Chapter 9 Flashcards
What is enthalpy H?
a measure of the heat energy in a chemical system
What does the chemical system refer to?
the atoms, molecules, or ions making up the chemicals
Can enthalpy be measured?
no
Can enthalpy changes be measured?
yes
What is the enthalpy change ΔH?
the difference in enthalpies
ΔH =
H(products) - H(reactants)
Is ΔH positive or negative?
it can be either
What does the value of ΔH depend on?
whether the products contain more or less energy than the reactants
What is the law of conservation of energy?
energy cannot be created or destroyed
In a chemical reaction involving an enthalpy change, where is heat energy transferred between?
the system and the surroundings
What is the system?
the chemicals - the reactants and products
What are the surroundings?
the apparatus, the laboratory, and everything that is not the chemical; system
What is the universe?
everything, including the system and surroundings
Can an enthalpy change ΔH be determined experimentally?
yes
How can an enthalpy change ΔH be determined experimentally?
by measuring the energy transfer between the sustem and the surroundings
What two directions can energy transfer be?
1) exothermic - from the system to the surroundings
2) endothermic - from the surroundings to the system
What do enthalpy profile diagrams show?
1) the relative enthalpies of the reactants and products
2) enthalpy chaneg ΔH
What are the characteristics of an exothermic energy change?
1) chemical system releases heat energy to surroundings
2) energy loss by chemical system is balanced by same energy gain in surroundings
3) ΔH is negative
4) temperature of the surroundings increase as they gain energy
What are the characteristics of an exothermic energy change?
1) chemical system takes in heat energy from surroundings
2) energy gain in chemical system is balanced by same energy loss by surroundings
3) ΔH is positive
4) temperature of surroundings decreases as they lose energy
What are atoms and ions held together by?
chemical bonds
What are the bonds in chemical reactions broken by?
an input of energy
What is activation energy Eₐ?
the minimum energy required for a reaction to take place
Why do reactions with small activation energies take place very quickly?
the energy needed to break bonds is readily available from the surroundings
What does a standard enthalpy change ΔH⦵ refer to?
an enthalpy H change Δ under standard conditions ⦵
What are the units of enthalpy change?
kJmol⁻¹
What is the standard pressure?
100 kPa
What is the standard temperature?
298K (25°C)
What is the standard concentration?
1 mol dm⁻³
What is the standard state?
the physical state of a substance under standard conditions
What is the standard enthalpy change of reaction ΔᵣH⦵?
the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states
What is the standard enthalpy change of formation ΔfH⦵?
the enthalpy change that takes place when one mole of a compound is formed from its elements, with all reactants and products in their standard states
What is the standard enthalpy change of combustion ΔcH⦵?
the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states
What is the standard enthalpy change of formation ΔneutH⦵?
the enthalpy change that accompanies the reaction of an acid by a base to form one mole f H₂O(l), under standard conditions, with all reactants and products in their standard states
What three quantities are used to calculate the energy change of the surroundings?
1) mass
2) specific heat capacity
3) temperature change
What is mass measured in?
grams (g)
What is specific heat capacity?
the energy required to raise the temperature of 1g of a substance by 1K
What is the specific heat capacity of water?
4.18 J g⁻¹K⁻¹
How is the temperature change ΔT determined?
T(final) - T(initial)
What is q?
heat energy
q =
mcΔT
