Section 5 - Physical Chemistry P1 Flashcards
Define an exothermic reaction:
An exothermic reaction is one which gives out energy to the surroundings, usually in the form of heat and usually shown by a rise in temperature of the surroundings.
Define an endothermic reaction:
An endothermic reaction is one which takes in energy to the surroundings, usually in the form of heat and usually shown by a fall in temperature of the surroundings.
What is enthalpy change?
The overall change in energy in a reaction is called the enthalpy change.
It has the symbol ΔH.
What are the units for enthalpy change?
kJ/mol
What is the enthalpy change value in an exothermic reaction?
-ΔH
because energy is given out
What is the enthalpy change value in an endothermic reaction?
+ΔH
because energy is taken in
What can you use to find out enthalpy change?
Calorimetry
-it allows you to measure the amount of energy transferred in a chemical reaction
What are the two different types of experiments you can do using calorimetry for measuring enthalpy change?
- Dissolving, displacement and neutralisation reactions
- combustion
Describe a practical for how you can use calorimetry for dissolving and displacement reactions:
You take a temperature of the reactants, mix them and take a temperature at the end of the reaction.
-mix the reactants in a polystyrene cup
What is the biggest problem with using a polystyrene cup for a calorimetry experiment?
The amount of energy lost to the surroundings
How can the accuracy of a calorimetry - dissolving, displacement and neutralisation reaction be improved?
- put the polystyrene cup in a beaker with cotton wool (more insulation)
- put a lid on the polystyrene cup (to reduce energy lost by evaporation)
Describe a practical for how you can use calorimetry for a neutralisation reactions:
- put 25cm³ of HCl and NaOH in separate beakers
- place both beakers in a 25⁰C water bath
- add the HCl and NaOH into a polystyrene cup
- take the temperature every 30secs, record the highest temperature
Describe a practical for how you can use calorimetry for a combustion reaction:
You can burn the fuel and use it to heat up water.
- put 50g of water in a copper can and record its temperature
- weigh the spirit burner and lid
- light the spirit burner under the copper can, stir the water constantly until the temperature is 50⁰C
- put out the flame and measure the final temperature of the water
- weigh the spirit burner and lid again
- calculate enthalpy change
What are the issues with using a calorimetry for a combustion reaction?
Heat loss to the environment
-so you have to reduce draughts as much as possible,
using draught excluders, put the flame close to the copper container of water, have a lid on the copper container
What is the equation for how to calculate heat energy transferred?
Q = m x c x ΔT Q - heat energy transferred (J) m - mass of the liquid being heated (g) c - specific heat capacity (J/g/⁰C) ΔT - change in temperature of the liquid (⁰C)
What is the specific heat capacity of water?
4.2J/g/⁰C
How do you calculate molar enthalpy change?
- calculate heat energy change in kJ
- calculate how many moles of the substance there is (moles = mass/Mr)
- molar enthalpy change = kJ/mol
Define the rate of a reaction:
it is how fast the reactants are changed into products
What is an example of a slow reaction?
rusting
What is an example of a moderate speed reaction?
magnesium reacting with an acid to produce a gentle stream of bubbles
What is an example of a fast reaction?
burning is fast
explosions are faster
What are the two ways you can find the speed of a reaction?
- the amount of product formed over time
- the amount of reactant used up over time
At the the start of a reaction what would the amount of reactants be? And at the the end of a reaction what would the amount of reactants be?
Start-high
End-low
What happens to the amount of reactants as a reaction takes place? And when does the amount of reactants change fastest?
It decreases
At the start
How could you sketch a graph to show what happens to the amount of reactants during a reaction?
Draw a graph of concentration(mol/dm³)[y] against time(s)[x]
The graph would be a curve starting high on the concentration curving down towards time and levelling out (like the shape of an elongated l)
How could you sketch a graph to show what happens to the amount of products during a reaction?
Draw a graph of concentration(mol/dm³)[y] against time(s)[x]
The graph would be a curve starting at the origin curving up and levelling out(like the shape of an f without the cross)
On a rate of reaction graph what can the gradient mean?
- the steeper the gradient the faster the reaction
- over time the gradient becomes less steep as the reactants are used up
- when the gradient = 0 the reaction has finished
What does the rate of a reaction depend on?
- collision frequency
- energy transferred during a collision
How does collision frequency affect the rate of reaction?
the more collisions, the faster the reaction is
-double the frequency, doubles the rate of reaction
How does the energy transferred during a collision affect the rate of reaction?
particles have to collide with enough energy for the collision to be successful
Define activation energy:
The minimum amount of energy that particles need to react is the activation energy
What factors affect the rate of reaction?
- temperature
- surface area
- concentration of a solution or pressure of a gas
- the presence of a catalyst
Hoes increasing the temperature affect the rate of reaction?
When temperature increases,,,
- particles gain kinetic energy and move faster, so they have more collisions
- particle collisions have more energy (reactions only happen if particles collide with enough energy)
- more successful collisions, so the rate of reaction increases
How does increasing the concentration or pressure affect the rate of reaction?
