Chem W3 and W4

• Collision model

A theory explaining reaction rates at the molecular level based on particles colliding to react.

• Orientation factor

The requirement that molecules must collide in the correct orientation for new bonds to form and a reaction to occur.

• Activation energy (Ea)

The minimum amount of energy required for a reaction to occur.

• Activated complex / transition state

The high-energy arrangement of atoms at the top of the energy pathway during a reaction.

• Exothermic process

A reaction that releases energy because the products are lower in energy than the reactants.

• Endothermic process

A reaction that absorbs energy because the products are higher in energy than the reactants.

• Reaction pathway

The sequence of energy changes and molecular arrangements that occur during a chemical reaction.

• Arrhenius equation

An equation relating the rate constant of a reaction to activation energy and temperature.

• Frequency factor (A)

A value in the Arrhenius equation related to the frequency of collisions and the likelihood of correct orientation during collisions.

• Rate constant (k)

A constant that relates the speed of a reaction to reactant concentrations.

• Reaction mechanism

The step-by-step sequence describing how reactants are converted into products.

• Elementary reaction

A reaction that occurs in a single step.

• Molecularity

The number of molecules involved in an elementary reaction step.

• Termolecular reaction

An elementary reaction involving the simultaneous collision of three molecules.

• Multistep mechanism

A reaction mechanism consisting of two or more elementary steps.

• Rate-determining step

The slowest step in a multistep reaction mechanism that limits the overall reaction rate.

• Intermediate

A substance formed in one step of a reaction mechanism and consumed in a later step.

• Rate law

An equation that expresses the relationship between reaction rate and reactant concentrations.

• Catalyst

A substance that changes the speed of a chemical reaction without undergoing permanent chemical change itself.

• Catalysis

The process by which a catalyst increases or decreases the rate of a reaction.

• Homogeneous catalyst

A catalyst present in the same phase as the reactants.

• Homogeneous catalysis

Catalysis in which the catalyst and reactants are in the same phase.

• Uncatalyzed reaction

A reaction occurring without the presence of a catalyst.

• Catalyzed reaction

A reaction whose rate has been altered by a catalyst.

• Equilibrium

A state in which forward and reverse reactions occur at equal rates.

• Concentration

The amount of a substance present in a given volume.

• Reaction rate

The speed at which reactants are converted into products.

Chemical Kinetics

It is the area of chemistry concerned with the speed, or rates of reactions.

Catalysts

they are agents that increase reaction rates without themselves being used up.

The speed of a chemical reaction

is the change in the concentration of reactants and products per unit of time

Instantaneous rate

is determined from the slope of the curve at a particular point in time.

Reaction Orders Zero Order

It is the one which the rate of disappearance of A is independent of A.

Reaction Orders, Second Order

Its rate depends on the concentration of a single reactant raised to the second power.

Reaction Orders, First Order.

A first order reaction is the one whose rate depends on the concentration of a single reactant raised to the first power.

Standard enthalpy change for a reaction or physical process

is the difference between the products in their standard states and the reactants in their standard states, all at the same specified temperature.

A solution

is a homogeneous mixture of two or more components which are not chemically reactive and soluble in each other.

The solute

is the material dissolved in a solvent to form a solution.

Solvent

is the substance in which another substance is dissolved to form a solution.

A saturated solution

is a solution that contains a maximum amount of solute dissolved in a solution at a given temperature.

A supersaturate solution

contains more dissolved solute then would normally be stable at that temperature. Cooling — Precipitation.

Protic solvents

have weakly acidic hydrogens capable of forming H bonds.

Solvation

is an interaction of a solute with the solvent, which leads to stabilization of the solute species in the solution.

Hydration number

Number of molecules that are influenced or affected by a different ion.

Miscibility

The ability of one liquid to mix with another

Vapour pressure

It happens when the escaped vapour exerts a pressure on the remaining liquid.

Raoult’s Law

The vapour pressure of a solution decreases if another substance is dissolved in the solution.

• Fractional crystallisation

A method of separating solutes based on differences in their solubilities.

• Saturated solution

A solution containing the maximum amount of dissolved solute possible at a given temperature.

• Supersaturated solution

A solution containing more dissolved solute than is normally stable at that temperature.

• Precipitation

The formation of a solid from a solution, often caused by cooling.

• Concentration

The amount of solute present in a given volume of solution.

• Relative permittivity (dielectric constant, εr)

A measure of how strongly a material reduces electric forces between charged particles compared with a vacuum.

• Polar solvent

A solvent with a relative permittivity greater than 15, capable of stabilising charged particles effectively.

• Non-polar solvent

A solvent with a low relative permittivity that poorly stabilises ions.

• Protic solvent

A polar solvent capable of forming hydrogen bonds (e.g. water, ethanol).

• Aprotic solvent

A polar solvent that cannot form hydrogen bonds (e.g. acetone).

• Solvation

The interaction between solute particles and solvent molecules that stabilises the solute in solution.

• Hydration

Solvation specifically involving water molecules.

• Hydration sphere (hydration shell)

Layers of water molecules surrounding an ion in aqueous solution.

• Hydration number

The number of water molecules affected or associated with an ion in solution.

• Solubility

The amount of a substance that can dissolve in a solvent at a specified temperature.

• Miscibility

The ability of one liquid to mix completely with another liquid.

• Immiscible liquids

Liquids that do not mix together, such as oil and water.

• Electrolyte

A substance that ionises in water and conducts electricity.

• Strong electrolyte

An electrolyte that dissociates almost completely into ions in solution.

• Weak electrolyte

An electrolyte that dissociates only partially into ions in solution.

• Degree of dissociation (Ka)

A measure of how much an electrolyte dissociates into ions.

• Conductivity

The ability of a solution to conduct electricity due to the presence of mobile ions.

• Saturated vapour pressure

The pressure exerted by a vapour in equilibrium with its liquid in a closed system at constant temperature.

• Partial vapour pressure

The pressure exerted by one component of a gas mixture.

• Vapour pressure

The total pressure exerted by all vapour phases in a system.

• Boiling point

The temperature at which a liquid’s vapour pressure equals the external pressure.

• Raoult’s Law

A law stating that the vapour pressure of a component in an ideal solution is proportional to its mole fraction.

• Mole fraction

The ratio of moles of one component to the total moles in a mixture.

• Ideal solution

A solution that obeys Raoult’s law due to similar intermolecular forces between components.

• Non-ideal solution

A solution that deviates from Raoult’s law.

• Ideal dilute solution

A very dilute solution where the solvent obeys Raoult’s law but the dilute solute does not.

• Henry’s Law

A law stating that the vapour pressure of a dilute solute is proportional to its concentration.

• Limiting law

A law that becomes increasingly accurate as concentration approaches zero.

• Partially miscible liquids

Liquids that mix only up to certain concentrations.