Last saved by: Luke

2.3.1 ENTHALPY CHANGES TEST MS
1.	(a)	(a reaction) that releases energy/ (a reaction) that releases heat/ a reaction with a
negative DH (1)	1
(b)	(i)	diagram to show
	upward hump (1)
	CO_2 + (2)H_2O/ carbon dioxide and water below
	reactants (1)	2
(ii)	E_a marked (1)
	if an arrowhead is included, it must be upwards	1
[4]
 
2.	(a)	(i)	bonds broken
	(N ¾ N) + (O == O) + (N ¾ H) = 163 + 497 + 4(390) = 2220 (kJ mol^-1) (1)
	bonds made
	(N  N) +  = 945 + 4(463) = 2797  (1)
	broken DH is +ve and made DH is -ve (1)
	enthalpy of reaction   577  (1)	4
(ii)		1
(b)	N-N bond is weak/ higher Ea for ammonia/ rate too slow for ammonia/
too much energy to break bonds in ammonia / hydrazine is liquid/ do
not need pressurised containers/ more moles/ lots of gas produced
by hydrazine/ more energy per mole produced by hydrazine (1)	1
[6]

3.	(i)	to break a bond energy has to be put in/ 
breaking bonds is endothermic	1
(ii)	energy needed to break 1 mole of bonds 
in the substance in the gaseous state 	2
(iii)	bonds broken:
3(C–H) + (C–O) + (O–H) + 1½ (O=O) = 2781 kJ 
bonds made:
2(C=O) + 4(O–H) = 3470 kJ 
DH_c = − 689  (kJ mol^−1)	3
(iv)	actual bond enthalpies may be different from average values 
	conditions are not standard / methanol/ water is a liquid under
standard conditions 	2
[8]

4.	(a)	(enthalpy change) when 1 mole of substance/ element/ compound (1)
NOT energy needed
is completely burnt (1)	2
(b)	C_3H_7OH(l) + 4½ O_2(g) ® 3CO_2(g) + 4H_2O(I)
correctly balanced equation (1)
state symbols (species must be correct) (1)	2
(c)	(i)	q = mcDT (1)
	DH = 50 ´ 4.18 ´ 12.8 = 2675 (J) = 2.68 (kJ) (1)	2
	ignore sign
(ii)	Mr propan-1-ol = 60 (1)
	number moles = 0.00167 (1)	2
(iii)	DH = - (1)	1
(iv)	heat losses (1)
	thermal capacity of beaker ignored (1)
	conditions were non-standard (1)
	combustion could be incomplete (1)
	propan-1-ol evaporates (1)
	water evaporates (1)	2
[11]

5.	(i)	4C(s) + 5H_2(g) ® C_4H_10(g)
reagents and products (1)
state symbols (1)	2
(ii)	4C + 5H_2  C_4H_10
4(-394) 5(-286) -2877
4CO_2 5H_2O
cycle (1)
correct values (1)
answer (1)
X – 2877 = 4(-394) + 5(-286)
X = -129 (kJ mol^−1)	3
[5]

6.	(a)	(heat/ energy change) when 1 mole of substance is
formed (1)
from its elements (1)	2
(b)	C(s) + 2H_2(g) ® CH_4(g)
balanced equation (1)
state symbols (1)	2
(c)	cycle drawn/ sum of enthalpy changes products – sum
of enthalpy changes reactants (1)
-75 – 242 + ´ = -110 (1)	3
DH = 207 (kJ mol^-1) (1)
(d)	any industrial use, examples include
manufacture of ammonia/ for Haber process	1
manufacture of margarine/ hydrogenation of alkenes
[8]

7.	(i)	(enthalpy/ energy change) when 1 mole of substance/compound formed 
from its elements 
	under standard conditions  (if conditions quoted must be
correct – 25  C/298 K, 1 atm/100 kPa/101 kPa)	3
(ii)	Mg(s) + N_2(g) + 3O_2(g)  Mg(NO_3)_2(s)
balanced species 
state symbols 	2
(iii)	cycle 
x – 791 = −602 – 2(33) 
x = 123 	3
[8]

Paddington Academy