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The Robot That Got Heavier: Atoms in a Fuel Cell

Grade 7 physical science: analyze hydrogen fuel-cell data from a school delivery robot and model atoms to explain conservation of mass in reactions.

Grade 7ScienceInteractive · Printable

Aligned toMS-PS1-5

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What’s included

Single worksheet
Auto-graded online
1 NGSS standard
Printable answer key
11 questions
Classroom-ready

Details

Format: Interactive · Printable
Grade: Grade 7
Subject: Science
Question types: Fill in the blank, Multiple choice, Matching, Short answer

About this worksheet

What students practice

Students use mass data to show that the total mass of reactants equals the total mass of products.
Students model how atoms are rearranged, not created or destroyed, in a chemical reaction.

How to use

Let students build the clay-ball model from paragraph 2 before question 5. The last question works as a written explanation or a class discussion starter.

Curriculum

NGSS standard covered

MS-PS1-5

Model how atoms are rearranged but not created or destroyed in a chemical reaction, so total mass is conserved.

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The Robot That Got Heavier: Atoms in a Fuel Cell

Use fuel-cell test data and atom models to show that mass is conserved.

The fuel-cell test runs

Read the passage and study Table 1. Then answer the questions.

A Delivery Robot That Makes Water

1The robotics team at Cedar Hollow Middle School is building a robot that carries library books between the school's two buildings. Instead of using only a battery, the team is testing a small hydrogen fuel cell. Inside the fuel cell, hydrogen gas from a metal canister reacts with oxygen gas from the air. The reaction makes water and gives off electrical energy that runs the robot's motors. Coach Daniel Mwangi connects and removes the hydrogen canisters, and team members never open them.

2The reaction can be written as 2 H₂ + O₂ → 2 H₂O. Each hydrogen molecule (H₂) has two hydrogen atoms, each oxygen molecule (O₂) has two oxygen atoms, and each water molecule (H₂O) has two hydrogen atoms joined to one oxygen atom. Team member Zainab Rahimi built a model of the reaction with clay balls: white balls for hydrogen atoms and red balls for oxygen atoms.

3To test the robot, the team clipped a sealed plastic bag over the fuel cell's exhaust tube so that every drop of water the fuel cell made was caught. Before and after each test run, they weighed the hydrogen canister on a scale that reads to 0.1 g to find how much hydrogen was used. After each run, they also weighed the water in the bag. The oxygen came from the air around the robot, so the team could not weigh it directly. Their results are in Table 1.

4Before Run 2, team member Haruto Kim also placed the whole robot on a large lab balance that also reads to 0.1 g, with its canister and the empty bag attached. The robot's total mass was 3,200.0 g. After the run, with the water still sealed in the bag, Haruto weighed the whole robot again.

5The team also fixed a cracked bracket with two-part epoxy glue. Coach Mwangi squeezed 6.0 g of liquid resin and 6.0 g of liquid hardener into a 4.5 g plastic cup on the scale and stirred them. The two liquids reacted, the cup grew warm, and a day later the glue had hardened into a solid. Nothing was added to or taken out of the cup.