IB Chemistry HL - CHAPTER 5

Temperature

Temperature is a measure of the average kinetic energy of the particles of a substance.

The law of conservation of energy

The law of conservation of energy states that the total energy of a process cannot change.

Exothermic (-∆H)

A reaction is exothemic (-∆H) when the products have less enthalpy than the reactants.

Endothermic (+∆H)

A reaction is endothermic (+∆H) when the products have more enthalpy than the reactants.

Standard conditions

• A pressure of 100 kPa

• A concentration of 1 mol dm^{-3} for all solutions

• Each substance involved is in its standard state

• A temperature of 298 K

The standard enthalpy change of reaction

The standard enthalpy change of reaction (∆H_{r}^{⦵}) is the enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions.

The standard enthalpy change of formation

The standard enthalpy change of formation (∆H_{f}^{⦵}) is the enthalpy change when one mole o a compound is formed from its elements under standard conditions.

The standard enthalpy change of combustions

The standard enthalpy change of combustion (∆H_{c}^{⦵}) is the enthalpy change when one mole of a substance is burnt completely in excess oxygen under standard conditions.

The standard enthalpy change of neutralization

The standard enthalpy change of neutralization (∆H_{neut}^{⦵}) is the enthalpy change when one mole of water is formed by reacting an acid and alkali under standard conditions.

The specific heat capacity

The specific heat capacity (c) is the amount o heat required to raise the temperature of one gram of a substance by one degree Kelvin.

Q

mc∆T

The principle of experiments to find enthalpy changes for reactions in solutions

Carry out the reaction with an excess of one reagent and measure the temperature change over the course of a few minutes.

The setup of experiments to find enthalpy changes for reactions in solutions

The errors of experiments to find enthalpy changes for reactions in solutions

• The density of the solution is assumed to be 1 g cm^{-3}

• Incomplete reaction

• Heat losses

Temperature correction graphs

The principle of experiments to find enthalpy changes for reactions in solutions

Use the heat released by a combustion reaction to increase the heat content of water.

The setup of experiments to find enthalpy changes for reactions in solutions

The errors of experiments to find enthalpy changes for reactions in solutions

• Heat loss to the surroundings and the calorimeter

  • Minimised by placing a lid over the calorimeter and having a small distance between the coppor calorimeter and the flame.

• Incomplete combustion

Enthalpy equation

∆Hr = Σ∆Hf(products) - Σ∆Hf(reactants)

Two important rules for solving Hess’s Law-cycles.

1. If you follow the direction of the arrow you add the quantity

2. If you go against the arrow you subtract the quantity

The steps to solve Hess’s law-equations

1. Identify which given equation contains the product you want.

2. Adjust the equation if necessary, to give the same product. If you reverse it, reverse the sign of the ∆H value.

3. Adjust the equation if necessary, to give the same number of moles of product.

4. Identify which given equation contains your reactant.

5. Adjust the equation if necessary,to give the same reactant.If you reverse the ∆H value.

6. Adjust the equation if necessary, to give the same number of moles of reactant.

7. Add the two equations together.

8. Cancel the common items.

9. Add the two ΔH values together to get the one you want.

The standard enthalpy of formation

The standard enthalpy of formation (∆H_{f}^{⦵}) is defined as the enthalpy change when one mole of a compound is formed from its elements under standard conditions.

Average bond energy

The average bond energy is the energy needed to break one mole of bonds in a gaseous molecule averaged over similar compounds.

Stability in endothermic processes

In endothermic processes (+∆H) reactants are more stable than the products.

Stability in exothermic processes

In exothermic processes (-∆H) reactants are less stable than the products.

Bond enthalpy calculations

Transition state

The transition state is a stage during the reaction at which chemical bonds are partially broken and formed. The transition state is vey unstable.

Activation energy

The activation energy (E_a) is the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction.

Does endothermic or exothermic reactions have higher activation energies?

Endothermic reactions have a higher activation energy compared to exothermic reactions.

The steady state of the ozone cycle

Increased depletion of ozone when other chemicals are introduced