NEBOSH DIPLOMA UNIT C UPDATED EXAM QUESTIONS
AND VERIFIED ANSWERS GRADED A+
Chemical reaction definition - CORRECT ANSWER✅✅✅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 ANSWER✅✅✅- 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 ANSWER✅✅✅- 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 ANSWER✅✅✅- 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 ANSWER✅✅✅- Endothermic reactions - reactions which
take in heat energy
- Exothermic reactions - reactions which give out heat energy
Runaway reactions - CORRECT ANSWER✅✅✅- 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 ANSWER✅✅✅- combustion
- Rusting
- Setting of cement
- Epoxy resin
- Quicklime
Bhopal tragedy - CORRECT ANSWER✅✅✅- 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 ANSWER✅✅✅-
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
ANSWER✅✅✅- 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 ANSWER✅✅✅- 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
- Can equipment be designed to totally contain the maximum pressure generated
- Can equipment be designed so it is difficult or impossible to create a hazardous situation due to
operating error
, - Can processes be located to reduce or eliminate adverse impacts from other adjacent hazardous
installations
- Can semi batch processes be modified to reduce accumulation of reactants
- Has the heating effect of the surroundings been considered under failure conditions when
working below ambient temps or using reactor heating
Controlling methods - reactions - CORRECT ANSWER✅✅✅Can be of two types
- Preventative I.e. they reduce the likelihood of a hazardous situation occurring, and designing
and operating safe chemical reaction process
- Protective I.e. they reduce the consequences of a hazardous situation
Methods to control pressure - CORRECT ANSWER✅✅✅- Particularly important where
there is a risk of explosion
- System should withstand the maximum pressure generated by the runaway reaction
- Controls will include early detection mov pressure rise measured by a detector and displayed in
a similar way as with temp
- Will rely on a rapid automatic pressure release, frequently using spring loaded pressure release
valves or bursting discs venting to a safe area
Controlling pressure - Emergency pressure relief or venting or dumping - CORRECT
ANSWER✅✅✅- Think about where contaminant is vented to - if it's hazardous it cannot be
relieved direct to atmosphere
Methods of control of pressure:
- Knock out drums
- Quench tanks
- Flare stacks
- Scrubbers
Controlling pressure - crash cooling - CORRECT ANSWER✅✅✅- In some cases may be
possible to bring a runaway reaction back under control (usually in early stages) by using
additional cooling
AND VERIFIED ANSWERS GRADED A+
Chemical reaction definition - CORRECT ANSWER✅✅✅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 ANSWER✅✅✅- 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 ANSWER✅✅✅- 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 ANSWER✅✅✅- 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 ANSWER✅✅✅- Endothermic reactions - reactions which
take in heat energy
- Exothermic reactions - reactions which give out heat energy
Runaway reactions - CORRECT ANSWER✅✅✅- 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 ANSWER✅✅✅- combustion
- Rusting
- Setting of cement
- Epoxy resin
- Quicklime
Bhopal tragedy - CORRECT ANSWER✅✅✅- 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 ANSWER✅✅✅-
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
ANSWER✅✅✅- 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 ANSWER✅✅✅- 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
- Can equipment be designed to totally contain the maximum pressure generated
- Can equipment be designed so it is difficult or impossible to create a hazardous situation due to
operating error
, - Can processes be located to reduce or eliminate adverse impacts from other adjacent hazardous
installations
- Can semi batch processes be modified to reduce accumulation of reactants
- Has the heating effect of the surroundings been considered under failure conditions when
working below ambient temps or using reactor heating
Controlling methods - reactions - CORRECT ANSWER✅✅✅Can be of two types
- Preventative I.e. they reduce the likelihood of a hazardous situation occurring, and designing
and operating safe chemical reaction process
- Protective I.e. they reduce the consequences of a hazardous situation
Methods to control pressure - CORRECT ANSWER✅✅✅- Particularly important where
there is a risk of explosion
- System should withstand the maximum pressure generated by the runaway reaction
- Controls will include early detection mov pressure rise measured by a detector and displayed in
a similar way as with temp
- Will rely on a rapid automatic pressure release, frequently using spring loaded pressure release
valves or bursting discs venting to a safe area
Controlling pressure - Emergency pressure relief or venting or dumping - CORRECT
ANSWER✅✅✅- Think about where contaminant is vented to - if it's hazardous it cannot be
relieved direct to atmosphere
Methods of control of pressure:
- Knock out drums
- Quench tanks
- Flare stacks
- Scrubbers
Controlling pressure - crash cooling - CORRECT ANSWER✅✅✅- In some cases may be
possible to bring a runaway reaction back under control (usually in early stages) by using
additional cooling
