OCR A Chemistry A-Level Paper 1

Define a transition element

d-block elements that form at least on ions with a partially filled d-orbital

What are the three properties of transition elements?

1. Form compound with different oxidation states

2. Formed coloured compounds

3. The elements and their compounds can act as catalysts

Name examples of processes catalysed by transition elements

Haber Process (nickel)

Hydrogen Peroxide Decomposition (manganese)

Hydrogenation (nickel)

What is a ligand?

A molecule or ion that donates a pair of electrons to a central metal ion to form a dative covalent bond

What are the four possible shapes of complex ions and what are their coordination numbers

Octahedral - 6

Tetrahedral - 4

Square Planar - 4

Linear - 2

What is a coordination number? (Transition Metals)

The number of dative covalent bonds (coordinate bonds) formed in a complex ion

What is stereoisomerism?

When two compounds have the same molecular formula but different spatial arrangements

Define optical stereoisomerism

Compounds which are non-superimposable mirror images

How is stereoisomerism used in medicine?

Cis-platin was found to prevent the growth of E.coli cultures. It was also found that cis-platin attacks tumours and in may cases the tumour seemed to shrink. However, there are many unpleasant side affects and its us can lead to kidney damage.

What are the observable changes when aqueous copper (II) ions react with ammonia?

Solution is initially a pale blue \[Cu(H₂O)₆\]²⁺

A pale blue precipitate is formed, which darkens the colour Cu(OH)₂

The precipitate dissolves in excess ammonia to form a dark blue solution \[Cu(NH₃)₄(H₂O)₂\]²⁺

What are the observable changes when aqueous copper (II) ions react with chloride ions?

Initially pale blue solution \[Cu(H₂O)₆\]²⁺

The solution turns yellow when an excess of HCl is added \[CuCl₄\]²⁻

If water is added to the solution, a blue solution is formed, although more dilute than the original one

If acid is added, a green solution is formed \[CuCl⁴(H₂O)₂\]²⁻

What are the observable changes when aqeuous chromium (III) ions react with ammonia?

Chromium (III) potassium is a pale purple solution dissolved in water \[Cr(H₂O)₆\]³⁺

When a small amount of ammonia is added, a grey-green precipitate form s Cr(OH)₃

If excess ammonia is added and left to stand, the precipitate dissolves to form a purple solution

How is ligand substitution involved in carbon monoxide poisoning?

Haemoglobin contains haem group, which has a Fe²⁺ ion as the central ion which can bind to oxygen gas forming oxyhaemogobin, which releases oxygen to the body cells when required. Carbon monoxide is able to bind to the haem group forming carboxyhaemoglobin. If carbon is breathed in a ligand substitution can take place where oxygen is replaced by carbon monoxide, which binds more strongly than oxygen to the haem group in an irreversible reaction. If carboxyhaemoglobin concentrations become too high, oxygen transport is prevented leading to death

What happens when Fe²⁺ and Fe³⁺ are respectively reacted with sodium hydroxide?

Fe²⁺ - pale green solution forms green, insoluble precipitate

Fe³⁺ - yellow solution forms orange-brown, insoluble precipitate

What happens when Cu²⁺ reacts with NaOH and with NH₃?

NaOH - blue solution forms blue, insoluble precipitate

NH₃ - blue solution forms blue precipitate, soluble in excess ammonia

What happens when Cr³⁺ reacts with NaOH and with NH₃?

NaOH - purple solutions forms grey precipitate, soluble in excess to form green solution

NH₃ - purple solution forms green precipitate, soluble in excess to form purple solution

What happens when Mn²⁺ reacts with NaOH?

Pale pink solution forms light brown precipitate, insoluble in excess

What is a disproportionation reaction?

A reaction where the same element is oxidised and reduced

What is a voltaic cell?

A type of electrochemical cell which converts chemical energy into electrical energy (e.g cells and batteries)

Define the standard electrode potential

The tendency of a cell to be reduced and gain electrons

How is the standard electrode potential calculated?

E° Cell \= E° reduction reaction - E° oxidation reaction

What is the overall reaction for a fuel cell?

H₂ + 1/2O₂ → H₂O

What are the three types of cells?

Primary - non-rechargeable and single use (irreversible reaction)

Secondary - rechargeable and reversible (e.g. lead-acid car batteries)

Fuel Cells - use energy from the reaction of a fuel with oxygen to create a voltage

What are the two half equations for an alkaline fuel cell?

