Higher Chemistry - Traffic Lights
Unit 3 CHEMISTRY IN SOCIETY
I know
Getting the most from reactants
1. The chemical industry makes a huge contribution to our quality of life.
2. When making new products, industrial chemists have to think about the most
economical way to make them in order to maximize profit.
3. Key considerations when making a new product are: availability, sustainability
and cost of feedstocks; product yield; marketability of by-products; recyclability
of reactants; energy requirements; use of processes that reduce or eliminate
the use and production of hazardous substances (green chemistry).
4. The key points in green chemistry are: waste prevention; safer product design
and products; use of renewable feedstocks; fewer steps in a process; use of
more specific catalysts.
5. The percentage yield is a way of measuring the efficiency of a chemical
process: percentage yield (%) =
actual yield
x 100
theoretical yield
6. Atom economy is a measure of the efficiency of a reaction. The proportion of
reactant atoms that end up in a useful product are compared to the number
ending up as waste.
7. Atom economy (%) = mass of desired product x 100
total mass of reactants
8. The mole ratio of atoms/ionic units in a compound can be worked out from
chemical formula.
9. The mole ratio of reactants and products can be calculated from balanced
equations.
10. Masses/moles reacting and produced can be calculated from mole ratios in
balanced equations.
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11. If the cost of feedstocks and the percentage yield are known for a reaction, the
cost of the reactants can be calculated.
12. Masses, moles, concentrations and volumes of solutions reacting and being
produced can be calculated from mole ratios in balanced equations.
13. Reactants in excess and the limiting reactant can be calculated from mole ratios
in balanced equations.
14. One mole of gas occupies the same volume when measured at the same
temperature and pressure and is known as molar volume (Vm).
15. Molar volume is measured in litres mol-1.
16. The volume of a gas can be calculated from the number of moles and vice
versa, given the molar volume.
17. Volumes of reactant and product gases can be worked out from mole ratios in
the balanced equations.
Equilibria
1.
Many reactions are reversible.
2.
Reversible reactions attain a state of dynamic equilibrium when the rates of the
forward and backward reactions are equal.
3.
Dynamic equilibrium will only be attained in a closed system where no products
can escape or reactants be added.
4.
At equilibrium, the concentration of reactants and products remain constant, but
rarely equal.
5.
Whenever a change is imposed on an equilibrium mixture, the position of
equilibrium will always move to offset the change.
6.
Increasing the concentrations of reactants shifts the equilibrium to the right
(products).
7.
Increasing the pressure shifts the equilibrium in the direction which results in a
decrease in the number of moles of gas reactant or product.
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8.
Increasing the temperature shifts the equilibrium in the direction of the
endothermic reaction (H = positive).
9.
Catalysts do not shift the equilibrium position but allow it to be reached quicker.
10.
In industry, reaction conditions are changed to maximize product yield at the
lowest cost.
11.
In industrial reactions, a balance has to be reached between shifting the
equilibrium position towards products and reaching the equilibrium position as
quickly as possible.
Chemical energy
1. It is essential that industrial chemists can calculate the amount of heat given out
or taken in by each chemical reaction in a chemical process.
2. Endothermic reactions require heat to be supplied, which incurs cost.
3. The heat produced during exothermic reactions often needs to be removed to
avoid the temperature rising too far.
4. The heat produced during exothermic reactions is not wasted and can be used
elsewhere in a process, e.g. to heat catalysts or produce steam to drive the
pumps needed to compress gases.
5. The enthalpy of combustion is the enthalpy change when 1 mole of substance
burns completely in oxygen.
6. The enthalpy of combustion is always exothermic.
7. The heat energy released, Eh, or taken in, during an enthalpy experiment can
be calculated using Eh = c x m x T, where m = mass of water (kg); c = specific
heat capacity of water (4.18 kJ kg-1 oC-1); T= change in temperature.
8. The enthalpy of a reaction can then be calculated using H = Eh /n, where n is
the number of moles of stuff e.g. burned or dissolved.
