C9- Enthalpy

Question 1

What is enthalpy?

Answer 1

Enthalpy (H) is a measure if the heat energy in a chemical system.

• can be thought of as the energy stored within bonds.
• enthalpy cannot be measured, but enthalpy changes can.

Question 2

What is a chemical system?

Answer 2

Refers to the atoms, molecules or ions making up the chemicals.

Question 3

What is enthalpy change and how to calculate it?

Answer 3

Is the difference in enthalpies of the reactants and products.
🔺H = H(products) - H(reactants)
- can be +ve or -ve depending on whether the products contain more/less energy than reactants.

Question 4

What does the law of conservation of energy state?

Answer 4

That energy cannot be created of destroyed.
- when a chemical reaction involving enthalpy change takes place, heat energy is transferred between the system and the surroundings.
System= the chemicals (reactants and products)
Surroundings = apparatus, lab or anything that is not the chemical system.

Question 5

Exothermic reaction.

Answer 5

Energy (heat) transferred from system to surroundings.

• energy loss by system= energy gained by surroundings.
• 🔺H = negative
• H(r) > H(p)

Question 6

Endothermic reaction.

Answer 6

Energy transferred to system from surroundings.

• 🔺H = positive.
• H(p) > H(r)
• temp of surroundings decreases as energy is lost.

Question 7

Explain activation energy.

Answer 7

In a chemical reaction, the bonds in the reactants need to be broken in order for new bonds in the products to form.
Activation energy= minimum energy required for a reaction to take place.

Question 8

What are the standard conditions?

Answer 8

Standard pressure = 100kPa
Standard temp= 298K (25*C)
Standard conc = 1mol dm-3 (solutions only)
Standard state = the physical state of the substance under the above conditions (100kPa and 298K)

Question 9

Standard enthalpy change of reaction.

Answer 9

Enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions & standard states
- it always refers to a stated equation and its value depends on the balancing numbers.

Question 10

Standard enthalpy change of formation.

Answer 10

Is the enthalpy change thats takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard state.
Compounds- equation must be balanced in a way that 1 mol is formed. Usually involves balancing with fractions.
Elements- formation of one mole of an element from its element has no change so enthalpy change of formation is always 0 kJmol-1.

Question 11

Standard enthalpy change of combustion.

Answer 11

Is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.
-Balance equation so that reactant is present in one mole.

Question 12

Standard enthalpy change of neutralisation.

Answer 12

Is the energy change that accompanies the reaction of an acid by a base to form one mole of H20, under standard conditions with reactants and products in their standard states.
- involves reaction between H+ and OH-. The value is the same for all neutralisation reactions= -57 kJmol-1.

Question 13

What are the 3 ways that energy change of the surroundings can be calculated?

Answer 13

1. Mass
2. Specific heat capacity.
3. Temperature change.

Question 14

How do you calculate energy change of surroundings using mass?

Answer 14

Identify the materials that are changing temp.
Weigh them before and after the reaction.
Measure in g.

Question 15

How do you calculate energy change of the surroundings using SHC?

Answer 15

Specific heat capacity is the energy required to raise the temp of 1g of a substance by 1 kelvin.

• every substance has a specific heat capacity. Good conductors have a small value. Insulators have a large value.
• water = 4.18 J-1K-1.

Question 16

How do you calculate energy change of the surroundings using temperature?

Answer 16

Using thermometer readings:

🔺T = T(final) - T(initial)

Question 17

What is the equation used for calculating an energy change?

Answer 17

q = mc🔺T

q= heat energy (J)
m= mass (g)
c = SHC 
🔺T = change in temp (K)

Question 18

Why is the experimental value for enthalpy change of combustion not always accurate?

Answer 18

• heat loss to surroundings.
• incomplete combustion, produces CO instead.
• evaporation of the alcohol from the wick.
• non-standard conditions
  Use of draught screens and input of O2 can minimise these.

Question 19

What is meant by average bond enthalpy?

Answer 19

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

• energy is always required to break bonds.
• bond enthalpies are always endothermic.
• bond enthalpies always have a positive enthalpy value.

Question 20

Is bond breaking and bond making endo or exothermic?

Answer 20

• Bond breaking= energy is required = endo = positive🔺H

- Bond making = energy released= exo = negative 🔺H

Question 21

How can you calculate enthalpy change from average bond enthalpies?

Answer 21

(Sum of bond enthalpies of reactants)-( sum of bond enthalpies of products)

Question 22

What are the two ways of indirectly calculating enthalpy change of reaction? (Hess’ law)

Answer 22

1. 🔺rH = (sum of enthalpy change of formation of products) - (sum of enthalpy change of formation of reactants)
2. “ = (sum of enthalpy change of combustion of reactants) - (sum of enthalpy change of combustion of products).