Hess's Law and Enthalpy of Formation
Students will apply Hess's Law to calculate enthalpy changes for reactions and use standard enthalpies of formation.
Key Questions
- Explain why enthalpy is a state function, allowing for the use of Hess's Law.
- Construct calculations to determine the enthalpy change of a reaction using Hess's Law.
- Calculate the standard enthalpy change of a reaction using standard enthalpies of formation.
Common Core State Standards
About This Topic
Scientific notation is a way to express very large or very small numbers using powers of ten. In 9th grade, students apply their knowledge of exponent laws to perform calculations with these numbers in the context of science. This topic is a key Common Core standard that connects 'Number and Quantity' to real-world applications in astronomy, biology, and chemistry.
Students learn that multiplying numbers in scientific notation involves multiplying the coefficients and adding the exponents. This makes it possible to compare the scale of a single cell to the scale of the entire Milky Way galaxy. This topic comes alive when students can engage in 'scale simulations' or collaborative investigations where they use scientific notation to solve 'Fermi problems', complex estimation challenges that require thinking in powers of ten.
Active Learning Ideas
Simulation Game: Scaling the Universe
Groups are given objects ranging from an atom to the sun. They must research their sizes, write them in scientific notation, and then 'order' them on a giant classroom timeline of scale, explaining the massive jumps in powers of ten between them.
Inquiry Circle: Fermi Problems
Students work together to solve 'impossible' questions like 'How many grains of sand are on a beach?' They must use scientific notation to make reasonable estimates for each part of the problem and multiply them to find a final answer.
Think-Pair-Share: Significant Figures Check
Give students a calculation in scientific notation. One student performs the math, while the other checks that the final answer is written correctly (with only one digit before the decimal) and discusses how many 'significant figures' should be kept.
Watch Out for These Misconceptions
Common MisconceptionStudents often think that a larger negative exponent means a larger number (e.g., thinking 10^-5 is bigger than 10^-2).
What to Teach Instead
Use the 'Scaling the Universe' activity. Peer discussion about 'place value' helps students realize that 10^-5 means the decimal is five places to the left, making it a much smaller fraction than 10^-2.
Common MisconceptionForgetting to adjust the coefficient after a calculation (e.g., leaving an answer as 15 x 10^4).
What to Teach Instead
Use 'Think-Pair-Share' to reinforce the 'standard form' rule. Students must check each other's work to ensure the coefficient is always between 1 and 10, adjusting the exponent accordingly (e.g., 1.5 x 10^5).
Suggested Methodologies
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Frequently Asked Questions
Why do scientists use scientific notation?
How can active learning help students understand scientific notation?
How do you add or subtract numbers in scientific notation?
What is a 'power of ten'?
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