Nebosh Diploma unit C UPDATED Study
Guide QUESTIONS AND CORRECT
ANSWERS
- Chemical reaction definition - CORRECT ANSWERS A
process in which one or more substances, the reactants are converted
into one or more different substances, the products. Substances are
either chemical elements or compounds. A chemical reaction
rearranges the constituent atoms of the reactants to create different
substances as products.
Effects of temperature - CORRECT ANSWERS - rate of
reaction will increase with increases in the temp of the reactants
- rate of reaction is exponential with temp and increases by a factor of
at least 2 for every 10 degree rise
- the reaction rate will also increase with increasing concentration of
the reactants
- chemical reactions that are exothermic (generate heat energy)
caution has to be taken to keep the reaction rate (exponential) within
control
- the removal of heat by the use of cooling water acts at a much
slower transfer rate (linear process)
Effects of pressure - CORRECT ANSWERS - Increasing the
pressure on a reaction involving reacting gases increases the rate of
reaction.
- Changing the pressure on a reaction which involves only solid or
liquids has no effect on the rate
,- The relationship between pressure and concentration, increasing the
pressure of a gas is exactly the same as increasing its concentration
- If you have a given mass of gas, you increase its pressure is to
squeeze it into a smaller volume, if you have the same mass in a
smaller volume, then it's concentration is higher.
Effects of catalysts - CORRECT ANSWERS - A catalyst is a
substance that speeds up a chemical reaction but it is not consumed by
the reaction
- Hence a catalyst can be recovered chemically unchanged at the end
of the reaction it has been used to speed up or catalyse
- Only a very small amount of catalyst is needed to increase the rate
of reaction amongst large amounts of reactants
Heat in reactions - CORRECT ANSWERS - Endothermic
reactions - reactions which take in heat energy
- Exothermic reactions - reactions which give out heat energy
Runaway reactions - CORRECT ANSWERS - Of most
concern in chemical processing is exothermic reactions
- If the heat released cannot be removed effectively it presents a
potential hazard by remaining in the reaction mass
- with the consequence of raising the reaction mass temperature, and
hence reaction rate, resulting in a runway reaction
Examples of exothermic reactions - CORRECT ANSWERS -
combustion
- Rusting
,- Setting of cement
- Epoxy resin
- Quicklime
Bhopal tragedy - CORRECT ANSWERS - Setup to produce
intermediate product
- Surplus causing a large quantity of gas to be stored
- Valves in poor quality
- Several vent gas scrubbers had been out of service as well as the
steam boiler intended to clean the pipes
- Water entered tank E610 because of some manual failure
- Exothermic reaction increased the temp inside the tank to over 20
degrees and raised the pressure
- This forced the emergency venting of pressure from the holding
tank, releasing a large volume of toxic gas
- Next morning thousands of people were found dead
- No official word from the company regarding the chemicals and
antidotes
- CEO charged with manslaughter
Causes of exothermic runaway and decomposition - CORRECT
ANSWERS - Mischarging of reactants e.g. addition of the
wrong material or the wrong amount, addition in the wrong order or at
the wrong rate or omission of a reactant
- Thermochemistry e.g. poor appreciation of the heat of reaction,
unknown thermal instability of reactants, intermediates or products
- Temperature control e.g. misreading of temps, failure of
thermocouples,
, - Inadequate agitation e.g. omission to start agitation, incorrect spec
- Maintenance e.g. unauthorised modification, build up of residues,
blockages leaks
- Poor control of raw materials e.g. contamination, viable raw material
spec
- Others e.g. human error, not following procedures, tiredness, cutting
corners
Underlying causes of exothermic runaway and decompositions -
CORRECT ANSWERS - An inadequate knowledge of
reaction chemistry/ thermochemistry
- Inadequate engineering design for heat transfer
- Inadequate process control
- Inadequate procedures and training
Questions to consider when controlling reactions - CORRECT
ANSWERS - Eliminate hazardous raw materials,
intermediates or by products
- Substitute less hazardous raw materials
- Minimise inventories of hazardous materials in storage tanks
- Is process equipment located to minimise the length of hazardous
material piping
- Is it possible to generate hazardous reactions in situ from less
hazardous materials rather than to have to store on site
- Is it possible to use heating and cooling fluids that limit the
maximum and minimum temps
- Can a catalyst be used
Guide QUESTIONS AND CORRECT
ANSWERS
- Chemical reaction definition - CORRECT ANSWERS A
process in which one or more substances, the reactants are converted
into one or more different substances, the products. Substances are
either chemical elements or compounds. A chemical reaction
rearranges the constituent atoms of the reactants to create different
substances as products.
