Chapter 9 - Enthalpy Flashcards
Define the conversation of energy
- energy cannot be made/destroyed
* only converted from one form to another
Define Enthalpy
•the heat content is stored in a chemical system (chemical energy)
Define enthalpy change
- heat exchanges with surroundings during chemical change
* difference between the enthalpy of the product and reactants
Calculate enthalpy change out of the reactants and the products enthalpy change
🔺H = H (products) - H (reactants)
Calculate the percentage of uncertainty
(n x maximum uncertainty) / (size of measurement) x 100
Define exothermic
Enthalpy of products is smaller than enthalpy of reactants - heat lost to surrounding (negative)
Define endothermic
Enthalpy of products is greater than the enthalpy of the reactants - heat taken in by surrounding (positive)
What do enthalpy profiles involve?
- enthalpy on y-axis
- progress of reaction on x-axis
- reactants
- products
Does an EXOTHERMIC enthalpy profile got down to up or up to down?
Up to down
Does an ENDOTHERMIC enthalpy profile go down to up or up to down?
Down to up
Define activation energy (Ea)
It’s the minimum energy required to start a chemical reaction by breaking of bonds
What are the standard conditions?
- 🔺H(s/c/f/r) ^Ø
- T = 25°C / 298 K
- P = 100kPa = 1 atm
- C = 1 moldm^-3
Calculate the energy exchange
q = mc🔺T
- q = energy exchange with surrounding (J)
- m = mass of surrounding (g)
- c = specific heat capacity of surrounding (Jg^-k^-)
- 🔺T = temperature change
Calculate enthalpy change
🔺H = -q/n •🔺H = enthalpy change (Jmol^-) •-q = enthalpy change with surrounding (J) •n = mol
Define the observer effect
Changes in act of observation will make on phenomenon being observed. The result of the instruments that alter the state of what they measure in some manner
E.G a thermometer may heat or cool the solution
Define the standard enthalpy change of combustion
Enthalpy change takes place when one mole of a substance reacts completely with oxygen, understand the standard conditions, all of the reactants and products in standard state
Define bond dissociation enthalpy (🔺Hd^ø)
Enthalpy change that takes place when breaking, by homily’s is fission, 1 mol of a given bond in the molecules of gaseous species
Define homolytic fission
- same number of electrons on either atom
- covalent bond
- endothermic
Overall enthalpy change = ………………………………
Bond broken - bonds formed
Define average bond enthalpy
Enthalpy change when one mole of bond of gaseous bond is broken
Why may the data book value of enthalpy bond be different from enthalpy bond calculated?
Bond enthalpy are averages of bond energies in a variety of compounds
Define Hess’ Law
If a reaction can take place by more than one route and the initial and final concentration are the same, the total energy change is the same for each route
Define enthalpy cycle
Allows indirect determination of enthalpy change from other enthalpy changes using Hess’ law
Calculation using enthalpy cycle
Route 1 = route 2
🔺H1 = 🔺H2 + 🔺H3
Define enthalpy changes of combustion
Enthalpy change takes place when one mole of a substance reacts completely with oxygen under standard conditions
State why the standard enthalpy of formation for hydrogen gas is zero
It is an element
Define enthalpy change of formation
enthalpy change takes place when one mole of a compound is formed from its constituent elements in standard states
Suggest two reasons why endothermic reaction needs heating to a higher temperature
- to overcome activational energy
* higher temperature is needed to break the bonds
Define enthalpy change of neutralisation
Energy change that accompanies the reaction of an acid by a base to form one mole of water, under standard conditions
Give two examples of exothermic reactions
Combustion
Oxidation, e.g. of carbohydrates
Give two examples of endothermic reactions
Thermal decomposition
Photosynthesis