9. The units of enthalpy are kJ mol-1.
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10. Enthalpies of formation can be used to calculate enthalpies of other reactions.
11. Hess’s Law can be used to calculate enthalpy changes.
12. Hess’s Law states that the enthalpy change for a chemical reaction is
independent of the route taken from reactants to products, so long as the
starting and finishing conditions are the same for each route.
13. For a diatomic molecule, XY, the molar bond enthalpy is the energy required to
break 1 mole of XY bonds.
14. For a diatomic molecule, XY, the molar bond enthalpy is the energy released
when 1 mole of XY bonds form.
15. The mean molar bond enthalpy is the average bond enthalpy of bonds which
exist in different molecular environments, e.g. C-H.
16. The difference between the energy needed to break bonds in gaseous
reactants and the energy released when new bonds are made gives an
estimate of the enthalpy change for the reaction.
.
Oxidising and reducing agents
1. An oxidising agent is a substance which accepts electrons thus letting
something else lose electrons.
2.
A reducing agent is a substance which donates electrons, thus allowing
something else to be reduced.
3. Oxidising agents are reduced; reducing agents are oxidised.
4. Elements can act as reducing and oxidising agents.
5. Elements with low electronegativity values, like the group 1 metals, are strong
reducing agents.
6. Elements with high electronegativity values, like the group 7 elements, are
strong oxidising agents.
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7. Reduction and oxidation ion-electron equations involving elements can be
written.
8. Compounds and group ions can act as oxidising and reducing agents.
9. Reduction and oxidation ion-electron equations can be written for reactions
involving compounds and group ions.
10. Redox ion-electron equations can be written by combining reduction and
oxidation equations.
11. The reduction and oxidising agents in a redox equation can be recognized.
12. Hydrogen peroxide, acidified permanganate and acidified dichromate solutions
are strong oxidising agents.
13. The oxidising property of hydrogen peroxide makes it useful as a bleach to
remove colour from materials.
14. The oxidising property of the permanganate ion makes it effective in killing fungi
and bacteria and it can inactivate viruses.
15. The electrochemical series lists substances in order of how well they act as
reducing/oxidising agents.
16. Strong reducing agents are found at the top right of the electrochemical series.
17. Strong oxidising agents are found at the bottom left of the electrochemical
series.
18. Ion-electron equations in the electrochemical series are written as reductions
but can be reversed to obtain the oxidation equation.
19. The position of substances in the electrochemical series can be used to predict
whether a redox reaction will take place.
20. If an oxidising agent is below the reducing agent in the electrochemical series, a
reaction will occur.
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Chemical analysis
1. There is a range of chromatography techniques including paper, thin layer, gas
and gas-liquid chromatography.
2. Size and polarity of molecules are important factors in separating substances
by chromatography.
3. Rf values can be calculated by dividing the distance travelled by a component
by the distance travelled by the solvent front.
4. Volumetric analysis is a quantitative analysis technique.
5. A standard solution is a solution of accurately known concentration.
6. Volumetric analysis uses an accurately known concentration of one substance
to find the concentration of another.
7. Volumetric analysis includes the use of acid/base titrations and redox titrations.
8. The end point of a titration is the point where the reaction has just reached
completion.
9. An indicator is a substance that changes colour at the endpoint of a reaction.
10. In redox reactions, some substances, e.g. potassium permanganate and
potassium dichromate, can act as self-indicators.
Researching chemistry
1. Skills of scientific inquiry include: researching current literature; carrying
out practical investigative work; communicating findings clearly.
2. You should know the correct use of: conical flask; beaker; measuring cylinder;
delivery tubes; dropper; test tubes/boiling tubes; evaporating basin; pipette
with safety filler; burette; volumetric flask; funnel; thermometer.
3. Knowledge of: filtration; distillation; use of a balance; titration; methods for the
collection of a gas over water using a gas syringe; safe methods for heating;
Bunsen burners; water baths/heating mantles.
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