chemical reaction

when chemical reactions occur

occur when 1+ new substances are formed. during a chemical reaction:

- atoms in the reactants r rearranged

- some chemical bonds r broken

- new chemical bonds form

- new substances r produced

sign of a chemical reaction

change in colour/temperature, production of light/sound, disappearance of a solid, formation of a new solid, formation of gas (shown by fizzing or a new odour)

physical changes

• Rule: Do not produce new substances.

• Particles: Same particles remain, but their position, arrangement, movement, or state may change.

Why are changes of state considered physical changes?

Reason: Substance remains chemically the same. Example: Ice → liquid water; the particles are still H20 molecules.

chemical changes

produce at least one new substance. the atoms r rearranged and new chemical bonds form

reactants

substances present @ the start of a chemical equation, written on the left-hand side of an equation

products

new substances formed by the reaction, written on the right hand side.

What happens over time as a reaction proceeds?

• Reactants: Decrease over time.

• Products: Increase over time.

• Monitoring: Measured by checking reactant or product amounts at different times.

how does temperature affect reaction rate

• The Effect: Higher/lower temperature speeds up or slows down the rate products form.

• Core Rule: Changes the rate only—does not change the fundamental identity of the products.

subscripts in chemical formulas

• Definition: Small number within a formula showing the exact number of atoms of that element in one molecule/unit.

• Golden Rule: Part of the formula and must not be changed when balancing equations.

state symbols

• (s): Solid

• (l): Liquid

• (g): Gas

• (aq): Aqueous (dissolved in water)

composition reactions

occur when 2+ reactants combine to form a product (general pattern: A+B→AB; many substances → one combined substance)

decomposition reactions

occur when 1 reactant breaks down into 2+ more products (general pattern: AB→A+B; one substance → multiple simpler substances)

law of conservation of mass

mass is neither created nor destroyed during a chemical reaction. therefore, mass of reactants = mass of products, because atoms r rearranged not created/destroyed

conservation of atoms

because atoms r conserved, there must be the same number of each type of atom on both sides (i.e. 4 carbon atoms in the reactants and 4 carbon atoms in the products)

balanced equations

contains the same no. of atoms of each element of both sides (i.e. 2H₂+O₂ → 2H₂O)

unbalanced equations

consider: H₂+O₂ → H₂O has a different number of oxygen atoms, meaning it does not obey conservation of mass and is unbalanced.

coefficient

the large number placed in front a chemical formula, stating how many molecules/formula of units of that substance r involved.

how to balance an equation

1. write the correct formulas - make sure you know the correct chemical formulas of all reactants and products

2. count the atoms - count each type of atom on both sides

3. add coefficients - put numbers in front of formulas to make the numbers of atoms equals

4. recount - check every element

5. repeat - continue changing coefficients until the number of every atom is equal on both sides.