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A-Level Chemistry > Energetics > Flashcards

Energetics Flashcards

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1
Q

what is energy?

A

the ability of a system to do work

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2
Q

what is enthalpy? what is its symbol?

A

name used for chemical energy, symbol is H

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3
Q

what is an enthalpy change?

A

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

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4
Q

what happens when an enthalpy change occurs?

A

energy is transferred between system and surroundings (the system = the chemicals, the surroundings is everything other than the chemicals)

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5
Q

what happens during an exothermic reaction?

A
  • energy transferred from system to surroundings
  • products have less energy than reactants
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6
Q

list 2 examples of exothermic reactions

A
  • combustion
  • neutralisation
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7
Q

what happens during an endothermic reaction?

A
  • energy transferred from surroundings to system
  • products have more energy than reactants
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8
Q

what are standard conditions?

A
  • 100kPa
  • 298K (25°c)
  • solutions at 1 mol/dm³
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9
Q

define standard enthalpy change of formation

A

energy transferred when 1 mole of the compound is formed from its elements under standard conditions, all reactants are in their standard states

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10
Q

what is the symbol for standard enthalpy change of formation?

A

ΔHf

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11
Q

define standard enthalpy change of combustion

A

enthalpy change that occurs when 1 mole of a substance is combusted completely under standard conditions, all reactants and products being in their standard states

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12
Q

what is the symbol for standard enthalpy change of combustion?

A

ΔHc

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13
Q

what are the products of incomplete combustion? will it be more/less exothermic than complete combustion?

A
  • soot (carbon), carbon monoxide and water
  • it will be less exothermic than complete combustion
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14
Q

equation to calculate the energy change for an experimental reaction

A
  • energy change (J) = mass of solution (thing being heated) (g) x specific heat capacity (J/g/K) x temperature change (K)
  • Q=M x Cp x ΔT
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15
Q

how do you convert Celsius to Kelvin?

A

add 273 to the Celsius value

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16
Q

what is the SHC of a solution when the reactants are dissolved in water?

A

SHC = 4.18
because in any reaction where the reactants are dissolved in water we assume the SHC is the same as pure water

17
Q

what are the general steps to calculate the enthalpy change of a reaction

A
  1. Using Q = m x SHC x ΔT calculate energy change for quantities used
  2. Work out the moles of the reactants used moles=mass/mr
  3. Divide q by the number of moles of the reactant not in excess to give ΔH
  4. Add a sign and unit (if temp inc then add a negative sign)
18
Q

what are some potential errors when measuring enthalpies of combustion using calorimetry

A
  • 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
19
Q

describe a method of a practical using the calorimetric method to measuring enthalpies of combustion

A

1) wash equipment (cup and pipettes etc) with the solutions used. dry cup after washing
2) put polystyrene cup in a beaker for insulation and support
3) measure out desired volumes of solutions with volumetric pipettes + transfer to insulated cup
4) clamp thermometer into place making sure thermometer bulb is immersed in solution
5) measure initial temps of the solution (or both solutions if 2 are used) repeat every minute for 2-3 minutes
6) after 3 mins transfer 2nd reagent to cup if a solid reagent is used then add solution to cup first and then add the solid weighed out on a balance. If using a solid reagent then use ‘before and after’ weighing method
7) stir mixture (ensures that all of the solution is at same temp)
8) record temp every minute after addition for several minutes

20
Q

if the reaction is slow, why may it be difficult to obtain accurate results of enthalpy change? how can this be counteracted?

A
  • exact temperature rise can be difficult to obtain as cooling occurs simultaneously with the reaction
  • take readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together
  • take temp of the reactants for a few mins before they are added together to get a better avg temp. If the two reactants are solutions then temp of both solutions need to be measured before addition + an average temperature is used
21
Q

what potential errors are there when carrying out a method of a practical using the calorimetric method to measuring enthalpies of combustion

A
  • energy transfer from surroundings (usually loss)
  • approximation in SHC of solution (method assumes all solutions have SHC of water)
  • neglecting SHC of calorimeter- we ignore any energy absorbed by apparatus
  • reaction or dissolving may be incomplete or slow
  • density of solution is taken to be the same as water
22
Q

what is Hess’s law?

A
  • Hess’s law states that total enthalpy change for a reaction is independent of the route by which the chemical change takes place
  • it is a version of the first law of thermodynamics, which states that energy is always conserved
23
Q

is standard enthalpy change of formation reaction - products or products - reactants?

A

products - reactants

24
Q

is standard enthalpy change of combustion reactants - products or products - reactants?

A

reactants - products

25
Q

why do elements have a ΔHc but not a ΔHf?

A
  • elements are already in their standard states: they are not formed from anything so their ΔHf is just 0
  • when calculating ΔHc don’t forget to multiply by the number of moles
26
Q

does breaking bonds require or release energy?

A

requires energy

27
Q

does making bonds require or release energy?

A

release

28
Q

what is the energy change equation?

A

energy change = energy change of reactants - energy change of products

A-Level Chemistry (23 decks)

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