R1.1: Measuring Enthalpy Change Flashcards
(19 cards)
What is temperature?
Measure of the average kinetic energy of particles
Measured in °C or K
What is heat?
Measure of the energy content of a substance
Measured in joules (J)
What is energy?
Energy is a measure of the ability to do work
Many different types (ex: heat)
How do chemical reactions relate to systems?
3 types of systems: open, closed, isolated
Chemical reactions -> energy flow in/out of reaction vessels
Most chemical reaction: open system
- energy + mater exchanged with surroundings
Energy can be exchanged between open <-> closed
- but TOTAL ENERGY of process cannot change (law of conservation of energy)
What is enthalpy/enthalpy change?
Also known as heat content
- The total chemical energy inside a substance (internal energy stored in chemical bonds)
- kJ/mol
Enthalpy change:
Chemical reaction = change in chemical energy = enthalpy change (ΔH)
- pos or neg (cannot measure absolute value)
- always at constant pressure (or else work done)
Explain the movement of energy in chemical reactions
No overall energy change -> energy conserved
Break bonds -> energy in
Make bonds -> energy out
Energy out > energy in -> exothermic
Energy in > energy out -> endothermic
What is an exothermic reactions?
Product have les enthalpy than reactants
Heat given off by the system to the surroundings
- temp or surrounding increase
- temp of system decreases
Enthalpy decreases -> ΔHnegative
Exothermic reactions are thermodynamically possible BUT if rate is too slow -> reaction might not occur
What is an endothermic reaction?
Product have more enthalpy that reactants
Heat is absorbed by the system from the surroundings
- temp of surroundings decrease
- temp of system increases
enthalpy increases -> ΔHpositive
What is a energy/reaction profile?
Shows how the potential energy of the reactants and products changes as the reactions proceeds
(the transition states -> stage during the reaction were chemical bonds are partially broken and formed- very unstable)
X-axis: reaction coordinate
Y-axis: potential energy
What is activation energy?
The minimum amount of energy needed for reactant particles to react to form the product
- amount of energy to reach transition state
Exothermic:
Reactant more energy than production
Closer in energy to transition state
Lower Ea
Endothermic:
Reactant less energy than production
Further in energy to transition state
Higher Ea
What is standard enthalpy change?
The enthalpy change under standard conditions including standard states and constant pressure
ΔHº = standard enthalpy change
- concentration of 1 mol/dm3 for all solutions
- kJ/mol
Standard state:
Normal, most pure stable state of a substance
100 kPa
- temperature is not part of the definition but 298K is commonly used
What is the enthalpy change of formation?
ΔHfº
Enthalpy change when 1 mole of a substance in its standard state is formed from its elements in their standard states at a pressure of 100kPa and a temperature of 298K
What is enthalpy change of combustion?
ΔHcº
Enthalpy change when 1 mole of a substance in its standard state is completely combusted in oxygen under standard conditions of 100kPa and a temperature of 298K (
What is standard enthalpy change of reaction?
ΔHꝊr
The enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions
What is standard enthalpy change of neutralization?
ΔHꝊneut
The enthalpy change when one mole of water is formed by reaction an acid and alkali under standard conditions
What is specific heat capacity (c) ?
The amount of energy required to change the temperature of 1 kg of a substance by 1 K (or 1°C)
Change in temp of surroundings -> depend on mass of material making up the surroundings and their c
- pure water: 4.18 kJ/kg/K
- assume dilute aqueous solutions of acids,bases,salts -> same c + density as pure water
What is calorimetry?
Way of measuring enthalpy changes
Simple calorimeter:
Polystyrene cup, vacuum flask/metal can
Explain an experiment for finding the enthalpy change for reactions in solution
Principle: carry out with excess of one reagent and measure the temp change over time (few min)
Assumptions made:
c of solution: 4.18 kJ/kg/K
Density of solution: 1 g/cm3
c of container ignored
Reaction is complete
There is negligible heat loss
Rate of cooling is constant
Temperature correction graphs:
Some reaction that aren’t instantaneous -> delay before max
- during delay -> lose heat to surroundings
Need to find true max
Steps:
1. Take temp before adding reactants
2. Add second reactant -> continue recording temp and time
3. Plot the graph and extrapolate until you intersect the where second reactant added
Explain an experiment for finding the enthalpy change for combustion
Principle: use the heat released by a combustion reaction to increase the heat content of water
- usually simple calorimeter used to measure the temp changes of the water
Not all heat produced by combustion reaction is transferred to water:
Lost to surrounding
Absorbed by calorimeter
Minimize heat losses:
- Copper calorimeter placed close to flame + use lid
- shielding
Main sources of error:
- Heat loss
- Incomplete combustion
- Mass and specific heat capacities of thermometer, stirrer, beaker, etc need to be taken into account
- Incorrect recording of mass of substance burned due to evaporation
