Unit 2: Chemistry In Action

Question 0

Define endothermic

Give an example

Answer 0

A reaction in which heat is taken in as the reactants change to products
This the temperature drops

Breakdown if limestone (CaCO3) to lime (CaO) and carbon dioxide
Or heating blue copper sulphate to break into anhydrous copper sulphate and water

Question 1

Define exothermic

Give an example

Answer 1

A reaction in which heat is given out as the reactants change to products
Thus the temperature rises

E.g. Neutralising an acid with an alkali
Or anhydrous copper sulphate and water to give blue copper sulphate

Question 2

Define enthalpy change

Answer 2

A measure of heat change in which energy is taken in or given out when a chemical or physical change occurs at a constant pressure

Question 3

What is thermochemistry

Answer 3

The study of heat changes during chemical reactions

Question 4

What must happen for bonds to be broken and formed

Answer 4

Energy must be put in to break bonds
Energy must be given out when bonds are formed

So most chemical reactions involve an energy change

Question 5

If a reaction is endothermic in one direction what will it be in reverse?

Answer 5

Exothermic

Question 6

What are the standard conditions for measuring enthalpy changes

Answer 6

100kPa pressure (approx normal atmospheric pressure)
298K temperature

Question 7

What is the standard state of an element

Answer 7

The state in which it exists at 298K and 100KPA

Question 8

What does a negative enthalpy change indicate?

Answer 8

The reaction is exothermic as it has lost energy to the surroundings

Question 9

Standard molar enthalpy of formation definition

Answer 9

The enthalpy change when one mole of substance is formed from it constituent elements under standard conditions and when all reactants and products are in their standard states

Question 10

Standard molar enthalpy of combustion definition

Answer 10

The enthalpy change when one mole of substance is completely burned in oxygen under standard conditions and when all reactants and products are in their standard states

Question 11

How can we measure heat change?

Answer 11

To measure the enthalpy change we arrange for heat to be transferred into a particular mass of a substance, often water

Q=mass x specific heat capacity x temp change

Question 12

What is a calorimeter

Answer 12

An instrument for measuring get changers that accompany chemical reactions

Question 13

What is the flame calorimeter?

Answer 13

An improved version of ten simple calorimeters used for measuring enthalpy changes of combustion

It incorporates the following to reduce heat loss: spiral chimney made of copper, enclosed flame, fuel burns in our oxygen rather than air

Question 14

Why can the hydrolysis of a haloalkane by sodium hydroxide solution not be carries our in water?

Answer 14

Haloalkanes do not mix with Waite do ethanol is used as a solvent in which the haloalkane and the aqueous sodium hydroxide both mix
This called a hydrolysis reaction

Question 15

State reactivity trend of Haloalkanes

Answer 15

Less reactive as we go down the halogen group

Because C-X bond gets stronger

Question 16

Boiling point trends haloalkanes

Answer 16

Increases with increased chain length
Increases as we down the halogen group

Because more van der waals forces due to larger molecules and more electrons

Question 17

Define nucleophile

Answer 17

An ion/group of atoms with a negatively charged area/partially negative charged area that takes part in an organic reaction by attacking an electron deficient area in another reactant
Has a lone pair of electrons which it can use to form a covalent bond

Question 18

What is nucleophilic substitution

Answer 18

An organic reaction in which a molecule with a partially positive charged carbon atom is attacked by a reagent with a negative charge (a nucleophile)
It results in the replacements of one of the groups or atoms on the original molecule by the nucleophile

Question 19

What is the leaving group?

Answer 19

In organic substitution reaction, the leaving group is an atom that is ejected from the starting material, normally taking with it an electron pair forming a negative ion
(Halide ion)

Question 20

Name 3 common nucleophile a

Answer 20

Hydroxide ion -:OH
Ammonia :NH3
Cyanide ion -:CN

Question 21

In elimination reactions, what acts as a base?

Answer 21

OH- group

Removes H+ ion from the haloalkane

Question 22

What is a CFC

Answer 22

Chlorofluorocarbons are haloalkanes containing both chlorine and fluorine atoms but not hydrogen

E.g. CCL3F , CCl2F2 etc.

Question 23

General formula of haloalkane

Answer 23

CnH2n+2X shortened to R-X

Question 24

How are haloalkanes formed?

Answer 24

Chain reaction

Free radical substitution

Question 25

What are the 3 steps of free radical substitution for chlorine

Answer 25

Initiation: Cl-Cl uv>light 2Cl•
Propagation: Cl• +Ch4 > HCl + •CH3
•CH3 +CL2 > CH3Cl + Cl•
Termination: Cl• + Cl• > Cl2. or 
•CH3 + •CH3 > C2H6. Or
Cl• + •CH3 > CH3Cl

Question 26

Equations for ozone molecules being attacked by chlorine free radicals

Answer 26

Cl• + O3 > O2 + ClO•

ClO• + O3 > 2O2 + Cl•

Question 27

How do the conditions of OH- ions affect mechanism on haloalkane

Answer 27

At room temp. Dissolved in water favour substitution

At High temp. Dissolved in ethanol favour elimination

Question 28

What is a nucleophile? Give examples

Answer 28

Negatively charged atom or an atom with a delta negative charge with a lone pair of electrons

E.g. :OH- , :NH3- , :CN-

Question 29

Trend and properties of haloalkanes

Answer 29

Bonds get weaker down group
Bond enthalpies decrease down group 
Boiling point increases with increased chain length and down the group 
Polarity of bond decreases down group 
Insoluble in water - not polar enough