Chem Exam Main Topics Flashcards
What is Polymerisation?
When alkenes join together
When alkenes join together, what happens to their double bond?
Their double bond opens up
How do you find the monomer used to make an addition polymer?
You find the repeat unit, add a double bond between the carbon atoms and remove the single bonds from each end
What do all alkenes have?
At least one C=C double bond
What are single covalent bonds in organic molecules?
Sigma bonds
When are sigma bonds formed?
When two orbitals overlap in a straight line, in the space between 2 atoms, giving the highest possible electron density between the 2 atoms
Why are sigma bonds the strongest type of covalent bond?
There is a high electron density between the nuclei so there is strong electrostatic attraction between the nuclei and shared pair of electrons, so there is high bond enthalpy
What is a double bond made up of?
A sigma bond and a pi bond
How is a pi-bond formed?
When 2 lobes of p-orbitals overlap sideways
In a pi-bond, how is the electron density spread out?
Above and below the nuclei
Why do pi-bonds have lower bond enthalpy than sigma bonds?
The electron density in a pi-bond is spread out above and below the nuclei, causing the electrostatic attraction between the nuclei and shared pair of electrons to be weaker in pi-bonds
In alkenes, what does the C=C bond contain?
A pi bond and a sigma bond
What causes alkenes to form sterioisomers?
The restricted motion around the C=C double bond
Ethene is said to be ‘completely planar’, what does this mean?
All the H-C-H bond angles are all the same. The atoms all lie in the same plane
What are sterioisomers?
Molecules which have the same structural formula, but different arrangement in space
When do stereoisomers occur?
When two double bonded carbon atoms each have 2 different groups or atoms attatched to them
What is the ‘E-isomer’?
The isomer which has the same groups positioned across the double bond
What is the ‘Z-isomer’?
The isomer which has the same groups positioned either both above or both below the double bond
What happens to the E/Z system when all the groups are different around the C=C double bond?
You assign each group/atom a number of priority. The 2 atoms/ groups with the highest mass number are labelled 1, whilst the 2 atoms or groups with the lowest mass number are labelled 2.
When do you use the cis-trans naming system?
If the carbon atoms have at least one group in common
When do you use the ‘cis’ naming system?
The the groups are on the same side of the double bond
When do you use the ‘trans’ naming system?
When the same groups are on opposite sides of the double bond
When does the cis-trans naming system not work?
When the carbon atoms have totally different groups attached to them
What happens in an alkene electrophilic reaction?
The alkene double bond opens up and atoms are added to the carbon atoms
Why do electrophilic addition reactions happen to alkenes?
Because the carbon double bond has plenty of electrons
What are electrophiles?
Electron pair acceptors-attracted to areas where there are lots of electrons
What are the 2 types of electrophiles?
Positively charged ions and polar molecules (as the delta+ is attracted to electrons)
What is required for the reaction between ethene and hydrogen?
A nickel catalyst, 150 degrees
What happens when halogens react with alkenes?
They form dihalogenoalkanes
What happens when you shake an alkene with bromine water?
The brown bromine water is decolourised as bromine is added across the double bond to form dibromoalkane
How do you form ethanol from ethene?
React ethene with steam with a solid phosphoric acid catalyst, 300 degrees and a pressure of 60-70 atm.
What colour is acidified potassium manganate?
Purple
What happens when you shake an alkene with acidified potassium managnate?
The purple solution is decolourised, and the alkene is oxidised to form a diol.
What is formed when alkenes react with hydrogen halides?
A halogenoalkane
What happens when hydrogen halides are added to an unsymmetrical alkene?
There are two possible products
What makes a carbocation more stable?
Having more alkyl groups attached to it
Why do alkyl groups attached to a carbocation make it more stable?
Because the alkyl groups feed electrons towards the positive charge
What is more likely to form, the stable carbocation or the unstable carbocation?
The stable carbocation
What is Markownikoff’s law?
The major product from an addition reaction of a hydrogen halide(HX) to an unsymmetrical alkene is the one where the hydrogen adds to the carbon with the most hydrogens already attached
What is the first way of determining whether an organic substance is an alkene?
Add bromine water to the substance and it will turn from brown to colourless
What is the second way of determining whether an organic substance is an alkene?
Add acidified potassium manganate (purple) and shake which will decolourise the manganate as a diol is formed
How do you test for a carbonyl group?
Add small amount of 2,4-dinitrophenylhydrazine and a yellow/orange precipitate will slowly form
What is Brady’s reagent?
2,4-dinitrophenylhydrazine dissolved in methanol and conc sulfuric acid
What is the main difference between aldehydes and ketones?
