Chapter 9 - Enthalpy

Question 1

define enthalpy

Answer 1

“enthalpy, H, is the measure of the heat energy in a chemical system”

Question 2

equation for enthalpy change

Answer 2

delta H = H(products) - H(reactants)

Question 3

what occurs in exothermic reactions

Answer 3

energy transferred from the system —> the surroundings
any energy loss by the system = energy gain by surroundings
the temperature increases
delta H is negative

Question 4

what does the enthalpy diagram look like for an exothermic reaction

Answer 4

the line for reactants is higher than the line for products

Question 5

what to remember when drawing enthalpy diagrams

Answer 5

the line for activation energy Ea goes up only

the line for delta H goes in only one direction

Question 6

what happens in endothermic reaction

Answer 6

energy transferred from the surroundings —> the system
any energy gain of the system = energy loss of surroundings
temperature decreases

Question 7

what does the enthalpy profile look like for an endothermic reaction

Answer 7

the line for reactants is lower than the line for products

Question 8

define activation energy and what it actually means

Answer 8

“the activation energy is the minimum energy required for a reaction to take place”

it is the energy for the bonds in the reactants to break

Question 9

what are standard conditions

Answer 9

standard pressure = 100KPa
standard temperature = 298K
standard conc. = 1moldm^-3
standard state = the state of any substances under these conditions

Question 10

define enthalpy change of reaction

Answer 10

“the enthalpy change of a reaction under standard conditions and with the molar quantities shown in the equation and the reactants and products in their standard states”

always refers to a stated equation

Question 11

define enthalpy change of formation

Answer 11

“the enthalpy change that takes place when ONE mole of a compound is formed from its ELEMENTS under standard conditions with all reactants and products in their standard states”

enthalpy change of formation for an element is always 0

Question 12

define enthalpy change of combustion

Answer 12

“the enthalpy change that takes place when ONE mole of a substance reacts COMPLETELY with oxygen under standard conditions and with all products and reactants in their standard states”

Question 13

define enthalpy change of neutralisation

Answer 13

“the energy change of a reaction of an acid with a base to form one mole of H2O under standard conditions and with all products and reactants in their standard states”

Question 14

how do we calculate the energy change of surroundings

Answer 14

q = M x C x deltaT

where q is energy in joules, m is mass in grams, c is specific heat capacity, and T is temperature in Kelvin

Question 15

what is the method for the determination of enthalpy change of combustion

Answer 15

1) measure a known volume/mass of water
2) Add a reactant to the spirit burner and weigh
3) place the spirit burner below the water and light, stir the water
4) measure the temperature change and mass change after the experiment
- - calculate energy change using q = mct
- - calculate mass and mol of reactant and thus energy per mole

Question 16

why might the experimental value of enthalpy changes differ from the book value

Answer 16

• heat loss to the surroundings
• incomplete combustion
• evaporation of reactant from wick
• non-standard conditions

Question 17

calculating the enthalpy change of reaction

Answer 17

for reactions using 2 solutions or one solution and a solid do a similar method to before but use a polystyrene cup to mix them in

Question 18

what is the purpose of a cooling curve

Answer 18

when conducting the above experiments for combustion and reaction you can plot the temperature change on a graph and extrapolate the curves to give the true max value. this can then be used

Question 19

what is average bond enthalpy

Answer 19

the amount of energy required to break one mole of specific bonds IN A GASEOUS MOLECULE

Question 20

what are the limitations of using average bond enthalpies

Answer 20

it is simply an average and thus true values can vary depending on the specific molecule which the bond is in.

Question 21

energy released/ required for bond breaking/making

Answer 21

energy is required to break bonds and energy is released when making bonds

Question 22

State Hess’ law

Answer 22

“if a reaction can take place by two or more routes and the starting and finishing conditions are the same then the total enthalpy change is the same for each route”

Question 23

things to remember when doing bond enthalpies

Answer 23

they’re always positive and watch out for significant figures

Question 24

for bond enthalpy equations, what should you put if the products are usually diatomic e.g. H2 or Br2 from HBr

Answer 24

just leave them as H or Br
so
HBr —> H + Br

Question 25

what to remember about bond enthalpy definitions

Answer 25

IN A GASEOUS MOLECULE