3.2.1 enthalpy changes Flashcards
First law of thermodynamics?
You can’t create or destroy energy you can only transfer energy
What piece of equipment can scientists use to identify if a reaction is endothermic or exothermic.
thermometer
define the term exothermic reaction. Give examples.
Energy released into surroundings from system
Respiration, Combustion, Neutralisation
Define the term endothermic reaction.
Give examples.
Energy is absorbed by entering the system
Thermal decomposition and photosynthesis
Give the definition of enthalpy and enthalpy change with units.
Enthalpy (h) - Amount of energy stored in a system
Enthalpy change (delta h) - change in energy when reactants become products
How is enthalpy change calculated?
Enthalpy of products - enthalpy of reactants
Enthalpy change sign for Exothermic and endothermic.
+ = Exothermic
- = Endothermic
Rules for drawing enthalpy profile diagrams?
Ea arrow = Start from reactants line to the peak
Delta H = reactants line to product lines
Reactants vs products = reactants higher then products: exothermic
reactants lower then products: endothermic
Define the term activation energy:
The minimum amount of energy required for the reaction to take place.
define the term catalyst and explain how it works?
speeds up the rate of reaction without being used up in the overall equation by providing an alternative pathway that minimises the activation energy
What does ecat stand for?
activation energy without a catalyst
How can using a catalyst make a reaction more cost effective?
Can be reused at low temperature and pressure
What happens to the enthalpy change if a reaction with same products and reactants is flipped?
Delta H is also flipped
What does absolute 0 mean?
No internal energy
What us calorimetry?
Practical determination of enthalpy change
if water cools - endothermic
if water heated -exothermic
3 piece of data need to calculate enthalpy change using calorimetry?
Mass of water
temperature change of water
specific heat capacity
In calorimetry what is the units for q,m,c and delta T
Q = j
M = g
c = j/g
Delta t = Change in temp c
What is the main steps to calculate the enthalpy change experimentally?
1)Use your results to calculate enthalpy change using q=mcdeltat and convert to kj by dividing by 1000
2)Calculate mol using the required equation
3)Caculate enthalpy change /mole
4) know if its endo or exothermic w +/-
What is enthalpy of neutralisation
The enthalpy change for the formation of 1 mole of H2O from neutralisation
Ionic equation with state symbols for neutralisation reaction and what is the sign?
H+ (aq) + OH– (aq) → H2O (l)
The sign for enthalpy change of neutralisation = -
A student carries out an experiment to measure the enthalpy change of neutralisation. plan an experiment that a student could carry out to measure this change. (6 marks)
1) calculate the volume of acid and alkali
2) measure the initial temperature of acid and alkali which have to be the same at room temp
3) mix both acid and alkali and calculate the temperature change and combined volume.
4)then using q=mc delta T and 1cm^3 = 1g calculate the energy change (c = 4.18 as its in an aq solution)
5)calculate 1 mol and use q/n
What is hess law?
Overall enthalpy change of a reaction stays the same whichever route is taken.
What type of reaction is enthalpy of neutralisation?
Exothermic so negative -
why is it not possible to measure the enthalpy change of a reaction directly
1)high activation energy
2)slow reaction rate
3)more then one reaction taking place.
