Enthalpy changes Flashcards
What is the principle of conservation of energy?
Energy cannot be created or destroyed, but may only be converted from one form to another.
What is Hess’s law?
The energy change of any chemical reaction is the same regardless of the route taken.
What is the enthalpy change of a reaction?
A measure of the energy change of any chemical or physical reaction.
What is the standard enthalpy change of a reaction?
The enthalpy change that occurs in a reaction between molar quantities of reactants in their standard states under standard conditions.
What are standard states and conditions?
298K temperature, 1moldm-3 concentration, 1atm pressure
Standards states- the physical state of a substance under standard conditions
What is the standard enthalpy change of atomisation?
The enthalpy change that occurs when one mole of atoms of an element in the gas phase are formed from in the element in its standard states under standard conditions.
What is the standard enthalpy change of lattice formation?
The enthalpy change that occurs when one mole of an ionic compound is formed from ions of the elements in the gas phase.
What is the standard enthalpy change of lattice breaking?
The reverse of lattice formation, the energy change that occurs when one mole of an ionic compound is broken up into ions of the elements in the gas phase.
What is the standard enthalpy change of hydration?
The enthalpy change that occurs when one mole of an ionic compound in solution is formed from ions of the elements in the gas phase.
What is electron affinity?
The enthalpy change that occurs when one mole of negative ions are formed from gaseous atoms of a substance by gaining an electron.
What is ionisation energy?
The enthalpy change that occurs when one mole of gaseous positive ions are formed from gaseous atoms of a substance by losing an electron.
How can ionic substances dissolve?
Partially negative O atoms in water are attracted to the positive ion and partially positive H atoms in water are attracted to the negative ion, making the ions hydrated.
What is the standard enthalpy change of solution?
The enthalpy change that occurs when one mole of a substance is dissolved completely in a solvent under standard conditions to form a solution. It is the sum of the enthalpy of lattice breaking and the enthalpy of hydration.
What happens when the enthalpies of hydration and lattice breaking change?
If the enthalpy of hydration is greater, the salt dissolves but if the enthalpy of lattice breaking is greater, the salt will not dissolve. The more exothermic the total, the more soluble the salt.
What factors affect both enthalpy of hydration and lattice breaking?
Both are increased by increasing charge on the ions
Both are decreased by decreasing size of the ions
What are Born-Haber cycles?
Energy cycles with lots of steps. It involves turning all the initial elements into gas phase atoms, ions and then combining these to form a solid or a solution.
How to calculate an overall enthalpy change in a Born-Haber cycle?
Any time the path goes against the direction of the value you are trying to find, subtract it. Any time it goes with the value, it is added.
What happens if the enthalpy change of formation of a compound is negative?
Energy is given out as the compound is formed from its elements, telling us the compound is stable compared to its elements.
What happens if. the enthalpy change of formation is positive?
Energy is required to make the compound as energy is taken in as the compound is formed from its elements. This tells us the compound is unstable compared to its elements.
