Enthalpy

Question 1

what is enthalpy?

Answer 1

a meaure of the energy stored in a chemical system

Question 2

what is the conservation of energy?

Answer 2

energy cannot be created or destroyed only transferred

Question 3

How do you calculate the enthalphy change?

Answer 3

product enthalpy - reactants enthalpy

Question 4

does exothermic reactions have a positive or negative enthalpy change?

Answer 4

negative

Question 5

does endothermic reactions have a positive or negative enthalpy change?

Answer 5

positive

Question 6

What does exothermic mean?

Answer 6

the products have less energy than the reactants as heat is given out to the surroundings

Question 7

what dpes endothermic mean?

Answer 7

the products have more enrgy than the reactants as energy has been taken in from the surroundings

Question 8

what are examples of exothermic reactions?

Answer 8

condensation reaction
liquid solidifies
neutralisation
vapour condensing to liquid

Question 9

what are examples of endothermic reactions?

Answer 9

decomposition reaction
liquid vapourising
solids melting

Question 10

what is the standard state of a substance?

Answer 10

the substance’s physical state under standard conditions

Question 11

What are the standard conditions?

Answer 11

1 atm / 100000 KPA
25 degrees / 298K

Question 12

what is the standard enthalpy change of reaction?

Answer 12

the enthalpy change for a reaction in the molar quantities shown in a chemical equation, under standard conditions

Question 13

what is the standard enthalpy change of combustion?

Answer 13

the enthalpy change for the complete combustion of 1 mole of a substance under standard conditions

Question 14

What is the standard enthalpy chnage of formation?

Answer 14

the enthalpy change for the formation of 1 mole of a compound from its elements in their standard states, under standard conditions

Question 15

what is the standard enthalpy change of neutralisation?

Answer 15

the enthalpy change for the formation of 1 mole of water from the neutralisation of an acid by a base under standard conditions

Question 16

How do you calculate the heat evolved/ absored (q) in a reaction?

Answer 16

q = M x C x (T1 -T2)

m = mass of substance (g)
c = specific heat capacity of water = 4.18 Jg-1K-1
triangleT = temp change (degrees or kelvin)
q (J)

Question 17

How do you calculate the enthalpy change from q = MCtriangleT

Answer 17

use equation
covert to KJ (divide by 1000)
divide by moles reacting
decide if exo or endo
decide if + or -

Question 18

What measurements do you take for calorimetry?

Answer 18

weigh mass of spirit burner with and without fuel
mass of beaker with and without water

Question 19

what are reasons why the calorimetry enthalpy change is different to the calculated enthalpy change?

Answer 19

losing heat to the surroundings
not under standard conditions
incomplete combustion
evapouration of alcohol from wick

Question 20

What are the assumptions with enthalpy changes of solutions?

Answer 20

the density is the same as water = 1gcm-3
volume (cm3) = mass (g)
the specific heat capacity of the solution is the same as water = 4.18 Jg-1K-1

Question 21

What is a chemical system?

Answer 21

the atoms, molecules or ions making up the chemicals

Question 22

How could you minimise errors in calorimetry?

Answer 22

draught screens
input of oxygen gas

Question 23

Why is a polystyrene cup used in measuring the enthalpy change on reaction?

Answer 23

offers some insulation against heat loss to the surroundings

Question 24

What is average bond enthalpy?

Answer 24

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

Question 25

Are bond ethalpies endothermic or exothermic?

Answer 25

require energy - endothermic = + value

Question 26

How do you calculate the enthalpy changes of reaction? (combustion)

Answer 26

tot bond enthalpies in reactants - tot bond enthalpies in products

Question 27

How do you calculate the average bond enthalpies of the enthalpy change of formation?

Answer 27

tot bond enthalpy in products - tot bond enthalpy in reactants

Question 28

What is Hess’ law?

Answer 28

if a reaction can take place by two routes, and starting and finishing conditions are the same, the total enthalpy change is the same for each route

Question 29

Which way to the arrows point in a reaction route with formation?

Answer 29

upwards

Question 30

Which way to the arrows point in a reaction route with combustion?

Answer 30

downwards

Question 31

Why can you only calculate approximate values for enthalpy changes on reaction?

Answer 31

the average value will not be exactly correct for any specific compound

Question 32

Why cant you use average bond enthalpies to predict the enthalpy change of formation for water?

Answer 32

the standard state of water is liquid and bond enthalpies can oonly be used for gas phase reactions as they do not take into account intermolecular forces

Question 33

Would you expect the enthalpy change of formation of water to be more or less exothermic than the calculated enthalpy change of water and hydrogen using bond enthalpies?

Answer 33

the enthalpy change of formation of water will be more exothermic than expected as there will be extra negative contribution to the enthalpy change from the formation of H bonds in liquid water
H bonds are not covered in bond enthalpies

Question 34

If an enthalpy change is negative, what does that tell you about the bond enthalpies in the reactants and products?

Answer 34

the sum of bond enthalpies in the products is greater than in the reactants