C7 - Energy Changes

Question 1

Conservation of Energy

Answer 1

Energy is conserved in chemical reactions; it is neither created nor destroyed, only transferred.

Question 2

Exothermic Reaction

Answer 2

A reaction in which energy is being transferred from the reacting substances to their surroundings.

Question 3

Endothermic Reaction

Answer 3

A reaction in which energy is being transferred to the reacting substances from their surroundings

Question 4

Required Practical - Investigating temperature differences

Answer 4

Use the large measuring cylinder to put a known volume dilute hydrochloric acid into the polystyrene cup.

Stand the cup inside the beaker. This will make it more stable.

Use the thermometer to measure the temperature of the acid. Record it in the first blank column of the table on the back of this sheet.

Put a 5cm3 sodium hydroxide solution into the small measuring cylinder.

Pour the sodium hydroxide into the cup, quickly fit the lid and gently stir the solution with the
thermometer through the hole. When the reading on the thermometer stops changing, write the temperature in the next space in the table.

Repeat steps 4 and 5 to add further 5cm3 portions of sodium hydroxide to the cup until a total of 40cm3 has been added. The last few additions should produce a temperature fall rather than a rise.

Repeat the whole investigation (steps 1 – 6) and record the results in the second blank column of the table.

Calculate the mean maximum temperature reached for each of the sodium hydroxide volumes and record it in the third blank column.

Plot a line graph of total volume of sodium hydroxide added in cm3 (x axis) against mean maximum temperature in oC (y axis). Draw two straight lines of best fit - one through the points which are increasing, and another through those which are decreasing. Ensure the two lines are extended so they cross each other.

Use the graph to estimate how much sodium hydroxide solution was needed to neutralise 25cm3 dilute hydrochloric acid.

Question 5

Examples of Exothermic Reactions

Answer 5

 Non-reusable hand warmers (exothermic): iron is oxidised, last for hours.
 Reusable hand warmers (exothermic): sodium ethanoate salt is dissolved in hot water to make supersaturated solution, last for 30min.

Question 6

Examples of Endothermic Reactions

Answer 6

 Self-heating cans (exothermic): when seal on base of can breaks, calcium oxide and water react.
 Cold-packs (endothermic): ammonium nitrate dissolves in water and absorbs energy from surroundings, last for 20min.