Aldehydes are easily oxidised, whilst ketones aren’t
How do you make Tollens’ reagent?
Dissolve silver nitrate in aqueous ammonia
What happens when an aldehyde is heated in a test tube with Tollens’ reagent?
A silver mirror is formed
What is the ionic equation of the formation of a silver mirror?
Ag(NH3)2 + +e- —-> Ag +2NH3
What is Fehling’s solution?
A blue solution of complexed copper (2) ions dissolved in NaOH
What happens when Fehling’s solution is heated with an aldehyde?
The blue copper(2) ions are reduced to a brick-red precipitate of copper(1) oxide. Cu2+ +e- –> Cu+
What is Benedict’s solution?
The same as Fehling’s solution, however the copper (2) ions are dissolved in sodium carbonate instead of sodium hydroxide
What happens when Benedict’s solution is heated with an aldehyde?
The blue copper (2) ions are reduced to a brick-red precipitate of copper(1) oxide
What happens when you heat an aldehyde with acidified dichromate ions?
A carboxylic acid is formed
When an aldehyde is oxidised by acidified dichromate ions, what is the colour change of the dichromate ions?
Orange to green
If something contains a methyl carbonyl group, what must it be?
Either ethanal or a ketone with at least one methyl group
What happens when carbonyls which contain a methyl carbonyl group are heated with iodine in the presence of an alkali?
If there is a methyl carbonyl group a yellow precipitate of triidomethane will form and an antiseptic smell
How do you test for primary or secondary alcohols?
Warm with potassium dichromate and add dilute sulfuric acid and the substance will change from orange to green
How do you test for a carboxylic acid?
Add some sodium carbonate solution and carbon dioxide will be formed
How do you test for an acyl chloride?
Add substance to water and hydrogen chloride will be formed
What happens when phenol is added to sodium carbonate solution?
It dissolves without the evolution of carbon dioxide
What happens when you add phenol to water, shake and then add a few drops of iron chloride solution?
A purple colour is seen
What happens when you add phenol/phenylamine to brown bromine water and shake?
Bromine colour goes and white precipitate is formed
What is the empirical formula?
The simplest whole number ratio of atoms in a compound
When an organic compound containing carbon and hydrogen (and oxygen) is completely burned, what are the products?
Carbon dioxide and water
When a completely combusted and carbon dioxide is formed, where has all the carbon in the carbon dioxide come from?
The organic compound
What is produced in an SN1 mechanism?
A racemic mixture of 2 optical isomers which won’t rotate plane-polarised light
How many steps are there in an SN1 mechanism?
2 steps
What happens in the first step of an SN1 mechanism?
A group breaks off, leaving a planar (flat ion)
What happens in the second step of an SN1 mechanism?
The planar ion can be attacked by either side, resulting in 2 optical isomers
What is the rate determining step of an SN1 mechanism, hence what is the rate equation of an SN1 mechanism?
The rate determining step is the first step, so the rate equation is r = k X (concentration of halogenoalkane)
Do Primary halogenoalkanes react by SN1 or SN2?
Only by SN2
Do Secondary halogenoalkanes react by SN1 or SN2?
Both
Do Tertiary halogenoalkanes react by SN1 or SN2?
Only SN1
What is produced in an SN2 mechanism?
A single enantiomer product, which rotates plane-polarised light differently to the reactant
How many steps are there in an SN2 mechanism?
1 single step where the nucleophile always attacks the opposite side to the leaving group, so only 1 product is produced
What is the rate equation of an SN2 mechanism?
r = k X (conc of halogenoalkane) X (conc of OH-)
What equation relates concentration of hydrogen ions and the pH value?
pH = -log(conc of H+)
For strong, monoprotic acids, what does the concentration of H+ equal?
The acid concentration
To find the pH of a weaker acid, what do you use?
Ka, the acid dissociation constant
What is Ka?
An equilibrium constant, which applies to a particular acid at a specific temperature regardless of the concentration
What is Kw?
The ionic product of water, Kw = (H+)(OH-)
For pure water, what is the ratio of OH- ions to H+ ions?
1 to 1
At room temperature, what is the value of Kw?
10^-14
How do you calculate pKa?
pKa = -log (Ka)
List in acidic order, sodium hydoroxide, hydrochloric acid, ammonia, sodium chloride, propanoic acid:
1-Hydrochloric acid, 2- Propanoic acid, 3-Sodium chloride, 4 -Ammonia, 5-Sodium hydroxide
When you dilute a strong acid, what does the pH increase by?
A factor of 1
When you dilute a weak acid by 10, what does the pH increase by?
0.5
At the half equivalence point, what does pH equal?
pKa
What does the pH of a buffer solution equal?
pH = pKa + log(A- / HA)
Why does water have an abnormally high melting and boiling point?
