chapter 9 enthalpy Flashcards
what is enthalpy?
•a measure of heat energy in a chemical system
•sometimes thought of as energy stored in bonds
what is an enthalpy change?
in a reaction reactants and products are likely to have different enthalpies and the difference is the enthalpy change
ΔH = H(products) - H(reactants)
what is a chemical system?
atoms, molecules or ions making up chemicals
what is the law of conservation of energy?
fundamental rule of science stating that energy cannot be created or destroyed
what is an exothermic reaction?
reaction in which the enthalpy of products is smaller than the enthalpy of the reactants resulting in heat loss to surroundings (-ΔH)
what is an endothermic reaction?
reaction in which enthalpy of products is greater than the enthalpy of the reactants resulting in heat being taken in from surroundings (+ΔH)
what is activation energy (Ea)?
minimum energy required to start a reaction by breaking the bonds
what does a very large activation energy mean for the rate of reaction?
a large energy barrier means that a reaction may take place extremely slowly or not at all
what sign is used to show standard conditions and what are they usually?
•298 K (25°C) and 100 kPa (1 atm)
•the symbol is ⦵
•1moldm^-3 for solutions and standard state is the physical state of a substance under these conditions
what is the standard en of reaction (ΔrH⦵)?
enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions with all reactants and products in standard state.
what must you do to equations to find ΔrH⦵?
balance them ensuring it is to the amount of moles the question asks for
e.g. Mg + 0.5O2 ~> MgO
what is the standard enthalpy change of formation (ΔfH⦵)?
enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard states
why do all elements have ΔfH⦵ of 0KJmol^-3?
it means one mole of an element is formed from its element so there is no change in enthalpy
what is the standard enthalpy change of combustion (ΔcH⦵)?
enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions with all reactants and products in standard states
what is the standard enthalpy change of neutralisation (ΔneutH⦵)?
enthalpy change that accompanies the reaction of an acid by a base to form one mole of water under standard conditions with all reactants and products in standard states
what is ΔneutH⦵ for all reactions and why is it the same?
-57kJmol^-3 and it is the same because all reactions involve H+ and OH- forming one mole of H2O
what are the three things needed to calculate an energy change?
mass, specific heat capacity, temperature change
what is mass measured in for enthalpy?
grams
what is specific heat capacity? and what is the shc of water?
•energy required to raise the temperature of 1g of a substance by 1kelvin.
•for water c=4.18g^-1K^-1
what is the equation for heat energy?
q=mcΔT
what are the three steps required for calculating ΔcH?
• calculate energy using q=mcΔT and convert to kJ
•calculate the amount in mol of substance burned
•do q(in kJ) divided by mol to calculate amount of energy lost per mol
how accurate are experimental ΔcH values?
less accurate than data book values because methods are less sophisticated
what are some possible reasons for less accurate experimental ΔcH values?
•heat loss to surroundings other than water
•incomplete combustion of fuel
•evaporation of fuel from wick of spirit burner
•non standard conditions
how do we determine ΔrH?
using polystyrene cups and a thermometer to measure temp change
