QCEChemistry

Chemical equilibrium

recognise that chemical systems may be open (allowing matter and energy to be exchanged with the surroundings) or closed (allow energy, but not matter, to be exchanged with the surroundings)

- An open system is a reaction vessel that has no lid, meaning that reactants or products can be lost to the atmosphere

-  A closed system is a reaction vessel that is closed off, or has a lid, meaning that no reactant or product particles can escape

• In open systems, matter and energy can be exchanged with the surroundings (steady state)

• In closed systems, energy can be exchanged with surroundings, but not matter (dynamic equilibrium)

understand that physical changes are usually reversible, whereas only some chemical reactions are reversible

- An irreversible reaction is one where the reactants form products and these products cannot be converted back to reactants

- A reversible reaction is one where products can react together to re-form reactants

Explanation: Why are physical changes usually reversible, whereas only some chemical reactions are reversible?

A physical change are changes affecting the form of a chemical substance, but not their chemical composition, that is there is no rearrangement of bonds. Chemical changes occur when a substance combines with another to form a new substance and changes their chemical properties or composition.

Physical changes are usually reversible because the molecules in the substance do not change, meaning the changes can be easily reversed. A clear example is when salt is dissolved in water. When dissolved, salt's physical state changes from solid to aqueous with cations splitting form anions. However, if left outside in an open system, the water will evaporate, forcing the anions and cations to recombine.

Only some chemical changes are reversible because a chemical change involves the formation and breakage of chemical bonds. For a chemical reaction to occur, sufficient amount of energy to exceed the activation energy is required. As the activation energy for a reverse reaction may be significantly higher than for the forward reaction, the reaction would not be reversible, or be difficult to reverse. Additionally, some reactions have products that may escape the system, or reverse pathways that are not possible.

appreciate that observable changes in chemical reactions and physical changes can be described and explained at an atomic and molecular level

- Changes in chemical reactions and physical changes can be measured through volumetric analysis, chromatography or spectroscopy

- If the equilibrium reaction involves acids or base, the change in pH can be measured

- Observation - such as colour change or temperature - can be used to identify chemical and physical changes

symbolise equilibrium equations by using = in balanced chemical equations

Example: Equilibrium system inside water bottle

- Equilibrium equation: H2O(I) ⇌ H2O(g)

- The energy of the molecules on the surface of water is greater than the intermolecular hydrogen bonding attracting the molecules to one another

- Surface molecules break free as gas and reside as water vapour (at the top of the bottle)

- Similarly, water vapour enters the liquid phase when they no longer have sufficient energy to overcome the hydrogen bonding in the liquid water

- If the walls of the drink bottle are cooler than the liquid water, water droplets accumulate on the sides of the bottle, producing condensation.

- On a hot day, water evaporates more readily because the molecules have more energy and can overcome the hydrogen bonding between water molecules in the liquid phase

○ Results in pressure building up in the bottle

○ Means gaseous side of the equilibrium equation is favoured at higher temperatures

Why is the energy of molecules on surface of water greater than H-bonding ?

The kinetic energy of some if the molecules is greater than the energy required to break the IMF (H-bonding). Remember the Maxwell Boltzmann distribution curve. Some particles have a lot of kinetic energy. There is also a transfer of KE between the air particles and the water particles at the surface. High energy molecules within the sample have to break through and in jostling with other molecules will lose some of the KE to them.

understand that, over time, physical changes and reversible chemical reactions reach a state of dynamic equilibrium in a closed system, with the relative concentrations of products and reactants defining the position of equilibrium

- Dynamic equilibrium is that state a reaction reaches when the rates of the forward and reverse reactions are equal

○ Reaction must be incomplete - does not consist of only products

○ Bonds are broken and re-formed constantly

- The relative concentrations of products and reactants determine the equilibrium position

explain the reversibility of chemical reactions by considering the activation energies of the forward and reverse reactions

- When particles collide, the energy associated with collisions can break bonds in reacting particles, allowing them to rearrange to form new products

○ Energy required to break or weaken the bonds of the reactants is the activation energy

- In the reverse reaction, once the products form, if the product particles collide with enough energy to break their bonds (=activation energy of the reverse reaction) it is possible to re-form reactants

analyse experimental data, including constructing and using appropriate graphical representations of relative changes in the concentration of reactants and product against time, to identify the position of equilibrium.

Explanation: Identifying the difference between the extent of a reaction and the rate of reaction

- The rate of reaction refers to how fast a reaction takes places

• On the above graphs, a faster rate of reaction would mean that it would take less time to establish equilibrium

- The extent of a reaction refers to how much product is formed when the system reaches equilibrium

• Extent does not give any information about how fast a reaction will proceed.
• On the above graphs, a higher extent would mean that the concentration of the products would be higher than in a reaction with a lower extent

Factors that affect equilibrium

apply Le Châtelier's principle to predict the effect changes of temperature, concentration of chemicals, pressure and the addition of a catalyst have onthe position of equilibrium and on the value of the equilibrium constant.

