2.1 - Thermochemistry Flashcards
What 2 forms of energy are there?
Kinetic energy and potential energy
What is kinetic energy?
Energy of motion at a molecular level.
What is potential energy?
Positions of the atoms relative to one another (involves bond breaking and making)
What is the equation for an internal energy system?
the sum of kinetic energy + the sum of potential energy
What is an exothermic reaction?
One that releases energy to the surroundings, there is a temperature rise and delta H is negative.
Examples
Acids+metals
Hand warmers
Thermite reactions
What is a endothermic reaction?
One that takes in energy from its surroundings, there is a temperature drop and delta H is positive.
Examples
Melting ice
Cold packs
Thermal decomposition of group 2 carbonates
What is enthalpy (H)?
Heat content of a system at constant pressure.
What is enthalpy change? (Delta H)
The heat added to a system at constant pressure.
What is the equation for enthalpy change? (Delta H)
Delta H = H(products)-H(reactants)
Describe an energy profile for an exothermic reaction.
Reactants have higher enthalpy than products, therefore delta H is negative.
Describe an energy profile for an endothermic reaction.
Products have higher enthalpy than reactants, therefore delta H is positive.
What is the principle of conservation of energy?
Energy cannot be created nor destroyed, only changed from one form to another.
What are the standard conditions of enthalpy change?
- all substances have to be within their standard states
- a temperature of 298K (25*c)
- a pressure of 1atm (101,000Pa)
What is the definition for the standard enthalpy change of formation? delta fH
- when one mole of a substance is formed from its constituent elements in their standard states under standard conditions
- all elements in their standard state have a standard enthalpy change of formation of 0Kj/mol-1.
- per mole regards to its products
What is the definition of the standard enthalpy change of combustion? delta cH
Enthalpy change when one mole of a substance is completely combusted in oxygen under standard conditions.
- per mole regards to its reactants
What is the equation for calculating enthalpy change of a reaction?
Delta rH = the sum of delta formation enthalpy (products) - the sum of delta formation enthalpy (reactants)
- remember any exothermic values (making bonds) need to be changed to a negative
What is Hess’s law?
The total enthalpy change for a reaction is independent of the rate taken from the reactants to the products.
delta H1 = delta H2 + delta H3
energy cycles :)
In formation enthalpy cycles, which way do the arrows face?
Arrows face up (elements to the equation), meaning usually left side doesnt match cycle so thats the number to reverse.
In combustion enthalpy cycles, which way do the arrows face?
Arrows face down (equation to the elements), meaning usually right side doesn’t match so thats the number to reverse.
What is the definition of bond enthalpies?
Enthalpy required to break a covalent x-y bond into x atoms and y atoms, all in a gaseous state.
Is breaking bonds endothermic or exothermic?
Endothermic, requires energy put in by surroundings
Is making bonds endothermic or exothermic?
Exothermic, energy released to its surroundings
What is the defintion of an average bond enthalpy?
The energy needed to break one mole of a specific bond in a molecule in the gaseous state.
Its an average as for example C-H has different values in methane compared to methanol, so average is created.
Why does average bond enthalpy often not reflect standard enthalpy change?
Elements must be gaseous in average bond enthalpy, whilst in standard enthalpy change the elements must be in their standard state, which could be a liquid or a solid.
What is the definition of the standard enthalpy change of a reaction?
The enthalpy change when a reaction takes place in the molar quantities shown by the balanced chemical equation under standard conditions with all the chemicals in their standard states.
How do you find the temp change in a time/temp reaction graph?
Draw a line vertical as to where the reacting substance was added, draw a line following the decrease in temperature at the top. Where the 2 lines meet gives where the temp would have reached with no cooling. Then minus starting temp to find the temperature change.
What is the equation for the energy change of water and what do the symbols mean?
q = m c deltaT
q = energy change in water (J)
m = mass of water (g)
c = specific heat capacity of water (4.18 Jg-1k-1)
deltaT = temperature change of the water
