Energetics

Question 1

Exothermic vs endothermic processes

Answer 1

Exo- energy is released from reaction. Temp of surroundings increases
Endo- energy is taken in by the chemical from surroundings. temp ov surroundings decreases
Breaking= endo
Making=exo

Question 2

Enthalpy chang definition

Answer 2

the change in heat energy at a contant pressure.

Question 3

Standrad conditions for enthaply change

Answer 3

100KPa (1 atm pressure)
298 K
1.0 mol dm-3 conc

Question 4

Activation energy definition

Answer 4

the minimum energy needed to start a reaction

Question 5

Mean bond enthalpy def

Answer 5

Energy required to break one mole of a covalent bond into gas atoms, averaged over a range of different compounds.

Question 6

Standrad enthaply deltafHø

Answer 6

the enthaply change when 1 mole of substance is formed from its cnstituent elements under standard conditions with all reactants and products beibg in the standard state.

Question 7

Formation of ammonia

Answer 7

Reaction would form 2 moles, so have to divide by 2to form 1 mole.

Question 8

Enthaply of Na(s)

Answer 8

0- Na is an element
Na(l) would not be 0 because its not the standard state of Na

Question 9

Why may the enthalpy of combustion of a compund be hard to measure.

Answer 9

Inomplete combution may occur.

Question 10

Hess Law definition

Answer 10

The enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.

Question 11

Give one reason why the bond enthalpy that you calculated above is differnt from the mean bond enthalpy quoted in a data book

Answer 11

The data book value is averaged over a range of differnt compounds.

Question 12

Q=mcdeltaT

Answer 12

Q=energy (J)
M= mass of water (g)
c= Specific heat capacity (4.18)
Delta T= change in temp.

Question 13

Delta H= Q/n

Answer 13

Delta H= Enthaply change
Q=energy taken in by reaction
n= number of moles

Question 14

Combustion reactions (type 1)

Answer 14

Equipment: Thermometer, calorimeter, water, combusting fuel
Method: the energy released by the fuel is transferred to the water and temperature rise of the water is measured. The specific heat capacity of water allows us to use temp rise of water to calculate energy transferred.

Question 15

CR (type 2)

Answer 15

Two solutions reacting- solution B reacts with solution A to produce a reaction mixture of B and A
use the total water volume when calculating mass.

Question 16

CR (type 3)

Answer 16

Adding a solid to a solution
Add solid B to solution A to produce reaction mixture A and B. Limiting reagent is always used for the moles in calculating Q

Question 17

Assumption made in calorimetry

Answer 17

The energy transferred to the water from the reacting chemicals is equal to the energy released by the reaction.

Question 18

Sources of error of combustion calorimetry and Improvements to minimise error

Answer 18

Heat lost to surroundings
Incomplete combustion
Heat energy transferred to the metal calorimeter
Some fuel evaporates
Improvements: Add a lid- reduces heat loss, insulate sides of calorimeter- reduces heat loss, reduce distance between flame and beaker- reduces heat loss, put sleeve around flame to protect it from draughts.

Question 19

Sources of error with solution calorimetry and Improvements to minimise error

Answer 19

Heat lost to surroundings
Improbements: add a lid- reduces heat loss
Insulate calorimeter-reduces heat loss.

Question 20

Steps to measure enthalpy change using a cooling curve

Answer 20

Record the temperature for a suitable time (3 mins) before adding reactants together to establish an accurate initial temp. mix reactants then record temp every minute until a trend is seen. Plot a graph of tep against time, extrapolate the cooling curve back to the point of addition, to establish a theorectical temp change accounting for heat loss.