Unit 5 Flashcards
Temperature vs heat
temperature is a measure of the average kinetic energy of the particles whereas heat is a measure of the energy content of a substance
Laws of conversation of energy
Although energy can be exchanged between open and closed systems and the surroundings, the total energy of the process cannot change
Enthalpy
Total amount of energy of a particle, regardless of its form.
Enthalpy change
Amount of enthalpy (energy) change in a system. This equals to the heat.
Exothermic reaction
Products have less enthalpy than the reactants. Heat energy is given off by the system to the surroundings
- The temperature of the surroundings increases
- The temperature of the system decreases
Endothermic reactions
Products have more enthalpy than the reactants. Heat energy is absorbed by the system from the surroundings
- The temperature of the surroundings decreases
- The temperature of the system increases
Energy profiles
shows the energies of the reactants, the transition state(s) and the products of the reaction with time
Transition time
stage during the reaction at which chemical bonds are partially broken and formed. It is very unstable – it cannot be isolated and is higher in energy than the reactants and products
Activation energy
the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction’
Standard enthalpy change
heat transferred at constant pressure under standard conditions and states
- A pressure of 100 kPa
- A concentration of 1 mol dm-3 for all solutions
- Each substance involved in the reaction is in its standard state
- Temperature of 298K
Standard enthalpy change of reaction
The enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions
Standard enthalpy change of formation
The enthalpy change when one mole of a compound is formed from its elements under standard conditions
Standard enthalpy change of combustion
The enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions
Standard enthalpy change of neutralisation
The enthalpy change when one mole of water is formed by reacting an acid and alkali under standard conditions
Calorimetry
technique used to measure changes in enthalpy of chemical reactions. Made up of polystyrene cup, vacuum flask, metal can
Specific heat capacity formula and definition
energy required to raise 1 g of a substance’s temperature by 1.
Q = m x c x Delta T
Assumptions about calorimetry experiments
That the specific heat capacity of the solution is the same as pure water, i.e. 4.18 J g-1 K-1
That the density of the solution is the same as pure water, i.e. 1 g cm3
The specific heat capacity of the container is ignored
The reaction is complete
There are negligible heat losses
Enthalpy of combustion experiments
- Use the heat released by a combustion reaction to increase the heat content of water. Usually used to measure temperature changes to water.
- Not all heat produced by combustion reaction transferred to water, some heat loss to surroundings, some absorbed by calorimeter. To Minimise this, calorimeter shouldn’t be placed too far above the flame and lid placed over it.
Bond enthalpy / Formation
Average energy required to break / make 1 mol of gaseous bond into gaseous atom
Overall enthalpy changes in enthalpy reactions
- If more energy is released when new bonds are formed than energy is required to break bonds, the reaction is exothermic. The products are more stable than the reactants
- If more energy is required to break bonds than energy is released when new bonds are formed, the reaction is endothermic. The products are less stable than the reactants
Average bond energy
The energy needed to break one mole of bonds in a gaseous molecule averaged over similar compounds
Bond enthalpy formula
Enthalpy change for bonds broken + Enthalpy change for bonds formed
ΔHrxn = BE reactant - BE formula
Why is bond enthalpy less accurate
because bond enthalpy values are averaged values, Delta H value calculated becomes less accurate than by using other methods.
What are sources of invalidity of bond enthalpy reactions
- Bond enthalpy is invalid to use when there are Non-gaseous state substance in a reaction.
- Bond enthalpy is measured for covalent bonds. So, if there are metal or ionic compound in a reaction, it is not valid to use.
Hess’s law
The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same.”
