energetics chemistry edexcel IGCSE

what is an exothermic reaction?

a reaction which gives out heat energy to the surroundings

how can you tell an exothermic reaction has occured?

a temperature increase in the surroundings

what is an endothermic reaction?

a reaction which takes in heat energy from the surroundings

how can you tell that an endothermic reaction has occurred?

the temperature of the surroundings decreases

what is the overall change in energy in a reaction called?

the enthalpy change (ΔH)

what are the unites of enthalpy change?

KJ/mol

is the value of the enthalpy change positive or negative if the reaction is exothermic?

negative - giving out energy

is the value of the enthalpy change positive or negative if the reaction is endothermic?

positive - taking in energy

is bond breaking exothermic or endothermic?

endothermic - taking in energy to break bonds

is bond making exothermic or endothermic?

exothermic - giving out energy so bonds are formed

what is the general reason for why an exothermic reaction is exothermic (enthalpy change negative) ?

more energy is given out making bonds than is taken in breaking bonds

what is the general reason for why an endothermic reaction is endothermic (enthalpy change positive)?

more energy is taken in breaking bonds than is given out making bonds

what does a reaction profile diagram show?

they show the relative energies of the reactants and products in a reaction, and how the energy changes

what does a reaction profile diagram for an exothermic reaction look like?

• reactants higher than products

• curve goes down

what does a reaction profile diagram for an endothermic reaction look like?

• products higher than reactants

• curve goes up

what is the enthalpy change (ΔH) on a reaction profile diagram?

height difference between the reactants and products

what is the activation energy on a reaction profile diagram?

the height difference between the reactants and the top of the curve

what is calorimetry?

a method of experimentation allowing you to find the amount of energy transferred in a chemical reaction

what are the two kinds of calorimetry you can do?

• dissolving, displacement and neutralisation reactions

• combustion

how would you carry out a calorimetry experiment for dissolving, displacement or neutralisation?

• take the temperature of the reactants - make sure they are the same temperature - a water bath may be necessary

• mix a set volume of them together in a polystyrene cup

• take the temperature of the mixture every 10/20/30 seconds, until the temperature no longer changes/begins to go in the opposite direction

why do you use a polystyrene cup for dissolving, displacement or neutralisation calorimetry experiments?

it is a good insulator

what is the main problem with calorimetry experiments?

energy is lost to the surroundings so the temperature or mass change measurement is inaccurate

how can you reduce energy loss to the surroundings in neutralisation, dissolving or displacement calorimetry experiments?

• placing the polystyrene cup in a beaker of cotton wool to give more insulation

• putting a lid on the cup to reduce energy loss by evaporation

how would you perform a combustion calorimetry experiment?

• put 50g of water in a copper can and record the starting temperature

• weigh the spirit burner and lid with fuel

• place the spirit burner under the can and light the wick

• heat the water, stirring constantly, until the temperature reaches about 50C

• put out the flame using the burner lid, and measure the final temperature of the water

• weight the spirit burner and lid and calculate mass change

how can you reduce energy loss to the surroundings when performing a combustion calorimetry experiment?

• reduce draughts - use a screen to act as a draught excluder

• use a metal can as it is a good conductor of heat

what equation can you use for combustion calorimetry experiments?

heat energy transferred

what is the equation for heat energy transferred?

• Q = m x c x ΔT where:

  • Q is heat energy transferred in Joules,

  • m is mass of liquid being heated in grams,

  • c is the specific heat capacity in J/g/C,

  • ΔT is the change in temperature of the liquid in C

what is the specific heat capacity of water?

4.2 J/g/C

what is the equation for molar enthalpy change?

ΔH = -Q/moles where:

• -Q is in kilojoules

• ΔH is in KJ/mol

what is the equation for normal enthalpy change?

ΔH = total energy absorbed to break bonds (reactants) - total energy released in making bonds (products)

how can you calculate the total energy absorbed to make or break bonds?

add up the individual bond energies of each chemical bond in the compound using displayed formula