Effects of temperature - CORRECT ANSWERS - rate of
reaction will increase with increases in the temp of the reactants
- rate of reaction is exponential with temp and increases by a factor of
at least 2 for every 10 degree rise
- the reaction rate will also increase with increasing concentration of
the reactants
- chemical reactions that are exothermic (generate heat energy)
caution has to be taken to keep the reaction rate (exponential) within
control
- the removal of heat by the use of cooling water acts at a much
slower transfer rate (linear process)
Effects of pressure - CORRECT ANSWERS - Increasing the
pressure on a reaction involving reacting gases increases the rate of
reaction.
- Changing the pressure on a reaction which involves only solid or
liquids has no effect on the rate
,- The relationship between pressure and concentration, increasing the
pressure of a gas is exactly the same as increasing its concentration
- If you have a given mass of gas, you increase its pressure is to
squeeze it into a smaller volume, if you have the same mass in a
smaller volume, then it's concentration is higher.
Effects of catalysts - CORRECT ANSWERS - A catalyst is a
substance that speeds up a chemical reaction but it is not consumed by
the reaction
- Hence a catalyst can be recovered chemically unchanged at the end
of the reaction it has been used to speed up or catalyse
- Only a very small amount of catalyst is needed to increase the rate
of reaction amongst large amounts of reactants
Heat in reactions - CORRECT ANSWERS - Endothermic
reactions - reactions which take in heat energy
- Exothermic reactions - reactions which give out heat energy
Runaway reactions - CORRECT ANSWERS - Of most
concern in chemical processing is exothermic reactions
- If the heat released cannot be removed effectively it presents a
potential hazard by remaining in the reaction mass
- with the consequence of raising the reaction mass temperature, and
hence reaction rate, resulting in a runway reaction
Examples of exothermic reactions - CORRECT ANSWERS -
combustion
- Rusting
,- Setting of cement
- Epoxy resin
- Quicklime
Bhopal tragedy - CORRECT ANSWERS - Setup to produce
intermediate product
- Surplus causing a large quantity of gas to be stored
- Valves in poor quality
- Several vent gas scrubbers had been out of service as well as the
steam boiler intended to clean the pipes
- Water entered tank E610 because of some manual failure
- Exothermic reaction increased the temp inside the tank to over 20
degrees and raised the pressure
- This forced the emergency venting of pressure from the holding
tank, releasing a large volume of toxic gas
- Next morning thousands of people were found dead
- No official word from the company regarding the chemicals and
antidotes
- CEO charged with manslaughter
Causes of exothermic runaway and decomposition - CORRECT
ANSWERS - Mischarging of reactants e.g. addition of the
wrong material or the wrong amount, addition in the wrong order or at
the wrong rate or omission of a reactant
- Thermochemistry e.g. poor appreciation of the heat of reaction,
unknown thermal instability of reactants, intermediates or products
- Temperature control e.g. misreading of temps, failure of
thermocouples,
, - Inadequate agitation e.g. omission to start agitation, incorrect spec
- Maintenance e.g. unauthorised modification, build up of residues,
blockages leaks
- Poor control of raw materials e.g. contamination, viable raw material
spec
- Others e.g. human error, not following procedures, tiredness, cutting
corners
Underlying causes of exothermic runaway and decompositions -
CORRECT ANSWERS - An inadequate knowledge of
reaction chemistry/ thermochemistry
- Inadequate engineering design for heat transfer
- Inadequate process control
- Inadequate procedures and training
Questions to consider when controlling reactions - CORRECT
ANSWERS - Eliminate hazardous raw materials,
intermediates or by products
- Substitute less hazardous raw materials
- Minimise inventories of hazardous materials in storage tanks
- Is process equipment located to minimise the length of hazardous
material piping
- Is it possible to generate hazardous reactions in situ from less
hazardous materials rather than to have to store on site
- Is it possible to use heating and cooling fluids that limit the
maximum and minimum temps
- Can a catalyst be used
