Enthalpy, Hess Cycles and Calorimetry: Module 3: Periodic Table and Energy: Chemistry OCR A A Level

Why might experimental methods for enthalpy determination not be accurate?

● Heat is lost to the surroundings.

● Not in standard conditions.

● Reaction may not go to completion.

Define enthalpy change of formation.

The energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard state under standard conditions.

What is an endothermic reaction?

A reaction with an overall positive enthalpy change (+ΔH) → enthalpy of products > enthalpy of reactants.

What does activation energy mean?

The minimum energy required for a reaction to take place.

Give an example of an equation which represents standard enthalpy of formation.

There are many e.g. H2 (g) + ½ O2 (g) → H2O (l).

What does system mean in a chemical reaction?

The atoms and bonds involved in the chemical reaction.

What energy change is breaking bonds associated with?

Energy is taken in to break bonds → Endothermic reaction

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 whereas standard enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate.

What does "in standard state" mean?

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

Which way does the arrow for activation energy point on an enthalpy profile diagram?

Always points upwards.

What are the advantages of using a bomb calorimeter?

● Minimises heat loss.

● Pure oxygen used → ensures complete combustion.

What does enthalpy change of reaction mean?

The energy change associated with a given reaction.

What is an exothermic reaction?

A reaction with an overall negative enthalpy change (-ΔH) → enthalpy of products < enthalpy of reactants.

How can you calculate enthalpy change from experimental data?

Use the equation Q = mcΔT, where m is the mass of the substance being heated (usually water), c is the specific heat capacity of that substance (water’s SHC = 4.18g⋅J^-1⋅K^-1) and ΔT is the change in temperature.

What are the standard conditions?

100 kPa

298 K

Give an example of an equation which represents standard enthalpy of combustion.

E.g. C (s) + O2 (g) → CO2 (g)

Explain the law of conservation

The amount of energy in an isolated system remains the same. Energy cannot be destroyed or created, It can only be transferred from one form to another.

How to calculate enthalpy change of reaction using average bond enthalpies?

ΔH = Σ(bond enthalpies of reaction) - Σ(bond enthalpies of products)

Define enthalpy change of neutralisation.

The energy change that takes place when 1 mole of water is formed from a neutralisation reaction.

What does average bond enthalpy mean?

The mean energy required to break 1 mole of bonds in gaseous molecules.

What energy change is making bonds associated with?

Energy is released to make bonds → Exothermic reaction

Define enthalpy change of combustion.

The energy change that takes place when 1 mole of a substance is completely combusted.

Enthalpy

• a measure of the heat content of a substance at constant pressure

• you cannot measure the actual enthalpy of a substance

• you can measure an ENTHALPY CHANGE at CONSTANT PRESSURE

• written as the symbol ∆Η , “delta H

First law of thermodynamics

Energy can be neither created nor destroyed but it can be converted from one

form to another.