7.2 The position of equilibrium

7.2.1 –Deduce the equilibrium constant expression (K_r) from the equation for a homogenous reaction.

Answer the following questions:

a.      What can change the value of K_c -Temperature

b.     The reaction must be at equilibrium for the value of K_c to be calculated

c.      Define the term homogeneous:

A substance or material that contains only one kind of compound or one element can be defined as homogeneous. Homogeneous is Latin for "the same kind". So for an equilibrium reaction, the reactants are homogeneous since they are the same state, so the products will be the same state as the reactants.

Topic 6: Kinetics Test Correction

Which one of the following factors does not affect the rate of a chemical reaction?

• The enthalpy change of the reaction
• The nature of the reactants (Nature means Orientation)
• The concentration of the reactants
• The presence of a catalyst

The rates of many reactions approximately double with a temperature increase of 10˚C. This is best attributed to a doubling of the fraction of molecules that have the energy needed for reaction.

Reactions between aqueous solutions of sodium thiosulfate and acid can be followed by timing the appearance of the solid sulphur that is produced. The time required for the appearance of the sulphur would be increased by which of the following changes?

·        Raising the temperature

·        Diluting the solution (this is because of the appearance of the solid sulphur)

·        Adding a catalyst

·        Increasing the concentration of the sodium thiosulfate

Define what is meant by the rate of reaction. Describe how the rate changes with time for the above reaction, and explain your answer using the Collision Theory:

·        Rate decreases as time increases.

·        The more concentrated the reactants, the more collisions there will be per second per unit volume.

·        As reactants get used up, concentration decreases.

·        This is why rate greatest at start rather than at the end.

·        Exothermic reaction therefore heat given out will also speed up reaction rate due to increasing collision frequency. Also kinetic energy of particles > E_a.

7.1: Dynamic equilibrium

7.1.1 -Outline the characteristics of chemical and physical systems in a state of equilibrium

Definition of equilibrium: At equilibrium, the rate of the forward reaction is equal to the rate of the backward reaction.

Physical System
An example is bromine (placed in a sealed container at room temperature) Since this liquid is volatile, with a boiling point closer to the room temperature.

Diagram shows a layer of bromine gas formed above the liquid bromine. This occurs in both forms as bromine's boiling point is closer to room temperature, some particles have more kinetic energy in order to be in a gas form (evaporation), however when these gas particles collide with the surface of the liquid they lose energy and turn back into liquid (condensation). Therefore Equilibrium is only reached when rate of evaporation is equal to the rate of condensation

Chemical System
Whereas in hydrogen iodide, there are the elements Hydrogen and Iodine. Iodine (released as a purple gas) and hydrogen and hydrogen iodide (colourless gas). Equilibrium has been reached since the rate of reverse reaction is between the Iodine and Hydrogen gas. This is considered an equilibrium mixture

Heating Copper Sulfate (CuSO4) crystals
By heating the blue crystals, the hydrated CuSO4 crystals are converted to anhydrous CuSO4 crystals(turning from blue to white). This is a reversible reaction.

The hydrated (blue) Copper Sulfate crystals

Heating the blue Copper Sulfate

The anhydrous (white) Copper Sulfate

Topic 6: Practice Questions

B -D as as pH can be measured as well

A

D -definition of rate of reaction (the concentration per unit time)

A -B. It is not A because magnesium is excess and the concentration of the hydrochloric acid will affect the reaction (increasing the concentration will speed up the reaction.

D -C because Adding a catalyst: the activation energy decreases as a catalyst offers an alternative route with a different (but lower) activation energy.

D -B because the size of the CaCO3 does not mean the surface area but the actual size. So the surface are is not increased.

C

a). Measuring the mass of reactants (g) using a top pan balance
The Volume of CO₂ being produced, collected in a gas syringe (cm³) and using a stopwatch to measure time (seconds)
Measuring the pH by the using the pH probe of solution and how long it takes to reach 7 (hitting neutralization)

b). Increase in temperature -This will increase the aerage kinetic energy of particles, resulting an increase of collision frequent. Therefore there is a higher number of particles with average kinetic energy > Ea

 Adding a catalyst -An alternative route is provided with lower Activation energy so more particles has the value of average kinetic energy that is higher than Ea so there is more collisions.

Increase of surface are -since there is more particles, the chances is higher for successful collisions

c).  i) Stay the samesince the MgCO3 is excess and HCl is the limiting factor.

ii). Increase. -Stay the same because the concentration of the reactant stays the same so the volume of the product stays the same.

Collision Theory (3)

6.2.5 Sketch and explain qualitatively the Maxwell–Boltzman energy distribution curve for a fixed amount of gas at different temperatures and its consequences for changes in reaction rate.

This graph shows the rate of reaction depends on the proportion of particles that have values of kinetic energy greater than activation energy. –The red line shows that the kinetic energy of the particle is lower, so the green shaded area has more particles than the blue area at the end of the reaction.

Maxwell-Boltzman Distribution

The distribution of the kinetic energy is shown by the Maxwell-Boltzman Distribution. The speed of the particles’ movement show the amount of kinetic energy they possess. The greater the speed of the particles shows they have more kinetic energy. The total of particles remains constant, despite the temperature increasing the area under the curve. The higher velocity the particles move show the higher average kinetic energy.

6.2.6 Describe the effect of a catalyst on a chemical reaction.

A catalyst “is a substance that increases the rate of reaction without itself undergoing permanent damage” –It provides the reaction an alternative route that has a lower activation energy.

In a reaction with a catalyst, without increasing the temperature, a larger number of particles has the value of kinetic energy greater than the activation energy and so will be able to undergo successful collisions.

6.2.7 Sketch and explain Maxwell– Boltzmann curves for reactions with and without catalysts.

Without a catalyst: there are less particles that has the value of kinetic energy greater than the activation energy. Therefore, the rate of reaction will be slower since there are fewer number of particles that will undergo successful collisions.

With a catalyst: there are more particles that have the value of kinetic energy than the activation energy. [activation energy is lower] So the rate of reaction will be faster as a greater number of particles will be able to undergo successful collisions.

6.2: Collision (2)

6.2.4 –Predict and explain, using the collision theory, the qualitative effects of particle size, temperature, concentration and pressure on the rate of a reaction.

1.      What was the independent variable?

The surface area of the marble chips

2.      What was the dependent variable?

The time it takes for the marble chips to react in order to produce 100cm³ of carbon dioxide
3.      What variables were controlled?

The temperature of the room remained the same, the volume of acid used (50cm³) and the mass of the calcium carbonate (around 6.1 grams)
4.      Using collision theory, explain the following the shape of graphs at the start of the reaction.

The shape of the graphs shows that the reaction is an increasing slope.

5.      What does the gradient of the graph at any one point represent?

The gradient shows the volume of carbon dioxide produced over time in seconds.
6.      What are the units for the gradient of the graph?

The unit is cm³ over seconds.
7.      Discuss the reasons for the differences in the shape of the graphs.

The graph shows that the rate of reaction for the marble chip (in powder form) increases since the surface area is more. So the more surface area there is, the higher chances for the particles to collide. It also shows how the smallest-sized chips reacts the slowest. The slow rate of reaction is followed by the medium-sized chips and the large-sized chips. However, it appears the rate of reaction for small-sized chips is faster than the rest.  Decreasing the size of the particle increases the rate of reaction since it has more surface area. There could be some error since it could be that the number of marble chips are not the same in order to keep the mass the same.