physical chemistry Flashcards
describe the changes in energy store during an exothermique reaction
Chemical energy from the bonds in the reaction
Are transferred to heat energy in the surroundings
Describe the changes in energy store in an endothermic reaction
Heat energy from the surroundings
Is absorbed and converted into the chemical energy store of the reaction
The enthalpy change is negative for an … reaction
And explain why
Exothermique réaction,
Because energy transferred to the surroundings
As the reactants contain more chemical energy than the products
An …. Reaction has less chemical energy in the products than in the reactants
Endothermic
Give the units for enthalpy change
Kj/Mole
Enthalpy change definition
Amount of energy given in or taken out in a chemical reaction
And is the difference between the energy in the reactants and in the products
In an exothermic reaction, does the heat given off warm the reaction or the surroundings or both
Both
In an endothermic reaction, does the temperature of the reaction, or the surroundings cool down, or both
Both the surroundings and reactions temperate cools down
Give 4 examples of types of reactions that are exothermic
-displacement reactions
-neutralization reactions
-metal and acid reactions
-combustion reactions
Give an exmoale of a type of reaction that is endothermic
Thermal decomposition
For the example, CaCo3(s) —-) CaO(s) + CO2(g),
What does an enthalpy change of +178KJ/Mol mean
That for every mol of calcium carbonate, 178KJ of heat energy must be absorbed to produce calcium oxide and carbon dioxide
(Remember that this is an endothermic reaction)
Specific heat capacity definition
The amount of heat energy required to raise the temperature of 1g of a substance by 1degree
What are the units for the specific heat capacity
J/g/C
(Joules per gram per degree)
What is the specific heat capacity of water
4.2 J/g/C
What is the equation for enthalpy change
Q= MCT
Heat energy transferred = mass x specific heat capacity x temperature change
Activation energy definition
The minimum amount of energy that the reactant particles need inorder to collide with each other and react
What are the units for molar enthalpy change
KJ
Give the formula for molar enthalpy change
Molar enthalpy change =Heat energy / number of moles
Triangle H = Q/N
Exothermic or endothermic?
More energy is released when new bonds are formed than energy required to break the bonds in the reactant
Exothermic
More energy is required to break the bonds than gained to make them.
Endothermic or Exothermic
Endothermic
Endothermic of Exothermic?
The energy used to break the bonds is greater than the energy released by forming them
Endothermic
Endothermic or Exothermic?
The energy released to make the bonds
Is greater than the energy required to break them
Exothermic
Units for bond energy
Kj/ mol
How do you calculate energy change, when doing bind energies
Total Energy in reactants - total energy in products
Bond energy meaning
Total amount of energy required to break one mole of a particular covalent bond
Explain how to investigate temperate change, using neutralization reaction of sodium hydroxide and hydrochloric acid
1)heat both solutions of hydrochloric acid and sodium hydroxide in separate beakers in a water bath until they are the same temperature
2)pour hydrochloric acid into a polystyrene cup to keep it insulated, and in a beaker to keep it from falling.
Add 5cm3 sodium hydroxide solution and then place a lid to stop it from evaporating
3) place the order inside and measure the starting teletype and then the temperature every 30 secs until there is no change
4) rinse the cup and repeat with higher volume of sodium hydroxide solution added, eg 10cm3.
5) repeat for a few values of sodium hydroxide, and then repeat the entire experiment again and calculate the average mean temperature change, using the highest value reached for each volume
Why does the temperature eventually begin to decrease when excess amounts of sodium hydroxide is added to hydrochloric acid
Because there is a larger volume of solution than there was before
Therefore the heat energy is spread out over a larger volume
Definition of a catalyst
Substance that speeds up a reaction without being consumed
Give the name of the type of metals that are good catalysts and explain why they are good
The transition metals
Because they have the ability to change oxidation state (so donate or accept electrons)
So they can absorb substances onto their surface and activate them
Explain how catalysts speed up the rate of reaction
They provide an alternative route for the reaction to occur
Which has lower activation energy
Therefore a more numbers of particles with have atleast the minimum amount of energy required for the reaction to occur
Increasing the rate of reaction
state the name of the catalyst , and give an example of a real life situation where the catalyst in required.
