Module 3.2

Question 1

what is enthalpy change?

Answer 1

heat change in a reaction at a given pressure
given symbol deltaH
units = Kjmol-1

Question 2

What are standard conditions?

Answer 2

100kPa
298K/25 degrees

Question 3

What is an example of an endothermic reaction?

Answer 3

thermal decomposition of calcium carbonate

Question 4

What is a endothermic reaction?

Answer 4

reactions absorb energy from the surrounds
have a positive enthalpy change

Question 5

What are exothermic reactions?

Answer 5

reactions that release energy to the surroundings
have a negative enthalpy change

Question 6

What is an example of a exothermic reaction?

Answer 6

combustion of ethane

Question 7

What is activation energy?

Answer 7

minimum amount of energy required for a reaction to take place

Question 8

What is the definition of enthalpy of neutralisation?

Answer 8

enthalpy change when an acid and alkali react to form one mole of water, under standard conditions

Question 9

What is the definition of enthalpy change of combustion?

Answer 9

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

Question 10

What is the definition of enthalpy change of formation?

Answer 10

enthalpy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions

Question 11

Bonds breaking is….

Answer 11

exothermic

Question 12

Bonds forming is …

Answer 12

exothermic

Question 13

How do you work out enthalpy change with mean bond enthalpies?

Answer 13

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

Question 14

What is the errors when using calorimetry to work out the enthalpy of combustion of a fuel?

Answer 14

incomplete combustion of fuel
incomplete transfer
heat energy absorbed by tripod

Question 15

Outline method for calorimetry?

Answer 15

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

Question 16

What are the errors in calorimetry experiment for solutions?

Answer 16

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

Question 17

What is Hess’s law?

Answer 17

total enthalpy change for a reaction is independent of the route by which the chemical change takes place

Question 18

What is the effect of increasing concentration and increasing pressure on the rate of reaction?

Answer 18

at higher concentrations
there are more particles per unit volume
frequency of successful collisions increases

Question 19

What is the rate of reaction?

Answer 19

change in concentration of a substance in unit time
mol dm-3 s-1

Question 20

What are catalysts?

Answer 20

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

Question 21

What is a homogenous catalyst?

Answer 21

same state as reactants

Question 22

What is a heterogenous catalyst?

Answer 22

different state from reactants

Question 23

What are the benefits of catalysts?

Answer 23

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

Question 24

What are ways in measuring reaction rates?

Answer 24

change in volume of gas
change in mass
titrations (acid, alkali and sodium thiosulphate)
colorimetry
change in electrical conductivity

Question 25

Describe the Maxwell Boltzmann distribution curve?

Answer 25

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

Question 26

How can a reaction go to completion if few particles have greater energy than the EA?

Answer 26

particles can gain energy through collisions

Question 27

What effect does temperature have on rate of reaction?

Answer 27

increases the rate of reaction
more kinetic energy
over activation energy
frequency of collisions increases

Question 28

What effect does surface area have on rate of reaction?

Answer 28

collisions will occur more frequently between the reactions and this increases the rate of reaction

Question 29

What is a dynamic equilibrium?

Answer 29

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

Question 30

What is the effect of temperature on equillibrium?

Answer 30

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

Question 31

What is the effect of pressure on equillibrium?

Answer 31

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

Question 32

What is the effect of concentration of equilibrium?

Answer 32

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

Question 33

What is the effect of catalysts on the position of equillibrium?

Answer 33

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

Question 34

What does it mean with a large Kc value?

Answer 34

the greater the amount of reactants

if Kc is small we say equ favours the reactants

Question 35

What effects Kc?

Answer 35

TEMPERATURE
not pressure
not catalysts
not concentrations