Question
Define the term activation energy, \({E_{\text{a}}}\).
State two conditions necessary for a reaction to take place between two reactant particles.
Sketch an enthalpy level diagram to describe the effect of a catalyst on an exothermic reaction.
Answer/Explanation
Markscheme
(minimum) energy needed for a reaction to occur / (minimum) energy difference between reactants and transition state;
particles must collide;
appropriate collision geometry/orientation;
\(E \geqslant {E_{\text{a}}}\);
Diagram showing:
correct labelling of axes (enthalpy/H/(potential) energy for y-axis and time/progress/course of reaction/reaction coordinate for x-axis) and H (products) line shown below H (reactants) line;
correct labelling of the two curves, catalysed and uncatalysed;
correct position of \({E_{\text{a}}}\) shown with lines for a catalysed and uncatalysed reaction;
the correct label \(\Delta H\) /change in enthalpy;
Do not penalize if reactants and products are not labelled.
If an endothermic reaction is shown, award [2 max] if all other parts are shown correctly.
Examiners report
Most candidates gave the correct definition of activation energy in (a).
The two conditions needed for a reaction to take place were given by the majority of candidates.
In (c) some of the enthalpy level diagrams had many labels missing. Axes weren‟t always labelled, one of them was wrongly labelled as delta H, and the curves of Ea with and without catalyst were not properly indicated. A few answers showed an endothermic reaction instead.
Question
Factors that affect the rate of a chemical reaction include particle size, concentration of reactants and the temperature of the reaction.
Propan-1-ol and propan-2-ol are two structural isomers of \({{\text{C}}_{\text{3}}}{{\text{H}}_{\text{8}}}{\text{O}}\).
Define the term rate of a chemical reaction.
List the three characteristic properties of reactant particles which affect the rate of reaction as described by the collision theory.
On the axes below sketch two Maxwell-Boltzmann energy distribution curves for the same sample of gas, one at a temperature \(T\) and another at a higher temperature \(T’\). Label both axes. Explain why raising the temperature increases the rate of a chemical reaction.
Explain why coal dust burns much faster than a large piece of coal with the same mass.
State the equation for the complete combustion of \({{\text{C}}_{\text{3}}}{{\text{H}}_{\text{8}}}{\text{O}}\).
Both propan-1-ol and propan-2-ol can be oxidized in aqueous solution by potassium dichromate(VI). State any necessary conditions for the oxidation to occur and describe the colour change during the oxidation process.
State the name(s) and structure(s) of the organic product(s) that can be formed when each of the alcohols is oxidized and suggest why one of the alcohols gives two organic products and the other only gives one organic product.
Answer/Explanation
Markscheme
increase in concentration of product per unit time / decrease in concentration of reactant per unit time;
Accept change instead of increase/decrease and mass/amount/volume instead of concentration.
frequency of collisions;
kinetic energy/speed of reactant particles;
collision geometry/orientation;
correctly labelled axes showing number of particles/frequency against (kinetic) energy;
correctly shaped graph for \(T\) (curve must not touch or cross x axes);
\(T’\) curve to the right of \(T\) and with a peak lower than \(T\);
increasing the temperature increases the (kinetic) energy of the particles / more particles will possess the necessary activation energy;
there will be more collisions per unit time / the frequency of collisions increases / there are more successful collisions;
the dust has a greatly increased surface area / more of the coal can come into contact with the oxygen molecules when it is in dust form / OWTTE;
\({{\text{C}}_3}{{\text{H}}_8}{\text{O}} + {\text{4}}\frac{1}{2}{{\text{O}}_2} \to {\text{3C}}{{\text{O}}_2} + {\text{4}}{{\text{H}}_2}{\text{O}}/{\text{2}}{{\text{C}}_3}{{\text{H}}_8}{\text{O}} + {\text{9}}{{\text{O}}_2} \to {\text{6C}}{{\text{O}}_2} + {\text{8}}{{\text{H}}_2}{\text{O}}\)
Award [1] for correct products and reactants and [1] for correct balancing.
