with steam gos by an alternative route (catalyst) with lower activation energy
c) the fraction of particles with enegry greater than the activation energy is higher in the recation with steam
(a and b are also correct but c is more correct)
This reaction is used to manufacture ammonia:
N2 + 3H2 ⇌ 2NH3
△H = -x kJ/mol
Bond N≡N has a dissociation energy (kJ/mol) of 944
Bond H-H has a dissociation energy (kJ/mol) of 436
Bond N-H has a dissociation energy (kJ/mol) of 388
calculate the energy change occuring during the reaction to manufacture ammonia
944 + (3 x 436) ⇌ 2 x (388 x 3)
2252 ⇌ 2328
2252 - 2328
= - 76
to obtain a reasonable yield of ammonia the reaction is carried out at a temperature of 4500C and a pressure of 250 atmospheres
predict what will happen to the yield of ammonia in the equilibrium mixture if the temperature is increased
yield will decrease
to obtain a reasonable yield of ammonia the reaction is carried out at a temperature of 4500C and a pressure of 250 atmospheres
predict what wil happen to the yield of ammonia in the equilibrium mixture if the temperature is increased
yield will decrease
the temperature of a mixture of nitrogen, hydrogen and ammonia gases is decreased until all the gases have liquefied
describe two canges in the movement of gas molecules as a gas liquefies
molecules will get closer together
molecules will lose kinetic energy (slow down)
molecule N2 has a heat of vaporisation (kJ/mol) of 2.8
molecule H2 has a heat of vaporisation (kJ/mol) of 0.45
molecule NH2 has a heat of vaporisation (kJ/mol) of 23
which of these gases will be the last to liquefy?
NH3
you have to take the most energy out of the NH3 gas
This reaction is used to manufacture ammonia in the Haber process:
N2 + 3H2 ⇌ 2NH3
△ = -92 kJ/mol
a temperature of 4500C and a pressure of 200 atmospheres are often used
if you decrease the temperature what will happen to the rate of reaction and yield of ammonia?
rate of reaction will decrease
yield of ammonia will increase
This reaction is used to manufacture ammonia in the Haber process:
N2 + 3H2 ⇌ 2NH2
△ = -92 kJ/mol
a temperature of 4500C and a pressure of 200 atmospheres are often used
if you add a catalyst what will happen to the rate of reaction and yield of ammonia?
rate of reaction will increase
yield of ammonia will not be affected (no change)
state and explain, using the kinetic theory, the effect on the rate of reaction of increasing the concentration of nitrogen in the Haber process
increases the number of particles (of nitrogen) in the system
particles collide more frequently
increases proportion of successful collisions because of the higher rate of reaction
under the conditions used in the Haber process the yield of ammonia is about 15%
what happens to the unreacted nitrogen and hydrogen?
it is reused
The equation for the complete combustion of methane is:
CH4 + 2O2 –> CO2 + 2H2O
the bond C-H has a dissociation energy (kJ/mol) of 412
the bond O-H has a dissociation energy (kJ/mol) of 463
the bond O=O has a dissociation energy (kJ/mol) of 496
the bond C=O has a dissociation energy (kJ/mol) of 743
use these values to calculate the energy change occuring during the complete combustion of methane
(412 x 4) + (496 x 2) –> (743 x 2) + (463 x 4)
2640 –> 3338
2640 - 3338
- 698 kJ/mol
the biggest error in this experiment is heat loss
suggest how the apparatus could be modified to reduce heat loss
put a lid of op
put it in an insulator - e.g. polystyrene or plastic
suggest why it is important to stir the chemicals thoroughly
so they react and mix and collide with each other
which one of these experiments was probably carried out on a different day to the others and why?
experiment 2
it has a different room temperature
suggest ehy experiment 4 should not be used to calculate the average change
its temperature change is anomalous
calculate the average temperature change from the first three experiments
(7.2 + 7.0 +6.8) ÷ 3
= 7
use the following equation to calculate the energy change for this reaction:
energy change in joules = 100 x 4.2 x average temperature change
100 x 4.2 x ((7.2 + 7.0 +6.8) ÷ 3)
100 x 4.2 x 7
= 2940 J
in the equilibrium below, what effect would an increase in temperature have on the system (rate and yield)?
2H2S + SO2 ⇌ 3S + 2H2O
△H is negative
increase in rate
decrease in yield
in the equilibrium below, what effect would an decrease in pressure have on the system (rate and yield)?
2H2S + SO2 ⇌ 3S + 2H2O
△H is negative
decrease in rate
increase in yield
