What are hydrogen bonds? | 16-18 years

These activities introduce hydrogen bonds as intermolecular bonds made between specific permanent dipoles. Students work through a cognitive conflict exercise, a simple experiment and then carry out research on a material containing hydrogen bonds.

Learning objectives

Students will understand that hydrogen bonds:

• Are intermolecular bonds formed between hydrogen atoms and an atom of nitrogen, oxygen or fluorine in a separate molecule.
• Are present in a wide range of chemicals.
• Require more energy to break than other types of intermolecular bond.

Sequence of activities

Introduction

1. Give each student a small piece of paper. Ask them to watch while a beaker of water is boiled in front of them.
2. When the water is fully boiling, ask them to write on the paper what they think is in the bubbles.
3. Take in the pieces of paper and write the range of responses on a board.
4. Indicate that there can only be one correct answer. (Responses are most likely to say ‘hydrogen and oxygen’, ‘carbon dioxide’, ‘oxygen’, ‘hydrogen’. A few will say ‘water vapour’ or ‘steam’ – either is correct.)
5. Divide the class into groups of three or four, and ask the groups to:
   • Discuss the responses and decide on the one correct answer, with reasons.
   • Work out explanations for why the remaining answers are incorrect or incomplete.

Plenary 1

In a plenary:

1. Hear each group’s responses.
2. Indicate the correct answer clearly after hearing all groups.
3. Use molecular models of water molecules to show what happens when water boils.
4. Introduce the term ‘hydrogen bond’ to explain how water molecules bond together.
5. Ask students to write a short explanation about what happens when water boils.
6. Describe the learning objectives.

Making slime

1. Explain that other molecules besides water contain hydrogen bonds and that ‘slime’ is a polymer containing hydrogen bonds that can be made easily.
2. Give each student a copy of the ‘Making slime’ worksheet.
3. Supervise the practical work as students follow steps 1–4 of the procedure for making a PVA polymer slime, before investigating its properties, eg using tests 1–4 from the second part of the procedure.

Plenary 2

In a plenary:

1. Invite students to describe the properties of slime and what uses it may have.
2. Introduce the idea that other materials with hydrogen bonding may be more useful!

Research activity

Explain the next activity, to research one material that contains hydrogen bonding. Hand out the ‘Finding out more’ sheet. Support students as they:

1. Re-form into groups.
2. Decide which material to investigate (ensure that each group investigates a different material).
3. Carry out their research.
4. Prepare their presentation.
5. Agree criteria for assessing each other’s presentations, such as:
   • Clarity of information presented.
   • Quality of explanations.
   • Details about the material.
   • Extent to which the brief was covered.

Presentations

1. Arrange for each group to make its presentation. Support them as they subsequently assess the presentations and give feedback to each other.
2. Ask the students to write, and hand in, a summary about hydrogen bonding based on the information presented.

Feedback

Give written feedback to students, indicating the extent to which they have grasped the key ideas, together with advice on how to develop their understanding.

Commentary

The introductory exercise uses cognitive conflict and peer discussion to prompt students to correct misconceptions about state change, and to introduce hydrogen bonds. Sharing the objectives is necessarily delayed until after this.

By its nature, the experiment continues the hydrogen bond theme in a lively way. The research task uses peer assessment to learn and apply knowledge about hydrogen bonds to unknown situations.

Teacher feedback on written summaries can highlight what individuals understand, together with pointers for development.

Answers

Making slime

1. Slime can stretch and snap.
2. See diagram.
3. Hydrogen bonds and instantaneous dipole-induced dipole bonds.
4. Any valid suggestions.
5. If slime was not water soluble it could be used in modelling (eg like Fimo).

See the ‘Structure of slime’ sheet for more information.