Energy Changes
What type of reaction transfers thermal energy to the surroundings and causes the temperature to increase?
An exothermic reaction.
What is the chemical name for rust?
Iron oxide.
Which gas is produced when a reactive metal reacts with water?
Hydrogen.
Complete the general word equation: metal + dilute acid → ______ + ______.
Salt + hydrogen.
In an investigation comparing different metals, what is the independent variable?
The type of metal.
A reaction starts at 19°C and finishes at 16°C. Is it exothermic or endothermic?
Endothermic, because the temperature decreased by 3°C.
Which two substances must be present for iron to rust?
Oxygen and water.
Complete the word equation: calcium + water → ______ + ______.
Calcium hydroxide + hydrogen.
Give two observations that may show a metal is reacting with dilute acid.
Any two: bubbles of gas, temperature change, colour change or the metal changing/disappearing.
In an investigation measuring the temperature rise when magnesium reacts with acid, how is temperature change calculated?
End temperature − start temperature.
What is the difference between an endothermic reaction and an endothermic process?
Both absorb energy, but a reaction forms new substances, while a process does not form new substances.
Explain how painting an iron object helps to prevent rusting.
The paint forms a barrier that prevents oxygen and water from reaching the iron.
How is hydrogen gas tested?
Hold a lighted splint near the gas. Hydrogen burns with a squeaky pop.
The times needed to collect the same volume of hydrogen are: magnesium 21 s, aluminium 27 s, iron 49 s, zinc 54 s and lead 69 s. Put the metals in order from most reactive to least reactive.
Magnesium → aluminium → iron → zinc → lead. A shorter collection time means a faster reaction and greater reactivity.
State three variables that should be controlled when comparing the reactions of different metals with dilute acid.
Any three: acid type, acid volume, acid concentration, mass of metal, form or surface area of metal, starting temperature or method.
Why does a wet cloth placed over bottles help keep the drinks cool in hot weather?
Water evaporates from the cloth. Evaporation takes in energy from the surroundings, cooling the cloth, water and bottles.
Three identical iron nails are placed in: dry air, water and air, and boiled water covered with oil. Predict which nail will rust and explain what the results prove.
Only the nail in water and air should rust. The dry-air tube has oxygen but no water, while the boiled-water tube has water but almost no oxygen. This proves that iron needs both oxygen and water to rust.
Put these metals in order from most reactive to least reactive with water: copper, potassium, zinc and magnesium.
Potassium → magnesium → zinc → copper.
Two students compare metals using different acids, different acid concentrations and different acid volumes. Why can they not compare the metals’ reactivities reliably?
More than one variable changed, so differences may have been caused by the acid conditions rather than the type of metal.
Why is a bar chart more suitable than a line graph for comparing temperature rises produced by four different metals?
Metal type is a discrete or categorical variable, so the values should be shown as separate bars.
Four reactions have these temperatures: A: 21→45°C, B: 18→22°C, C: 19→16°C, D: 18→20°C. Identify the only endothermic reaction and the reaction with the greatest temperature change.
C is endothermic. A has the greatest temperature change: 24°C.
A student investigates whether iron nails rust faster in warm water than in cold water. State the independent variable, dependent variable and two control variables.
Independent variable: temperature of the water. Dependent variable: amount or rate of rusting. Control variables: same type and size of nail, same water volume, same time, and same access to oxygen.
Explain why copper is suitable for water pipes but iron is less suitable.
Copper does not react with water. Iron can rust when oxygen is present with water, producing iron oxide that can colour the water and weaken the pipe.
Complete the equation: magnesium + sulfuric acid → ______ + ______.
Magnesium sulfate + hydrogen.
A student compares metal X and metal Y in 10 cm³ of the same hydrochloric acid. Metal X has a small mass and raises the temperature by 2°C. Metal Y has a larger mass and raises the temperature by 6°C. Can the student conclude that Y is more reactive or releases more energy?
No. The masses of metal were different, so it was not a fair comparison. The same mass and form of each metal should be used, and the tests should be repeated.