IB Chemistry Topic 5/15

What is the enthalpy change when a reaction is endothermic

What is the enthalpy change when a reaction is endothermic

positive

What is the enthalpy change when a reaction is exothermic

negative

What are the conditions for standard enthalpy change

100 kPa

All solutions have a concentration of 1 mols/dm³

All substances in their standard states

How can you calculate the heat loss/gain of a sample

Q = heat change

m = mass

c = specific heat capacity

ΔT = change in temperature

Q = mcΔT

What is Hess’s Law

The overall enthalpy change of a chemical reaction is independent of the pathway taken. It allows us to calculate the enthalpy change of a reaction by combining the enthalpy changes of other reactions. Provided that the starting and finishing conditions are the same in reactants and products

What is the standard enthalpy change of formation

The standard enthalpy change of formation is the heat energy released or absorbed when one mole of a compound is formed from its elements in their standard states in standard conditions. It is a measure of the stability and energy content of a compound.

What is the lattice enthalpy of a substance

The lattice enthalpy of a substance is the energy released when one mole of an ionic compound is formed from its constituent ions in the gaseous state.

What are the steps of a born haber cycle

• Enthalpy of formation

• Enthalpy of atomisation (of all reactants)

• (potentially bond enthalpy)

• Ionization

• Affinity

• Lattice enthalpy

What affects lattice enthalpy

Increasing ionic radius, decreases attraction, therefore decreases lattice enthalpy

Increasing ionic charge, increases attraction, therefore increases lattice enthalpy

What is entropy

Measure of the disorder or randomness in a system. It increases with the number of possible arrangements of particles in a system.

How do you calculate the entropy of a reaction

Entropy of products - entropy of reactants

What is the enthalpy change of breaking bonds

positive (endothermic)

What is the enthalpy change of making bonds

negative (exothermic)

What does the hydration enthalpy give

The strength of interaction between the polar water molecules and the separated ions

How do exothermic reactions change the entropy

increase in the surroundings entropy

What is the equation to calculate entropy of surroundings

Entropy of surroundings (ΔS) is calculated using the equation ΔS = -ΔH/T, where ΔH is the change in enthalpy of the system and T is the temperature in Kelvin.

What are the units of entropy

J K^-1 mol^-1

What is the second law of thermodynamics

For a spontaneous change

ΔS (total) = ΔS system + ΔS surroundings

Is greater than 0

How can endothermic reactions occur

If the change in entropy is great enough to compensate for the negative entropy change of the surroundings as heat flows surroundings to system

What is the Gibbs free energy equation

ΔG = ΔH - TΔS, where ΔG represents the change in Gibbs free energy, ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy.

What does Gibbs free energy show us

ΔG must be negative for a reaction to be spontaneous.

How does temperature affect the feasibility of a reaction according to Gibbs free energy

At a low temperature all exothermic reactions can occur

All reactions are feasible at high temperature due to it being high enough to make the effect of ΔH negligible