3.1.4 Energetics

Question 1

What are some major errors that can occur during calorimetry?

Answer 1

• Measurement errors

• Unwanted heat loss to surroundings

• Incomplete mixings/reactions

• Incomplete combustion

Question 2

How to find the heat energy change in calorimetry?

Answer 2

You measure the temperature change in a fixed value of water.

Question 3

What is the equation to work out the heat energy?

Answer 3

q = m x c x ΔT

[q] = heat energy (J)
[m] = mass (g)
[c] = heat capacity constant (given in Q)
[ΔT] = temperature change (c° or K)

Question 4

What equation do you use to work out the enthalpy change?

Answer 4

ΔH = (q ÷ 1000) ÷ n

[ΔH] = enthalpy change (KJ mol^-1)
[q] = heat energy change (J)
[n] = number of moles (mol)

Question 5

What is mean bond (dissociation) enthalpy?

Answer 5

The enthalpy change when one mole of bonds of the same type is broken averaged over a wide variety of compounds.

Question 6

What is enthalpy?

Answer 6

It is a measure of the heat content of a substance.

Question 7

Is bond breaking endothermic or exothermic?

Answer 7

Endothermic.

Question 8

Is bond making endothermic or exothermic?

Answer 8

Exothermic.

Question 9

In exothermic reactions is the energy change positive or negative?

Answer 9

The energy change is negative.

As the reaction gets hotter.

Question 10

In endothermic reactions is the energy change positive or negative?

Answer 10

The energy change is positive.

As the reaction gets colder.

Question 11

How do you calculate mean bond enthalpy?

Answer 11

• Draw out all of the molecules in the equation, showing every bond.

• Count how many of each bond there is in each compound and add up their bond enthalpies from the data table.

• Calculate the answer of the reaction using the formula:

ΔH = Σ bonds broken - Σ bonds formed
(total enthalpy of reactants - total enthalpy of products)

(You may be given the total bond enthalpy and asked to calculate a specific bonds mean enthalpy. E.g. C=C.)

Question 12

What is enthalpy of formation (ΔHf)?

Answer 12

It is the minimum enthalpy change when one mole of a compound is formed in their standard states from their constituent elements (the elements that make them up).

Question 13

What is enthalpy of combustion (ΔHc)?

Answer 13

It is the minimum enthalpy change when one mole of a compound is completely burned in their standard state.

Question 14

What does Hess’s Law state?

Answer 14

It states that enthalpy change for a reaction is independent of the route taken.