C5 - energy changes

Question 1

Define an exothermic reaction

Answer 1

Energy is given out to the surroundings because more energy was given out by making bonds than was taken in for breaking bonds

Question 2

Does an exothermic reaction get warmer or colder?

Answer 2

warmer

Question 3

define an endothermic reaction

Answer 3

Energy is taken in from the surroundings because more energy was taken in for breaking bonds than was given out by breaking bonds

Question 4

does an endothermic reaction get warmer or colder?

Answer 4

colder

Question 5

Give two examples of an exothermic reaction

Answer 5

combustion, neutralization reactions

Question 6

Give two examples of endothermic reactions

Answer 6

the reaction between citric acid and sodium hydrogencarbonate, thermal decomposition (photosynthesis also counts but it isn’t in the book)

Question 7

give two uses of exothermic reactions

Answer 7

some hand warmers and self-heating cans

Question 8

give a use of endothermic reactions

Answer 8

some sports injury packs

Question 9

what are reaction profiles?

Answer 9

diagrams that show the relative energies of the reactants and products in a reaction, and how the energy changes over the course of the reaction

Question 10

If the reactants are higher than the products in a reaction profile, is the reaction exothermic or endothermic?

Answer 10

exothermic

Question 11

if the reactants are lower than the products in a reaction profile, is the reaction exothermic or endothermic?

Answer 11

endothermic

Question 12

what is the name of the initial rise of energy in a reaction profile?

Answer 12

activation energy

Question 13

what is activation energy?

Answer 13

the minimum amount of energy the reactants need to collide with each other and react. The greater the activation energy, the more energy needed to start the reaction.

Question 14

what does the overall difference in height of a reaction profile represent?

Answer 14

the overall energy change during the reaction

Question 15

Does breaking bonds require energy or release energy? Is it exothermic or endothermic?

Answer 15

It requires energy - it is endothermic

Question 16

does forming bonds require energy or release energy? is it exothermic or endothermic?

Answer 16

It releases energy - it is exothermic

Question 17

H-H: +436 kJ/mol; Cl-Cl; + 242 kJ/mol; H-Cl: +431kJ/mol

work out the total energy change for the reaction H2 + Cl2 -> 2HCl

Answer 17

-184 kJ/mol

Question 18

describe an experiment you could do to measure the amount of energy released by a chemical reaction. What is the biggest problem with this reaction? What could this method be used for?

Answer 18

1. you can measure the amount of energy released by a chemical reaction (in a solution) by taking the temperature of the reagents, mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction
2. the biggest problem with energy measurements is the amount of energy lost to the surroundings
3. you can reduce the energy lost by putting the polystyrene cup into a beaker of cotton wool to give more insulation, and putting a lid on the cup to reduce energy lost by evaporation
4. this method works for neutralisation reactions or reactions between metals and acids, or carbonates and acids
5. you can also use this method to investigate what effect different variables have on the amount of energy transferred - e.g. the mass or concentration of the reactants used.

Question 19

describe an experiment to test the effect of acid concentration on the energy released in a neutralisation reaction between hydrochloric acid (HCl) and sodium hydroxide

Answer 19

1. put 25 cm^3 of 0.25 mol/dm^3 of hydrochloric acid and sodium hydroxide in separate beakers
2. place the beakers in a water bath set to 25 degrees C until they are both 25 degrees 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
5. repeat steps 1-4 using 0.5 mol/dm^3 and then 1 mol/dm^3 of hydrochloric acid

Question 20

what is the equation for the overall energy change of the reaction?

Answer 20

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

Question 21

is a negative overall energy change endothermic or exothermic?

Answer 21

exothermic

Question 22

would an endothermic reaction have a negative or positive overall energy change?

Answer 22

positive