Sec 4 Combined Science (Chemistry) Chemical Energetics topical questions

State what is meant by an exothermic reaction, and give the observation that identifies one.

State whether each change is exothermic or endothermic: (a) burning natural gas; (b) ammonium nitrate dissolving in water.

A mixture of citric acid solution and sodium hydrogencarbonate cools from 22.0 °C to 16.2 °C. State the temperature change and the type of energy change.

Name one piece of safety equipment and one measuring instrument needed for a temperature-change experiment with dilute acid and alkali.

Classify each change as exothermic or endothermic: (a) quicklime reacting with water inside a self-heating can; (b) a cold pack cooling a sports injury.

A thermometer in a reaction mixture rises from 22.0 °C to a maximum of 30.6 °C. State the temperature change, the direction of energy flow, and the type of energy change.

25.0 cm³ of dilute hydrochloric acid at 21.0 °C is mixed with 25.0 cm³ of sodium hydroxide solution, also at 21.0 °C. The maximum temperature reached is 29.5 °C. (a) Calculate the temperature rise. (b) State whether the reaction is exothermic or endothermic, with a reason. (c) Name the type of chemical reaction that occurred.

A student wants to test whether dissolving ammonium nitrate in water is exothermic or endothermic. Describe the experiment and the observation for each possible outcome.

A self-heating can mixes water with calcium oxide when a button is pressed: CaO + H₂O → Ca(OH)₂. (a) Explain why this reaction suits a self-heating can. (b) State one observation, apart from the warming, that shows a chemical reaction has happened. (c) The coffee inside reaches 68 °C but the can wall outside also becomes hot. Explain why.

Two neutralisation experiments are done in identical insulated cups; in each experiment the sodium hydroxide solution is in excess. Experiment 1: 25.0 cm³ of hydrochloric acid with excess sodium hydroxide, rise 6.5 °C. Experiment 2: 25.0 cm³ of sulfuric acid of the same concentration with excess sodium hydroxide, rise 9.8 °C. (a) State what the temperature changes show about the energy transferred in the two experiments. (b) Suggest one reason, in terms of the amounts of acid and alkali, why the second experiment released more heat.

25.0 cm³ of dilute hydrochloric acid at 20.5 °C is mixed with 25.0 cm³ of sodium hydroxide solution at 20.5 °C in an insulated cup. Temperature readings are taken every 30 s: 20.5, 24.1, 26.3, 27.0, 27.0, 26.4 °C. (a) State the maximum temperature and when it is first reached. (b) Calculate the temperature rise. (c) Explain why the readings fall slightly after the peak. (d) Name the type of energy change.

Design an experiment to show that the reaction between zinc and dilute sulfuric acid is exothermic. (a) Name the apparatus and the measurements you would take. (b) State the result that confirms the conclusion. (c) Explain one safety precaution, apart from wearing goggles.

A student investigates an unknown white solid X. When X is added to water, the temperature falls from 23.1 °C to 18.4 °C. When a flame is applied directly to a dry sample of X, it melts without decomposing, and no gas is released. (a) State, with a calculation, the type of energy change when X dissolves. (b) Explain why the student's results do not prove that X reacted chemically with the water. (c) State one additional observation that WOULD show a chemical reaction had taken place.

A cold pack contains a pouch of ammonium nitrate crystals next to a chamber of water. Squeezing breaks the barrier and the crystals dissolve. (a) Explain, in terms of energy flow, why the pack gets cold enough to numb a sprain. (b) Explain why the same pack would NOT work as a hand warmer. (c) The instructions say to squeeze and shake the pack. Suggest why shaking helps the cooling happen faster.

Calcium carbonate is heated strongly in a roasting tube: CaCO₃ → CaO + CO₂. A student claims: "The tube is being heated, so the reaction must be exothermic." (a) Explain the error in the student's claim, using the definition of an exothermic reaction. (b) State the observation that would confirm carbon dioxide is escaping, and name the test. (c) State what would happen to the decomposition if the flame were removed, and what this shows.

Four experiments are carried out in identical insulated cups, each starting at 20.5 °C: 1 — magnesium powder + dilute hydrochloric acid; 2 — ammonium nitrate + water; 3 — sodium hydroxide solution + dilute hydrochloric acid; 4 — calcium carbonate granules + cold water. The highest or lowest temperatures recorded are: 1 — 31.2 °C; 2 — 16.8 °C; 3 — 27.9 °C; 4 — 20.5 °C. (a) Classify each experiment as exothermic, endothermic or no energy change, with the temperature change for each. (b) Explain why experiment 4 shows no temperature change.

50.0 cm³ of dilute hydrochloric acid at 22.0 °C is mixed with 50.0 cm³ of potassium hydroxide solution at 24.0 °C; the maximum temperature reached is 32.5 °C. (a) Calculate the starting temperature the rise must be measured against. (b) Calculate the temperature rise. (c) A second student mixes acid at 22.0 °C with alkali at 26.0 °C, records the same maximum of 32.5 °C, and claims the rise is larger because one solution started hotter. Explain the error and calculate the correct second rise.

A student compares the heat released when equal masses of magnesium ribbon and zinc powder react with dilute sulfuric acid. Each experiment uses 25.0 cm³ of acid at 21.0 °C in an identical insulated cup, with the acid in excess. Temperature rise with magnesium: 9.2 °C. Temperature rise with zinc: 5.1 °C. (a) State two variables the student controlled. (b) Explain why comparing temperature rises is a fair way to compare energy released in these two experiments. (c) Which metal's reaction transferred more heat, and how do the data show it? (d) A classmate says: \u0027The zinc experiment was slower, so it must have released less heat.\u0027 Evaluate this claim. (e) State why having the acid in excess in both experiments matters for the comparison.