7.2.2 I can deduce the extent of a reaction from the magnitude of Kc

What does the word magnitude mean?

Magnitude is the size of something.

Explain why the three reactions below do not have units for Kc?

The three reactions do not have a unit for Kc because the sum of the number of moles of the products equals the sum of the number of moles of the reactants. 

Deduce the extent of the reaction if Kc is
a. Significantly larger than 1

The reaction is considered to go almost to completion (very high conversion of reactants into products).

b. Between 0.01 and 100

Both the reactants and products are present and are in significant amounts.

c. Extremely small

The reaction hardly proceeds (very low conversion of reactants into products).

7.2.1 I can deduce the equilibrium constant Kc for homogenous reactions

What can change the value of the equilibrium constant? 

The only thing that changes the value of the equilibrium constant for a reaction is the temperature.

The reaction must be at ________ for the value of the equilibrium constant to be calculated. 

Equilibrium - when the concentrations used in the equation are the equilibrium concentrations for all reactants and products.

Define the term homogeneous 

A mixture which has uniform composition and properties throughout.

The Units Of Kc

aA + bB <----------> cC + dD

If (c+d) - (a+b) = 0     then there are no units

If (c+d) - (a+b) = 1     then Kc -----> mol dm^-3

If (c+d) - (a+b) = -1     then Kc -----> mol^-1 dm^3

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

Dynamic Equilibrium

Physical Systems

When a liquid has reached its boiling point a significant number of particles will have enough energy to escape from the liquid state and form vapour by evaporating.

At the same time, some of these vapour molecules will collide with the surface of the liquid, lose energy and become liquid by condensing.

There will come a time when the rate of evaporation is equal to the rate of condensation and at this point there's no net change in the amounts of liquid and gas present. The system has reached equilibrium.

Chemical Systems

In a chemical system we are dealing with reversible reactions. There's the forward reaction (reactants to products), backward reactions (products to reactants) and the reverse reaction.

When a chemical system reaches equilibrium the reverse reaction equal the backward reaction and there's no net change observed even though both reactions are still occurring. The concentrations of both reactants and products remain constant over time and this is referred to as the equilibrium mixture.

Characteristics Of The Equilibrium State

At equilibrium state the rate of the forward reaction is equal to the rate of the backward reaction.

Feature of equilibrium state:

• Equilibrium is dynamic

The reaction hasn't stopped but both forward and backward reactions are still occurring.

• Equilibrium is achieved in a closed system

A closed system prevents exchange of matter with the surroundings, so equilibrium is achieved where both reactants and products can react and recombine with each other.

• The concentrations of reactants and products remain constant at equilibrium

They are being produced and destroyed at an equal rate.

• At equilibrium there's no change in macroscopic properties

This refers to observable properties such as colour and density. These don't change as they depend on the concentrations of the components of the mixture.

• Equilibrium can be reached from either direction

The same equilibrium mixture will result under the same conditions, no matter whether the reactions is started with all reactants, all products, or a mixture of both.

Even though the concentrations of reactant and product are constant and equilibrium, this doesn't imply that they're are equal. Most commonly there will be a higher concentration of either reactant or product in the equilibrium mixture, depending both on the reaction and on the conditions. 

The proportion of reactant and product in the equilibrium mixture is referred to as its equilibrium position. Reactions where the mixture contains more products are said to 'lie to the right', and reactions with more reactants are said to 'lie to the left'. 

Kinetics Heinemann Questions

Pg. 126 SL Chemistry Heinemann

1. The reaction between calcium carbonate and hydrochloric acid, carried out in an open flask, can be represented by the following equation:

CaCO2(s) + 2HCl(aq) ----------> CaCl2(aq) + H2O(l) + CO2(g)

Which of the measurements below could be used to measure the rate of reaction?

I   The mass of the flask and contents

II  The pH of the reaction mixture

III The volume of carbon dioxide produced 

Answer: I, II, III 

The pH of the reaction mixture could be used to measure the rate of reaction because the mixture will go from acidic to neutral, and the rate at which this happens can be measured. 

2. For a given reaction, why does the rate of reaction increase when the concentration of the reactants are increased? 

The frequency of the molecular collisions increases.

3. Based on the definition for the rate of reaction, which units are used for a rate?

mol dm-3 time -1

4. Excess magnesium was added to a beaker of aqueous hydrochloric acid on a balance. A graph of the mass of the beaker and contents was plotted against time (line 1)

What change in the experiment could give line 2?

