Reactants are on the LEFT of the arrow; products are on the RIGHT.
Before you read
Test yourself first: 2 questions
Question 1 of 2
A chemical reaction that releases heat is described as:
Answer: B. Exothermic reactions release heat to the surroundings; endothermic reactions absorb heat.
Question 2 of 2
A catalyst affects a reaction by:
Answer: A. Enzymes are biological catalysts: they accelerate reactions at body temperature and emerge unchanged, ready to act again.
Keep the streak going with more questions like these, each with a full rationale.
What is reactions cheat sheet?
That single principle — conservation of mass — is what makes an equation need balancing, and it is the reasoning most reaction questions test.
Key points
- Reactants are on the LEFT of the arrow; products are on the RIGHT.
- A balanced equation has the same number of each type of atom on both sides, because matter is conserved.
- A CATALYST speeds a reaction and is NOT consumed by it. Enzymes are biological catalysts.
- Exothermic reactions RELEASE heat; endothermic reactions ABSORB it.
- A chemical change makes a new substance (rusting, burning); a physical change does not (melting, dissolving, cutting).
Study first
- 2H₂ + O₂ → 2H₂O: four hydrogen and two oxygen atoms on the left, and the same four and two on the right. Balanced.
- An enzyme lets a reaction in the body run at body temperature that would otherwise need high heat — and the enzyme is still there afterwards, ready to act again.
Commonly tested
- Count atoms element by element in a column under each side. Balancing becomes bookkeeping rather than guesswork.
- To classify a change, ask one question: is there a NEW substance? If not, it is physical.
Watch out for
- Changing a chemical formula (H₂O → H₂O₂) to balance an equation. You may only change the COEFFICIENTS in front.
- Believing a catalyst is used up; if it were, it would be a reactant.
- Calling dissolving or melting a chemical change — no new substance forms, so both are physical.
Reactions Cheat Sheet: FAQs
What should I focus on first when studying Reactions Cheat Sheet?
2H₂ + O₂ → 2H₂O: four hydrogen and two oxygen atoms on the left, and the same four and two on the right. Balanced. An enzyme lets a reaction in the body run at body temperature that would otherwise need high heat — and the enzyme is still there afterwards, ready to act again.
What is most often missed on Reactions Cheat Sheet?
Changing a chemical formula (H₂O → H₂O₂) to balance an equation. You may only change the COEFFICIENTS in front. Believing a catalyst is used up; if it were, it would be a reactant. Calling dissolving or melting a chemical change — no new substance forms, so both are physical.
What do I need to know about Reactions Cheat Sheet for the HESI A2?
Reactants are on the LEFT of the arrow; products are on the RIGHT. A balanced equation has the same number of each type of atom on both sides, because matter is conserved. A CATALYST speeds a reaction and is NOT consumed by it. Enzymes are biological catalysts. Exothermic reactions RELEASE heat; endothermic reactions ABSORB it.
What are the most commonly tested points on Reactions Cheat Sheet?
Count atoms element by element in a column under each side. Balancing becomes bookkeeping rather than guesswork. To classify a change, ask one question: is there a NEW substance? If not, it is physical.
