heat energy changes in chemical reactions

Question 1

recall that changes in heat energy accompany the following changes

Answer 1

• salts dissolving in water
• neutralisation reactions
• displacement reactions
• precipitation reactions
  when these reactions take place in a solution, temperature changes can be measured to reflect the heat changes

Question 2

what happens when salts dissolve in water in terms of change in temperature

Answer 2

• you can measure the temperature change when dissolving salts in water by adding the salt to a polystyrene cup of water and measuring the change in temperature when the salt has dissolved
  a) dissolving ammonium chloride decreases temperature of the reaction mixture - it is endothermic
  b) dissolving calcium chloride increases temperature - it is exothermic

Question 3

what happens to heat energy in neutralisation reactions

Answer 3

• in a neutralisation reaction an acid and a base react to form a salt and water
• most are exothermic
• neutralisation reaction between ethanoic acid and sodium carbonate is endothermic

Question 4

what happens to the heat in displacement reactions

Answer 4

• in a displacement reaction a more reactive element displaces a less reactive element in a compound
• releases energy and is exothermic
• zinc powder and copper sulphate react in a displacement reaction forming zinc sulfate and copper

Question 5

what happens to heat in precipitation reaction

Answer 5

• precipitates are insoluble solids which can sometimes form when two solutions are mixed together
• exothermic - an increase of temperature of the surroundings

Question 6

what is an exothermic reaction or change

Answer 6

where heat energy is given out

Question 7

what is an endothermic reaction or change

Answer 7

where heat energy is taken in

Question 8

is the breaking of bonds endothermic or exothermic

Answer 8

endothermic

Question 9

is the making of bonds endothermic or exothermic

Answer 9

exothermic

Question 10

what is the overall heat energy change for a reaction

Answer 10

• exothermic if more heat energy is released in forming bonds in the products than is required in breaking bonds in the reactants
• endothermic if less heat energy is released in forming bonds in the products than is required in breaking bonds in the reactants

Question 11

explain the term activation energy

Answer 11

• minimum amount of energy needed for bonds to break
• particles need to collide with at least the activation energy for collisions to be successful

Question 12

analyse reaction profiles for endothermic reactions

Answer 12

• products are at a higher energy than the reactants
• difference in height represents the energy taken in during a reaction
  ( )
  ( ————-
  —–

Question 13

analyse reaction profiles for exothermic reactions

Answer 13

• products are at a lower energy than the reactants
• the difference in height represents the energy given out in the reaction

    (     )   ------           )
                  )  
                    ---------

Question 14

what happens to activation energy in reaction profiles

Answer 14

• the activation energy is the energy difference between the reactants and the highest point on the curve
• if the energy input is less than the activation energy there won’t be enough energy to start the reaction so nothing will happen

Question 15

Calculate the energy change in a reaction given the energies of bonds (in kJ mol–1)

Answer 15

overall energy change = energy required to break bonds - energy released by forming bonds

Question 16

what means a positive or negative energy change

Answer 16

• positive = endothermic reaction
• negative = exothermic reaction