Energetics

Question 1

2 types of reaction

Answer 1

Endothermic
Exothermic

Question 2

Enthalpy change

Answer 2

Heat energy change measured under conditions of constant pressure

Question 3

standard enthalpy change symbol

Answer 3

ΔH°

Question 4

What are standard conditions for standard enthalpy changes

Answer 4

100kPa
And a stated temp eg 298 Kelvin

Question 5

Enthalpy of formation

Answer 5

ΔfH is the enthalpy change when one mole of compound is formed from its constituent elements in their standard states under standard conditions (298K and 100 kPa).

Question 6

enthalpy of combustion

Answer 6

ΔcH is the enthalpy change when one mole of substance in its standard state is burnt in excess oxygen under standard conditions.

Question 7

Exothermic definition

Answer 7

Heat energy given out
Reactants have more energy than products
🔼H = neg

Question 8

Endothermic def

Answer 8

Heat given in
Reactants have less energy
🔼H = positive

Question 9

Enthalpy change units

Answer 9

Kj/mol
Kjmol^-1

Question 10

Axis of Enthalpy change diagrams

Answer 10

X axis = progress of reaction
Y axis = energy

Question 11

Activation energy on a energy profile graph

Answer 11

From highest point to reactants

Question 12

Making bonds is …

Answer 12

Exothermic
Releases energy
-ve 🔼H

Question 13

Breaking bonds is …

Answer 13

Endothermic
Req energy
🔼H = positive

Question 14

Whne is a reaction exothermic

Answer 14

When energy rel from bond making is greater than energy taken in from bond breaking

Question 15

calorimetry equation

Answer 15

q = m x c x ΔT

Question 16

what is calorimetry

Answer 16

Calorimetry is the measurement enthalpy changes in chemical reactions

Question 17

q ?

Answer 17

heat change (j)

Question 18

m?

Answer 18

mass (g)
of water whereby 1cm3 = 1g

Question 19

c?

Answer 19

specifc heat capacity
4.18

Question 20

ΔT

Answer 20

change in temp
K

Question 21

After calc q you …

Answer 21

convert to kj
divide by number of mol

Question 22

hess law states

Answer 22

enthalpy change is independent of the route taken

Question 23

how to calc enthalpy change of combustion

Answer 23

arrows point down toward
co2 and h2o at bottom
enthalpy chage = reac - proud

Question 24

State the meaning of the term average bond Enthalpy for the _____ bond

Answer 24

bond Enthalpy change to break 1 mol of _____ bonds

Question 25

How to calc Enthalpy of formation

Answer 25

arrows point upwards Elements at the bottom Prod - reactants

Question 26

define mean bond enthalpy

Answer 26

The average heat energy required to break one mol of covalent bonds over a range of compounds

Question 27

How to calculate mean bond enthalpy

Answer 27

Change in H = sum bonds broken - sum bonds formed This gives 1 mark

Question 28

Why may Enthalpy change of an element thag you would expect to be 0 not be 0

Answer 28

Element not in its standard state

Question 29

State why standard Enthalpy of formation of oxygen is 0

Answer 29

By definition 🔼Hf is formation from an element

Question 30

describe how you would carry out reaction to find enthalpy of solution od mgcl2

Answer 30

(1a) Measures water with named appropriate apparatus (1b) Suitable volume/mass / volume/mass in range 10 – 200 cm3 /g (1c) Into insulated container / polystyrene cup (NOT just ‘lid’) (1d) Add known mass of MgCl2(s) (1e) Use of ‘before and after’ weighing method. NOT ‘added with washings’ Stage 2 Measurements (could mark from diagram) (2a) Record initial temperature (min 2 measurements) (2b) Record T at regular timed intervals for 5+ mins / until trend seen (2c) Plot T vs time Stage 3 Use of Results (3a and 3b could come from diagram) (3a) Extrapolate lines to when solid added (to find initial and final T) (3b) Tfinal – Tinitial = ΔT / idea of finding ΔT from graph at point of addition