Exercises: Producing and testing common gases

Match each gas to its laboratory test:

Gases: hydrogen, oxygen, carbon dioxide.

Tests: (i) turns limewater milky; (ii) relights a glowing splint; (iii) burns with a squeaky pop when a lit splint is held to it.

Marble chips (calcium carbonate) are added to dilute hydrochloric acid and bubbles form.

1. a) Name the gas given off.

1. b) Describe how you would test for it.

1. c) What would you observe in a positive test?

Name a reaction that can be used in school to produce hydrogen gas. State the reactants and the test for hydrogen.

Magnesium ribbon reacts with dilute hydrochloric acid.

1. a) Write a word equation for the reaction.

1. b) Write a balanced symbol equation: $\text{Mg} + \text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$.

1. c) Describe the test that proves the gas is hydrogen.

1. d) Name a piece of apparatus that could be used to measure the volume of gas produced over time.

Baking soda (sodium hydrogencarbonate) is added to vinegar in a flask connected by a delivery tube to a test tube of limewater.

1. a) What do you observe in the flask?

1. b) What happens in the limewater, and what does this tell you?

1. c) Explain why a balloon fitted over the mouth of a flask of this reaction inflates.

1. d) Suggest why collecting the gas over water might lead to a lower measured volume than expected, given that carbon dioxide is slightly soluble in water.

In a school in Kerry, $3.0\ \text{g}$ of marble chips react with excess acid and the total volume of $\text{CO}_2$ collected is $480\ \text{cm}^3$ in $4$ minutes.

1. a) Calculate the average rate in $\text{cm}^3/\text{min}$.

1. b) Calculate the average rate in $\text{cm}^3/\text{s}$.

1. c) State two changes that could make the reaction faster.

A student is asked to identify three colourless gases, X, Y and Z, collected in separate test tubes. A lit splint is placed in each. In X the splint goes out and a limewater test turns milky. In Y the splint burns with a squeaky pop. In Z the splint burns more brightly.

1. a) Identify X, Y and Z.

1. b) For X, state which test is the specific confirmatory one and explain why the splint going out alone is not sufficient.

1. c) Suggest a reaction from which gas Z could be prepared in the laboratory, including a catalyst.

1. d) Explain one safety precaution when preparing and testing gas Y.

The following equation represents the reaction of calcium carbonate with hydrochloric acid:

$\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$

1. a) Explain why the mass of the flask and contents on an open balance decreases during the reaction.

1. b) Explain why the mass would not decrease if the flask were sealed (take care with pressure) and the reaction occurred in a strong closed container.

1. c) A student's graph of mass against time is steep at the start and flattens. Explain the shape.

1. d) Explain how the graph would change if the acid were warmed.

A student wants to compare the effectiveness of two catalysts, $\text{MnO}_2$ and a piece of fresh liver, on the decomposition of hydrogen peroxide. She measures the volume of oxygen collected in $60\ \text{s}$.

1. a) State four variables that must be kept constant for a fair comparison.

1. b) Which catalyst is more effective? Justify using the data.

1. c) Suggest why the liver catalyst might stop working if the mixture is heated to $80\ °\text{C}$.

1. d) Calculate how many times faster the reaction is with liver compared with no catalyst.