I   The same mass of magnesium but in smaller pieces 

II  The same volume of a more concentrated solution of hydrochloric acid

III A lower temperature 

Answer: II only

5. The rate of a reaction between two gases increases when the temperature is increased and a catalyst is added. Which statements are both correct for the effect of these change son the reaction?

Increasing the temperature - Activation energy doesn't change

Adding a catalyst - Activation energy decreases 

6. Consider the reaction between solid CaCO3 and aqueous HCl. The reaction will be speeded up by an increase in which of the following conditions?

I   Concentration of HCl

II  Size of the CaCO3 particles

III Temperature 

Answer: I and III only

An increase in the size of the CaCO3 particles will slow down the reaction as there will be a smaller surface area of one of the reactants. 

7. Which of the following is important in determining whether a reaction occurs. 

I   Energy of the molecules

II  Orientation of the molecules 

Answer: Both I and II 

11. When excess lumps of magnesium carbonate are added to dilute hydrochloric acid the following reaction takes place:

MgCO3(s) + 2HCl(aq) ----------> MgCl2(aq) + CO2(g) + H2)(l)

(a) Outline two ways in which the rate of this reaction could be studied. 

The volume of carbon dioxide produced could be measured consistently per unit of time by using a gas syringe or an inverted measuring cylinder filled with water. A stop clock would be used to keep track of the time. 

The change of mass of the reactants could be measured consistently per unite of time as the reactions carries out by placing the reactants in a conical flask on a top pan balance. A stop clock is used to record the time. 

(b) State and explain three ways in which the rate of reaction could be increased. 

Increasing the temperature increases the frequency of collisions and the energy of the reacting molecules. 

Increasing the concentration of HCl will increase the frequency of collisions as there would be more reactant HCl particles. 

Using small pieces of solid magnesium carbonate will increase the surface area and result in an increase in the frequency of collisions as reacting particles will have more area to collide with each other. 

(c) State and explain whether the total volume of carbon dioxide produced would increase, decrease or stay the same if: 

(i) More lumps of magnesium carbonate were used.

The volume of carbon dioxide would stay the same as the same volume of HCl is used and it acts as a limiting factor.

(ii) The experiments were carried out at a higher temperature.

The volume of carbon dioxide would stay the same as the amount of reactants are used.

Kinetics Objectives 6.2.5 to 6.2.7

6.2.5 Sketch and explain qualitatively the Maxwell-Boltzmann energy distribution curve for a fixed amount of gas at different temperature and its consequences for changes in reaction rate

The Maxwell-Boltzmann Distribution Curve

The fact that particles in a gas at a particular temperature show a range of values of kinetic energy is express by the Maxwell-Boltzmann distribution curve.

This shows the number of particles that have a particular value of kinetic energy (or the probability of that value occurring) plotted against the values for kinetic energy. The area under the curve represents the total number of particles in the sample.

6.2.6 Describe the effect of a catalyst on a chemical reaction

A catalyst is a substance that increases the rate of a chemical reaction without itself undergoing permanent change.

Most catalysts work by providing an alternate route for the reaction, which has lower activation energy.

This means that 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 catalyst

Without a catalyst there are a smaller number of particles that has the value of kinetic energy greater than the activation energy. This means that a fewer number of particles will able to undergo successful collisions and the rate of reaction will be slower.

With a catalyst the activation energy is lowered and there is therefore a greater number of particles that has the value of kinetic energy than the activation energy. This means that a greater number of particles will be able to undergo successful collisions and the rate of reaction will be faster.


This can be seen in the reaction of aluminium and iodine catalyzed using water, shown in this video:

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

Factors affecting rate of reaction:

Temperature

Increasing the temperature increases the rate of all reactions. This is because temperature is a measure of the average kinetic energy of the particles and so a higher temperature represents an increase in their average kinetic energy. This means that a larger number of particles will have energies exceeding the activation energy. 

An increase in temperature results in:

• An increase in collision frequency
• More collisions involving particles with higher values of kinetic energy, specifically higher than the activation energy

An increase in the number of successful collisions and hence and increase in the rate of reaction. 

Many reactions double their reaction rate for every 10K increase in temperature. 

Changes in temperature affect collision theory but it should be understood that this is less significant than an increase in the number of particle with sufficient energy (Ea)

Concentration

Increasing the concentration of reactants increases the rate of reaction. This is because as concentration increases, the frequency of collisions between reactant particles increase, so that the frequency of successful collisions also increases. 