Because of the hydrogen bonds between the water molecules
Why is the density of ice far less than the density of water?
In ice a lattice is formed which wastes a lot of space. When ice melts some of the hydrogen bonds are broken and the lattice breaks down, allowing molecules to fill their spaces
In order for a substance to dissolve in another, what 3 things must happen?
- Bonds in substance must break
- Bonds in solvent must break
- New bonds have to form between substance and solvent
What is the case when a substance dissolves?
The strength of the new bonds formed are about the same as, or greater than the strength of the bonds broken
What are the two main types of solvent?
Polar and non-polar solvents
What are polar solvents?
Solvents made up of polar molecules, such as water
Why do many polar molecules have limited solubility in water?
Because they either do not form hydrogen bonds with the water, or the hydrogen bonds they form are weak compared with the hydrogen bonds in water
What is the ‘rule of thumb’ for what dissolves in what?
Like dissolved in like, e.g. crude oil is made up of alkanes dissolved in one another
Why can alcohols dissolve in water?
Hydrogen bonds can form
Why does the solubility of alcohols decrease as the length of the carbon chain increases?
The carbon chain part of the alcohol isn’t attracted to the water, so the more carbons there are, the less soluble it is
What type of solvent is water?
A polar solvent
When an ionic substance is mixed with water, what happens?
The ions in the ionic substance are attracted to the oppositely charged ends of the water molecules. The ions are pulled away from the ionic lattice by the water molecules, which surround the ions.
What is hydration?
The process in which water molecules pull apart ions from an ionic lattice and surround it
Why do some ionic substances not dissolve?
Because the bonding between their ions is too strong
Why can’t halogenoalkanes dissolve?
They contain polar bonds but their dipoles aren’t strong enough to form hydrogen bonds with water
As halogenoalkanes can dissolve can form permanent dipole-permanent dipole bonds, what does this mean they can dissolve in?
Polar solvents that form permanent dipole-permanent dipole bonds
What happens to the solubility of group 2 hydroxides as you go down the group?
The solubility increases
What happens to the solubility of group 2 sulfates as you go down the group?
The solubility decreases
Is barium sulfate soluble or insoluble?
Insoluble
What are compounds that have very low solubilities said to be?
Sparingly soluble
What is thermal decomposition?
When a substance breaks down when heated
What does it mean if a substance is very thermally stable?
More heat is required to break it down
What are carbonate and nitrate ions examples of?
Large negative ions (cations)
How are large negative ions made unstable?
By the presence of a positively charged ion ( a cation)
What happens when a cation polarises an anion?
It is distorted, the greater the distortion, the less stable the compound
Why do large cations cause less distortion than small cations?
They have a lower charge density, so as you go down the group the cations distort the anion less, so the more stable the carbonate/nitrate compound is
Why does thermal stability increase down a group?
Larger cations cause less distortion, so the more thermally stable the nitrate/carbonate compound is
Why are group 2 compounds less thermally stable than group 1 compounds?
The greater the charge on the cation, the more distortion and less stable the carbonate/nitrate compound is
Do group 1 carbonates thermally decompose?
No, they are thermally stable other than lithium carbonate which decomposes to lithium oxide + carbon dioxide
Do group 2 carbonates thermally decompose?
Yes, and they form the oxide + carbon dioxide
Do group 1 nitrates thermally decompose?
Yes to form the nitrite (NO2) and oxygen
Do group 2 nitrates thermally decompose?
Yes, to form the oxide, nitrogen dioxide and oxygen
How do you test how easily nitrates decompose?
- Measure how long it takes to produce a certain amount of oxygen (e.g. relight a glowing splint)
- How long it takes form a certain amount of brown gas (NO2) to be produced, which needs to be done in a fume cupboard as NO2 is toxic
How do you test how easily carbonates decompose?
How long it takes for a certain amount of carbon dioxide to be produced
How do you test for carbon dioxide?
Use lime water, which will turn cloudy with carbon dioxide
What is the enthalpy change?
The heat energy change in a reaction at a constant pressure, units kJ/mol
What is an exothermic reaction?
A reaction which gives out energy, so delta H is negative, so the tempreature often goes up
What is an endothermic rection?
A reaction which absorbs heat energy, so delta H is positive, and the temperature often falls
What is the relationship between stability and enthalpy?
The less enthalpy a substance has, the more stable it is
What is the activation energy?
The minimum amount of energy needed to begin breaking reactant bonds and start a chemical reaction
Why do we use standard conditions (100kPa and 298K) when measuring enthalpy change?
