3.2.1 Enthalpy

Question 1

What is enthalpy?

Answer 1

measure of heat energy in a chemical system (atoms, ions, molecules making up the chemicals)

Question 2

What is the enthalpy change of a reaction?

Answer 2

Enthalpy of products - enthalpy of reactants

Question 3

What is an exothermic reaction? What does the energy profile diagram look like?

Answer 3

chemical system releases heat energy to surroundings
enthalpy change is NEGATIVE
temp of surroundings increases as they gain energy

line for products lower than line for reactants

Question 4

What is an endothermic reaction? What does the energy profile diagram look like?

Answer 4

chemical system takes in heat energy from the surroundings
enthalpy change is positive
temp of surroundings decreases as they lose energy

line for products higher than line for reactants

Question 5

What does activation energy mean?

Answer 5

the minimum energy required for a reaction to take place

Question 6

What are the standard conditions?

Answer 6

standard pressure: 100kPa
standard temp: 298K or 25 degrees celsius
standard concentration: 1 mol dm-3
standard state: physical state of a substance under standard conditions. E.g. C is actually solid

Question 7

Define enthalpy change of formation (enthalpy change associated with a stated equaton, ∆f H)

Answer 7

enthalpy change when 1 mole of a compound is formed from its elements under standard conditions, with all reactants and products being in their standard states

Question 8

Define enthalpy change of combustion (complete combuston of 1 mol of a substance, ∆c H)

Answer 8

enthalpy change when 1 mole of a substance reacts completely with oxygen under standard coniditions, with all reactants and products being in their standard states

Question 9

Define enthalpy change of neutralisaton (formaton of 1 mol of water from neutralisaton, ∆neutH)

Answer 9

enthalpy change when the reaction of an acid with a base forms 1 mole of water (l) under standard conditions, with all reactants and products being in their standard states

Question 10

Is the thermometer part of system or surroundings?

Answer 10

Surroundings

Question 11

WHat is the formula for determing enthalpy changes directly and what are the units?

Answer 11

q = mcat

m in kg
q in joules

(at = change in temperature)

usually its mass and specific heat capacity of WATER
when solid and solution is mixed together, just take the mass of the solution

Question 12

How do you convert from energy released to enthalpy change?

Answer 12

You do Q divided by 1000 to get kJ
Then divide by number of moles
AND LOOK AT RATIO IN EQUATION!

MAKE SURE TO PUT A NEGATIVE SIGN!

Question 13

For enthalpy change of combustion, why are the data book values different?

Answer 13

In experiment:

non standard conditions
incomplete combustion (so input of O2 gas)
evaporation from wick (use lid)
other heat losses to surroundings (use draught screens)

Last 3 make the experiment less exothermic

Question 14

What is average bold enthalpy? Formal definition isn’t required.

Answer 14

breaking of 1 mol of covalent bonds in gaseous molecules - always positive values since they’re endothermic
An actual bond enthalpy may differ from average depending on chemical environment of bond.

Question 15

What is bond breaking and bond making?

Answer 15

Breaking = endothermic
Making = exothermic

Question 16

What determines if an overall reaction is exothermic or endothermic?

Answer 16

BOND ENTHALPIES IN REACTANTS - BOND ENTHALPIES IN PRODUCTS

difference between energy required for breaking and energy released by making

Question 17

How do you calculate enthalpy changes indirectly?

Answer 17

Hess’ Law and enthalpy cycles
If a reaction can take place by 2 routes, and the starting and finishing conditions are the same, then the enthalpy change is the same for each route (conservation of energy)

Question 18

Using enthalpy changes of formation, what is the enthalpy change of reaction equation?

Answer 18

Total enthalpy changes of formation of PRODUCTS - total enthalpy changes of formation of reactions

Question 19

Using enthalpy changes of combustion, what is the enthalpy change of reaction equation?

Answer 19

Total enthalpy changes of combustion of REACTANTS - total enthalpy changes of combustion of products?

Question 20

What is the procedure done to determine enthalpy change of combustion using a wick burner?

Answer 20

1. measure out certain volume of water and pour into beaker, record initial temp
2. add fuel to spirit burner and weigh
3. place under beaker, light and stir water with thermometer
4. after 3 min extinguish flame and immediately record temp of water
5. reweigh spirit burner (assume wick hasnt been burnt)

Question 21

What can be used to correct heat loss? How?

Answer 21

cooling curves

add a volume of known concentration of 1 reactant e.g. copper sulfate solution to polystyrene cup.

Start stopwatch and take temp of solution every 30s until temp stays constant
Add solid to solution and stir. record temp every 30s until it has fallen for several min

plot a graph of temp against time
extrapolate cooling curve part of graph back to time when solid was added and draw vertical line from this time (on the x axis) to extrapolated curve

Question 22

A student carried out an experiment to measure the enthalpy change of combustion of methanol.
The energy from the combustion of methanol was used to heat a beaker containing water.
The student’s calculated enthalpy change of combustion was more exothermic than the value in
data books.
Which error could have caused this difference?

A Some methanol had evaporated from the wick before the final weighing.
B In the calculation, the student used the molar mass of ethanol instead of methanol.
C There was incomplete combustion.
D The water boiled for 5 minutes before the final temperature was taken.

Answer 22

B

Question 23

The student’s calculated enthalpy change was less exothermic than the value in data books.
Which of the following errors could have contributed to this result?

Error 1:
After the final temperature was recorded, the student removed the burner
from under the beaker. The flame burnt for a further 5 minutes before
weighing the spirit burner.
Error 2: The student recorded the final temperature 5 minutes after removing the
burner.
Error 3: The student spilt some water on the bench when pouring the water from the
measuring cylinder into the beaker.
A. 1, 2 and 3
B. Only 1 and 2
C. Only 2 and 3
D. Only 1

Answer 23

B