- Le Chatelier's Principle states that if an equilibrium system is subjected to a change, the system will adjust itself to partially oppose the effect of the change

• Results in the change in the equilibrium position (the relative amounts of reactants and products at equilibrium). Can be changed by:

▪ Adding or removing reactant or product

▪ Changing pressure by changing the volume of the sealed container (for gaseous equilibria)

▪ Dilution (for equilibria in solution)

▪ Changing temperature

- Adding a catalyst does not change the relative concentrations of the reactants and products, and does not change the position of equilibrium

• Catalyst lowers the activation energy of the forward and reverse reaction, increasing the number of successful particle collisions

▪ Increases rate of reaction because more particles have energies greater than the activation energy

explain the effect of changes of concentration and pressure on chemical systems at equilibrium by applying collision theory to the forward and reverse reactions

Explanation: effect of changing concentration

- Concentration is the amount of solute dissolved in a substance

Predicting the effect using Le Chatelier's Principle

- When the concentration of a reactant is increased, the system will partially oppose the change by moving to decrease the reactant concentration. The forward reaction will be favoured and the equilibrium position will shift to the right.

Predicting the effect using collision theory and reaction rates

- Collision theory states that for a chemical reaction to occur, the reacting particles must collide with one another with sufficient energy to exceed the activation energy and collide with the proper orientation

- If the reactant concentration increases, the frequency of collisions increases, increasing the likelihood of a successful collision (where there is sufficient energy to exceed the activation energy and collision in the proper orientation).

• The rate of the forward reaction instantly increases. As the concentration of the products subsequently increases, there are more frequent product collisions, increasing the rate of the reverse reaction
• Eventually, the rate of the forward and reverse reactions equal and equilibrium is re -established

Explanation: effect of changing pressure

- Pressure is the force exerted per unit area, by one substance upon another substance

- Pressure can be changed by:

○ Adding or removing reactants or products

○ Changing the volume of the system

○ Adding an inert gas

▪ Adding inert gas does not affect the equilibrium position because there no reaction with inert gas molecules. Therefore the reactant or product concentration would not change

Predicting the effect using Le Chatelier's Principle

- When the pressure of a system increases, the equilibrium position will shift to reduce the pressure, by moving in the direction of the fewest gas particles.

Predicting the effecting using collision theory

- If the volume of the system decreases, the gas molecules are closer to each other and collisions between molecules become more frequent.

• Rate of reaction increases, and to counteract this change, the equilibrium position will favour the side of the reaction with the least number of particles

Important: notes regarding conditions for pressure changes

- Pressure changes do not affect equilibrium position of systems in the liquid or solid phases

• Particles are too tightly packed for an increase in pressure to have a noticeable effect
• Negligible change in concentration, and no effect on equilibrium position

Explanation: effect of dilutions

- Dilution by adding water reduces the number of particles per volume

• Results in shift in position towards the side that produces the greater number of dissolved particles
• Addition of water reduces all concentration of products and reactants instantaneously

explain and predict the effect of temperature change on chemical systems at equilibrium by considering the enthalpy change for the forward and reverse reactions

- In endothermic reactions, products have more energy than reactants (energy is absorbed from environment and positive enthalpy)

• energy + reactants ⇌ products

Predicting using Le Chatelier's Principle

- An increase to temperature results in the reaction working to remove extra energy

• Le Chatelier's Principle states that the equilibrium position will move to side where energy is absorbed and stored within the molecules (favouring of forward reaction)

Predicting using enthalpy change

- The activation energy of the forward reaction is greater than the reverse reaction
- As temperature increases, the kinetic energy of the particles increases, meaning a greater proportion of the particles have the necessary energy to overcome the activation energy barrier for the exothermic reverse reaction.

Explanation: effect of temperature change on exothermic reactions

- In exothermic reactions, reactants have more energy than products (energy is released to the environment and negative enthalpy)

• reactants ⇌ products + energy

Predicting using Le Chatelier's Principle

- An increase to temperature results in the reaction working to create extra energy

• Le Chatelier's Principle states that the equilibrium position will move to side where energy is released from the molecules (favouring of reverse reaction)

Predicting using enthalpy change

- The activation energy of the forward reaction is less than the reverse reaction

- As temperature increases, the kinetic energy of the particles increases, meaning a greater proportion of the particles have the necessary energy to overcome the activation energy barrier for the endothermic reverse reaction.

