Energetics

Question 1

what is an exothermic reaction?

Answer 1

one which gives OUT energy to the surroundings, usually in the form of heat and usually shown by a rise in temperature of the surroundings

Question 2

what is an endothermic reaction?

Answer 2

one which takes IN energy from the surroundings, usually in the form of heat and usually shown by a fall in temperature of the surroundings.

Question 3

what is enthalpy change?

Answer 3

the overall change in energy in a reaction. it has the symbol ΔH.

Question 4

what value can enthalpy change have?

Answer 4

enthalpy change can have a positive or negative value.

• if the reaction is EXOTHERMIC, the value is negative because the reaction is GIVING OUT energy.
• if the reaction is ENDOTHERMIC, the value is positive because the reaction TAKES IN energy.

Question 5

what is a reaction profile?

Answer 5

• (sometimes called energy level diagrams)
• show the relative energies of the reactants and products in a reaction
• and how the energy changes over the course of the reaction.

Question 6

how is an exothermic reaction shown on a reaction profile?

Answer 6

• the products are at a LOWER energy than the reactants.
• the difference in height represents the energy given out (per mole) in the reaction
• ΔH is negative here.

Question 7

how is an endothermic reaction shown on a reaction profile?

Answer 7

• the products are at a HIGHER energy than the reactants.
• the difference in height represents the energy taken in (per mole) during the reaction
• ΔH is positive here.

Question 8

what is enthalpy change measured in?

Answer 8

• the units of ΔH are kJ/mol

- so its the amount of energy in kilojoules per mole of a reactant.

Question 9

how can you find out enthalpy changes?

Answer 9

• using calorimetry
• calorimetry allows you to measure the amount of energy transferred in a chemical reaction with a simple set of equipment

Question 10

how do you carry out calorimetry for DISSOLVING, DISPLACEMENT and NEUTRALISATION?

Answer 10

• to measure the amount of energy transferred in these reactions (in solution) you just take the temperature of the reactants, mix them and measure the temperature of the solution at the end of the reaction.
• mix the reactants in a polystyrene cup.
• reduce energy loss to the surroundings by insulating with cotton wool or putting a lid on the cup to reduce loss by evaporation.

Question 11

example of measuring energy transferred in a neutralisation reaction

Answer 11

• between hydrochloric acid and sodium hydroxide
  1. put 25cm3 of hydrochloric acid and sodium hydroxide in separate beakers
  2. place the beakers in a water bath and set to 25°C until they are both at the same temperature (25°C)
  3. add the HCl followed by the NaOH to a polystyrene cup with a lid.
  4. take the temperature of the mixture every 30 seconds and record the highest temperature

Question 12

how to carry out calorimetry for combustion?

Answer 12

• to measure the amount of energy transferred when a fuel is burnt, you can simply burn the fuel and use the flame to heat up some water. this method uses a metal container, usually made of copper because copper conducts heat so well.
  1. v important to make as much heat as possible go into heating up the water. reducing draughts is the key, use a draught excluder.
  2. put 50g of water in the copper can and record its temperature.
  3. weigh the spirit burner and lid
  4. put the spirit burner underneath the can and light the wick. heat the water, stirring constantly, until the temperature reaches about 50°C.
  5. put out the flame using the burner lid and measure the final temperature of the water.
  6. weigh the spirit burner and lid again. calculate the ΔH

Question 13

how do you calculate heat energy transferred?

Answer 13

• if you measure (i) how much fuel you’ve burned (ii) the temperature change of the water, you can work out how much energy is supplied per gram of fuel.
• use the equation:
  Q = m x c x ΔT
• you also need to know the water’s specific heat capacity (c) which is 4.2 J/g/°C

Question 14

how do you calculate molar enthalpy change?

Answer 14

• once you’ve calculated heat energy change (Q) you can use it to work out the molar enthalpy change ΔH (the enthalpy change given out by one mole of the reactant).
• you need the same info as before and the Mr of the fuel
  1. first calculate heat energy change Q
  2. next find how many mole of fuel produced this energy change. moles = mass/Mr
  3. ΔH/moles = answer.

Question 15

what must be supplied to break bonds?

Answer 15

• energy must be supplied

- so bond breaking is an ENDOTHERMIC process

Question 16

what happens when bonds are formed?

Answer 16

• energy is released

- so bond formation is an exothermic process

Question 17

what happens in endothermic reactions?

Answer 17

the energy used to break bonds is greater than the energy released by forming them

Question 18

what happens in exothermic reactions?

Answer 18

the energy released by forming bonds is greater than the energy used to break them.

Question 19

how do you use bond energy to calculate enthalpy change during a chemical reaction?

Answer 19

• each type of chemical bond has a particular bond energy associated with it.
• this bond energy can vary slightly depending on what compound the bond is in, so you’ll be given avg bond energies in the exam.
• use these to calculate enthalpy change for a reaction.
• add up the energy of the bonds that are broken and subtract the energy of the bonds that are made.
  ΔH = total energy absorber - total energy released
• a positive energy means an endothermic reaction and a negative energy change means an exothermic reaction.