Advanced Higher Chemistry questions and answers
Graded A+
Why does having an excess reactant help to convert costly reactants into
products? - ✅✅It provides the best chance for the limiting reactant to react
completely.
Normally the cheaper reactant would be the one in excess and can be recycled
back into the reaction to save cost and avoid wasting materials.
What are 3 different transition metals that can act as catalysts, give the names of
the reactions? - ✅✅Iron (Fe), Haber Process
Platinum (Pt), Ostwald Process, Catalytic converters in car exhausts
Nickel (Ni), hardening of veg oils to fats
Why are so many catalysts derived from transition metals? - ✅✅Because the
vacant d orbitals are readily available for (reversible) bonding. (They show
variable oxidation states)
Explain in terms of electron orbitals, how transition metal catalysts lower
activation energy of a reaction? - ✅✅A reactant may be adsorbed (held by
covalent bonds often within the d orbitals of the catalyst) in a suitable orientation
for a more probably successful reaction.
The reactant may be adsorbed in a way that the bonds within the reactant are
weakened which lowers activation energy.
,What is the molar gas volume? - ✅✅The volume of all gases is the same under
the same conditions of temperature and pressure.
One mole of any gas has a volume of approx 24 litres at room temperature
What is the difference between theoretical yield and actual yield? -
✅✅Theoretical - what we expect to obtain from a reaction based on the
number of moles of the limiting reactant and actual - actual amount of product
that forms in a chemical reaction
What are the reasons that a reaction may not achieve a 100% percentage yield? -
✅✅- Mass transfer/ mechanical losses (product may be left in glassware etc)
- Purification of product
- Other side reactions may have occurred with impurities that may be present in
the reaction mixture.
- Many reactions are reversible and a dynamic equilibrium is established,
therefore when the product is formed it can revert back to reactants.
- Insufficient time for the reaction to complete.
What is an empirical formula? - ✅✅An empirical formula is the simplest ratio
of atoms in the compound.
What is enthalpy change? - ✅✅The difference in the heat energies between
the products and the reactants
What is combustion? - ✅✅When one mole of a substance burns completely in
oxygen (kJ per mole)
,What is Hess' Law? - ✅✅The enthalpy change of a chemical reaction depends
only on the chemical nature and physical properties of the reactants and products
and is independent of any intermediate steps.
Why is Hess' Law important? - ✅✅It allows us to calculate enthalpy changes
which cannot be determined in an experiment
What is enthalpy of formation? - ✅✅Enthalpy change when one mole of a
substance is formed from its elements
How do we calculate enthalpy of formation and why? - ✅✅Since enthalpy of
formation is hard to obtain, we use enthalpy of combustion to calculate it
How do we calculate Hess' Law? - ✅✅Step 1) Write out the combustion
equations for each substance
Step 2) Multiply each equation by what is required for it to match the original
Step 3) Flip around the final one as the direction is the wrong way
Step 4) Add up all the corrected enthalpy of combustions to obtain the answer
When do bonds break/form? - ✅✅Bonds break in reactant molecules and form
in product molecules
What is molar bond enthalpy? - ✅✅The enthalpy change when one mole of a
bond in gaseous molecules is broken or formed.
, Is bond breaking endothermic or exothermic? - ✅✅Endothermic
Is bond making endothermic or exothermic? - ✅✅Exothermic
What state does bond making/breaking occur? - ✅✅In the gaseous state
What is the definition of a dynamic equilibrium? - ✅✅When the rates of the
forward and reverse reactions are equal
What is required for a dynamic equilibrium to occur? - ✅✅It only occurs in a
closed system
What is a closed system? - ✅✅When none of the chemicals can escape from
the system
What is Le Chatelier's principle? - ✅✅If a system at equilibrium is subjected to
any change then the system re-adjusts itself to counteract the applied change
What conditions favour an endothermic reaction? - ✅✅Rise in temperature
What conditions favour an exothermic reaction? - ✅✅Low temperatures
When dealing with pressure in equilibriums, what should only be considered? -
✅✅Only reactants/ products involving gas molecules
Graded A+
Why does having an excess reactant help to convert costly reactants into
products? - ✅✅It provides the best chance for the limiting reactant to react
completely.
