Topic 5.1: Measuring energy changes Flashcards
Definition of heat
A form of energy that flows from a body at high temperature to a body at a low temperature
Definition of temperature
A measure of the average kinetic energy of particles in a substance
Definition of internal energy
Total amount of energy in a sample of a substance
Definition of system
Part of the universe chosen for study
Types of system
a) Open
b) Closed
c) Isolated
Definition of surroundings
Part of the universe outside the system which the system interacts with
First law of thermodynamics (Law of conservation of energy)
Energy can be converted from one form to another, but it is neither created nor destroyed
Definition of enthalpy
Internal energy contained in a substance
Definition of enthalpy change (units)
Amount of heat exchanged with the surroundings at constant pressure (kJ mol-1)
Can enthalpy be measured?
Enthalpy changes can be measured, no absolute values of enthalpy
Exothermic reactions
a) Heat is released to the surroundings
b) Enthalpy change has negative values
c) Products are more stable than reactants
Examples of exothermic reactions
a) Bond formation
b) Combustion
c) Gas => Liquid => Solid
Endothermic reactions
a) Heat is absorbed from the surroundings
b) Enthalpy change has positive values
c) Products are less stable than the reactants
Examples of endothermic reactions
a) Bond breaking
b) Solid => Liquid => Gas
c) Photosynthesis
Standard enthalpy change
Enthalpy change of a reaction carried out under standard conditions with everything in its standard state
Standard conditions
a) Pressure
b) Temperature
c) Molarity
a) 101.3 kPa
b) 298 K
c) 1 mol dm-3
Standard enthalpy change of formation
Enthalpy change when one mole of substance in its standard state is formed from its elements in their standard states under standard conditions
Formula to calculate enthalpy change from formation
Enthalpy change of reaction = Sum of the enthalpy of formation of products - Sum of the enthalpy of formation of reactants
What is the value of the enthalpy of formation of elements in their standard state?
0
Standard enthalpy of combustion
Enthalpy change when one mole of a substance in its standard state is burned completely in oxygen under standard conditions.
Common products of combustion reactions
CO2 + H2O
What type of reaction are combustion reactions?
Exothermic
Formula to calculate enthalpy change from combustion
Enthalpy change of reaction = Sum of the enthalpy of combustion of reactants - Sum of the enthalpy of combustion of products
Requirements of thermochemical equations
a) Value of enthalpy change
b) State symbols
c) Balanced equation
Formula to calculate heat change from temperature change
q = mc∆T
Specific heat capacity
Energy required to raise the temperature of 1g of a substance by 1K
Formula to calculate enthalpy change
∆H = q / n
Enthalpy change of neutralization
Enthalpy change when an acid and base react together to form one mole of water under standard conditions
What type of reaction are neutralization reactions?
Exothermic
Steps to calculate enthalpy change of neutralization
a) Calculate the number of moles of acid and base using n=cv
b) Determine the limiting reactant.
c) Add the volumes of acid and base together (where 1cm3 = 1g) to get the mass
d) Use Q=mc∆T to calculate enthalpy change
e) Use ∆H=Q / n where n is number of moles of water produced
Assumptions at calculating enthalpy change of solutions
a) No heat loss from the system
b) All heat is transferred to water
c) The density of solutions are the same as water, so 1 cm3 of solution has 1 g.
d) The specific heat capacity of the mixture is the same as that of water.
Definition of calorimetry
Process of measuring the amount of heat released or absorbed during a chemical reaction
Assumption of calorimetry of combustion reactions
a) All heat is transferred to the water
b) Mass of water remained constant
c) Unlimited source of oxygen
d) Done under standard conditions
Errors of calorimetry of combustion reactions
a) Incomplete combustion
b) No all heat transferred to water
c) loss of heat from water to surroundings
d) Mass and specific heat capacities of container taken into account
e) Incorrect recording of mass of substance burned due to evaporation
How to compensate heat loss of water in a displacement reaction
Extrapolate the graph recorded