The effect of concentration can be seen by following the rate of a reaction as it progresses. As reactants are used up, their concentration falls and the rate of the reaction decreases, giving a typical rate curve. 

Changes in concentration affect the collision frequency only.

Particle Size

Decreasing the particle size increases the rate of reaction. This is because subdividing a large particle into smaller parts increases the total surface area and therefore allows more contact and a higher probability of collisions between reactants. 

In reactions involving solutions, stirring may help to decrease particle so and so increase the rate. 

Pressure

For reactions involving gases, increasing pressure increases the rate of reaction. This is is because the higher pressure compresses the gas, effectively increasing its concentration - this will increase the frequency of collisions.

The Harber process illustrates the effect of high pressure on reactions that involve gaseous reactants

Additional Notes:

• Light (EMR) can promote some reactions e.g. dye fading, photosynthesis, skin tanning, methane/chlorine explosion, skin cancer, photography, vitamin D in skin and hydrogen peroxide photodecomposition (it is kept in brown bottles)

Calcium Carbonate Chips & Hydrochloric Acid Reaction

Independent variable: The surface area of the marble/calcium carbonate chips used

Dependent variable: The volume (cm³) of carbon dioxide produced  

Control variables:

• Volume of HCl acid used
• The concentration of the HCl acid used
• Mass of calcium carbonate used 
• The volume of carbon dioxide produced was noted every 10 seconds 
• The temperature of the water and surroundings

The graph generally shows that the smaller the size of the calcium carbonate chips, the faster the rate of reaction. This is because with smaller calcium carbonate chips there is an increase in the total surface area, which allows more contact with the HCl particles and therefore a higher probability of collision between the two reactants. 

The smallest calcium carbonate chips shows an extremely fast production of carbon dioxide gas in a short amount of time, while chips #4 (the largest calcium carbonate chips) line on the graph has a much smaller gradient as it produces less gas in a greater amount of time. This correlates with the idea that the smaller the particle size, the faster the rate of reaction. However, the powdered calcium carbonate shows a much smaller gradient than expected and produces less gas in a longer amount of time when it was expected to have the fastest rate of reaction. This may be because the powder sticks together causing the pieces to act as one large clump of calcium carbonate, therefore decreasing the total surface area which allows less contact with the HCl particles. 

The marble chips also came from different sources and this could mean that there are different impurities mixed in with the marble. The level of these impurities aren't controlled and could account for the differences in the rate of reaction.

The gradient of the graph at any one point represents the rate of reaction, measured in centimeters cubed of carbon dioxide produced per second (cm³/sec). 

6.2.2. & 6.2.3. I can define activation energy and describe collision theory using three factors which affect the rate of reaction

Definition of activation energy	Minimum value of kinetic energy which particles must have before they're able to react.
Three factors that affect the rate of reaction	1. Collision frequency
	2. Number of particles with a greater kinetic energy than the activation energy
	3. Collision geometry or orientation

6.2.2.   Define the term activation energy

Activation energy is defined as the minimum value of kinetic energy which particles must have before they're able to react.

6.2.3.    Describe the collision theory

The rate of reaction will depend on the frequency of collisions which occur between particles possessing both:

• Values of kinetic energy greater than the activation energy
• Appropriate collision geometry

When reactants are laced together, the kinetic energy that the particles possess causes them to collide with each other. The energy of these collisions results in some bonds between the reactants being broken and new bonds being formed. 

The rate of the reaction will depend on the number of collisions between particles which are successful - which lead to the formation of products. Not all collisions will be successful and there are two reasons for this: energy of collision and geometry of collision. 

1. Energy Of Collision

The particles must have a certain minimum value for their kinetic energy in order for a collision to lead to a reaction. This energy is necessary to overcome repulsion between molecules and to break some bonds in the reactants before they can react. 

When this energy is supplied, the reactants achieve the transitions state from which products can form. The energy required represents an energy barrier for the reaction and is known as the activation energy.

The value of activation energy varies greatly from one reaction to another and the magnitude of this value plays an important part in determining the overall rate of reaction. The rate of the reaction depends on the proportion of particles that have values of kinetic energy greater than the activation energy.

2. Geometry Of Collision

Because collisions between particles are random, they're likely to occur with the particles in many different orientations. This can be crucial in determining whether or not the collisions will be successful and therefore what proportion of collisions will lead to a reaction.