Kognity questions 6.1
In a reaction, the concentration of reactant X decreases from 1.0 mol dm-3 to 0.50 mol dm-3 in 10.0 seconds. Calculate the average rate of reaction.
0.050 mol dm-3 s-1 Explanation: Average rate of reaction = change in concentration / time = 0.50 / 10.0 = 0.050 mol dm-3 s-1
Examine the graph below for an experiment involving the collection of a carbon dioxide over time. Trials I and II were for the same reaction, but under different conditions. Calculate the average rate of reaction, in cm3 s−1, for Trial I for the first 5 seconds of the reaction. Give the answer to one significant figure and units are not required in the answer.
3 cm^3 s^-1 15/5 = 3
Which of the following letters represents the activation energy for an endothermic reaction in the presence of a catalyst?
C
Which of the following factors determine whether a collision between reactant particles will be a successful collision? I. particles must collide with kinetic energy ≥ Ea II. particles must collide with correct orientation III. particles must collide with kinetic energy ≤ Ea
I and II only Explanation: Collision theory states that the factors that determine if a collision is successful or not are that the particles must collide with the correct orientation and that the particles must collide with KE ≥ Ea.
would adding a catalyst affect kinetic energy and the change of the curve
The shape of the curve remains the same, since the catalyst does not have an effect on the amount of kinetic energy present in the reacting particles.
Deduce the initial rate of reaction, at t = 0 s, in mol dm-3 s-1, given the graphical data of the concentration of reactant over time.
0.020 mol dm-3 s-1 Explanation: In order to calculate the instantaneous rate of reaction, the gradient of the tangent line at that particular instant in time needs to be calculated. For the initial rate of reaction, draw a tangent line at t = 0 s and extend the line outwards; the two points that fall along the tangent line are (0, 1.0) and (10, 0.8) Calculating the gradient: GradientGradientGradient=y2−y1x2−x1=1.0−0.80−10=−0.020mol dm−3 s−1 Gradient=y2−y1x2−x1Gradient=1.0−0.80−10Gradient=−0.020mol dm−3 s−1 Since the reactant concentration is decreasing the rate can be interpreted as a positive value (concentration of reactant decreases by 0.020 mol dm-3 each second as the initial, instantaneous rate).
How is the rate of reaction affected by an increase in temperature compared with an increase in concentration?
An increase in temperature increases both the frequency of collisions and the average kinetic energy of the particles, whereas an increase in concentration only increases the frequency of collisions. Explanation: An increase in temperature causes an increase in the average kinetic energy of the particles and an increase in the frequency of collisions between reactant particles. An increase in concentration causes more frequent collisions, as the particles are closer together, but does not increase the average kinetic energy of the particles.
Which of the following factors causes an increase in the rate of a reaction when the temperature is increased?
An increase in the number of particles with KE ≥ Ea Explanation: Temperature is proportional to the average kinetic energy of the particles. Increasing the temperature increases the average kinetic energy, therefore, more particles have KE ≥ Ea which leads to an increase in the rate of reaction.
According to collision theory, which of the following factors affect the rate of reaction? I. Number of particles with KE ≥ Ea II. The geometry of collisions III. The frequency of collisions
I, II and III Explanation: Collision theory states that the factors that affect the rate of reaction are the geometry of collisions, that the particles collide with KE ≥ Ea, and the frequency of collisions.
Which of the following change(s) for a catalysed reaction? I. The activation energy for the forward reaction only. II. The activation energy for both the forward and reverse reactions. III. The enthalpy change for both the forward and reverse reaction.
II only Explanation: A catalyst decreases the activation energy for both the forward and reverse reactions (by providing an alternative reaction pathway with a lower activation energy), but does not affect the enthalpy of either reaction.
An experiment was conducted for the reaction below between excess calcium carbonate and100.0 cm3 of hydrochloric acid: CaCO3(s) + 2HCl (aq) → 2CaCl2(aq) + H2O(l) + CO2(g) Which factor(s) would cause a greater decrease in mass? I. Conducting the reaction at a higher temperature II. Using the same volume of HCl but with a higher concentration III. Sealing the flask
II only Explanation: When using a change in mass to measure the rate of reaction involving the production of a gas, the mass will decrease if the reaction is conducted in an open container, so sealing the flask and trapping the gas inside would result in no mass changes. Conducting the reaction at a higher temperature will result in the same mass change, but more rapidly. Using a high concentration and same volume of HCl, the limiting reagent, will cause a greater quantity of carbon dioxide to be produced, therefore a greater mass change will be observed.
