# Module 3 – Periodic table and energy

## Hess’ Law and Enthalpy Cycles

Experiments: The experiments in previous section allow enthalpy changes to be determined directly. Hess’ Law determines it indirectly. • Difficult to determine standard enthalpy change of combustion directly as… - Carbon dioxide and water would also be formed. -...

## Periodic Table

Mendeleev: He arranged elements in order of atomic mass and lined up in groups with similar properties. Left gapsif group properties did not fit for elements not discovered and if some atomic mass measurements were wrong. Potassium and argon are not arranged in...

## Periodic Trends in Bonding and Structure

Semi- Metals: Elements near the diagonal divide are semi- metals or metalloids with in- between properties. • Metallic Bonding: The electrostatic attraction between positive metal ions and delocalised electrons. • Giant Metallic Lattice: Each atom has donated its...

## Reaction Rates

Rate of Reaction: Change in concentration of a reactant used up or product formed in a given time. • Calculating Rate of Reaction: Rate (mol dm-3 s -1 ) = change in concentration (mol dm-3 ) / time (s). Time on x axis. • Working out rate of anything is change in...

## Catalysts

Catalyst: A catalyst provides an alternative reaction pathway for reaction with lower activation energy. Catalyst regenerated/ not used up. Increases rate of reaction. Enthalpy Profile Diagrams: Reactants and product should be shown at correct levels with respect to...

## The Boltzmann Distribution

Elastic Collisions: Molecules in gas move at high speed and collide with each other. Collisions are elastic as molecules do not slow down as a result of collision so no energy lost. • Energy of Molecules: In gas, liquid, or solution, some molecules move slowly with...

## Dynamic Equilibrium and le Chatelier’s Principle

Reversible Reactions: A reaction that takes place in both forward and reverse directions if reversible. Equilibrium only in reversible reactions. • Dynamic Equilibrium System: Equilibrium that exists in a closed system when rate of forward reaction is equal to rate of...

## The Equilibrium Constant Kc

Equilibrium Constant: Kc. A measure of the position of equilibrium. • Equilibrium Law: Square brackets shorthand for ‘concentration of’. a, b, c and d are balancing numbers- the powers. A, B, C and D are the equilibrium concentrations of products and reactants. •...

## Group 2

Group 2: Aka alkaline earth metals. • Redox Reactions: They are the most common type of reaction of Group 2 elements. Each metal atom is oxidised, losing two electrons to form a X2+ ion- the same electron configuration as noble gas. Another species will gain these two...

## The Halogens

Halogens: Group 17. Reactive. All halogens exist as diatomic molecules. • Characteristics: Fluorine is yellow gas. Chlorine is green gas. Bromine is red/ brown liquid. Iodine is grey solid. • Boiling Point Increases Down: - As you go down the group, greater number of...

## Qualitative Analysis

Quantitative Analysis: Example is titration. • Qualitative Analysis: Relies on observations. • Test for Anions: Carbonate, sulphate and halide. • Carbonate and Acids: Carbonates react with acids to form carbon dioxide. Na2CO3 (aq) + 2HNO3 (aq) -> 2NaNO3 (aq) + CO2...

## Enthalpy Change

Enthalpy: H. The heat content that is stored in a chemical system. • Enthalpy Change: ∆ = − . • Conservation of Energy: Energy cannot be created or destroyed. Heat transferred between system and surroundings. • (Chemical) System: The chemical compounds being studied-...

## Measuring Enthalpy Changes

Thermometer: It is part of surroundings so can measure temp change in surroundings. • Kelvin Scale: Scale starts at absolute zero 0 K or -273oC. 1 K rise = 1 oC rise. oC + 273 = K. • Calculating Energy Change: Energy change of surroundings is calculated using q =...