2H₂O + 2e⁻ ⇌ H₂ + 2OH⁻

1/2O₂ + H₂O + 2e⁻⇌ 2OH⁻

Define entropy

A quantitative measure of the degree of disorder in a system

How do you calculate entropy change?

ΣS(products) - ΣS(reactants)

For a reaction to take place, does the free energy change (∆G) need to be positive or negative?

Negative

How do you calculate if a reaction is feasible at a specific temperature?

T \= ∆H/∆S

Define lattice formation enthalpy

The enthalpy associated with the formation of one mole of ionic lattice from its constituent gaseous ions

List two factors which affect the strength of ionic bonding

Ion charge

Radius

(Charge density)

Define atomisation enthalpy

The enthalpy change when one mole of gaseous ion is formed from the element in its standard state

Define hydration enthalpy

The enthalpy change when one mole of gaseous ion dissolves in water

Define enthalpy of solution

The enthalpy change when one mole of ionic substance dissolves in water

Define first ionisation energy

The energy required to remove one mole of gaseous electron to form a 1+ ion

What are the properties of metallic bonding?

Conductive

High melting and boiling points

Insoluble

What are the properties of a giant covalent lattice?

Does not conduct (mostly)

High melting and boiling points

Insoluble in most solutions

Give an example of a conductive giant covalent lattice?

Graphite - a series of delocalised electrons between layers of hexagonally arranged carbons allow graphite to conduct

Why does boiling point increase from group 1 to 4 but decrease between 4 and 5?

The change from giant to simple molecular substances

Group 2 compounds react with water to form what?

Metal hydroxides

Reactivity of group 2 increases in which direction?

Down the group

Give an example of a use of group 2 compounds in medicine

Used as antacids for treating indigestion, using magnesium and calcium carbonates as the stomach is mostly hydrochloric acid

All halogens exist as what type of molecules?

Diatomic

Bromine can be displaced by which other halogen, chlorine or bromine?

Bromine

How do you distinguish between the solution colours of halogens?

By adding cyclohexane (an organic, non-polar solvent). The colours are more distinct because they dissolve more readily in the solvent

Reactivity increases in which direction in group 7 elements?

Upwards

How does chlorine react with cold, aqueous sodium hydroxide?

Cl₂ + NaOH → NaCl + NaClO + H₂O

How does chlorine react with water?

Cl₂ + H₂O → HClO + HCl

What are the benefits and risks of the use of chlorine?

Beneficial - disinfects water so it's fit to drink, overall risk of not adding chlorine to water is greater than risk posed by chlorinated hydrocarbons

Risks - toxic gas, respiratory irritant, fatal in large concentrations, can react with organic hydrocarbons from decaying vegetation, chlorinated hydrocarbons are suspected of causing cancer

How do you test for halide ions?

React with aqueous silver nitrate

Chloride - white ppt

Bromide - cream ppt

Iodide - yellow ppt

What is the sequence for testing for anions?

Carbonate - no others react to produce effervescence

Sulphate - carbonate ions also produce white precipitate

Halide - silver carbonate and silver sulphate re also precipitates

Define enthalpy

A measure of the heat energy in a chemical system

Define enthalpy change of reaction

The enthalpy change that accompanies a reaction in the molar quantities expressed in the chemical equation, under standard conditions

Define enthalpy change of formation

The enthalpy change when one mole of compound is formed from its elements in their constituent states, under standard conditions

Define enthalpy change of combustion

The enthalpy change when one mole of substance reacts completely with oxygen, under standard conditions

Define enthalpy change of neutralisation

The enthalpy change when one mole of water is produced in the reaction between an acid and an alkali, under standard conditions

How is energy change calculated?

Q \= mc∆T

What does n represent in ∆H \= q/n

The number of moles of the limiting agent

Define average bond enthalpy

The average enthalpy change that takes place when breaking one mole of gaseous molecules by homolytic fission

Explain Hess' Law

If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route

What is an enthalpy cycle?

An indirect way to determine enthalpy change, showing two possible routes for a reaction

Define rate of reaction

The change in concentration of reactants per unit time

How is rate of reaction calculated from a graph?

Using the gradient

Define catalyst

A substance that increases the rate of reaction without being consumed by the overall reaction and lowers the required activation energy

How does a catalyst work?

1. Reactants are adsorbed onto the catalyst surface, forming weak bonds

2. The bonds within the reactants are weakened

3. New bonds form between the reactant compounds

4. The new products desorb from the catalyst surface

Why do catalyst have great economic importance?