Equilibrium constants

understand that equilibrium law expressions can be written for homogeneous and heterogeneous systems and that the equilibrium constant (K), at any given temperature, indicates the relationship between product and reactant concentrations at equilibrium

- Equilibrium Law is the law that states that the concentrations of products to their power of their coefficients, divided by the concentration of reactants to the power of their coefficients is equal to the equilibrium constant (Kc)

• The equilibrium constant is the ratio of the reactants to products in a reaction when it is at equilibrium

Ensure to not use water in the equilibrium calculation if water is an aqueous solution

Equilibrium constant for exothermic vs endothermic reactions

- In exothermic reactions, the forward reaction is favoured (lower Ea)

• Product concentration is higher than reactant concentration, meaning Kc > 1

- In endothermic reactions, reverse reaction is favoured

• Reactant concentration is higher than reactant concentration, meaning Kc < 1

Equilibrium constant for homogeneous vs heterogeneous systems

- A homogeneous system is one where all reactants and products are in the same phase

• Equilibrium constant is calculated using all reactant and product concentrations

- A heterogeneous system is one where reactants and products are in different phases

• For solids in heterogeneous systems, concentration is considered constant, so it is removed from the equilibrium expression
• Only aqueous or gas reagents are included

Explanation: impact of temperature on the equilibrium constant

- As temperature increases for exothermic reactions, the amount of products present at equilibrium decreases, and so Kc decreases

• From the Maxwell-Boltzmann Distribution curve, it can be seen that the number of particles exceeding the Ea of the reverse reaction is smaller than the number of particles exceeding the Ea of the forward reaction. An increase to the temperature (which increases the number of particles exceeding both activation energies, a shift of the curve to the right), means there is proportionally more reverse reaction, as adding more particles to a small quantity has a more significant impact than adding the same number of particles to a larger quantity. Therefore, the concentration of the products decreases, and the concentration of the reactants increases, decreasing Kc.

- For endothermic reactions, the opposite occurs

deduce the equilibrium law expression from the equation for a homogeneous reaction and use equilibrium constants (Kc), to predict qualitatively, the relative amounts of reactants and products (equilibrium position)

deduce the extent of a reaction from the magnitude of the equilibrium constant

Explanation: magnitude of the equilibrium constant

Explanation: reaction quotient (Qc) vs Kc

Explanation: dependency of Kc on equation

- If one equation is the reverse of another, Kc is inverse of each other

- If coefficients of equation are doubled, Kc is squared

- If coefficients of equation are halved, Kc is square rooted

use appropriate mathematical representation to solve problems, including calculating equilibrium constants and the concentration of reactants and products.

- Types of questions:

○ Write equilibrium expression

○ Determine Kc from concentrations

○ Calculate concentrations from given Kc

○ Determine if a system is in equilibrium

Properties of acids and bases

understand that acids are substances that can act as proton (hydrogen ion) donors and can be classified as monoprotic or polyprotic depending on the number of protons donated by each molecule of the acid

- Acids are substances that can act as a proton (hydrogen ion) donor

• A monoprotic acid is an acid that can donate one hydrogen ion per molecule,

▪ Hydrogen bromide (HBr), nitric acid (HNO3), ethanoic acid (CH3COOH)

• A polyprotic acid is an acid that can donate more than one hydrogen ion per molecule

▪ Diprotic (sulfuric acid - H2SO4)

▪ Triprotic (phosphoric acid - H3PO4)

distinguish between strong and weak acids and bases in terms of the extent of dissociation, reaction with water and electrical conductivity and distinguish between the terms strong and concentrated for acids and bases.

Difference between strong and weak acids

- A strong acid is an acid that completely ionises in water, producing hydronium (or H+ ) ions

• Examples of strong acids include:

▪ Hydrochloric acid (HCl), nitric acid (HNO3) and sulfuric acid (H2SO4)

- A weak acid is an acid that does not completely ionise in water, producing an equilibrium system with hydronium ions (or H+ ) and undissociated acid molecules

• Examples of weak acids include:

▪ Carboxylic acid (acetic acid - CH3COOH), carbonic acid (H2CO3), aqueous carbon dioxide (CO2)

Differences between strong and weak bases

- A strong base is a base that completely ionises in water, producing hydroxide ions (OH-)

• Examples of strong bases include:

▪ All group 1 hydroxides (e.g sodium hydroxide - NaOH) and barium hydroxide

- A weak base is a base that does not completely ionise in water, producing an equilibrium system with hydroxide ions (OH- ) and undissociated base molecules

• Examples of weak bases include:

▪ Ammonia (NH3) and amines

Electrical conductivity

- Electrical conductivity is the degree to which a material conducts an electric current

• Acids and bases are electrolytes because they can conduct an electric current
• This is because the substance dissociates to form a cation and anion, and migrate towards the oppositely charged terminals (cathode and anode) of an electric circuit
• If a light bulb is connected, the brighter the light, the greater number of ions present stronger the acid

Differences between strong and concentrated, weak and diluted

- Diluted and concentrated refers to the amount of acid or base molecules in a solution

• Concentrated solutions have more solute than dilute solutions

- Strong and weak refers to the extent to which the acid or base reacts with water to form hydronium ions (for acids) or hydroxide ions (for bases)