Enthalpy Changes

Question 1

What is enthalpy

Answer 1

(H) is a measure of the thermal energy stored in a chemical system

T

Question 2

What is the chemical system

Answer 2

Te atoms , molecules or ions making up the chemicals- everything else in the surroundings

Question 3

Enthalpy change

Answer 3

(Change in H) , the change/difference in thermal energy between that stored in products and stored in reactants

Question 4

Units of enthalpy

Answer 4

KJ mol-1

Question 5

How do you work out change in enthalpy

Answer 5

H products - H reactants

Energy stored in the system - not making or breaking bonds

Positive or negative , depending on whether the products contain MORE or LESS energy than the reactants

Question 6

Why do you use triangle H *

Answer 6

To sho the measurements were made under standard conditions and that the elements were in their standard states

1 atm/ 100kPA and 25 degrees/ 298K

Question 7

Conservation of energy

Answer 7

Energy cannot be created or destroyed

Question 8

Exothermic reaction

Answer 8

Change in enthalpy is NEGATIVE
Chemical system loses energy
Surroundings gain energy
Temperature of surroundings increases

E.g. Combustion of fuels

Question 9

Endothermic reaction

Answer 9

Change in enthalpy is positive 
Chemical system gains energy 
Surroundings loose energy 
Temperature of surroundings decreases
E.g. Thermal decomposition of CaCO3

Question 10

Standard enthalpy change of reaction

Answer 10

The energy change associated with a. Givern reaction under standard conditions

Occurs in the molar quantities in the equation

Question 11

HStandard enthalpy change of formation

Answer 11

Energy change that take place when one mole of a compound is formed from its elements in their standard condition

Question 12

Enthalpy of formation of an element

Answer 12

0 kJ mol-1

Question 13

Standard enthalpy change of combustion

Answer 13

Energy change hat takes pace when one mole of a substance is completely combusted

Question 14

Standard enthalpy change of neutralisation

Answer 14

Energy change associated with the formation of 1 mole of water from neutralisation

Question 15

Bond dissociation enthalpy

Answer 15

Amount of energy needed t break a bond per mole

Question 16

Average bond enthalpy

Answer 16

Energy needed to break me mole of bonds in the gas phase averaged over many different compounds

Question 17

Calorimertry?

Answer 17

Using q=mc change in T

Question 18

What do q, m, c and change in t stand for

Answer 18

Q= heat exchanges with the surroundings, joules

M= mass of the substrate heated or cooled, grams

C= the specific heat capacity of the substance that is heated or cooled-energy required to raise the temperature of 1g of a substance by 1k, expressed as Jg-1k-1

Change in temp , K

Question 19

273K

Answer 19

0 degrees C

Question 20

How do you use calorimetery to measure enthalpy change

Answer 20

Use q=mct to calculate energy change for quantities used

Work out the moles of the reactants used
Divide q by the number of reactant not in excess to give change in H
Add a sign and a unit (kJmol-1)

Question 21

How do you change Jmol-1 to kJ mol -1

Answer 21

Divide by 1000

Question 22

Bomb calorimeter

Answer 22

Equipment that MINIMISES heat loss
Uses pure oxygen , to ensure COMPLETE combustion is achieved
Ensures it accurate

Question 23

Specific heat capacity of water

Answer 23

4.18 J g-1k-1 in any reaction where the reactants are dissolved in water

Question 24

How to work out density of solution

Answer 24

SAME as water - 1 gcm-3

E.g. 25cm 3= 25G

Question 25

What is calorimeter

Answer 25

Type of experiment where stances are mixed in insulated container and temp rise is measured

Question 26

Errors in calorimetric method

Answer 26

• energy transfer from surroundings (loss)
  -approximation in specific heat capacity of soltiuion - assumes all solutions have same as water
  Ignoring energy absorbed by apparatus
  Reaction may be in compete or sow
  Density f solution is taken to be the same as water

Question 27

3 ways to measure enthalpy changes

Answer 27

Calorimetric method - using experiments

Hess’ law - can determine them INDIRECTLY

Bond enthalpies - no experiment has to be done

Question 28

Average bond enthalpy

Answer 28

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

Strength of caovalent bond

Question 29

Why are bond enthalpies always endothermic

Answer 29

Positive

• breaking bond - energy is always required and more energy is used then given out

Question 30

Limitations of bond enthalpies to calculate enthalpy change

Answer 30

An average bond enthalpy si calculated from actual bond enthalpies in different chemical environments

BUT - it can depend

Question 31

How to work out enthalpy change from bond enthalpies

Answer 31

= sum of bond enthalpies in REACTANTS- PRODUCTS

Question 32

WHAT IS hess’ law

Answer 32

The total enthalpy hinge for a reaction si independent of the route by which the chemical change takes place

If a reaction can take place by 2 routes+the starting and finishing conditions are the same - enthalpy change is Same for each route

Question 33

How do you work out enthalpy change of e.g. Formation

Answer 33

Write a balanced symbol equation for the equation

Draw enthalpy cycle- elements at the bottom and all arrows going to equation

Add enthalpies

Follow the arrows and add them

OR I use the equation =sum of products-sum fo reactants

Question 34

Combustion elements products

Answer 34

Co2 adn h20

Question 35

What are standard conditions

Answer 35

100kPa and 25 degrees C