enphalpy

Question 1

what is thermochemistry

Answer 1

thermochemistry is the study of heat changes during a chemical reaction

Question 2

what happens when bonds are formed

Answer 2

energy is released when bonds are formed in the reactants.

Question 3

what is the definition for an exothermic reaction

Answer 3

an exothermic reaction releases heat energy to the surroundings . the energy required to break the bonds in the reactants is less than the energy released when the bonds in the products are formed (temp will increase )(energy change is negative) eg combustion

Question 4

what is the definition for an endothermic reaction

Answer 4

an endothermic reaction takes in heat energy from the surroundings . the energy required to break the bonds in the reactants is greater than the energy released when the bonds in the products are formed.(temp will decrease) (energy change is positive)

Question 5

what are some examples of exo reactions

Answer 5

combustion , respiration , freezing , rusting , fission , neutralisation

Question 6

what are some examples of endo reactions

Answer 6

evaporation , photosynthesis , ionisation , thermal composition

Question 7

what is enthalpy change

Answer 7

enthalpy change is the heat energy change measured under conditions of constant pressure. it is usually measured in kilojoules per mol KJmol-1

Question 8

what are the standard conditions of enphalpy changes

Answer 8

a pressure of 100kpa
a temperature of 298k
for chemicals in solution concentration of 1moldm-3
all chemicals must be in their standard states

Question 9

how do you calculate the heat change

Answer 9

q = mc(delta t)
where q= the heat change measured in joules
m is the mass of the substance being heated (in grams)
c= the heat capactity of the substance being heated (JK-1g-1)
delta h= the temp change of the substance being heated ( temp change in kelvin )

Question 10

how do you calculate the enphalpy change of a reaction

Answer 10

delta h = q/ n x 1000
where delta h is the enphalpy change in kilojoules per mol KJmol-1
n is the number of moles reacted of the limiting raegent in the reaction

Question 11

what is the definition for the enphalpy change of combustion

Answer 11

the enphalpy change when one mol of substance is completely burnt in oxygen under standard conditions , all products and reactants being in their standard state

Question 12

what is the definition for the enphalpy of formation

Answer 12

is the enphalpy change when one mol of compound is formed from its constituent elements under standard conditions , all products and reactants being in their standard state

Question 13

what is the definition for the standard molar enphalpy of reaction

Answer 13

is the enphalpy change associated with a stated reaction under standard conditions , all products and reactants being in their standard state

Question 14

what is the definition for the enphalpy of neutralisation

Answer 14

is the enphalpy change when 1 mol of water is formed from a neutralisation reaction under standard conditions , all products and reactants being in their standard state

Question 15

what is the formula for chloroethane

Answer 15

c2h5cl

Question 16

what is the formula for methyl propane

Answer 16

CH3CHCH3CH3

Question 17

what is an enphalpy change of formation that is also an enphalpy change of combustion

Answer 17

H2 + 1/2 O2—>H20

Question 18

what is the formula for propan - 2 - ol

Answer 18

CH3CHOHCH3

Question 19

what is the formula for ethanol

Answer 19

C2H5OH

Question 21

Why do we use polystyrene cups for enthalpy changes

Answer 21

Cheap , waterproof , lightweight ,and offer some insulation against heat loss to the surroundings

Question 22

What state are average bond enpalpies in

Answer 22

Gaseous

Question 23

What is the definition for average bond enthalpy

Answer 23

The energy required to break one mole of a specified type of bond in a gaseous molecule

Question 24

Is bond breaking endo or exothermic

Answer 24

Endothermic as energy is always put in to break a bond , energy change is positive

Question 25

Is bond forming endo or exothermic

Answer 25

Exothermic so energy change is negative

Question 26

Why is average bond enthalpy not a standard enthalpy change

Answer 26

Because they are determined for molecules in a gaseous state and under standard conditions not all molecules are gaseous

Question 27

How do you calculate enthalpy change from average bond enthalpies

Answer 27

Bonds broken - bonds formed ( sum of all energy stored in bonds in reactants - sum of all energy stored in products )

Question 28

What are the limitations of average bond enthalpies

Answer 28

As you are using average bond enthalpies , the actual energy involved in breaking and making individual bonds would be slightly different as the bonds may be in different environments

Question 29

What is the enthalpy change

Answer 29

The overall effect of these two changes ( bonds formed and broken ) if you need more energy to break bonds then it’s endothermic and if more energy is released it’s exothermic

Question 30

What is bond dissociation enthalpy

Answer 30

The energy needed to break the attraction between atoms and ions

Question 31

What is the enthalpy change of a reaction dependent on

Answer 31

The enthalpy change for a reaction is independent of the route taken , providing start and finish conditions are the same

Question 32

How do you do a Hess cycle for formation using formation data

Answer 32

Point arrrows up , bottom of Hess cycle is elements in standard states and enthalpy change = products - reactant

Question 33

What do you do with the arrows during combustion

Answer 33

Point them down , enthalpy change = reactants - products