QCAA Chemistry Unit 3

Reversible Reactions

Reactions which proceed in the 'normal way' (LHS->RHS) and then return in reverse (LHS<-RHS) only in CLOSED SYSTEMS.

Physical Changes

A type of change that alters the physical properties of a substance but does not change its composition. eg. state changes, separation of mixtures, physical deformation.

Chemical Changes

Changes which occur when bonds are broken and/or formed between molecules of atoms and by which new substances are formed. Can be reversible or irreversible.

Irreversible Chemical Reactions

When chemical reactions occur to form products which cannot be converted back into the reactants. eg. making a cake, combustion reactions

Reversible Chemical Reactions

Reactions in closed vessels which do not go to completion under a given set of conditions of temperature and pressure allowing the reaction to take place in both the forward and backwards direction.

Equilibrium Reaction

A state in which the rate of the forward reaction equals the rate of the reverse reaction.

Dynamic Equilibrium

Result of diffusion where there is continuous movement of particles but no overall change in concentration.

Open System

A system in which matter can be exchanged with the surroundings.

Closed System

A system in which energy, but not matter, is exchanged with its surroundings.

Isolated System

A system that can exchange neither energy nor matter with its surroundings.

Exothermic Reaction

A reaction that releases energy, usually in the form of heat.

Endothermic Reaction

A reaction in which energy is absorbed from its environment.

Equilibrium Law

If the concentrations of all substances are raised to the power of the number of moles, the aA+bB=cC+dD Kc=[C]^c[D]^d/[A]^a[B]^b is constant. (Kc= moldm^-3)

Kc>1

At equilibrium, the products will be favoured (right shift).

Kc<1

At equilibrium, the reactants will be favoured (left shift).

Kc

Equilibrium constant. Represents the constant ratio of reactions and products in a reaction once a system has reached equilibrium and is not affected by concentration, pressure or catalysts but is affected by temperature.

Q(c)

Reaction quotient. Measures the relative amounts of products and reactants present during a reaction at a particular point in time. When compared to Kc, aids in figuring out which direction a reaction is likely to proceed.

Le Chatelier's Principle

When a change is applied to a system in dynamic equilibrium, the system reacts in such a way as to oppose the effect of the change.

Equilibrium Factors

- adding or removing a reactant or products

- changing the pressure (for equilibria involving gases)

- dilution (for equilibria involving gases)

- changing the temperature

- effect of catalyst

Catalyst

Work by providing an alternative reaction pathway involving a lower activation energy.

Haber Process

An industrial process for producing ammonia from nitrogen and hydrogen by combining them under high pressure in the present of an iron catalyst.

Arrhenius Acids

Substances that ionise in water to produce hydronium ions (H30+).

Arrhenius Base

Substances that ionise in water to produce hydroxide ions (OH-).

Electrolyte

A substance that, when dissolved, can conduct electricity within a solution.

pH

Value used to express the acidity or alkalinity of a solution; defined as the log of the reciprocal of the concentration of hydronium ions (pH= -log(H3O+))

Self-Ionisation of Water

Water is both an acid and a base and is therefore amphiprotic. By the Bronsted-Lowry definition, water can donate protons and accept protons, this is the self-ionisation of water.

Ionisation Constant of Water (Kw)

(H30+)(OH-)=1.00x10^-14 at 25°C/298K.

Dilution

The process of adding solvent to lower the concentration of solute in a solution. M1V1=M2V2

Bronsted-Lowry Acid

A substance that donates a proton to a base (doesn't require deionisation in water).

Bronsted-Lowry Base

A substance that accepts a proton from an acid (doesn't require deionisation in water).

Conjugate Acid-base Pairs

Two substances that are related by the loss or gain of a single hydrogen ion.

Amphiprotic Substances

Species that can either donate protons or accept protons, depending on the chemical environment. eg. water

Ka

Acid dissociation constant.

Ka=([H3O+)(A-)/([HA])

- high = likely to dissociate (ionize), small = unlikely/less dissociation

Kb

Base dissociation constant.

Kb=([B+][OH-])/([BOH])

- high = likely to dissociate (ionize), small = unlikely/less dissociation

Monoprotic Acids

Acids which donate one proton only. eg. Hal, HF, HNO3

Polyprotic Acids

Acids which can donate more than one proton from each molecule. eg. H2S04, H2CO3, H3PO4

Triprotic Acids

Acids which donate three protons. e.g.. H3PO4, H3BO3

Concentration/Dilution

Describes the amount of acid or base in a given volume of solution.

Strength

Amount of dissociation occurring with the acid and base with the solution.

Buffer Solution

A solution made from a weak acid and its conjugate base that neutralizes small amounts of acids or bases added to it.

Titrations

Controlled neutralization reactions used to find the concentration of an acid or base sample.

Titrant

A solution of known concentration that is used to titrate a solution of unknown concentration.

Acid-Base Indicator

Organic substances which change in colour in solution when hydronium ion (H30+) concentrations reach a particular value.

End Point

The point in a titration at which an indicator changes colour.

Equivalence Point

The point at which the two solutions used in a titration are present in chemically equivalent amounts.

Titration Curves

Show change in pH that occurs when acids and bases are titrated against each other.

Indicators

Substances that change colour reversibly accordingly to the pH of the solution.

Volumetric Analysis (Titration)

Technique used to determine the concentration of a solution of accurately known volume, by titrating it against another solution of known concentration.

Standard Solution

A solution of accurately known concentration which is created via dissolving an accurately measured mass of a primary standard in an accurately measured volume of water.

Primary Standards

Substances that are so pure that the number of moles of a substance can be calculated from their mass.

Titre

Amount utilised out of a burette.

Aliquot

A portion of a well-mixed sample removed for testing.

Dilution Factor

The ratio of the initial and final solution volumes. (volume of diluted/ volume of undiluted)

Redox

A shortened name for a reaction that involves reduction and oxidation.

Redox Reactions

A chemical reaction involving the transfer of one or more electrons from one reactant to another; also called oxidation-reduction reaction.

Oxidation

A chemical change in which a substance combines with oxygen, as when iron oxidizes, forming rust; loss of electrons.

Reduction

The loss of oxygen from a compound; gain of electrons.

Electronegativity

A measure of the ability of an atom in a chemical compound to attract electrons which increases across a period and up a group; relative change between nucleus and valence electrons.

Ionisation Energy

The energy required to remove a mole of electrons from a mole of isolated gaseous atoms or ions.

Electron Affinity/ Electron Gain Enthalpy

The amount of energy released when an isolated gaseous atom accepts an electron to form a monovalent gaseous anion.

Oxidising Agent

The substance that is reduced and thereby causes the oxidation of some other substance in a redox reaction.

Reducing Agent/ Reductant

The substance that is oxidised and thereby causes the reduction of some other substance in a redox reaction.