What is the activation energy?
- It is the minimum energy required for a reaction to occur
- Minimum energy required to start a reaction by the breaking of bonds
What is an endothermic reaction?
- A reaction in which the enthalpy of products is greater than the enthalpy of reactants, resulting in heat being taken from the surroundings
- Delta H is positive
- Energy is being transferred from the surroundings to the system
- The temperature of the surroundings decreases as they lose energy
What is an exothermic reaction?
- A reaction in which the enthalpy of products is smaller than the enthalpy of the reactants resulting in heat loss tot he surroundings
- Delta H is negative
- Energy is being transferred from the system to the surroundings
- The temperature of the surroundings increases as they gain energy
What is enthalpy, H?
- The heat energy stored in a chemical system
- It is a measure of the eta energy in a chemical system
What is enthalpy change, Delta H?
- The difference between the enthalpy of the products and the enthalpy of the reactants
- Any heat energy lost/gained by the system is exchanged with the surroundings
- H(products)-H(reactants)
What is an enthalpy profile diagram?
Shows the enthalpy of the reactants and products along with the activation energy
What are the standard conditions?
100KPa, 1molperdmcubed, 298K, standard state is the physical state of the substance under standard conditions
What is average bond enthalpy?
- The energy required to break one mole of a specified type of bond in a gaseous molecule
- The average bond enthalpy change when 1 mol of a given type of bond is broken by homolytic fission in a gases state
What type of reaction is breaking bonds?
- Breaking bonds is an endothermic process, consequently average bon enthalpies are always positive values
- In an endothermic reaction, more bonds are broken than are made
- In an exothermic reaction, more bonds are made than are broken
- MEXO BENDO
How can bond enthalpies be used to calculate values for enthalpy changes of reactions?
DeltaHr = sum(bond enthalpies in reactants) - sum(bond enthalpies in products)
What is the first law of thermodynamics?
Energy can only be transferred, not created or destroyed (energy is conserved)
What are energy level diagrams?
They show the difference in energy of the reactant and products and do not include the activation energy
What are examples of exothermic reactions?
- Combustion
What are examples of endothermic reactions?
- Most decomposition
How would you measure enthalpy changes using a calorimetry experiment?
- The reactants are placed in an insulating polystyrene cup and a lid is fitted to minimise heat losses and the reaction is stirred with the thermometer
- The heat energy given out (or taken in) is used to heat (or cool) a known mass of water (the surroundings)
- Water has a specific heat capacity of 4.2 Jpergrampercelcius and in simple laboratory calorimetry, it is assumed that all solutions have the same specific heat capacity and density of water. Then use equations.
What are the equations?
- Q=mcdeltaT (mass of liquid!)
2. DeltaH = -Q/n (moles of solid) (Q must be in KJ)
What is the standard enthalpy change of reaction?
- The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactanst and products being their standard states
- DeltaHr
What is the standard enthalpy change of formation?
- The enthalpy change when one mole of product is formed from its constituent elements under standard conditions, all reactions and products being in their standard states
- DeltaHf
What is the standard enthalpy change of combustion?
- The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions, all reactants and products being in their standard states
- DeltaHc
What is the standard enthalpy change of neutralisation?
- The enthalpy change, under standard conditions, when one mole of water is produced as a result of the reaction between an acid and an alkali
- The energy changer that accompanies the reaction of an acid by a base to from one mole of H2O (l), under standard conditions with all reactants and products in their standard states
- The value is the same for all neutralisation reactions
- DeltaHneut
What could be wrong with your experiment if your DeltaH value is too high (not negative enough)? Delta Hc Less exothermic than expected
- Heat loss to the surrounding other than the water. This includes the beaker but mainly the air surrounding the flame
- (Incomplete combustion as there may be some incomplete combustion with carbon monoxide and carbon being. produced instead of carbon dioxide and you would see the carbon as a black layer of soot on the beaker)
- Evaporation of methanol from the wick, as the briner mist be weighed s soon as possible after extinguishing the flame, otherwise some ethanol may have evaporated from the wick. Spirit burners usually have a cover to reduce this error
- TO HELP: use of draught screen and an input of oxygen gas could minimise errors from the heat loss and incomplete combustion
What could be wrong with your experiment if your DeltaH value is too low (not positive enough)?
- Non standard conditions, the data book value is a standard delve and the conditions for this experiment are unlikely to be identical to standard conditions
What is the specific heat capacity (c)?
The energy required to raise the temperature of 1g of substance by 1K
What does Hess’ law state?
If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route
