chapter 4 - energetics

Question 1

calorimetry equation

Answer 1

q = m c change in temperature

q = heat change in joules 
m = mass of what has been heated (water) 
c = specific heat capacity (4.18)

Question 2

define enthalpy of formation

Answer 2

enthalpy change when one mole of a compound is formed from its elements in their standard states and under standard conditions

Question 3

define enthalpy of combustion

Answer 3

enthalpy change when one mole of a compound is burnt completely in oxygen with all reactants and products in their standard states and in their standard conditions

Question 4

enthalpy change

Answer 4

Is the amount of heat energy taken in or given out during any change in a system provided the pressure is constant

Question 5

exothermic reactions

Answer 5

1) Particles lose energy
2) when bonds are made energy is released
3) delta H value is negative
4) condensing and freezing are exothermic
5) combustion, most dissolving, neutralization, respiration and protein synthesis are exothermic
6) Temperature increases

Question 6

exothermic graph

Answer 6

reactants are higher than the products. reactants are less stable than the products reactions release energy into the surroundings so temperature increases. delta H is negative.

Question 7

Endothermic reactions

Answer 7

1) Particles gain energy
2) when bonds of broken energy is required
3) The Delta H value is positive
4) boiling and melting are endothermic reactions
5) The temperature decreases

Question 8

Endothermic graphs

Answer 8

The products are higher than the reactants. The products are less stable than the reactants. Reactions take in energy from the surroundings therefore the temperature decreases. delta H is positive

Question 9

What is calorimetry

Answer 9

if the energy released or absorbed by a chemical reaction is transferred to the water (surroundings) then this energy change can be measured.

Question 10

Assumptions on heat loss

Answer 10

1) lots of heat escapes to the surroundings in particular the air.
2) heat is absorbed by the apparatus more so in calorimetry as equipment is made out of metal, which is a good conductor
3) it is unlikely that the reaction will undergo complete combustion as sometimes carbon monoxide or smaller side products can be formed.

Question 11

Calculate bond enthalpy

Answer 11

delta H = sum of enthalpies of all bonds broken - sum of enthalpies of all bonds made

Question 12

Bond length and bond enthalpy

Answer 12

The smaller the bond length the stronger the attraction/bond. Therefore the higher the bond enthalpy

Question 13

relationship between bond [double/single/triple] and bond enthalpy

Answer 13

The more bonds that are [double/triple] The higher the bond enthalpy

Question 14

Why is the enthalpy value you have calculated different to the one from the data book

Answer 14

The data book uses a mean value for all (N-H) enthalpy bonds whereas we have only calculated one (N-H) bond type from one molecule [particular molecule]

Question 15

define enthalpy of neutralization

Answer 15

this is given off as the enthalpy change when an acid reacts with a base to form one mole of water

Question 16

Enthalpy of formation Hess cycle

Answer 16

delta f = sum of products - sum of reactants

both arrows point upwards

Question 17

enthalpy of combustion Hess cycle

Answer 17

delta c = sum of reactants - sum of products

both arrows point downwards (combust)

carbon combusts to form carbon dioxide (CO2)
hydrogen combusts to form water (H2O)
sulfur combusts to form sulfur dioxide (SO2)

Question 18

define Hesses law

Answer 18

if a change can be brought about in more than one way the overall enthalpy change is independent of the route taken