Module 5

define rate of reaction?

the change in concentration of a reactant or a product in a given time

define order?

the power to which the concentration of a reactant is raised in the rate equation

define overall order?

the sum of the individual orders of reactants in the rate equation: m + n

define rate constant?

k, the constant that links the rate of reactant with the concentrations of the reactants raised to the powers of their orders in the rate equation (rate=k[A]^m[B]^n)

define half-life?

the time taken for the concentration of a reactant to decrease by half

define rate-determining step?

the slowest step in the reaction mechanism of a multi-step reaction

how to deduct orders from experimental data?

zero order: when the concentration of a reactant has no effect on the rate

first order: if the concentration of A is doubled the reaction rate increases by a factor of 2^1, tripled then 3^1

second order: if the concentration of A is doubled the reaction rate increases by a factor of 2², tripled then 3²

how to calculate the rate constant?

step 1: determine the orders, overall order and rate equation

step 2: write the rate equation

step 3: calculate the rate constant by rearranging the rate equation and substituting experimental values

step 4: substituting units into k expression and cancel common units

how to deduce order of a reactant from a concentration-time graph?

zero order reaction produces a straight line with a negative gradient (gradient is equal to the rate constant), first order reaction produces a downward curve with a decreasing gradient over time, second order reaction produces a steeper downward curve at the start but tails off more slowly

how to calculate reaction rate of a conc-time graph?

measure the gradient

what tests could be used to generate data for a conc-time graph?

continuous rate monitoring e.g. gas collection, mass loss, colorimetry (amount of light absorbed by a solution)

what is colorimetry?

the wavelength of light passing through a coloured solution is controlled by a filter. the amount of light absorbed by a solution is measured which is directly linked to the concentration of the solution.

describe the half-life of first order reactants?

constant - exponential decay

how to determine order from a conc-time graph?

determine successive half-lives - if 3 successive half-lives have a constant value then the reaction is first order.

define lattice enthalpy?

the enthalpy change that takes place when 1 mole of ionic lattice is formed from gaseous ions under standard conditions

define enthalpy of solution?

the energy change when 1 mole of solute is completely dissolved in water under standard conditions

define enthalpy of hydration?

the enthalpy change when dissolving one mole of gaseous ions in water

factors that affect the size of lattice enthalpy?

• size of ions involved

• charges on the ions

• ionic bond strength

what would cause lattice enthalpy to be more negative?

smaller ions because they can get closer and so have stronger attraction

increasing ionic charge as ionic attraction increases

describe hydration?

when an ionic lattice is broken the ions become part of the solution

positive ions get attracted towards the partially negative oxygen and vice versa for the hydrogen

factors affecting the size of hydration?

• size of ions involved

• charge on the ions

calculation of lattice enthalpy from Born-Haber cycles?

1. follow the reactions to work out which enthalpy change occurs at each stage,

2. Set up equation - ∆H_f = ∆H_atm + IE + EA + LE

3. Rearrange equation for LE and substitute values - LE = ∆H_f - (∆H_atm + IE + EA)

Calculation of ∆H_sol from Born-Haber cycles?

1. follow the reactions to work out which enthalpy change occurs at each stage (LE is solid to gaseous)

2. Set up equation - ∆H_sol = LE + ∆H_hyd⁺ + ∆H_hyd^-

3. substitute values

define enthalpy change of formation?

The enthalpy change when 1 mole of a substance is formed from its constituent elements under standard conditions

define enthalpy change of atomisation?

the enthalpy change required to produce 1 mole of gaseous atoms from the element - always endothermic

define ionisation energy?

The energy required to remove one electron from the outermost shell of an atom in the a mole of atoms in the gaseous state - endothermic

define electron affinity?

The enthalpy change when one moles of gaseous atoms acquires one mole of electrons to form one mole of gaseous negative ions - exothermic

describe enthalpy change in solution?

when a solid ionic compound dissolves in water the ionic lattice breaks up and water molecules are attracted to the ions surrounding them

2 types of energy change:

• formation of gaseous atoms from the breaking up of the ionic lattice

• formation of hydrated aqueous ions from the interaction of gaseous atoms with polar molecules

if the lattice enthalpy is very exothermic (negative) what will melting point be?

very high

what would cause enthalpy of hydration to be more negative?

smaller ionic size and increased ionic charge as these increase the ionic attraction

what is necessary for ionic compounds to dissolve?

the attraction in the ionic lattice must be overcome so energy must equal lattice enthalpy

energy equal to hydration enthalpy is released when water molecules surround the ions

if the sum of hydration enthalpies is greater than the lattice enthalpy then the compound should dissolve with a enthalpy of solution (overall enthalpy) that is exothermic (not always true as entropy and temp. also affect solubility)