Module 3.2 Flashcards
what is enthalpy change?
heat change in a reaction at a given pressure
given symbol deltaH
units = Kjmol-1
What are standard conditions?
100kPa
298K/25 degrees
What is an example of an endothermic reaction?
thermal decomposition of calcium carbonate
What is a endothermic reaction?
reactions absorb energy from the surrounds
have a positive enthalpy change
What are exothermic reactions?
reactions that release energy to the surroundings
have a negative enthalpy change
What is an example of a exothermic reaction?
combustion of ethane
What is activation energy?
minimum amount of energy required for a reaction to take place
What is the definition of enthalpy of neutralisation?
enthalpy change when an acid and alkali react to form one mole of water, under standard conditions
What is the definition of enthalpy change of combustion?
enthalpy change of a substance when one mole of a substance is completely burnt in oxygen to make carbon dioxide and water, under standard conditions
What is the definition of enthalpy change of formation?
enthalpy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions
Bonds breaking is….
exothermic
Bonds forming is …
exothermic
How do you work out enthalpy change with mean bond enthalpies?
total energy to break bonds - total energy to form bonds
ΔH values will be less accurate than using formation or combustion data because the mean bond energies are not exact
What is the errors when using calorimetry to work out the enthalpy of combustion of a fuel?
incomplete combustion of fuel
incomplete transfer
heat energy absorbed by tripod
Outline method for calorimetry?
put polystyrene cup in beaker for insulation and support
measure desired volumes of solutions with volumetric pipettes and transfer to cup
clamp a thermometer into place
measure initial temp of solution every min for 3 mins
after 3 mins transfer second reagent into cup
stir mixture to ensure all solution is same temp
record temperature
What are the errors in calorimetry experiment for solutions?
energy transfer to surroundings
approx. of specific heat capacity
reaction may be slow or incomplete
density of solution is taken to be same as water
What is Hess’s law?
total enthalpy change for a reaction is independent of the route by which the chemical change takes place
What is the effect of increasing concentration and increasing pressure on the rate of reaction?
at higher concentrations
there are more particles per unit volume
frequency of successful collisions increases
What is the rate of reaction?
change in concentration of a substance in unit time
mol dm-3 s-1
What are catalysts?
increase the rate of reaction without being used up
by providing an alternative pathway with a lower activation energy so more molceules have sufficient energy above activation
What is a homogenous catalyst?
same state as reactants
What is a heterogenous catalyst?
different state from reactants
What are the benefits of catalysts?
speed up the rate of reaction
lower temperatures/pressures can be used
save energy costs = reduces energy demand for proving high temperature and less electrical pumping costs for high pressure
fewer carbon dioxide emissions from burning of fossil fuels
What are ways in measuring reaction rates?
change in volume of gas
change in mass
titrations (acid, alkali and sodium thiosulphate)
colorimetry
change in electrical conductivity
Describe the Maxwell Boltzmann distribution curve?
energy distribution should go through origin because there are no molecules with no energy
are under the curve represents the total number of particles present
the mean energy of the particles is not at the peak of the curve
only a few particles have greater energy than the EA
the energy distribution should never meet the X axis as there is no maximum energy for molecules
How can a reaction go to completion if few particles have greater energy than the EA?
particles can gain energy through collisions
What effect does temperature have on rate of reaction?
increases the rate of reaction
more kinetic energy
over activation energy
frequency of collisions increases
What effect does surface area have on rate of reaction?
collisions will occur more frequently between the reactions and this increases the rate of reaction
What is a dynamic equilibrium?
exists in a closed system
when the rate of the forward reaction is equal to the rate of the reverse reactions
concentrations of the reactants and products doesnt change
What is the effect of temperature on equillibrium?
increase temp = equ will shift to oppose the change and move in the endothermic direction to reduce temp by absorbing heat
decrease temp = equ will shift to oppose change and move in the exothermic direction to increase the temp by giving out heat
What is the effect of pressure on equillibrium?
increase pressure = equ will shift towards the side with fewer moles to oppose the change and therefore reduce pressure
decrease pressure = equ to shift towards the side with more moles to oppose change and therefore increase in pressure
same moles on both side = changing pressure has no effect on position of equ
What is the effect of concentration of equilibrium?
I2 + 2OH- = I- + IO- + H2O
increasing the conc of OH- ions causes the equ to shift to oppose the change and move in the forward direction to remove OH- ions. The position of equ will shift to the RHS, giving higher yield of I- and IO-.
Adding H+ ions reacts with OH- ions and reduces their conc = equ shifts to the LHS
What is the effect of catalysts on the position of equillibrium?
NO EFFECT
but will speed up the rate of reaction at which equ is achieved
it speeds up the rate of the forward and reverse reactions by the same amount
What does it mean with a large Kc value?
the greater the amount of reactants
if Kc is small we say equ favours the reactants
What effects Kc?
TEMPERATURE
not pressure
not catalysts
not concentrations
