Physical: Energetics

Question 1

What is Enthalpy?

Answer 1

The total chemical/ heat energy that is in a substance under constant conditions

Question 2

What is Enthalpy change

Answer 2

When energy is given to surroundings or absorbed to increase/ decrease overall energy of system

Question 3

How is Enthalpy change represented

Answer 3

∆H under constant conditions
units= KJ/Mol

Question 4

What is the enthalpy energy of an exothermic reaction and why

Answer 4

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

Question 5

What is the enthalpy energy of an endothermic reaction and why

Answer 5

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

Question 6

What are the constant conditions for enthalpy change

Answer 6

100KPa
298 Kelivn

Question 7

What is the definition of reaction

Answer 7

Enthalpy change in an equation react in standard conditions
ΔHr°

Question 8

What is the definition for formation

Answer 8

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

Question 9

What is the definition for combustion

Answer 9

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°

Question 10

What is the definition for neutralisation

Answer 10

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°

Question 11

Give an equation that represents a formation reaction

Answer 11

Mg (s) + Cl2 (g) ——> MgCl2 (s)
-1 mol of product
-in standard states

Question 12

Give an equation that represents a combustion reaction

Answer 12

CH4 (g) + 2 O2 (g) —–> CO2 (g) + 2 H20 (l)

Question 13

How do you calculate enthalpy change

Answer 13

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

Question 14

what is the equation for ΔH

Answer 14

Q=mc Δ T ΔH=Q/n
mass of surroundings (g)
c= 4.18 constant
+/- Kj per mol

Question 15

Required practices: explain the points of finding ΔH of a solution (calorimetry)

Answer 15

-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

Question 16

Describe Hess law

Answer 16

he enthalpy change for a chemical reaction is independent of the route taken.

Question 17

what is activation energy

Answer 17

The activation energy is the energy difference from reactants to the transition state PEAK

Question 18

what is calorimetry

Answer 18

Calorimetry is the measurement enthalpy changes in chemical reactions

Question 19

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?

Answer 19

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

Question 20

hess law cycles equations

Answer 20

ΔH2 = ΔH1 + ΔHr
l.’ΔHr = ΔH2 – ΔH1

Question 21

which directions do arrows point in formation and combustion hess cycles

Answer 21

formation- away from elements
combustions- towards elements

Question 22

calculate enthalpy change of reactions using bond energies

Answer 22

enthalpy chnagr for bonds broken + enthalpy change for bonds formed