Chemical Reactions
What are the substances that are present at the beginning of a chemical reaction called?
Reactants
Matter cannot be __________ or __________ during a chemical reaction.
created or destroyed
What do the numbers placed in front of chemical formulas in an equation represent?
The number of particles/molecules/formula units involved in the reaction. These are called coefficients.
What pH value is considered neutral?
pH 7
I am a substance with a pH of 2. I turn litmus paper red. What am I?
An acid.
What are the new substances formed during a chemical reaction called?
Products
What does the Law of Conservation of Mass tell us about the total mass before and after a chemical reaction in a closed system?
The total mass remains the same
Balance this equation:
___ H₂ + ___ O₂ → ___ H₂O
2H₂ + O₂ → 2H₂O
A solution has a pH of 3. Is it acidic, neutral or basic?
Acidic
I contain hydrogen and oxygen. I have two hydrogen atoms for every oxygen atom. What is my chemical formula?
H₂O
Name two observations that could provide evidence that a chemical reaction has occurred.
A reaction starts with 25 g of reactants in a sealed container. What should the total mass of the products be?
25g
Balance this equation:
___ Mg + ___ O₂ → ___ MgO
2Mg + O₂ → 2MgO
What happens to litmus paper when it comes into contact with an acid?
It stays red
A student has an unknown colourless liquid. They add universal indicator and it turns green. What can they conclude?
The solution is approximately neutral, with a pH around 7.
What happens to the atoms and the bonds between them during a chemical reaction?
Bonds between atoms in the reactants break and new bonds form. The atoms are rearranged to form new substances.
A student reacts two substances in an open container. The mass before the reaction is 50 g, but after the reaction they measure only 46 g. Has 4 g of matter been destroyed?
No. Matter has not been destroyed. Some matter, likely a gas, may have escaped from the open container. The mass of the substances remaining in the container is lower, but the total mass is still conserved.
A student tries to balance:
CH₄ + O₂ → CO₂ + H₂O
by changing CH₄ to CH₃. Why is this incorrect?
Changing the chemical formula changes the substance. CH₄ is methane, while CH₃ represents a different molecule. When balancing equations, the chemical formulas must stay the same; only the coefficients can be changed.
An acid and a base react together. What is this type of reaction called, and what are the two main products?
A neutralisation reaction, producing a salt and water.
I start a chemical reaction. I am changed during the reaction and end up as part of a new substance. What am I?
A reactant.
A student says, "When wood burns, the wood disappears, so the atoms in the wood must have been destroyed." Explain why the student is incorrect.
Atoms cannot be created or destroyed during a chemical reaction. The atoms in the wood are rearranged and combine with oxygen to form new substances, such as carbon dioxide and water vapour.
Explain the difference between how conservation of mass is observed in a closed system compared with an open system
In a closed system, matter cannot enter or leave, so the measured mass before and after the reaction remains the same. In an open system, matter can enter or leave, so the measured mass may change, even though mass is still conserved overall.
Balance the following equation:
___ Fe + ___ O₂ → ___ Fe₂O₃
4Fe + 3O₂ → 2Fe₂O₃
Solution A has a high concentration of H⁺ ions, while Solution B has a high concentration of OH⁻ ions. Which solution is acidic and which is basic?
Solution A is acidic because it has a higher concentration of H⁺ ions. Solution B is basic because it has a higher concentration of OH⁻ ions.
A student weighs a container before and after a chemical reaction. The mass is exactly the same both times. Was the container sealed?
Yes, the reaction occurred in a closed system, so no matter could enter or leave. The atoms were rearranged to form new substances, but no atoms were created or destroyed, so the total mass remained constant.