topic 8: energetics I

Question 1

what is enthalpy change

Answer 1

the amount of heat energy taken in or given out during any change in a system provided the pressure is constant

Question 2

what happens in exothermic change

Answer 2

energy is transferred from the system (chemicals) to the surroundings.
the products have less energy than the reactants

Question 3

is the delta H negative or positive for exothermic reaction

Answer 3

negative

Question 4

what are some examples of exothermic processes

Answer 4

the combustion of fuels and the oxidation of
carbohydrates such as glucose in respiration

Question 5

draw how a reaction profile of exothermic reaction looks like

Answer 5

progress of reaction = x axis
energy = y axis
product line lower than reactants as energy is lost
curve going over to the products
the part above the reactants line is activation energy and the part below is delta H

Question 6

what happens in endothermic change

Answer 6

energy is
transferred from the surroundings to the
system (chemicals)
the products have more energy than the reactants

Question 7

what is an example of an endothermic reaction

Answer 7

thermal decomposition of
calcium carbonate

Question 8

is the delta H positive or negative in a endothermic reaction

Answer 8

positive

Question 9

draw the reaction profile of an endothermic reaction

Answer 9

the curve that joins the reactants and products
the gap between the reactants and products are delta H
the activation energy goes from the reactants to the top of the curve
progress of curve=x axis
energy = y axis

Question 10

what is the standard enthalpy change of formation

Answer 10

the enthalpy change when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kpa), all
reactants and products being in their standard states
symbol deltafH

Question 11

what are some examples of standard enthalpy change of formation using MgCl2 and Fe2O3 as examples

Answer 11

Mg(s) + Cl2(g) —>MgCl2(s)
2Fe(s) + 1.5O2(g) —> Fe2O3(s)

Question 12

what is the standard enthalpy change of combustion

Answer 12

the enthalpy change that occurs when one mole of a substance is
combusted completely in oxygen under standard conditions.
(298K and 100kPa), all reactants and products being in their standard states
Symbol deltacH

Question 13

what are the standard conditions

Answer 13

• 100 kPa pressure
• 298 K (room temperature or 25oC)
• solutions at 1mol dm-3
• all substances should have their normal state at 298K

Question 14

what is an example of standard enthalpy of combustion using CH4

Answer 14

CH4(g) + 2O2(g) —> CO2(g) + 2 H2O (l)

Question 15

what is the enthalpy change of reaction

Answer 15

the enthalpy change
when the number of moles of reactants as specified in the
balanced equation react together

Question 16

what is the standard enthalpy change of neutralisation

Answer 16

the enthalpy change when solutions of an
acid and an alkali react together under standard conditions to produce 1 mole of water

Question 17

are enthalpy change of neutralisation always exothermic or endothermic

Answer 17

always exothermic

Question 18

what does Hess law state

Answer 18

total enthalpy change for a reaction is independent of the route by which the chemical change takes place

Question 19

how if the deltaH worked out using Hess’s law

Answer 19

deltaH reaction = ΣdeltafH products - ΣdeltafH reactants

Question 20

how do you draw and use Hess law to work out the enthalpy change of combustion

Answer 20

reactants in the top left, products in the top right and the combustion products at the bottom
the arrow from the reactants and products to the combustion products goes downwards

Question 21

how do you draw and use Hess Law to work out the enthalpy change to form a hydrate salt from an anhydrous salt

Answer 21

have the anhydrous salt(s) and water in the top left corner and have the hydrated salt in the top right corner
have the aqueous anhydrous salt at the bottom
have the arrows going from the reactants and products to the bottom

Question 22

what are Hess law cycles used to measure

Answer 22

to measure the enthalpy change for a reaction that cannot be measured
directly by experiments. instead alternative reactions are carried out that can be measured experimentally

Question 23

why can the formation of a hydrated salt
from an anhydrous salt not be done experimentally

Answer 23

because it is impossible to add the exact amount of water and it is not easy to measure the temperature change of a solid

Question 24

how do you draw and use the Hess law to work out the thermal decomposition of calcium carbonate

Answer 24

have calcium carbonate in the top left corner, have the CaO(s) + CO2(g) in the top right corner and have CaCl2(aq) + H2O(l) + CO2(g)
the arrows should go down towards the bottom box

Question 25

why can the thermal decomposition of calcium carbonate not happen experimentally and what happens instead

Answer 25

because it is impossible to add the heat required to decompose the solid and to measure the temperature change of a solid at the same time
instead both calcium carbonate and calcium oxide are reacted with hydrochloric acid to
form a solution of calcium chloride. the
temperature changes can be measured for
these reactions

Question 26

what is the equation to work out the enthalpy change for a reaction experimentally

Answer 26

energy change = mass of solution x heat capacity x temperature change
Q(J) = m(g) x cp(J g-1K-1) x deltaT( K)

Question 27

what is the general method of calorimetric method

Answer 27

 washes the equipment (cup and pipettes etc) with the solutions to be used
 dry the cup after washing
 put polystyrene cup in a beaker for insulation and support
 measure out desired volumes of solutions with volumetric pipettes and transfer to
insulated cup
 clamp thermometer into place making sure the thermometer bulb is immersed in solution
 measure the initial temperatures of the solution or both solutions if 2 are used. do this
every minute for 2-3 minutes
 at minute 3 transfer second reagent to cup. if a solid reagent is used then add the
solution to the cup first and then add the solid weighed out on a balance
 If using a solid reagent then use ‘before and after’ weighing method
 stirs mixture (ensures that all of the solution is at the same temperature)
 record temperature every minute after addition for several minutes

Question 28

what happens when the calorimetric reaction is too slow

Answer 28

then the exact temperature rise can be difficult to obtain as cooling occurs simultaneously with the reaction

Question 29

how do you counteract the reaction being too slow

Answer 29

we take readings at regular time intervals
and extrapolate the temperature curve/line back to the time the reactants were added together
we also take the temperature of the reactants for a few
minutes before they are added together to get a better
average temperature. if the two reactants are solutions then
the temperature of both solutions need to be measured
before addition and an average temperature is used

Question 30

what are some errors in the calorimetric method

Answer 30

• energy transfer from surroundings (usually loss)
• approximation in specific heat capacity of solution. The method assumes all
  solutions have the heat capacity of water
• neglecting the specific heat capacity of the calorimeter- we ignore any
  energy absorbed by the apparatus
• reaction or dissolving may be incomplete or slow
• density of solution is taken to be the same as water

Question 31

what are some errors of using the method of measuring enthalpies of combustion using calorimetry

Answer 31

• energy losses from calorimeter
• incomplete combustion of fuel
• incomplete transfer of energy
• evaporation of fuel after weighing
• heat capacity of calorimeter not included
• measurements not carried out under standard conditions as H2O is gas, not liquid, in this experiment

Question 32

what is the definition of the mean enthalpies

Answer 32

the mean bond enthalpy is the enthalpy needed to
break the covalent bond into gaseous atoms, averaged over different molecules
—– the definition only applies when the substances start and end in the
gaseous state

Question 33

what is the general formula of deltaH if all substances are gases

Answer 33

deltaH = sum of bond enthalpies broken - sum of bond enthalpies made

Question 34

for the value of the enthalpies of combustion in a homologous series, what happens as the number of carbon atoms increases

Answer 34

more negative

Question 35

when drawn on the graph what graph is higher and steeper for the Mr of alcohol by the enthalpy change of combustion and why

Answer 35

the calculated one because there will be significant heat loss and incomplete combustion
which will lead to less energy being
released

Question 36

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