Iron beads
Used to speed up the rate of reaction in the Huber process to make ammonia
Why are catalysts important in commercial terms
They speed up the rate of reaction,
So increase the production rate
And reduce energy costs
Describe an experiment to use to investigate the effect of surface area on reaction rate
1)pour hydrochloric acid into a beaker
2)submerge a measuring cylinder under water to avoid air bubbles from forming
3)connect a delivery tube in the beaker and under the measuring cylinder submerged upside down into a water trough
4)add calcium carbonate chips to the beaker and quickly place a bung
5) start the stop watch, and every ten seconds measure the volume of gas produced
6)keep repeating the experiment with different sizes of calcium carbonate chips
7) then when you have repeated with different sizes, repeat the whole experiment again and take an average
Why is it important in a downwards displacement reaction to fully submerge the measuring cylinder underwater
So that no air bubbles can form
(Bc if they form, they can cause further displacement of water, bc the more gas that there is, the more water that it is going to push away/displace)
Describe an experiment to investigate the effect of concentration on the rate of reaction
1)add 50cm3 of sodium thioaulfate to a flask
2)draw a cross underneath the flask
3) add hydrochloric acid to the flask
4)start the stop watch and by looking from above, measure the amount of time that it takes for the cross to disappear.
5)the two clear solutions will react together to form a yellow precipitate of sulfur
6)keep repeating with different concentrations of sodium thiosulfate, by adding different volumes of water inorder to dilute it
7)repeat the whole experiment again and then take an average
State the name and the colour of the precipitate when sodium thiosulfate and hydrochloric acid react
Precipitate of sulfur
Which is yellow
(This reaction is utilized in the experiment to investigate the the effect of concentration on reaction rates)
Explain how you can vary the concentration of a solution
Add different volumes of water to it in order to dilute it
Describe how to carry out an experiment to investigate the effect of temperature on the rate of reaction
1)pour hydrochloric acid into a beaker and heat in a water bath to a specific temperature
2)add a magnesium strip to the beaker
3)start the stopwatch and measure the amount of time that it takes for the magnesium strip to dissolve
4) keep repeating the experiment with different temperature of hydrochloric acid, by using the water bath
5) then repeat the entire experiment again and take an average
Describe how to carry out an experiment to investigate the effect of a catalyst on the rate of reaction
1) add hydrogen peroxide to a conical flask
2) attach a delivery tube to a gas syringe
3)add manganese(IV) oxide powder to the flask and attach the bung
4)measure the volume of gas produced in the gas syringe after every ten seconds
5)repeat the experiment without the catalyst, or then use different catalysts such as zinc oxide powder, or copper (ii) oxide
In the experiment to investigate the effect of catalysts on the rate of reaction, state three examples of catalysts that can be used
Manganese (IV) oxide powder
Zinc oxide powder
Copper (ii) oxide
Explain the effect of surface area on the rate of reaction
With increased surface area for the solid reactant
There is more surface area available for the other reactant to work on
Therefore there are more frequent successful collisions per second
Therefore a higher rate of reaction
Explain the effect of concentration on the rate of reaction
With an increased concentration
There are more particles in the same volume of solution of reactant
Therefore more frequent successful collisions per second
So a higher rate of reaction
Explain how gas pressure could affect the rate of reaction
The higher the gas pressure,
The higher the number of particles in the same volume,
as the particles will be more crowded together
Therefore more frequent successful collisions per second
So the rate of reaction increases
Give four factors that can affect the rate of reaction
Surface area
Concentration
Temperature
Presence of a catalyst
Explain how temperature can affect the rate of reaction
With an increased temperature,
The faster the movement of the particles,
And because higher temperature also increases the kinetic energy of the particles, more particles will have enough energy to react
Therefore there will be more frequent successful collisions per second
So a higher rate of reaction
(Remember temperature increases the kinetic energy aswell as the frequency of collisions)
Hydrogen and iodine are out in a closed jar to react and form hydrogen iodide.