Normally the cheaper reactant would be the one in excess and can be recycled
back into the reaction to save cost and avoid wasting materials.
What are 3 different transition metals that can act as catalysts, give the names of
the reactions? - ✅✅Iron (Fe), Haber Process
Platinum (Pt), Ostwald Process, Catalytic converters in car exhausts
Nickel (Ni), hardening of veg oils to fats
Why are so many catalysts derived from transition metals? - ✅✅Because the
vacant d orbitals are readily available for (reversible) bonding. (They show
variable oxidation states)
Explain in terms of electron orbitals, how transition metal catalysts lower
activation energy of a reaction? - ✅✅A reactant may be adsorbed (held by
covalent bonds often within the d orbitals of the catalyst) in a suitable orientation
for a more probably successful reaction.
The reactant may be adsorbed in a way that the bonds within the reactant are
weakened which lowers activation energy.
,What is the molar gas volume? - ✅✅The volume of all gases is the same under
the same conditions of temperature and pressure.
One mole of any gas has a volume of approx 24 litres at room temperature
What is the difference between theoretical yield and actual yield? -
✅✅Theoretical - what we expect to obtain from a reaction based on the
number of moles of the limiting reactant and actual - actual amount of product
that forms in a chemical reaction
What are the reasons that a reaction may not achieve a 100% percentage yield? -
✅✅- Mass transfer/ mechanical losses (product may be left in glassware etc)
- Purification of product
- Other side reactions may have occurred with impurities that may be present in
the reaction mixture.
- Many reactions are reversible and a dynamic equilibrium is established,
therefore when the product is formed it can revert back to reactants.
- Insufficient time for the reaction to complete.
What is an empirical formula? - ✅✅An empirical formula is the simplest ratio
of atoms in the compound.
What is enthalpy change? - ✅✅The difference in the heat energies between
the products and the reactants
What is combustion? - ✅✅When one mole of a substance burns completely in
oxygen (kJ per mole)
,What is Hess' Law? - ✅✅The enthalpy change of a chemical reaction depends
only on the chemical nature and physical properties of the reactants and products
and is independent of any intermediate steps.
Why is Hess' Law important? - ✅✅It allows us to calculate enthalpy changes
which cannot be determined in an experiment
What is enthalpy of formation? - ✅✅Enthalpy change when one mole of a
substance is formed from its elements
How do we calculate enthalpy of formation and why? - ✅✅Since enthalpy of
formation is hard to obtain, we use enthalpy of combustion to calculate it
How do we calculate Hess' Law? - ✅✅Step 1) Write out the combustion
equations for each substance
Step 2) Multiply each equation by what is required for it to match the original
Step 3) Flip around the final one as the direction is the wrong way
Step 4) Add up all the corrected enthalpy of combustions to obtain the answer
When do bonds break/form? - ✅✅Bonds break in reactant molecules and form
in product molecules
What is molar bond enthalpy? - ✅✅The enthalpy change when one mole of a
bond in gaseous molecules is broken or formed.
, Is bond breaking endothermic or exothermic? - ✅✅Endothermic
Is bond making endothermic or exothermic? - ✅✅Exothermic
What state does bond making/breaking occur? - ✅✅In the gaseous state
What is the definition of a dynamic equilibrium? - ✅✅When the rates of the
forward and reverse reactions are equal
What is required for a dynamic equilibrium to occur? - ✅✅It only occurs in a
closed system
What is a closed system? - ✅✅When none of the chemicals can escape from
the system
What is Le Chatelier's principle? - ✅✅If a system at equilibrium is subjected to
any change then the system re-adjusts itself to counteract the applied change
What conditions favour an endothermic reaction? - ✅✅Rise in temperature
What conditions favour an exothermic reaction? - ✅✅Low temperatures
When dealing with pressure in equilibriums, what should only be considered? -
✅✅Only reactants/ products involving gas molecules
