Physical Chemistry Flashcards
Symbol for enthalpy change and unit
ΔH (kj/mol)
Define enthalpy change
change in energy
When is enthalpy change positive or negative?
+ = Endothermic
- = Exothermic
How can you measure enthalpy change?
Through Calorimetry.
- Allows to meansure the amount of energy transferred
Explain the Calorimetry practical for dissolving, displacement and neutralisation reactions.
-Take temp of reactants (all the same)
-Mix them
-Measure temp at the end
-Equipment: Polysterene cup (it insulates), thermometer, lid.
Explain the calorimetry experiment for combustion
-Energy transferred when fuel is burnt.
-50g of water and record its temp
-Weigh the spirit burner and lid.
-Spirit burner underneath the can and light the wick. Heat the water, stir constantly until temp is about 50º.
-Put out flame + measure temp
-Weigh spirit burner and lid again
-Equipment: Thermometer, Lid, Copper Can, 50cm³ water, draught excluder, spirit burner
Formula for heat energy change
q(j)=m(g)xc(heat capacity)xΔT(ºC)
What the specific heat capacity of water
4.18/4.2 J/g/ºC
Steps to calculate molar enthalpy change
1.calculate q
2.number of moles produced this energy change (mass/mr)
3.q/mol
4.add signs
What is the rate of reaction?
How fast the reactants are changed into products.
How do you find the speed of reaction?
By recording the amount of product formed or the amount of reactant used up over time.
Characteristics of a graph of Rate of Reaction
-Steeper line = faster RoR
-Less steep = reactants are used up
-Flat = Reaction has ended
Describe collision theory
-The more collisions there are the faster the reaction is.
-Particles have to collide with enough energy for the collision to be successful.
-Minimum amount of energy = activation energy
What does the RoR depend on?
-Temperature
-Concentration
-Pressure
-Surface area
-Catalyst
How does temperature affect the RoR?
-Particles move faster
-More collisions
-More energy in collisions (reactions happen if the particles collide with energy
=More successful collisions
=↑RoR
How does the concentration affect the RoR?
-More particles of reactant in the same volume.
-Collisions are more likely
=↑RoR
How does Surface area affect the RoR?
-Larger Surface area to volume ratio
-PArticles around it will have more area to work on
=↑RoR
What does a catalyst do?
-Increases the RoR without being chemically changed
-Provides an alternative reaction pathway that requires a lower activation energy
Formula for the speed of a reaction
RoR= Amount of reactant used or amount of product formed/Time
What is a reversible reaction?
A reaction where the products of the reaction can react with each other and convert back to the original reactants.
What is a closed system?
Doesn’t allow for the transfer of matter in and out of the system.
What is equilibrium?
The concentrations of reactant and products will reach a certain balance and stay there.
How can the equilibrium change?
By changing the temperature and pressure. The position of equilibrium changes. It may give you more product and less reactants. Positioon shifts to right.
How would a reverible reaction look like in a graph?
It would look like a mirror.
What happens when a reversible formula is exothermic?
It would be endothermic in the other direction
3 methods to calculate the rate of reaction
1) Beaker over X. Only when both are aqueous or liquid. Calculate how long it takes for the x to be covered by the precipitate
2) Place reaction on a scale with cotton. Calculate the change in mass against time
3) Reaction connected to a syringe. Calculate the volume of gas produced against time.
What happens in a chemical reaction in terms of bond energies?
Energy is supplied to break bonds and energy is released when bonds are made
exothermic-energy released from forming bonds is greater than needed to break the bonds
endothermic-energy needed to break the bonds is greater than energy released making them
Equation to find enthalpy change in terms of bind energies?
Energy of reaction = sum of bond energies of bonds broken - sum of bond energies of bonds made
Units for RoR
-g/s
-mol/s
3 common ways to measure the RoR
Loss in mass of reactants, volume of gas produced, time for a solution to become opaque
How to find the RoR at some time from a graph f amount of reactant vs time
-Pick a point corresponding to time
-Find tangent to the curve at this point
-Tangent = gradient
Describe the effect of increasing pressure of a gas on the RoR
-Increasing the pressure of reacting gases is the same as increasing the conc.
-t increases the number of gas molecules in the same volume and so increases the frequency of collisions and therefore increases the RoR.
Describe Le Chatelier’s Principle
If the system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change and restore the equilibrium.
Describe the effect of changing the concentration of reactant and product on the position of the equilibrium
-If conc of one of the reactants or products is changed, it is not longer in equilibrium
-If the concentration of a reactant is increased, more products will be formed until equilibrium
-If the conc of product is decreased, more reactants will react until equilibrium
Describe the effect of a change in temp on the position of the equilibrium
-If the temp>
-Endothermic = products>
-Exothermic= products<
-If temp<
-Endothermic= products<
-Exothermic= products>
Describe the effect of changing pressure on the position of equilibrium
FOR GASES
-Equilibrium shifts towards the side with less molecules (moles)
Describe the effect of a catalyst on the position of the equilibrium
-No effect
-It just speeds up both forward and backward reactions equally
-equilibrium is achieved faster