Explain what would happen if the lid on the top was removed
The system would no longer be at equilibrium, because the reactants and products would be able to escape the system
And dynamic equilibrium can only occur in a closed system
Dynamic equilibrium defintion
In a closed system,
when the concentration of reactants and products are constant,
and when the rate of the forwards and backwards reaction are the same
Reversible reaction defintion
When the products can react together to form the reactants again
So the reaction can occur in the forwards direction, and the backwards direction
Explain how the reaction between solid ammonium chloride to become ammonia gas and hydrogen chloride can reach dynamic equilibrium
1)At the beginning of the reaction, there is more ammonium chloride and no ammonia and hydrogen chloride, therefore the forward rate of reaction is faster
2)then as the ammonium chloride begins to decrease, the backwards rate of reaction increases whilst the forwards decreases
3) eventually both rates of reaction becomes equal and so the concentration of reactants and products will be constant, so it will have reached dynamic equilibrium
What is le chateliers principal
When a change is made to the conditions of a system at equilibrium, the system automatically opposes the change in-order to restore the equilibrium
In the harber process, which direction is Exothermic
The forward reaction
In the haber process, the backwards reaction is,
Endothermic
In the haber process, how can you obtain more ammonia in terms of temperature
Decrease the temperature,
So that instead, the reaction will be endothermic,
And then the position of equilibrium will move to the right
(Remeber that in the haber process the forward reaction is Exothermic, so decreasing the temp will mean that the opposite will actually happenso the temp will basically be increased which is an Exothermic reaction)
What is the haber process
A process to make ammonia using hydrogen and nitrogen
in a reversible Exothermic reaction
Give the balanced symbol equation of the haber process
N2 + 3H2 —-)(—- NH3
In the haber process, what happens if you decrease the pressure
The position of equilibrium moves to the left,
because it will move in the direction that produces larger number of molecules
To increase the pressure again
So that concentration of nitrogen and hydrogen increase (bc they’re on the left
Describe the effect of changing temperature on the position of equilibrium
Increasing the temperature,
means that the equilibrium will move to the endothermic side
Therefore the products on that side will increase, and then the products in the Exothermic side will decrease
What is the postion of equilibrium
The side of the reaction which has a higher concentration of particles
Describe the effect of changing the pressure on the position of equilibrium,
Increasing the pressure,
means that the position of equilibrium will shift to the side with the smaller amount of molecules, as per its balanced symbol equation
So that the pressure is reduced again
Decreasing the pressure means the the pistons of equilibrium will shift to the side with the higher number of molecules, as per its balanced symbol equation so that the pressure is increased again
What happens when the pressure is increases in the haber process
the pressure is increased,
So the equilibrium position moves to the right,
as it has a smaller number of molecules
so the yield of ammonia increases.
Give two conditions for dynamic equilibrium to occur
Must be a reversible reaction
Must happen in a closed system
In the reaction between hydrated copper (IV) sulfate to become anyhdeous copper sulfate + water, explain which side of the reaction is Exothermic and why
Backwards reaction is Exothermic,
Because heat is released
In the reaction between hydrated copper (IV) sulfate to become anyhdeous copper sulfate + water, explain which side of the reaction is Endothermic and why
Forwards reaction
because heat energy is taken in
Effect of catalyst in the rate of reaction:
No effect because it only speeds up the forward and backwards reaction equally,
so that equilibrium is achieved faster
When investigating the amount of energy per mole of a particular substance by burning a spirit burner underneath some water, explain three reasons that the value you obtain is less than what you would see in a data book:
1)no drought exluder to prevent heat loss
2)incomplete combustion(bc lack of oxygen so therefore less energy released)
3)evaporation of water (so that then more energy is lost to surroundings)
Why is a bung placed on the conical flask when manganese (IV) oxide and hydrogen peroxide react together
So that the oxygen gas produced does not evaporate
What is the catalyst in the reaction between manganese (IV) oxide and hydrogen peroxide
Manganese (iv) oxide
Why is it better to use a gas syringe instead of a downwards displacement method to collect oxygen gas
Gas syringe gives you dry air.
Downwards displacement may contains traces of water vapour in the air
Catalyst defintion
Substance that increases the rate of reaction, and remains chemically unchanged at the end of the reaction
Give three characteristics of a reaction that is in dynamic equilibrium
1)forwards and backwards reaction happen simultaneously
2)rate if forwards reaction is the same as rate of backwards reaction
3)concentration of both reactants and products remains constant
How is ammonia separated from nitrogen and hydrogen in haber process
It is cooled
What happens to unreacted nitrogen and hydrogen in the haber process when ammonia is formed
It is recirculated
A mixture contains a solution of this:
2N0, (g) (brown)—-) (—-N204, (g), (colorless)
When pressure decreases, explain why the colourless turns brown
When pressure decreases,
The equilibrium moves to the left
bc the left has the higher number of molecules
so therfore more NO is formed
Give a source of error why the enthalpy change may be lower than the data book value for each of these reactions.
Neutralization enthalpy change:
Combustion enthalpy change:
-the acid or alkali may be impure, therefore affecting the temperature change
-incomplete combustion may have occurred, therfore affecting the temperature change again
