Chemistry: Enthalpy Change ERT

Simple Experiment : 

Simple Experiment Work in pairs – 1 test tube each Each student: put 5 mL of water in their test tube and measure the temperature. Student 1 Add a half spatula load of NaOH, stir until dissolved. Feel outside of test tube and measure temperature. Student 2: Add a half spatula load of solid ammonium nitrate, stir until dissolved. Feel outside of test tube and measure temperature. Compare results. Need coats and goggles

Discussion : 

Discussion NaOH(s)  NaOH(aq) NH4NO3 (s)  NH4NO3 (aq)

Energy changes : 

Energy changes Exothermic reaction – releases heat (chemicals lose energy) Endothermic – energy is absorbed (chemicals gain energy) Heat released or absorbed called: heat of reaction(kJ/mol)

Enthalpy : 

Enthalpy Energy stored in a sample of matter. Attractions & repulsions between protons & electrons Attractions (and some repulsions) between atoms in a molecule Movement of atoms & electrons Enthaply – heat content - H

Enthalpy : 

Enthalpy Can’t measure Can measure changes From heat released or absorbed Symbol enthalpy change: H“delta H” H = Hp – Hr Not interested in values of Hp or Hr Just interested in value of H

Exothermic reaction : 

Exothermic reaction Energy released

Energy change diagram : 

Energy change diagram H +ve or -ve ? Before rx H negative

Energy change diagram : 

Energy change diagram Before rx Exothermic

Energy change diagram : 

Energy change diagram E Reactants Products Before rx After rx Before rx

Your ERT – about enthalpy : 

Your ERT – about enthalpy

Enthalpy : 

Enthalpy The energy that is gained or lost from a chemical system when a reaction occurs Read textbook p 424 - 426

Enthalpy values : 

Enthalpy values Enthalpy of combustion Enthalpy of evaporation/ vaporisation Enthalpy of fusion / melting Enthalpy of formation

How are they determined? : 

How are they determined? Research this – experiments used Calculations used Compare methods

What is the use of them? : 

What is the use of them? Enthalpy is important in choosing fuels How can knowledge of enthalpy values help is choosing fuels? Which values would be most useful? What limitations are there?