Physical: Energetics Flashcards
What is Enthalpy?
The total chemical/ heat energy that is in a substance under constant conditions
What is Enthalpy change
When energy is given to surroundings or absorbed to increase/ decrease overall energy of system
How is Enthalpy change represented
∆H under constant conditions
units= KJ/Mol
What is the enthalpy energy of an exothermic reaction and why
Negative enthalpy since the products have less energy than the reactants as energy is dissipated to the surroundings and lost
Enthalpy decreases since reactants are higher than the products
What is the enthalpy energy of an endothermic reaction and why
Positive enthalpy since products have a higher energy than reactants as energy is absorbed from the surroundings
Enthalpy increases since reactants are less than the products
What are the constant conditions for enthalpy change
100KPa
298 Kelivn
What is the definition of reaction
Enthalpy change in an equation react in standard conditions
ΔHr°
What is the definition for formation
enthalpy change when i mole of a compound is formed from its elements under standard conditions and all products and reactants are in their standard states
ΔH°f
What is the definition for combustion
enthalpy change when 1 mole of a substance is burnt in excess oxygen under standard conditions and all products and reactants in their standard states
ΔHc°
What is the definition for neutralisation
enthalpy change when 1 mole of water is formed by reacting an acid and alkali under standard conditions and all reactants and products in their standard states
ΔHneut°
Give an equation that represents a formation reaction
Mg (s) + Cl2 (g) ——> MgCl2 (s)
-1 mol of product
-in standard states
Give an equation that represents a combustion reaction
CH4 (g) + 2 O2 (g) —–> CO2 (g) + 2 H20 (l)
How do you calculate enthalpy change
energy change = mass of solution/surroundings (g) x heat capacity (4.18 ) x temperature change (k) (minus start temp from end)
result will be in Kj, convert to joules
what is the equation for ΔH
Q=mc Δ T ΔH=Q/n
mass of surroundings (g)
c= 4.18 constant
+/- Kj per mol
Required practices: explain the points of finding ΔH of a solution (calorimetry)
-using measuring cylinder/ weighing boat measure solution/ solids, place solution/water in polystyrene cup for insulation
-record initial temp of water/solution, begin timer record temp at regular intervals each min continue stirring
-add solution/solid at forth minute and stir continuously, dont record temp at forth minute
- record temp after fixth-14th
-draw graph with temp on y axis and time on x axis, extrapolate from forth minute down from line of best fit for temp
this gives temp Δ, use to calculate Q= mc V T, then ΔH=Q/n
remember to divide by 1000 since ans will be in J and need in KJ
Describe Hess law
he enthalpy change for a chemical reaction is independent of the route taken.
what is activation energy
The activation energy is the energy difference from reactants to the transition state PEAK
what is calorimetry
Calorimetry is the measurement enthalpy changes in chemical reactions
Specific heat capacity calculations
In a calorimetry experiment, 2.50 g of methane is burnt in excess oxygen.
30% of the energy released during the combustion is absorbed by 500 g of water, the temperature of which rises from 25 °C to 68 °C
The specific heat capacity of water is 4.18 J g-1 °C−1
What is the total energy released per gram of methane burnt?
q = m x c x ΔT
m (of water) = 500 g
c (of water) = 4.18 J g-1 °C-
ΔT (of water) = 68 C - 25 C = 43 C
q = 500 x 4.18 x 43
= 89 870 J
This is only 30% of the total energy released by methane
Total energy x 0.3 = 89 870 J
Total energy = 299 567 J
his is released by 2.50 g of methane
Energy released by 1.00 g of methane = 299 567 ÷ 2.50
= 119 827 J = 120 000 J
= 120 kJ g-1
hess law cycles equations
ΔH2 = ΔH1 + ΔHr
l.’ΔHr = ΔH2 – ΔH1
which directions do arrows point in formation and combustion hess cycles
formation- away from elements
combustions- towards elements
calculate enthalpy change of reactions using bond energies
enthalpy chnagr for bonds broken + enthalpy change for bonds formed