If a solution is more concentrated…
-there are more particles of reactant in the same volume, this makes collisions more likely, so the rate of reaction increases
If the pressure of a gas is increased…
-particles as closer together so collisions of particles will be more frequent, so the rate of reaction increases
How does increasing the surface area (having smaller solid particles) the rate of reaction?
If a reactant is a solid & the surface area is increased…
- will increase its surface area to volume ratio
- particles around it will have more area to work on, frequency of collisions will increase
- so the rate of reaction is faster for solids with larger surface are to volume ratios
Define a catalyst:
A catalyst is a substance which increases the rate of reaction without being chemically changed or used up in the reaction.
Do you need a large or a small amount of catalyst in a reaction?
Only need a small amount because it isn’t used up
How do catalysts work?
- by decreasing the activation energy
- they provide an alternative reaction pathway that has a lower activation energy
- so more particles have at least the minimum amount of energy needed to start a reaction when the particles collide
What is the equation for how to calculate rate of reaction?
rate of reaction =
amount of reactant used or amount of product formed
÷ time
What are three experiments you can do to measure the rate of reaction?
- precipitation
- change in mess(usually gas given off)
- the volume of gas given off
Describe how you can use a precipitation reaction to measure rate of reaction:
- put a flask on a piece of paper with a cross on
- mix the two clear reactant solutions in the flask and start a timer
- observe how long you can see the cross for and stop once you can’t, the faster it disappears, the faster the reaction
When does a precipitation reaction work for measuring the rate of reaction?
For a reaction where you are mixing two clear solutions which produce a precipitate
What is the problem with the precipitation reaction for measuring the rate of a reaction?
The result is subjective
-different people might not agree on when the mark disappears
Describe how you can measure change in mass from a reaction to measure rate of reaction:
- need a reaction that produces a gas,place it on a mass balance, with a piece of cotton wool in the flask top
- the quicker the reading on the balance drops, the faster the reaction
- reaction has finished when the mass stops changing
- can use your results to plot a graph of change in mass against time
Why is cotton wool placed in the top of the flask for the mass loss experiment for measuring the rate of reaction?
- it lets gases through
- stops any solid, liquid or aqueous reactant flying out during the reaction
What is a safety precaution that may need to be taken depending on the product produced in a mass loss experiment?
If the gas released is harmful then the experiment should be carried out in a fume cupboard
Describe how you can use the volume of gas given off in a reaction to measure rate of reaction:
- use a gas syringe to measure the volume of gas given off
- reaction has finished when no more gas is produced
- use your results to plot a graph of gas volume against time elapsed
What is a safety precaution that may need to be taken depending on the reaction in a volume of gas given off reaction?
- need to use the right sized gas syringe
- if the reaction is too vigorous it could blow the plunger out of the end of the syringe
Describe a practical for how you can measure how surface area affects rate:
- put dilute HCl and marble chips into a flask attached to a bung with a gas syringe
- measure the volume of gas produced @ 20s intervals
- record your results in a table, plot a graph of volume against time
- repeat the experiment with the same volume and concentration of acid, and the same mass of marble chips, just more crunched up and then as a powder
Describe a practical for how you can measure how concentration affects rate:
- put dilute HCl and marble chips into a flask attached to a bung with a gas syringe
- measure the volume of gas produced @ 20s intervals
- record your results in a table, plot a graph of volume against time
- repeat the experiment with the same mass and size of the marble chips, and the same volume of acid just varying the concentration
Describe an experiment for how reaction rate is affected by temperature:
-have sodium thiosulphate and HCl in water baths at different temperatures (20, 30, 40)
-mix the two solutions of the same temperature together in a flask over a cross and time how long it takes for the yellow precipitate to cover it
-record the results in a table
[this can also be used to test the effects of concentration, vary the conc. instead of the temp.]
Describe a practical for how you can measure how using a catalyst affects rate:
The decomposition of hydrogen peroxide
- put hydrogen peroxide and MnO₂ powder into a flask and attach a gas syringe
- measure the volume of gas produced at regular time intervals, record the results in a table
- repeat the experiment with a different catalyst but keep the same volume and conc. of H₂O₂ and the same mass of catalyst
- plot a graph of volume against time
What is the equation for the decomposition of hydrogen peroxide?
2H₂O₂ (aq) -> 2H₂O (l) + O₂ (g)
What catalyst speed up the decomposition of hydrogen peroxide?
manganese(IV) oxide (MnO₂)
copper(II) oxide (CuO)
zinc oxide (ZnO)
What is a reversible reaction?
A reversible reaction is one where the products of the reaction can react with each other and convert back to the original reactants.
A + B ⇌ C + D
What is an example of a reversible reaction?
- The thermal decomposition of ammonium chloride
- The dehydration of copper(II) sulphate
Describe the thermal decomposition of ammonium chloride:
- ammonium chloride is a white solid, when heated it breaks down into the gases ammonia and hydrogen chloride (forward reaction)
- if the ammonia and hydrogen chloride cools they react to re-form the white solid (backward reaction)