They allow increases sustainability of reactions by decreasing the activation energy of the reaction, meaning that lower temperatures and pressures are required for successful reactions to take place. This means that less specialised, reinforced equipment is needed to reinforce these high conditions, and so less fossil fuels are consumed to provide the thermal energy to react.

In which direction does the Boltzmann distribution curve shift when temperature increases?

To the right

Define dynamic equilibrium

The forward and backward reactions take place at equal rates in a closed system

Explain Le Chatelier's principle

When a change is made to a system in equilibrium, the position of equilibrium moves to oppose the change

What conditions are required for the Haber process?

450°c

200 atm

Iron catalyst

Define electron affinity

The energy change when one mole of gaseous atoms each acquire an electron to form one mole of gaseous 1- ions

How do you set up a half cell?

Fill a beaker with an aqueous ion solution and insert a metal electrode of the same element or an inert electrode (e.g. platinum). A salt bridge (paper dipped in KNO) should be used to connect the two solutions

List some of the limitations of predictions using standard electrode potential

They give no indication of the rate of reaction

If the concentration is not 1 moldm⁻³ then the equilibrium of the half cells shifts and affects the overall cell potential

Conditions may not be stanard

How is rate of reaction calculated?

Rate \= Change in Concentration/Change in Time

Define order of reaction

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

Define half-life

The time taken for the concentration of a reactant to reduce by half

Define rate-determining step

The slowest step in a multi-step reaction sequence

How is the rate constant calculated?

K \= rate/[A][B]

In a time-concentration graph, what shape graph do zero order, first order, and second order reactions produce?

Zero - no gradient, straight line

First - downward curve with constant half-life (exponential decay)

Second - steep downward curve

In a rate-concentration graph, what shape graphs do zero order, first order, and second order reactions produce?

Zero - horizontal straight line

First - straight line through the origin

Second - upward curve

Explain the use of clock reactions

In a clock reaction, you are measuring an average rate of change in reactant over time. Over time, you can assume that the average rate of reaction is constant. The shorter the period of time over which an average is measured, the less the rate changes over that period of time. The initial rate measured during a clock reaction is reasonably accurate provided that less than 15% of the reaction has taken place

What is the effect of temperature on rate constants?

Increases the rate constant by increasing the chance of successful collision through kinetic energy

What does the Arrhenius equation show?

An exponential relationship between the rate constant and temperature (measured in kelvin)

How is the Arrhenius equation expressed in logarithmic form?

lnK \= -(Ea/RT) + lnA

Which part of the logarithmic expression of the Arrhenius equation represents the gradient of a graph?

-(Ea/R)

What anagram is used when calculating the equilibrium constant from experimental results?

ICE (initial rate, change in concentrations, equilibrium)

What stages are required to calculate Kp?

ICE

Mole fraction

Partial pressure

What does an equilibrium position of 1 suggest?

Equilibrium is halfway between reactants and products

What is the effect of concentration on the equilibrium constant?

Unaffected. The equilibrium shift results from the fact that the rate constant does not change.

How do you calculate pH changes after dilution?

1. Find the concentration of the diluted acid

2. Find the pH of the acid before and after it was diluted

3. Calculate pH change

How is the acid constant of a weak acid calculated?

Ka \= ([H⁺] [A⁻]) / [HA]

How can Ka be calculated experimentally?

Prepare a standard solution of a weak acid of known concentration and measure the pH using a pH monitor

What is the value of the water constant?

1.00 x10⁻⁷

How is pH calculated?

pH \= -log₁₀[H⁺]

How do you calculate the pH of strong bases?

Kw \= [H⁺][OH⁻] \= 10⁻¹⁴

Define buffer solution

A system that minimises pH changes when small amounts of an acid or a base are added

Which two components are contained in a buffer solution?

A weak acid and a conjugate base?

How could a buffer be prepared?

Weak acid and its salt - e.g. mixing ethanoic acid in water causes its partial dissociation and is the source of the weak acid. Sodium ethanoate is then mixed into the water, which dissolves completely providing a source of conjugate base ions

Partial neutralisation of an acid - e.g. NaOH mixed with an excess of a weak acid partially neutralises it, leaving some unreacted weak acid and forming conjugate base

What is a Bronsted Lowry acid and base?

Acid - a proton donor

Base - a proton acceptor

Explain conjugate acid-base pairs

Contains two species that can be intervconverted by the transfer of a proton, e.g. H₂O and OH⁻

Define dibasic acid

An acid which releases 2 hydrogen ions per molecule