Ignore state symbols.
acidic solution / \({{\text{H}}^ + }\) / sulfuric acid;
warm / heat / reflux;
(the solution changes) from orange to green;
\({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{CHO}}\) and propanal;
\({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{COOH}}\) and propanoic acid;
\({\text{C}}{{\text{H}}_3}{\text{COC}}{{\text{H}}_3}\) and propanone/acetone;
Award [1] for 2 or 3 correct names or structures, award [2] for 4 or 5 correct names or structures.
propan-1-ol gives propanal and propanoic acid and propan-2-ol gives propanone;
propan-1-ol has two H atoms bonded to the C containing the –OH whereas propan-2-ol only has one / propan-1-ol is a primary alcohol and propan-2-ol is a secondary alcohol;
Examiners report
This question began with kinetics and although many did well, there were also a lot of marks lost. Some did not have a correct definition of rate of reaction and many misread the question that asked for the properties of reactant particles that affect rate. Many candidates talked about surface area, concentration etc as opposed to collision frequency, collision geometry and reactant particle kinetic energy. The Maxwell-Bolzmann energy curves were drawn very badly and even candidates who could do it lost marks for the sloppy drawing of the curves e.g curves did not start at the origin or they crossed the x axis.
Also candidates could not label the axes correctly. However most could suggest that coal dust burns faster as it has a larger surface area.
This question began with kinetics and although many did well, there were also a lot of marks lost. Some did not have a correct definition of rate of reaction and many misread the question that asked for the properties of reactant particles that affect rate. Many candidates talked about surface area, concentration etc as opposed to collision frequency, collision geometry and reactant particle kinetic energy. The Maxwell-Bolzmann energy curves were drawn very badly and even candidates who could do it lost marks for the sloppy drawing of the curves e.g curves did not start at the origin or they crossed the x axis.
Also candidates could not label the axes correctly. However most could suggest that coal dust burns faster as it has a larger surface area.
This question began with kinetics and although many did well, there were also a lot of marks lost. Some did not have a correct definition of rate of reaction and many misread the question that asked for the properties of reactant particles that affect rate. Many candidates talked about surface area, concentration etc as opposed to collision frequency, collision geometry and reactant particle kinetic energy. The Maxwell-Bolzmann energy curves were drawn very badly and even candidates who could do it lost marks for the sloppy drawing of the curves e.g curves did not start at the origin or they crossed the x axis.
Also candidates could not label the axes correctly. However most could suggest that coal dust burns faster as it has a larger surface area.
This question began with kinetics and although many did well, there were also a lot of marks lost. Some did not have a correct definition of rate of reaction and many misread the question that asked for the properties of reactant particles that affect rate. Many candidates talked about surface area, concentration etc as opposed to collision frequency, collision geometry and reactant particle kinetic energy. The Maxwell-Bolzmann energy curves were drawn very badly and even candidates who could do it lost marks for the sloppy drawing of the curves e.g curves did not start at the origin or they crossed the x axis.
Also candidates could not label the axes correctly. However most could suggest that coal dust burns faster as it has a larger surface area.
Part (b) was based on organic chemistry and most candidates knew that the products of combusting propan-2-ol were carbon dioxide and water- although few could balance the equation correctly. In the next part of the question the colour change from orange to green was well known, but the necessary conditions of reflux and acidifying the dichromate were not. The final part of this question was often done very well and many candidates could draw the structures of the 3 oxidation products and name them.
Part (b) was based on organic chemistry and most candidates knew that the products of combusting propan-2-ol were carbon dioxide and water- although few could balance the equation correctly. In the next part of the question the colour change from orange to green was well known, but the necessary conditions of reflux and acidifying the dichromate were not. The final part of this question was often done very well and many candidates could draw the structures of the 3 oxidation products and name them.
Part (b) was based on organic chemistry and most candidates knew that the products of combusting propan-2-ol were carbon dioxide and water- although few could balance the equation correctly. In the next part of the question the colour change from orange to green was well known, but the necessary conditions of reflux and acidifying the dichromate were not. The final part of this question was often done very well and many candidates could draw the structures of the 3 oxidation products and name them.