topic 8

Question 1

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

exothermic reaction

Answer 2

energy is transferred from the system to surroundings
products have less energy than the reactants
ΔH is negative

Question 3

endothermic reaction

Answer 3

energy is transferred from surroundings to system. input of energy is required
products have more energy than the reactants
ΔH is positive

Question 4

enthalpy of formation

Answer 4

the enthalpy change when one mole of a substance is produced from its elements under standard conditions

Question 5

enthalpy of combustion

Answer 5

the enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions

Question 6

standard conditions

Answer 6

pressure - 100 kPa
room temperature - 298 K
solution - 1 mol dm^3
normal state at 298 K

Question 7

incomplete combustion products

Answer 7

soot
carbon monoxide
water
less exothermic than complete combustion

Question 8

enthalpy of reaction

Answer 8

enthalpy change when quantities of substances in standard states react completely under standard conditiosn

Question 9

enthalpy of neutralisation

Answer 9

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

Question 10

Hess’s Law

Answer 10

states that the total enthalpy change for a reaction is independent of the route by which the chemical changes take place

Question 11

ΔH reaction formula - enthalpy change of formation

Answer 11

ΔH reaction = ΔfH products - ΔfH reactants

Question 12

ΔH reaction formula - enthalpy change of combustion

Answer 12

ΔH reaction = ΔcH products - ΔcH reactants

Question 13

reaction in solution equation

Answer 13

Q = m x c x ΔT

energy change = mass of solution x heat capacity x change in temp

Question 14

calorimetric method

Answer 14

• wash equipment with the solutions to be used
• dry the cup after washing
• put polystyrene cup in a beaker for insulation and support
• measure desired volume of solutions with volumetric pipettes and transfer to insulated cup
• clamp thermometer into place making sure the thermometer bulb is immersed in solution
• measure initial temp
• do this every minute for a few minutes
• at final minute transfer 2nd reagent into cup
• for solid, measure before and after weight
• record temp every minute after addition for several minutes

Question 15

errors in method

Answer 15

• energy transferred from surroundings
• approximation in specific heat capacity of solution
• neglecting specific heat capacity of the calorimeter
• reaction may be incomplete or slow
• density of solution is taken to be same as water

Question 16

in general if all substances are gases ΔH = ?

Answer 16

bond enthalpies broken - bond enthalpies made

Question 17

mean bond enthalpies

Answer 17

the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules

Question 18

bond enthalpy

Answer 18

the amount of energy stored in a bond between atoms in the molecule

Question 19

why can enthalpy change not be measured directly

anydrous salt -> hydrated salt

Answer 19

• it is impossible to add the exact amount of water
• not easy to measure the temp change of a soild

Question 20

why can enthalpy change not be measured directly

thermal decomposition of CaCO3

Answer 20

• impossible to add the heat required to decompose the solid
• cant measure the temp change of a soild at the same time