8.1 [Heat energy and enthalpy] Flashcards
What are the 2 most important forms of energy?
Chemical energy
Heat energy
What is chemical energy made up of?
Kinetic energy which is a measure of the motion of particles
potential energy which is a measure of how strongly these particles interact with each other
What is heat energy?
Heat energy is the portion of the potential energy and the kinetic energy of a substance that is responsible for the temperature of a substance
The heat energy of a substance is directly proportional to its absolute temperature
What is enthalpy and enthalpy changes?
Enthalpy is a measure of the total energy of system
You cannot directly determine the enthalpy of a system but can measure the enthalpy change (delta H) that takes place during a physical or chemical reaction
The enthalpy change of a process is the heat energy that is transferred between the system and that surroundings at constant pressure
What are exothermic and endothermic reaction?
Exothermic - Where heat energy is transferred from the system to the surroundings. Exothermic reactions negative values
Endothermic - Where heat energy is transferred from the surroundings to the system Endothermic positive values
What are example of exo and endothermic reactions
Exothermic
Freezing
Condensing water vapour
Dissolve sodium hydroxide in water
Reaction between dilute hydrochloric acid and aqueous sodium hydroxide
Combustion
Endothermic
Melting ice
Evaporating water
Dissolving ammonium nitrate in water
Reaction between dilute ethanoic acid and solid sodium hydrogen carbonate
Photosynthesis
What will the values be for endothermic and exothermic reactions
Exothermic reactions have negative enthalpy change values
Endothermic reactions have positive enthalpy change values
What is the system in a reaction?
The system refers to the reaction mixture. Everything outside of the system is called the surroundings
How can endothermic and exothermic reactions be recognised?
Exothermic reactions can be recognised because they result in an increase in temperature. Like when hydrochloric is added to aqueous sodium hydroxide the temperature of the reaction mixture increases.
Endothermic reactions often produce a decrease in temperature of the reaction mixture, for example when solid sodium hydrogen carbonate is added to aqueous citric acid, Any reaction that has to be continually heated for a reaction to take place is endothermic.
What are standard conditions?
Standard enthalpy change conditions are 100kPa pressure and a stated temperature
How is a enthalpy change diagram constructed for an exothermic reaction?
In an exothermic reaction, the final enthalpy of the system is less than its initial enthalpy.
How is enthalpy change diagrams constructed for an endothermic reaction?
In an endothermic reaction, the final enthalpy of the system is more than its initial enthalpy
What is the change in enthalpy given by?
Energy of the products - Energy of the reactants
For an exothermic reaction the enthalpy of the reactants > than the enthalpy of the products
For an endothermic reaction the enthalpy of the products > than then enthalpy of the reactants
What are the key points to focus on when constructing an enthalpy change diagram?
Only need to label the vertical axis.
The formula for both the reactants and products should be given including their state symbol
The values for delta H should be given, including correct sign.
Not essential to show the activation energy in an enthalpy level diagram.
What is standard enthalpy change of combustion?
Standard enthalpy change of combustion is the enthalpy change measured at 100kPa and a specified temperature. When one mole of a substance is completely burned in oxygen.
How would the combustion of Hydrogen be written in an equation form
H2 + 1/2O2 > H2O
What is the experiment that determines that enthalpy changes of combustion of a liquid?
To find the enthalpy change of combustion of a liquid, a known mass of the liquid is burned and the heat energy produced is used to heat a known volume of water.
Following procedure:
A spirit burner containing the liquid under test is weighed.
A known volume of water is added to a copper can.
The temperature of the water is measured.
The burner is lit.
The mixture is constantly stirred with the thermometer.
When the temperature of the water has reached approximately 20oC above its initial temperature, the flame is extinguished and the burner is immediately reweighed
How is enthalpy change of combustion calculated?
Stage 1: Calculate the heat energy, Q transferred to the water using the equation
Q=mc delta T, m is mass of water c is specific heat capacity of the water approximately 4.18JgK.
Stage 2: Calculate the amount, n, of ethanol burned
Stage 3: Calculat the deltac H using equation delta H = -Q/n
What are some possible sources of error from the experimental determination of enthalpy change of combustion of a liquid?
Some of the heat energy produced in burning is transferred to the air and not the water
Some of the ethanol may not be burn completely to form carbon dioxide and water. (Incomplete combustion would cause soot to form on the bottom of the Copper can)
Some of the heat energy produced in burning is transferred to the copper can and not the water
The conditions are not standard. For example, water vapour not liquid water, is produced
As the experiment takes a long time not all of the heat energy transferred from the water to the surroundings is compensated for.
What is standard enthalpy change of neutralisation?
The standard enthalpy change of neutralisation is the enthalpy change measured at 100Kpa and a specified temperature, usually 298K, when one mole of water is produced by the neutralisation of an acid with an alkali.
Neutralisation of HCl
HCl + NaOH > NaCl + H2O
Why when strong acids react with strong alkalis is the standard enthalpy change of neutralisation very constant?
Making the assumption that strong acids and alkalis are fully ionised in aqueous solution, then the reaction between them is simplified to:
H+ + OH- > H20
What is the experimental determination of enthalpy change of neutralisation?
Wear safety glasses and a lab coat
Using a pipette fitted with a safety filter, place 25.ocm3 of 1.00 mol dm3 acid into an expanded polystyrene cup
Measure the temperature of the acid
Using a pipette fitted with a safety filter, place 25.0 cm of the alkali usually sodium hydroxide with a greater concentration than 1.0mol to make sure all acid is neutralised
Measure the temperature of the alkali
Add the alkali to the acid stir with thermometer and measure max temp reached
How is enthalpy change of neutralisation calculated?
calculate the mean starting temperature using the initial temperature of the acid and the initial temperature of the alkali. use this to work out the temperature change
Then work out the total volume of the solution heated so volume of acid + volume of alkali
Then work out the mass of the solution
Then use specific heat capacity to calculate Q = total mass of solution x SHC temperature change
What are the sources of error and assumptions made in this experiment?
Answers should be given to only 2 significant figures as delta T is given to only 2 significant figures.
There are the usual uncertainties of measurements involved with the use of the pipette and the thermometer. Additionally, some heat energy will be transferred to the thermometer and the polystyrene cup.