Which of the following changes will not lead to an increase in the rate of reaction of a gas phase reaction occurring in a high pressure reaction vessel?
Increasing the volume of the reaction vessel Explanation: The following lead to an increase in the rate of reaction: decreasing the volume of the reaction vessel will increase the pressure inside the vessel by increasing the rate of collisions between the gas particles and the sides of the vessel and, consequently, between the gas particles themselves injecting more reactants into the existing reaction vessel will increase the concentration of the reactants heating the reaction vessel - albeit highly dangerous - will speed up the reaction by increasing the average kinetic energy of the gas particles Increasing the volume of the reaction vessel will decrease the pressure inside the vessel by decreasing the rate of collisions between the gas particles and the sides of the vessel and, consequently, between the gas particles themselves. This results in a slower rate of reaction.
How will the presence of a catalyst affect the Maxwell Boltzmann distribution curve below?
The shape of the curve remains the same, but the activation energy shifts to the left, increasing the fraction of particles that have energy equal to, or greater, than the activation energy. Explanation: The presence of a catalyst provides an alternate reaction pathway with a lower activation energy, shifting the level of Ea on the graph to the left. The shape of the curve remains the same, since the catalyst does not have an effect on the amount of kinetic energy present in the reacting particles. As shown in the diagram below, the newly shaded area shows a greater number of particles that have energy equal, or greater to, the activation energy of the catalysed reaction, increasing the proportion of effective collisions and the rate.
Examine the graph below for an experiment involving the collection of carbon dioxide gas over time according to the reaction equation below. Na2CO3(aq)+2HCl(aq)→2NaCl(aq)+H2O(l)+CO2(g)Na2CO3(aq)+2HCl(aq)→2NaCl(aq)+H2O(l)+CO2(g) Trials I and II were for the same reaction, but under different conditions. Identify the correct statement below. (only the green and red lines)
Trial I was conducted with a higher concentration of acid but with the same amount of the limiting reactant. Explanation: Trial I has a greater initial rate of reaction compared with Trial II (because of the higher concentration of acid), but the average rates of both experiments are the same, indicating that the same quantity of gas had been produced over time. Therefore, both trials must have had the same quantity of the limiting reagent.
Copper(II) chloride forms a blue coloured solution whereas a solution of magnesium chloride is colourless. The two react together as shown: Mg (s) + CuCl2 (aq) → MgCl2 (aq) + Cu (s) A simple method to monitor the rate of this reaction is to monitor the change in the ______ of the copper(II) chloride solution over time.
concentration
As a reaction progresses, the rate of reaction _______ over time because the frequency of collisions between reactant particles decreases. When examining the effect of a factor on rate of reaction, the _______ rate of reaction is used for comparison, rather than the average rate.
decreases, instantaneous
The rate of a chemical reaction can be expressed as Δ[R]/tΔ, where R is a reactant. Which are the correct units for this expression?
mol dm-3 s-1 Explanation: Δ[R]/Δt= concentration/time = moldm−3/s= moldm−3s−1
A catalyst increases the rate of reaction as it:
provides an alternative pathway of lower activation energy so more particles have E ≥ Ea Explanation: A catalyst provides an alternative pathway of lower activation energy so more particles then have E ≥ Ea. This results in an increase in the frequency of successful collisions between reactant particles and a higher rate of reaction.
In examining the Maxwell-Boltzmann distribution curves at different temperatures on the graph below, which variable in the graph remains the same?
the area under each curve Since the number of particles remains constant, the area under each curve will also remain the same. The change of shape to the curve occurs because the distribution of particles at different amounts of energy will change, but the area under the curve, representing the total number of particles, does not change
Which of the following is the best definition of activation energy?
the minimum amount of energy that particles must possess for a collision that results in a chemical reaction Explanation: Activation energy (Ea) is defined as the minimum amount of energy that particles must possess for a collision to result in a chemical reaction. It can be thought of as the energy barrier to the reaction; only particles with kinetic energy equal to, or greater than, the activation energy will react.
If the pressure of a gaseous reaction is increased, what effect will it have on the rate of reaction?
the rate of reaction increases as the particles are closer together Explanation: There are more particles in the same volume, therefore, they are more likely to collide and so there are more collisions per time period. It is effectively the same as increasing the concentration of a reactant in reactions taking place in solution.
