Chapter 9 Enthalpy I

Question 1

What is enthalpy ?

Answer 1

Measure of heat energy in a chemical system

Question 2

What is enthalpy change?

Answer 2

The transfer of heat energy between the a chemical system and its surroundings at a constant pressure

Question 3

What is the activation energy of a reaction?

Answer 3

The minimum amount of energy for a reaction to take place

Question 4

What are the axis for an energy profile? Where would the activation energy go? Where are the products and reactants in endo and exothermic reactions?

Answer 4

X= Progress of Reaction Y= Enthalpy
The activation energy goes from the reactants to the high point on the curve
Endo= Reactants below products
Exo= Reactants above products

Question 5

What are the standard conditions?

Answer 5

100KPa
298 K/ 25 Degrees Celcius
1 mol/dm3 concentration
All elements in standard state under these conditions

Question 6

Define the enthalpy change of reaction

Answer 6

The enthalpy change that accompanies a reaction in its molar quantities shown in the chemical equation under standard conditions, with all elements in their standard states

E.g 2Mg + O2 —> 2MgO give state symbols

Question 7

Define the enthalpy change of formation

Answer 7

The enthalpy change that accompanies the formation of one mole of a compound from its elements under standard conditions with all reactants and products in their standard states

E.g 3 C (s) + 9/2 H2 (g) + 1/2 N2 (g) —> C3H9N (l) give state symbols

Question 8

Give an equation to show the breakage of a H-Br bond

Answer 8

H-Br (g) —> H (g) + Br (g) must be gas

Question 9

Define the enthalpy change of combustion

Answer 9

The enthalpy change that accompanies one mole of a substance completely reacting with oxygen (complete combustion) under standard conditions with all elements in their standard states

E.g CH4 (g) + 2 O2 —> CO2 + 2 H20

Question 10

Define the enthalpy change of neutralisation

Answer 10

The enthalpy change that accompanies the formation of 1 mol of water when an acid reacts with a base, under standard conditions with all reactants/products in their standard states

Question 11

Does the enthalpy change of neutralisation change?

Answer 11

No, even if more reactants are used, as there is a bigger volume, the temperature change is spread over a larger area, so the same average temperature is the same

Question 12

What is the equation for calorimetery?

Answer 12

q= mass x change in temperature x SHC

Question 13

What is the SHC of water? What is the density of water?

Answer 13

4.18 j/k/g
1g/cm3

Question 14

How do you find standard enthalpy change of reaction using calorimetery?

Answer 14

Calculate energy, converting units where necessary, and using measurements of the change in temperature of the water heated/ the surrounding
N=m/mr of limited reactants
Divide the energy in kj by the moles
ADD A MINUS SIGN IF EXOTHERMIC

Question 15

Why is calculating standard enthalpy changes using calorimetery not always accurate?

Answer 15

Heat loss to the surroundings
Incomplete combustion
(Evaporation from the wick)

Question 16

Define average bond enthalpy

Answer 16

The amount of energy required to break 1 mol of a specified bond in a gaseous molecule

Question 17

What are the limitations of average bond enthalpy?

Answer 17

Actual bond enthalpy values vary depending on the chemical environment of the bond
The average bond enthalpy is only an average from multiple chemical environments

Question 18

What are bond breaking and making in terms of energy changes?

Answer 18

Bond breaking is endothermic
Bond making is exothermic

Question 19

How can you find out the calculation for enthalpy change using bonds?

Answer 19

Hess Cycle, to atoms so arrows downwards
Or think about whether more energy is given out when bonds form or when broken , then add minus sign if exothermic

Question 20

Define Hess’ Law

Answer 20

The overall enthalpy change of a reaction is independent of the route of the reaction, linked to the first law of thermodynamics- energy canot be destroyed or created

Question 21

How do the arrows point in a Hess cycle when formation values are given?

Answer 21

Point upwards

Question 22

How do the arrows point in a Hess Cycle when combustion values are given?

Answer 22

Point downwards

Question 23

Why might a reaction not be able to be determine experimentally?

Answer 23

Reaction does not occur independently, multiple products will form
Rate is too slow
Activation energy is too high

Question 24

How would you create a Hess Cycle with unfamiliar equations?

Answer 24

Start with the largest equation
Draw arrows in the correct direction at what can be substituted in
Follow the cycle, reversing signs if necessary

Question 25

With calorimetry of an equation, which reactant do you use?

Answer 25

The one which is limiting
NOT BOTH, ONLY ONE

Also, if only one liquid and a solid, use the new mass if values given

Question 26

How do you calculate percentage uncertainty with regard to thermometers in calorimetry?

Answer 26

( Uncertainty thermometer/ change in temp) x 2 x 100

Question 27

How would spilling water from the measuring cylinder to a glass beaker affect the results of the calorimetry?

How would leaving the flame on for a few minutes then turn off and record affect results?

Answer 27

Spilling water would make it more exothermic. Measured higher so in the calculation higher value, even though should be lower. Plus likely higher temp achieved so overall larger energy.

Fuel used without heating water. Less exothermic.

Question 28

How would heating water in calorimetry for longer affect accuracy?

Answer 28

Larger change in temp, so reduces percentage uncertainty so could be more accurate

But larger timings mean more heat loss to the surroundings so could decrease accuracy

Question 29

If 5cm3 of A reacts with 5cm3 of B, the change in temp is 10 degrees celcius.

If 2.5cm3 of A reacts with 2.5cm3 of B, what is the temperature change? Conc same.

Answer 29

10 degrees Celsius

Even though half as much of the reactants, half the volume, so average temperature is still the same

Question 30

What do you need to check when doing Enthalpy questions?

Answer 30

What is the question asking?
If it is a standard Enthalpy change, change the number of moles associated with the equation
E.g if it’s combustion but 2 moles of reactant, need to divide the Enthalpy by two

Question 31

How do you reduce the error from a calorimetry experiment?

Answer 31

Burn in a plentiful supply of oxygen to reduce likelihood of incomplete combustion
Add lid to beaker, insulate, draft shield to reduce heat loss
Place cap over burner when not using
Use standard conditions
Use larger masses of water/heat for longer to reduce percentage uncertainties

Question 32

Why may isomers have different combustion values?

Answer 32

Different number and types of bond breaking, so some will be more endothermic than others

However, all produce the same combustion products, so same amount of energy released

So overall change in combustion may differ

Question 33

Why might the calculated enthalpy of neutralisation from a reaction of ethanoic acid and KOH be less exothermic than expected?

Answer 33

Ethanoic acid is a weak acid
Partially disassociates, more energy would be need to fully disassociate and then release more energy