Because changes in enthalpy are affected by temperature and pressure, so standard conditions means everyone knows exactly what the enthalpy change is describing
What is the standard enthalpy change of reaction?
The enthalpy change when the reaction occurs in molar quantities shown in the chemical equation, under standard conditions
What is the standard enthalpy change of formation?
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions
What is the standard enthalpy change of formation?
The enthalpy change when and acid and alkali react together, under standard conditions to form 1 mole of water
What is the standard enthalpy change of combustion?
The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions
How do you find the standard enthalpy changes of combustion?
Use a calorimeter to find out how much heat is given out by a reaction by measuring the temperature change of water
-As the fuel burns it heats the water, and ideally all all the heat given out by the fuel is absorbed by the water
What equation do you use to calculate energy changes?
q = mcT
Why is the actual combustion enthalpy often higher than observed?
- Heat can be lost through the calorimeter
- The fuel may not have burned completely
What is Hess’s law?
The total enthalpy change of a reaction is exactly the same, no matter which route is taken
When reactions happen, what happens with bonds?
Bonds are broken and product bonds are formed. The overall enthalpy change of a reaction is the overall effect of these 2 changes
Is bond breaking endothermic or exothermic?
Bond breaking is endothermic
Is bond forming endothermic or exothermic?
Bond breaking is exothermic
What is bond enthalpy?
The amount of energy required to break 1 mole of a type of bond in a molecules in the gas phase
What is the mean bond enthalpy?
The energy needed to break one mole of bonds in the gas phase, averaged over many different compounds
What is the enthalpy change of reaction in terms of bond enthalpies?
Sum of bond enthalpies of reactants - Sum of bond enthalpies of products
What is the lattice energy?
The energy change when 1 mole of an ionic solid is formed from its gaseous ions under standard conditions
What is the enthalpy change of hydration?
The enthalpy change when 1 mole of gaseous ions dissolves in water
What is the enthalpy change of solution?
The enthalpy change when 1 mole of a solute dissolves in water
What does the enthalpy change of solution equal?(Hess’s cycle)
-lattice energy + enthalpy change of hydration
What is entropy?
The measure of disorder of particles (the number of ways they can arrange themselves)
What happens to the value of entropy as you go from solid, to liquid to gas?
The entropy increases
What is the relationship between energy and entropy?
The more energy particles have, the more entropy they have
What is the relationship between number of particles and entropy?
The more particles a substance has, the more entropy it has
What happens to entropy as the temperature increases?
The entropy increases
Why is the big changes in entropy at the melting and boiling point?
Because the particles change state instead of just vibrating more
Why is the increase in entropy at the boiling point far greater than the increase in entropy at the melting point?
There are far more ways a gas can arrange itself than a liquid compared to the amount of ways a liquid can arrange itself compared to a gas
What are the units we use for entropy?
Joules / K / mol
What does the entropy change of a system equal?
Entropy of products - Entropy of reactants
Why do particles move to increase their entropy?
So that they are more energetically stable
Why are some endothermic reactions feasible(can happen without the addition of energy) when their energy change is endothermic?
Due to favourable entropy values
What does the total entropy change equal?
Entropy of system + Entropy of surroundings
What does the entropy of surrounding equal?
- delta H / Temperature
What sign will the entropy change of surroundings be for an endothermic reaction?
Negative
What does it mean when the total change in entropy is positive?
The reaction is kinetically feasible, meaning that it can react spontaneously at standard conditions
What does it mean when the total change in entropy is negative?
The reaction is not kinetically feasible, so therefore is not spontaneous at standard room temperature
What is the equilibrium constant Kc?
The ratio of products to reactants at equilibrium
For homogenous equilibria, what is included in the expression for Kc?
All the products and reactants
For heterogenous equilibria, what do you include in the expression for Kc?
Solids or pure liquids
Why don’t you include solids or pure liquids in the expression for Kc when dealing with heterogenous equilibria?
Because their concentrations stay constant throughout
Why don’t you include catalysts in the expression for the equilibrium constant?
Because they do not affect equilibrium concentrations, they only speed up the rate at which dynamic equilibrium is reached
What does the total pressure in a gas mixture equal?
The sum of the partial pressures of the individual gases
What is the equilibrium constant, Kp?
The constant you use when dealing with equilibria involving gases
To calculate Kp, what do you do?
You put in the individual partial pressures
What is the unit of partial pressure?
Atm
Why does changes in concentration and pressure not affect the value of Kc and Kp?
They affect the amount of products and reactants present at equilibrium, but the ratio stays the same
What does change in tempreature do to the value of Kc and Kp?
It alters the amounts of products and reactants present at equilibrium, and changes the value of Kc and Kp
