3.1.4 Energetics

What energy change is breaking & making bonds associated with?

• Breaking bonds: endothermic reaction (energy is required to break bonds)

• Making bonds: exothermic reaction (energy is released to form bonds

What is an endothermic & exothermic reaction?

• Endothermic reaction: energy in breaking bonds > energy out making bonds (positive enthalpy change)

• Exothermic reaction: energy in breaking bonds < energy out making bonds (negative enthalpy change)

What are examples of endothermic & exothermic reactions?

Endothermic:

• thermal decomposition

Exothermic:

• combustion of fuels

• neutralisation

Define enthalpy change

Heat energy change at constant pressure

Define standard enthalpy change

The enthalpy change when molar quantities of reactants react in their standard states under standard conditions

What are the standard states & conditions?

• Standard states: solid, liquid or gas at room temperature

• Standard conditions: 100kPa & 298K (25°C)

What does “in standard state” mean?

The state an element/compound exists at in standard conditions (100kPa, 298K)

Draw an enthalpy change diagram for an endothermic reaction

Draw an enthalpy change diagram for an exothermic reaction

What is the equation to calculate the enthalpy change using mean bond enthalpy?

Enthalpy change = bond breaking - bond making

Define standard enthalpy of reaction

The enthalpy change for a given reaction taking place in molar quantities in standard states under standard conditions

Define standard enthalpy of formation

The enthalpy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions

Define enthalpy change of combustion

The enthalpy change when one mole of a substance is burned completely in oxygen with all substances in their standard states

What is the equation to calculate enthalpy change (change in energy) & give the units?

∆E = mc∆T

• ∆E = heat energy lost or gained (kJ)

• m = mass of water/solution (kg)

• c = specific heat capacity (4.18)

• ∆T = change in temperature (K)

What is the equation to calculate the molar enthalpy change?

∆H = ∆E / n

Draw a diagram of a simple calorimeter

How could you improve the results for a simple calorimeter?

Reduce heat loss (e.g. using a lid or wind shields to prevent a draught moving the flame)

Why is a flame calorimeter better than a simple calorimeter?

To reduce heat loss it has the following features:

• spiral chimney made of copper

• the flame is enclosed

• the fuel burns in pure oxygen, rather than air to ensure complete combustion occurs

How do you measure the enthalpy change for a reaction occurring in (aq) ?

1. Use an expanded polystyrene cup as a calorimeter (good insulator to reduce heat loss)

2. Add acid first & measure the temperature change

3. Then add alkali/solid, stir & measure the temperature change again

What is Hess’s Law?

The total enthalpy change of a reaction is independent of the route taken

How should your Hess’ cycle be set up using enthalpy of formation?

arrows go up:

How should your Hess’ cycle be set up using enthalpy of combustion?

arrows go down:

What are the rules for the signs, regarding the arrows?

• Go with the arrow: sign stays the same (add)

• Go against the arrow: sign changes (minus)

What is the enthalpy of an element?

The enthalpy of all elements in their standard states (the states in which they exist at 100kPa & 298K) is defined as 0

Define bond dissociation enthalpy

The amount of energy required to break one mole of a specific covalent bond in the gaseous state

Define mean bond enthalpy

The average energy required to break one mole of a specific type of bond in the gaseous state, averaged across a range of different compounds

Why may experimental methods for enthalpy determination not be very accurate?

• Heat is lost to the surroundings

• Not in standard conditions

• Reaction may not go to completion

Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?

• Bond enthalpies are a mean for the same bond across different molecules

• Standard enthalpy of combustion & formation apply just to that molecule, so they are more accurate

How do you calculate an accurate temperature change in a calorimetry experiment (extrapolation) ?

1. During the experiment, record the temperature at regular intervals, beginning a couple of minutes before you start the reaction

2. Plot a graph of your results

3. Draw two lines of best fit: one going through the points from before the reaction started & one going through the points after it started

4. Extend both lines so that they both pass the time when the reaction started

5. The distance between the two lines at the time the reaction started (before any heat was lost) is the accurate temperature change for the reaction