ENTHALPY

OCR A A LEVEL CHEMISTRY

What does system mean in a chemical reaction?

the atoms and bonds involved in the reaction

Explain the law of conservation.

the amount of energy in an isolated system stays the same / energy cannot be created or destroyed, only transferred from one system to another

Breaking bonds is...?

endothermic

Making bonds is…?

exothermic

What is an endothermic reaction?

* reaction with an overall positive enthalpy change


* products > reactants

What is an exothermic reaction?

* reaction with an overall negative enthalpy change
* reactants > products

What type of reaction is shown in this enthalpy profile?

exothermic

What type of reaction is shown in this enthalpy profile?

endothermic

What is activation energy (Ea)?

the minimum energy required for a reaction to take place

Which way does the arrow for Ea point in an enthalpy profile diagram?

always points upwards

What are the standard conditions?

* 101 kPa / 1atm
* 298K / 25°C
* 1 moldm⁻³

What does ‘standard state’ mean?

the state an element or compound exists at under standard conditions

What is standard enthalpy change of formation?

enthalpy change when 1 mol of a compound is formed from its constituent elements in their standard stated under standard conditions

What is standard enthalpy change of combustion?

enthalpy change when 1 mol of a substance reacts completely with oxygen with all reactants and products in their standard states

What is standard enthalpy change of neutralisation?

enthalpy change when 1 mol of water is formed in the reaction between an acid and a base, under standard conditions with all reactants and products in their standard states

What is standard enthalpy change of reaction?

enthalpy change accompanying a reacion in the molar quantities shown in the chemical equation in standard conditions with all reactants and products in their standard states

How can you calculate enthalpy change from experimental data?

Q = mc∆T

What are the advantages of using a bomb calorimeter?

* minimises heat loss
* pure oxygen used ensures complete combustion

Why might experimental methods for enthalpy determination be inaccurate?

* heat lost to surroundings
* not under standard conditions
* reaction may not go to completion

What is average bond enthalpy?

the mean energy required to break 1 mol of bonds in gaseous molecules

How do you calculate enthalpy change of reaction using average bond enthalpies?

reactants - products (B.E. R-P)

Why are bond enthalpies less accurate than standard energy of combustion/formation?

enthalpies are a mean for the same bond in a different molecule whereas standard enthalpies are specific to the molecule