Enthalpy Changes | OCR A Level Chemistry

What is the difference between an exothermic and endothermic reaction regarding energy transfer?

Exothermic transfers energy from the system to surroundings; endothermic transfers energy from surroundings to the system.

How does the energy of products compare to reactants in an exothermic reaction?

Products have less energy than reactants.

How does the energy of products compare to reactants in an endothermic reaction?

Products have more energy than reactants.

What is the sign of ΔH for an exothermic reaction?

Negative.

What is the sign of ΔH for an endothermic reaction?

Positive.

Define activation energy (EA).

The minimum energy particles need to collide to start a reaction.

What are the standard conditions for measuring enthalpy changes?

100 kPa pressure, 298 K (25°C), and solutions at 1 mol dm⁻³.

What is the standard enthalpy change of reaction (ΔrH)?

The enthalpy change when the number of moles of reactants as specified in the balanced equation react together.

What is the standard enthalpy change of formation (ΔfH)?

The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions.

What is the enthalpy of formation of an element?

0 kJ mol⁻¹.

What is the standard enthalpy change of combustion (ΔcH)?

The enthalpy change when one mole of a substance is combusted completely in oxygen under standard conditions.

What is the standard enthalpy change of neutralisation (ΔneutH)?

The enthalpy change when solutions of an acid and an alkali react to produce 1 mole of water under standard conditions.

What is the formula for calculating energy change (Q) in calorimetry?

Q = m × cp × ΔT.

In the calorimetry equation Q = m × cp × ΔT, what does 'm' represent?

The mass of the solution.

What value is typically used for the specific heat capacity (cp) of solutions in these experiments?

4.18 J g⁻¹K⁻¹ (the heat capacity of water).

Why is a polystyrene cup used in calorimetry experiments?

To provide insulation and minimize heat loss to the surroundings.

How is the temperature rise determined if a reaction is slow and cooling occurs simultaneously?

By taking readings at regular intervals and extrapolating the temperature curve back to the time of addition.

What assumption is made regarding the density of solutions in calorimetry calculations?

The density is assumed to be the same as water (1 g cm⁻³).

What is a common source of error in calorimetry regarding the apparatus?

Neglecting the specific heat capacity of the calorimeter (energy absorbed by the apparatus).

When calculating ΔH per mole, which reactant's moles should be used?

The reactant that is not in excess.

How do you convert J mol⁻¹ to kJ mol⁻¹?

Divide by 1000.

What is the effect of incomplete combustion on the enthalpy change compared to complete combustion?

It is less exothermic.

What should be done with the equipment before starting a calorimetry experiment?

Wash it with the solutions to be used and dry the cup.

Why is the mixture stirred during a calorimetry experiment?

To ensure that all of the solution is at the same temperature.

If a solid reagent is used in a calorimetry experiment, how is its mass determined?

Using the 'before and after' weighing method.

What is the formula for calculating enthalpy change per mole?

ΔH = Q / number of moles

What is the sign convention for an exothermic reaction?

Negative sign

What are the three requirements for reporting enthalpy calculation answers?

Sign, unit, and 3 significant figures

What is the formula used to calculate the energy change in calorimetry?

Q = m cp ΔT

In a combustion calorimetry experiment, what does 'm' represent in the formula Q = m cp ΔT?

The mass of the water in the calorimeter

List three common sources of error in calorimetry experiments.

Energy losses from the calorimeter, incomplete combustion of fuel, and incomplete transfer of energy

Why are enthalpy changes calculated using mean bond energies often less accurate?

Mean bond energies are averages and not exact values for every specific bond

Define mean bond enthalpy.

The enthalpy change when one mole of gaseous covalent bonds is broken, averaged over different molecules

What is the state of substances when using mean bond enthalpy definitions?

Gaseous state

Does bond breaking absorb or release energy?

Absorbs energy

Does bond making absorb or release energy?

Releases energy

What is the relationship between bond breaking and bond making in an endothermic reaction?

More energy is absorbed breaking bonds than is released making them

What is the formula for ΔH using bond energies?

ΔH = Σ(bond energies broken) - Σ(bond energies made)

State Hess's Law.

The total enthalpy change for a reaction is independent of the route by which the chemical change takes place

What physical law is Hess's Law a version of?

The first law of thermodynamics (energy is conserved)

Why are Hess's Law cycles used?

To measure enthalpy changes for reactions that cannot be measured directly by experiment

What is the enthalpy of formation for elements in their standard states?

Zero

What is the formula for ΔH reaction using enthalpies of formation?

ΔH = ΣΔfH(products) - ΣΔfH(reactants)

What is the formula for ΔH reaction using enthalpies of combustion?

ΔH = ΣΔcH(reactants) - ΣΔcH(products)

Why is it difficult to measure the enthalpy of formation of a hydrated salt directly?

It is impossible to add the exact amount of water and difficult to measure the temperature change of a solid

In a reaction profile, what does the difference between the energy of reactants and the energy of products represent?

ΔH (enthalpy change)

What is the primary difference between the reaction profiles of exothermic and endothermic reactions?

In exothermic reactions, products are at a lower energy level than reactants; in endothermic, they are at a higher level

Why are measurements in combustion calorimetry often not under standard conditions?

Water is produced as a gas rather than a liquid in the experiment

What happens to the fuel in a calorimeter that causes an error after weighing?

Evaporation of the fuel

What is the significance of the 'mean' in mean bond enthalpy?

It accounts for the fact that the same type of bond has slightly different energies in different molecules

How do you calculate the number of moles of a fuel for enthalpy calculations?

Mass of fuel / Mr of fuel