Skip to Content Go to sign in Skip to Primary Navigation Skip to Secondary Navigation Skip to Page Navigation Skip to Header Navigation Skip to Footer Navigation Read more about accessability options and our navigation

Physics Classroom is making strides to make our site accessible to everyone. Our site contains 6 navigation areas. The Primary, Secondary, and Page Level navigations have a screen reader version of their nav structure that allows using the left and right keys to navigate sibling navigation items, and up or down keys to navigate parent or child navigation items. The others can be navigated using tabs. The Primary Navigation handles the first 2 levels of site pages. The Secondary (which is not always available) handles the 3rd and 4th level of structure. The Page level navigation allows you to navigate the current page's headings quickly. The Header Navigation contains the Light/Dark Mode toggle, Search, Notifications and account login. The Breadcrumb Navigation contains the breadcrumb of the current page. If the current page has a breadcrumb, you can get to it by skipping to the content and tabbing in reverse (shift plus tab). The Footer Navigation contains links such as Privacy, Contact, about and terms. Some resources contain an Audio Player that can be activated by holding down the T key for 3 seconds, and then using K to pause and resume. While not every area of Physics Classroom is usable purely from keyboard and screen reader, we are committed to continue work on making this possible. If you have questions or need additional help, please use this link to contact us.

Return to screen reader navigation
Home Concept Builders Chemical Reactions Writing Balanced Chemical Equations Help Help 15

Writing Balanced Chemical Equations - Questions 15 Help

A balanced chemical equation shows the number of particles of each substance involved in a chemical reaction. Such an equation includes formulas and coefficients. A formula includes symbols for each type of atom in the substance. The coefficients are the numbers present in front of the formulae. If you are given a verbal description of a reaction, then you can identify the reactant and product formulae and write a balanced chemical equation with the formulae and some coefficients.

There are two questions in this Question Group. The two questions are very similar or are the same type of reaction The question below is one of the questions.
 

Version 1:
The sentence below describes a chemical reaction. Write and balance the equation for this reaction.
 
Barium chloride and sodium sulfate undergo a double replacement reaction.

To be successful with this question, you need to understand what a double replacement reaction is, how to write formulas for ionic compounds, and how to balance a chemical equation.

A double replacement reaction involves the reaction of two ionic compounds to form two different ionic compounds. The positive ion in one compound replaces the positive ion in the other compound. When the reaction is over, the cations have replaced each other in the other formula. The generic equation for such a reaction is ...
AB + CD ==> AD + CB
 
Here A, B, C, and D are fictional elements or ions. A and C are the metal elements or positive ions. B and D represent nonmetals or negative ions. You will notice that A was orginally paired with B on the reactant side; but on the product side A is paired with D. Similarly, the positive ion C starts with the anion D on the reactant side but finishes paired with B on the product side of the equation. As such, A and C have switched places. You need to ....

Analyze the two reactants in your equation and identify the formulae of the two cations (positive ions) in the reactants. Also identify the formulae of the two anions (negative ions). Then to determine the products, switch the cations to the other anion. You'd be wise to use some paper, write some formulae down, and take your time. A minute spent writing down some formulae for the cations and anions will both build some good skill and probably save you time and frustration as you attempt to get this question correct. The formulae for the product is going to require some good skill in and of itself. Use the link below to assist with formula writing.

Writing Formualae for Ionic Compounds
 

Understanding this about double replacement reactions and formula writing allows you to get through Step 1 and Step 2 of the question. That leaves Step 3 in which you have to identify the lowest possible whole number coefficients.

The infographic below describes a step-by-step method for balancing a chemical equation. Study the method and then apply it to this question.
 
Concept Builder Teacher Notes Directions