Enthalpy

Question 1

what is enthalpy?

Answer 1

the heat content that is stored in a chemical system (units kJmol-1)

Question 2

what is the law of conservation of energy?

Answer 2

no heat energy lost in chemical reactions, it transfers from one place to another

Question 3

draw an exothermic energy profile diagram

Answer 3

temp increases and negative energy change- heat is transferred from system to surroundings.
enthalpy of products lower than enthalpy of reactants

Question 4

draw an endothermic energy profile diagram

Answer 4

temp decreases and positive energy change- heat is transferred from system to surroundings. enthalpy of products is higher than enthalpy of reactants

Question 5

what is activation energy?

Answer 5

the minimum energy required to start a reaction (Ea)

Question 6

what are the standard conditions for standard enthalpy changes?

Answer 6

100 kPa pressure
298 K (25 degrees)
1 moldm-3

Question 7

what is the standard enthalpy change of combustion?

Answer 7

the enthalpy change when one mole of substance reacts completely with oxygen under standard conditions- always exothermic

Question 8

what is the standard enthalpy change of formation?

Answer 8

the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states- always exothermic

Question 9

what is the standard enthalpy change of formation for all elements eg H2?

Answer 9

0 kJmol-1

Question 10

what is the standard enthalpy change of neutralisation?

Answer 10

the enthalpy change that accompanies the neutralisation of an aqueous acid by an aqueous base to form one mole of water under standard conditions- always 57.5 kJmol-1

Question 11

what is the standard enthalpy change of reaction?

Answer 11

the standard enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions- depends on moles in the equation

Question 12

what are the 2 methods of experimentally determining enthalpy change?

Answer 12

1) spirit burner for combustion
2) solutions in polystyrene cup for neut and reaction

Question 13

describe the method for using a spirit burner to determine enthalpy change?

Answer 13

you need to know:
1) water temp change
2) mass of water
3) mass of fuel burnt

Question 14

describe the method for using a polystyrene cup to determine enthalpy change?

Answer 14

for neut- mixing 2 solutions, acid + base
for reaction- mixing 2 solutions or a solid placed into a solution

we need to know:
1) temp change of solution
2) mass of solution
3) mass of stuff, depends lol

Question 15

what are the 2 equations to calculate enthalpy change from experimental data?

Answer 15

1) q= mc(delta)T
energy transferred= mass of surroundings x 4.18 x temp change

2) (delta) H= q / n
enthalpy change = energy transferred / moles

ALL ENTHALPY CHANGE VALUES NEED A + OR -

Question 16

what are the errors that could happen in calorimetry?

Answer 16

1) heat loss to surroundings
2) not done under standard conditions

Question 17

what are the errors that could happen in a combustion experiment specifically?

Answer 17

1) liquid fuel may evaporate
2) heat capacity of beaker not considered
3) incomplete combustion

Question 18

how do cooling curves work?

Answer 18

issue: we often get an incorrect max temp as solution starts to cool immediately.
1) initial temp readings are taken for a temp start
2) reactants mixed in a cup
3) temperature recorded at regular intervals

Question 19

is energy required or released to break bonds?

Answer 19

required

Question 20

is energy required or released to make bonds?

Answer 20

released

Question 21

what does mexo bendo mean?

Answer 21

making bonds= exothermic
breaking bonds= endothermic

Question 22

why are reactions exothermic (in terms of making and breaking bonds)?

Answer 22

more energy released when making bonds than required to break bonds

Question 23

what are reactions endothermic (in terms of making and breaking bonds)?

Answer 23

more energy is required when breaking bonds than released when making bonds

Question 24

what is bond enthalpy?

Answer 24

when one mole of bonds are broken in a gaseous molecule at 298K.

Question 25

what is the equation for enthalpy change (in terms of bond enthalpy)?

Answer 25

(delta)H= sum of bond enthalpies of bonds broken- sum of bond enthalpies of bonds made

Question 26

what are errors associated with bond enthalpy calculations?

Answer 26

1) using average bond enthalpies- not specific to the reaction
2) always in gaseous states- not standard for many substances

Question 27

what are the 3 reasons that reactions can’t experimentally be set up?

Answer 27

1) high activation energy
2) slow reaction rate
3) more than one reaction taking place

Question 28

what is hess’ law?

Answer 28

if a reaction can take place by more than one route and the initial + final conditions are the same, the total enthalpy change is the same